FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: 338Raptor on June 28, 2020, 12:26:12 AM
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Need some opinions on what to try next.
To begin, I think my accelerator pumps (both 50cc, yellow and brown cams) and nozzles (both 40’s) are sized right. When I quickly smash the throttle 1/2 way and all the way my AFR stays at about 13 to 1.
At WOT my AFR is a bit rich at 12.2 to 1. I’m not concerned about correcting this until I get my tip in problem resolved (see below).
Details about the carb:
780 double pumper
73 primary, 78 secondary jets
4.5 power valve. But I’ve also tried 5.5 and 6.5 with similar results.
PVCR’s drilled to .0625”
10” vacuum at idle, transition slots are not uncovered. Idle = 1000rpm
Burning non-ethanol premium fuel
Float levels are set to mid window. I started with level at bottom of the window and raising it midway did help a bit.
Motor details:
10:1 compression.
Iron 427 SOHC 4V (500ish Horsepower)
Close to stock 1967 NOS cam specs. (Crane says operating range is 2500-7000 rpm)
2800 lbs street car
Here’s my problem:
The engine goes lean (16-17 to 1) when slowly increasing RPM at about 1/4 throttle and doesn’t richen up until I quickly press into the secondaries. If I try to slowly increase throttle beyond 1/4 throttle the lean condition causes snapping and popping (bad noises).
Anything less than 1/4 throttle (cruising) is a bit too rich (12-13 to 1).
When I go WOT I’m a bit too rich (12.2 to 1) I can easily fix this but I think I need to focus on the 1/4 throttle “too lean” issue first.
My first inclination is to increase primary jets and decrease secondary jets. If I do this I’ll end up with close to the same size jets at all 4 corners with a power valve on the primary side only. Does this sound normal?
Should I be looking at increasing or decreasing emulsion jets?
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First, this is just my opinion after working with Holley carbs for the better part of 50+ years. A 780 cfm carb does not need 50 cc pumps for the street, nor does it need 40 squirters. You are causing the jetting to be totally wrong by covering up the off throttle with the large squirters and pumps and not getting the carb metering blocks to work properly. By running rich with the squirters and pumps you are having to lean the carb down too much, which is causing your lean condition at part throttle. I would prefer to start with gong back to 30cc pumps, 31 squirters, and 72/82 jetting and work from there to find a good street tune. Power valves should be a part of the final tune for street. Joe-JDC
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Similar to Joe. Find the specs for the list number and get it back to std. Tune from there. Make sure your ignition timing is what it needs to be first
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Add a vacuum gauge to your tuning kit,you’ll be amazed what you can learn about the carb circuits your running in.
If you want to play with some math, calculate the area of the PVCR and Main jet area and add them together. The total should be close to the area of the secondary jet.
That said your slow rolli in lean condition,I’m assuming your just rolling in here, that is working the Idle feed restrictor. To test richen the idle mixture and test again,it also is working the transition circuit .
Double pumpers struggle in this area when trying to tune for economy and day to day drivability, adding (dumping) fuel in this zone is the normal fix.
I would remove carb and clean/blow out every passage if the air bleeds are dirty that causes all kinds of weird problems,remove mixture screws too if they are ridged your going to have trouble.
You may end up with another carb to really tune that zone.
I read years ago that the last great tuned Holley carb was the OEM 780 on the 73 ‘Vette Tuned for drive ability ,not necessarily WOT. I once tuned a BG Demon 850 vacuum carb with the old 780 primary bleed and channel restrictions setting as a starting point and ended up with 433 FE that got 18mpg at 70-80mph 2400-2600rpm in my Mustang. My current FI set up gets 16 at best.
We’ve had good luck with Quick Fuel carbs but I fear Holley has bought them so things might have changed.
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Lance, 780’s have different venturi size for the secondaries, so calculating area doesn’t really work for this particular carb.
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Lance, 780’s have different venturi size for the secondaries, so calculating area doesn’t really work for this particular carb.
Certainly can calculate the area versus the original specs though, given a LIST number and before and after PVCRs
That being said, agree with all above, no need for 50cc and sewer pipe squirters. There are a few 4150 780s, so if you post your LIST number we can give you original specs to start from. My gut says dual 30cc pumps on the street, blue cam on the front if you need some oomph and 25s for squrters, 5.5 or 6.5 PVsr
I will say this, although I think the carb needs some un-loving. It doesn't seem that crazy that the engine would go backfire and popping lean. Are you 100% sure no crossfire, crossed wires, vacuum leak, tight intake valve, etc?
Also, is it a backfire through the carb (lean), or bangs in the primary pipe (rich)
I think I would run the valves, look at all the wires and plug insulators, double and triple check firing order, and be very sure you don't have a big vacuum leak somewhere, because although the ports are big on a SOHC, your build shouldn't need a crutch like that.
Another option would be to see if some buddies good running carb does the same thing without messing with anything other than idle speed
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Lance, 780’s have different venturi size for the secondaries, so calculating area doesn’t really work for this particular carb.
Certainly can calculate the area versus the original specs though, given a LIST number and before and after PVCRs
That being said, agree with all above, no need for 50cc and sewer pipe squirters. There are a few 4150 780s, so if you post your LIST number we can give you original specs to start from. My gut says dual 30cc pumps on the street, blue cam on the front if you need some oomph and 25s for squrters, 5.5 or 6.5 PVsr
I will say this, although I think the carb needs some un-loving. It doesn't seem that crazy that the engine would go backfire and popping lean. Are you 100% sure no crossfire, crossed wires, vacuum leak, tight intake valve, etc?
Also, is it a backfire through the carb (lean), or bangs in the primary pipe (rich)
I think I would run the valves, look at all the wires and plug insulators, double and triple check firing order, and be very sure you don't have a big vacuum leak somewhere, because although the ports are big on a SOHC, your build shouldn't need a crutch like that.
Another option would be to see if some buddies good running carb does the same thing without messing with anything other than idle speed
List: 14936
The guy I bought the car from said it’s a 780. That may be incorrect. I thought List numbers started with an R, this one doesn’t.
1. No vacuum leaks unless the one vacuum port hooked to the PCV valve counts.
2. Firing order is correct. It runs great otherwise.
3. Timing is at 29 total. Maybe it’s coming in too early?
4. I know the 50cc pumps and 40 squirters are big but watching my wide band the AFR stays perfectly steady at approx 13 to 1 when quickly smashing the throttle. It spiked 18 to 1 for 1/2 a second with the 30cc pump.
5. The snapping/popping noise occurs when AFR hits 18 or 20 to 1, not when rich.
6. Carb has been disassembled, cleaned, holes blown out etc. I’ve done this 2x.
7. Motor runs good at WOT and Cruising and smash throttle hard acceleration. It runs super lean when slowly easing into the throttle under light load (when the accelerator pumps aren’t a factor).
8. I haven’t checked valve lash. I need to do that anyway. I’ll do that next.
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29* total timing(that includes mechanical advance?). Why only 29*?
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29* total timing(that includes mechanical advance?). Why only 29*?
Cammer?
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Yes. Only 29.
1967 iron Cammer.
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Oops, sorry, skimmed over the details a bit too fast I suppose.
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Lance, 780’s have different venturi size for the secondaries, so calculating area doesn’t really work for this particular carb.
Certainly can calculate the area versus the original specs though, given a LIST number and before and after PVCRs
That being said, agree with all above, no need for 50cc and sewer pipe squirters. There are a few 4150 780s, so if you post your LIST number we can give you original specs to start from. My gut says dual 30cc pumps on the street, blue cam on the front if you need some oomph and 25s for squrters, 5.5 or 6.5 PVsr
I will say this, although I think the carb needs some un-loving. It doesn't seem that crazy that the engine would go backfire and popping lean. Are you 100% sure no crossfire, crossed wires, vacuum leak, tight intake valve, etc?
Also, is it a backfire through the carb (lean), or bangs in the primary pipe (rich)
I think I would run the valves, look at all the wires and plug insulators, double and triple check firing order, and be very sure you don't have a big vacuum leak somewhere, because although the ports are big on a SOHC, your build shouldn't need a crutch like that.
Another option would be to see if some buddies good running carb does the same thing without messing with anything other than idle speed
List: 14936
The guy I bought the car from said it’s a 780. That may be incorrect. I thought List numbers started with an R, this one doesn’t.
1. No vacuum leaks unless the one vacuum port hooked to the PCV valve counts.
2. Firing order is correct. It runs great otherwise.
3. Timing is at 29 total. Maybe it’s coming in too early?
4. I know the 50cc pumps and 40 squirters are big but watching my wide band the AFR stays perfectly steady at approx 13 to 1 when quickly smashing the throttle. It spiked 18 to 1 for 1/2 a second with the 30cc pump.
5. The snapping/popping noise occurs when AFR hits 18 or 20 to 1, not when rich.
6. Carb has been disassembled, cleaned, holes blown out etc. I’ve done this 2x.
7. Motor runs good at WOT and Cruising and smash throttle hard acceleration. It runs super lean when slowly easing into the throttle under light load (when the accelerator pumps aren’t a factor).
8. I haven’t checked valve lash. I need to do that anyway. I’ll do that next.
That LIST number doesn't match anything in my books, even with or without an R
As far as those accel pumps, don't let yourself be talked into that they are helping. Needing that much, you are likely not getting any transition fuel. Maybe the wrong gasket on the base plate between it an the throttle body? Could be blocking transition fuel. The 50cc pump combined with big shooters are a symptom, not a solution.
In the end, when you get that carb right, the accel pumps just need to give a tickle, maybe 1/2 of what you are jamming down there now. All your behavior seems to indicate an issue with the transition from idle to main jet. Need to figure out why that is happening before you just bathe all those parts in fuel
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Now that makes sense. I’ll check my base gasket.
I don’t think my transition slots are providing fuel.
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Like Ross said, the transition circuit seems to not be working.
Has this carb been drilled ?
If it has been “worked” you might save your sanity by punting that carb for a new one.
Having only touched a non running SOHC I don’t know what they want for a carb.
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Certainly can calculate the area versus the original specs though, given a LIST number and before and after
Sorry, should have been more clear. Just mentioning that primary and secondary have different requirements.
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I don’t know what had been done to the carb before I got it. I know the motor ran way worse than it does now. It had 80/87 jets in it and 35 nozzles. It ran 10 to 1 AFR at WOT. It also stumbled really bad at initial throttle, that’s why I increased the nozzle size to 40’s. It doesn’t stumble now but that’s probably making up for the fact the transition slots aren’t working.
When trying to get more mid throttle fuel I drilled the PVCR’s .002” over what I thought was standard. But now I’m unsure if I really know what this carb is without a List number. So I may have drilled them larger than I intended. If necessary I can epoxy and redrill smaller.
I’ll check the base gasket and if that doesn’t fix it I’ll be in the market for a new carburetor.
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i bought a 950 a few years ago,used.when i put it on my car and cranked it up it sounded awesome,but when i took off down the road it fell flat on its ass.he had 70s up front and 72s in the secondary with no power valves either place. i put 80s up front with a power valve and 88s in the secondarys with the power valve blocked.it screamed after that.
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I don’t know what had been done to the carb before I got it. I know the motor ran way worse than it does now. It had 80/87 jets in it and 35 nozzles. It ran 10 to 1 AFR at WOT. It also stumbled really bad at initial throttle, that’s why I increased the nozzle size to 40’s. It doesn’t stumble now but that’s probably making up for the fact the transition slots aren’t working.
When trying to get more mid throttle fuel I drilled the PVCR’s .002” over what I thought was standard. But now I’m unsure if I really know what this carb is without a List number. So I may have drilled them larger than I intended. If necessary I can epoxy and redrill smaller.
I’ll check the base gasket and if that doesn’t fix it I’ll be in the market for a new carburetor.
I think that's a good call. Sometimes carburetors get so much "super tuning" done to them you need to basically replace most of it to get back to the starting point.
Hopefully it was just a throttle body to base gasket blocking the transition circuit but if it isn't, I'd go with something nice. Considering the engine underneath it, it's worth a good carb
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I found more info on the carb.
It’s 860 CFM built by Pro Systems. It is built on a Holley HP format with some billet parts. They sent me all the specs so I can go back to a starting point after I install new gaskets. Hope to have the gaskets installed and road tested later today.
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I found more info on the carb.
It’s 860 CFM built by Pro Systems. It is built on a Holley HP format with some billet parts. They sent me all the specs so I can go back to a starting point after I install new gaskets. Hope to have the gaskets installed and road tested later today.
Did you find the incorrect gasket in there?
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No. The gasket was correct, all the holes lined up and all the transfer slots look good with no obstruction. I disassembled air bleeds and everything and blew it out with carb cleaner. No problems found. I’ll test drive it tomorrow but I seriously doubt anything will change. I put all the jets, pump cams, etc back to the original tune from the carb shop. I also verified all the orifice sizes and they all checked out.
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Nice work, hopefully it comes together.
How different was the original setup compared to yours, other than the accel pumps and shooters?
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Problem still exists.
The original tune was the same tune I started with.
I triple checked every port/slot/orifice and made damn sure everything was sealing and there where no cracks or leaks. I’m 99.99% confident everything is assembled correctly.
This is the result (same as before)
1. Idle is good at 1000 rpm and 11” vacuum at 13.7 to 1. Four corner idle mixture adjustment (1 +1/8 full turns out) and all 4 throttle plates are just barely exposing the transfer slots. This is where it has the most vacuum)
2. When I smash the throttle from idle it goes lean 20-1 for a split second. Like the squirters aren’t providing enough fuel to compensate for the blast of air. (Running black and orange cams)
3. WOT is 10.5 to 1. Still too rich. (Primaries and maybe secondaries too large)
4. Cruising is 12 to 1. Still too rich. (Primaries too large)
5. Roll into the throttle from cruising speed and it goes super lean 17-20 to 1. When this happens it snaps and crackles for 1-2 seconds then goes rich when I hit WOT.
6. I’ve tried 4.0, 5.0, and 6.5 Standard flow power valves. Same result.
It’s like the power valve isn’t richening it up under load. PV vacuum port is clear and flows freely. All PV ports are clear and PVRC is .0600 like the carb manufacturer recommended.
Carb manufacturer recommending sending to them for rebuild but that will be a 2 week turn around. I would like to avoid that.
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Problem still exists.
The original tune was the same tune I started with.
I triple checked every port/slot/orifice and made damn sure everything was sealing and there where no cracks or leaks. I’m 99.99% confident everything is assembled correctly.
This is the result (same as before)
1. Idle is good at 1000 rpm and 11” vacuum at 13.7 to 1. Four corner idle mixture adjustment (1 +1/8 full turns out) and all 4 throttle plates are just barely exposing the transfer slots. This is where it has the most vacuum)
2. When I smash the throttle from idle it goes lean 20-1 for a split second. Like the squirters aren’t providing enough fuel to compensate for the blast of air. (Running black and orange cams)
3. WOT is 10.5 to 1. Still too rich. (Primaries and maybe secondaries too large)
4. Cruising is 12 to 1. Still too rich. (Primaries too large)
5. Roll into the throttle from cruising speed and it goes super lean 17-20 to 1. When this happens it snaps and crackles for 1-2 seconds then goes rich when I hit WOT.
6. I’ve tried 4.0, 5.0, and 6.5 Standard flow power valves. Same result.
It’s like the power valve isn’t richening it up under load. PV vacuum port is clear and flows freely. All PV ports are clear and PVRC is .0600 like the carb manufacturer recommended.
Carb manufacturer recommending sending to them for rebuild but that will be a 2 week turn around. I would like to avoid that.
OK, so first, don't deduce anything, right now, let's use your post as gathering data. With all love and kindness, you seem to provide data with your decision already, let's just look at the data.
1 - Idle - You idle OK, but on a 4 corner idle circuit, generally you don't go 1 1/8 out like a primary only. That starts to tell me that maybe you could use a little bigger IFR, that would bring the screws in and more importantly, feed the idle and transition with more fuel. However, before we determine that, what is your IFR size?
2 - Throttle smash - you have a delay where it goes lean. How do you adjust your accelerator pump? Also, is it an umbrella or steel ball check valve?
3 - Ignore WOT for now, that will be the easy one
4 - What changes did the PV make, is 6.5 less lean than 4.5?, and when you are cruising, what is the vacuum?
5 - What are your current HSAB sizes?
6 - What if any markings are there on your throttle blades?
7 - What is the full setup of the carb now, jets, accel pump shooters, cams, PV value
Shoot those answers back, but I will ask you one more. If you grab the boosters, are they loose?
Something is happening here, it's either a lack of transition fuel or a booster that isn't starting when it should. Don't make any changes yet let's keep picking.
One last repeat, do you have any other known carb even if too small, that you can throw on there just to verify that it is the carb? Heck I have a known good 3310 if you need one to try, cheapo, not pretty, but could loan it if you promise not to go inside of it :)
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I have an 850DP I’ll put on it for a test.
I’ll clean it up and put new gaskets in it first. It’s been sitting for 20 years.
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An easy test but not 100% all the time back all the mixture screws out 1/2 turn more and drive,you goal is to roll in the throttle not stab it. See if it pulls smother.
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I’ll try that but I think the problem is after the transition. After the throttle plates have already uncovered the slots and the idle ports are not a factor.
But I’ll try anything.
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Work the throttle slow, you need to try to separate the circuits ,slamming the throttle open will not get the carb tuned you need to work it from bottom to top. Idle-WOT.
By richening the idle mixture you should see a change,you would be surprised how long a Holley is running in the transition circuit.
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Opened up the idle screws 1/2 turn and no difference other than 12 to 1 AFR at idle instead of high 13’s. It still has a super lean spot under light load (1/3 to 1/2 throttle). Everywhere else it’s still too rich. It acts like the Power Valve circuit isn’t working at all. I’ll put on my other carb and see how that works.
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I once fought a 3310 Holley with a lean spot in it like that and I eventually found that I had a gasket that blocked one of the fuel passages. Acted just like what you are seeing. Another one had something inside the metering block that had some crud or something in it. Used some aluminum cleaning acid in that circuit to “boil it out”
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I pulled a 26 year old down leg 850-DP out of my collection of dusty/dirty parts I keep around “just in case”. I cleaned it up, blew out all the orifices, installed new gaskets, PV’s, acc pumps, etc and put it on my engine. The engine runs perfectly now, no more lean spot.
So obviously there must be something wrong inside the fancy annular booster custom 850-HP. I’ll be sending it back to the carb builder. Hopefully he can find and fix the problem.
If nothing else, I now have a very good understanding of how all the circuits in a carburetor work.
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A lot of over engineering happens at carb shops.
I think I spend the majority of my time just setting stuff back to the original setting. Not that original is perfect, but it’s a much closer starting point
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I pulled a 26 year old down leg 850-DP out of my collection of dusty/dirty parts I keep around “just in case”. I cleaned it up, blew out all the orifices, installed new gaskets, PV’s, acc pumps, etc and put it on my engine. The engine runs perfectly now, no more lean spot.
So obviously there must be something wrong inside the fancy annular booster custom 850-HP. I’ll be sending it back to the carb builder. Hopefully he can find and fix the problem.
If nothing else, I now have a very good understanding of how all the circuits in a carburetor work.
Well, both knowing it is the carb and the lesson are big wins in my book. Lot's of guys don't like the 4781 DPs, but I enjoy them. I generally pull the big pump off the back (if it has them), lose the rear PV and jet accordingly and they are happy. Apparently venturi design slows booster responsiveness, but, I have really seen them work well on the street. Hopefully they can figure out what's killing the transition on the big dollar carb
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I ran a 4781 on my drag race Boss 302 for thirty years! It was VERY predictable and rarely needed any changes despite varying quality of engines underneath it. My contemporaries ALL said a 750dp would out run it but NONE did. I dumped the rear power valve too but kept the big pump . I too still have it "just because".
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I heard back from the carb builder today. His initial answer, without seeing the carb, is that the carb was calibrated for non-ethanol fuel of 17 years ago. He thinks modern non-ethanol fuel is different enough to cause the problems I experienced. Personally I don’t believe his theory but I’m sending it back to him for rebuild/recalibration anyway. After I get it back and get it fine tuned to my engine I will do a chassis dyno comparison between the two carburetors. Should be very interesting to see the drivability differences and actual power difference. I’ll update this thread after I get results.
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"Ethanol" laced fuels do require a bit richer calibration. Older #1850 Holleys especially.
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I heard back from the carb builder today. His initial answer, without seeing the carb, is that the carb was calibrated for non-ethanol fuel of 17 years ago. He thinks modern non-ethanol fuel is different enough to cause the problems I experienced. Personally I don’t believe his theory but I’m sending it back to him for rebuild/recalibration anyway. After I get it back and get it fine tuned to my engine I will do a chassis dyno comparison between the two carburetors. Should be very interesting to see the drivability differences and actual power difference. I’ll update this thread after I get results.
Hogwash LOL you were rich and lean depending on the circuit, regardless, hopefully he can get it worked out
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2X on hogwash. 100% gas or 90% gas, it should run fine on either.
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I think I found the problem.
The 4-1/4” Sidepipe mufflers have a 2” diameter hole thru the baffles. So I have 2” primary tubes flowing to a 3-3/8” collector to 2” ID mufflers. No wonder I can’t get the motor to run right at high rpm. It runs great down low. I’ll be replacing my Sidepipe mufflers ASAP.
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I think I found the problem.
The 4-1/4” Sidepipe mufflers have a 2” diameter hole thru the baffles. So I have 2” primary tubes flowing to a 3-3/8” collector to 2” ID mufflers. No wonder I can’t get the motor to run right at high rpm. It runs great down low. I’ll be replacing my Sidepipe mufflers ASAP.
No doubt sidepipes steal power, sometimes a LOT, so good call, but didn't the carb swap make it run better?
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Much more often than not, a misbehaving Holley can be spanked by un-experting whatever has been changed, a good cleaning, a genuine Holley Renew Kit, and proper adjustment of the pump(s), secondary throttle blade setting and idle screws. At that point it should run reasonable well, and quite well if it's the proper one for the application. Got a mild 390 in a car or 4x4? Plain Jane #1850 is hard to beat. Put a 4781 DP on it? Not so much. Chasing problems with an "experted" carb that has mods, by making more mods usually just results in tail-chasing. Un-f#$k it first, might get a nice surprise
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It runs great with the 850DP. But is does begin losing power after 5000 rpm. I’m sure the 2” exhaust pipe is responsible for that.
Below 5k all is well.
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Gene nailed it.
I hear “wow dunno what you did but it runs great now”
And most times all I did was put it back to factory spec
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It’s definitely not at factory spec.
It started there, but now:
74 primary (stock 78)
75 secondary (Stock 78)
6.5 primary power valve
Plugged secondary PV (stock 6.5)
PVRC .0620 (stock .0665)
35 primary nozzle (stock cam, don’t remember the color)
31 secondary nozzle (stock cam, don’t remember the color)
Idle/low speed jets are .027 stock, next change is to try .026
I tried .0247 and it was a little too lean. I think .026 will be perfect.
I’m going to wait for the new mufflers before I do any more tuning. The new mufflers have a 3” flow path so I’m sure the carb will need completely retuned.
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Idle feed restriction is .027??
I highly doubt that.
For a 750 I’d start with a ifr/iab like .033/.067
For an 850 more like .035/.070
.027 would be lean in a 600 on your engine.
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That carb seems to be the craziest conglomeration of changes. To be honest, if I couldn't go back to pure stock on the LIST number because of booster changes or something like that, I'd run the 850. It's a decent match anyway
As far as the exhaust, the core change should help, but it'll be loud! In the end, Cobra headers usually don't work well anyway, but it doesn't make them flatten out badly.
Have you verified that you are getting full ignition advance, full throttle and are you confident in your technique in degreeing of the cams, both in process and desired centerline? Any of those would flatten it out above 5000
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Plugging the secondary power valve AND pulling four jets out is a really radical lean move. Dont know what the secondary PVCR is, but that restriction in the exhaust might be the only thing keeping the pistons inside of the engine.
The "dig" on the old 4781 850s is/was a soft mid range. The reality is that they are low on signal due to the booster/venturi relationship. This makes them nonresponsive to the normal "jet change" mentality. You might not even feel a two jet change because the draw is softer.
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That is why we all love the 4780 so much
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That’s what it is.
Idle feed is 27 on the stock 850DP. I bought it new 25 years ago and didn’t modify it until now.
Idle feed was 33 on the custom 850HP, it’s back at the builder now.
I didn’t degree the cams (original builder did that) so it’s possible that could be an issue.
I’m getting full advance and full throttle. AFR at WOT is 12.7 to 13.0. I only have one wide band so I’m only reading 1 side of the engine. I’m assuming the other side is doing the same thing.
As a reminder I’m Burning non-ethanol.
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That’s what it is.
Idle feed is 27 on the stock 850DP. I bought it new 25 years ago and didn’t modify it until now.
Idle feed was 33 on the custom 850HP, it’s back at the builder now.
I didn’t degree the cams (original builder did that) so it’s possible that could be an issue.
I’m getting full advance and full throttle. AFR at WOT is 12.7 to 13.0. I only have one wide band so I’m only reading 1 side of the engine. I’m assuming the other side is doing the same thing.
As a reminder I’m Burning non-ethanol.
I thought the problem carb was a 780? Did I mix things up? If so, in general (lots of things play into it) an 850 would be about 80/88 jetting without a rear PV.
IMO I think you are putting more stock into the fuel blend than really matters. Even ethanol blends vary widely, and don't cause the craziness you had been seeing, and although there can be some jetting required when alcohol is involved LOL I have never heard anyone complain that new versus old non-ethanol caused issues.
With that being said, I realize I am a comment or two from you telling me to butt out, but as a long time carb guy who dove pretty deep into the HP series, I feel your pain. They can be funky, but when it comes back, remember that fixing them with pump shot is not where you want to be. I also think you have to remember that your "old shelf carb" which was designed for old fuel, did pretty good as-is, that's a good indicator that it is a carb problem, not a fuel problem.
As far as the cams, Jay can talk MUCH smarter on this than me, but due to the design of the SOHC valve train, cams are much milder than the numbers seem in a SOHC. If your cams are milder and advanced, that could be killing off the top end charge (assuming you don't have some odd air filter choking it or the lid too close to the carb)
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When I started the thread I thought the custom carb was a 780 because that’s what the original owner told me. After a little research I learned it was a custom 850-HP. After a dozen unsuccessful adjustments I pulled that carb and installed a 25 year old 850 double pumper. Both carbs started on the engine is stock form.
The 850-HP I started with (that I originally thought was a 780) had the builders original settings in it and it would run 20 to 1 lean at mid throttle then 10 to 1 rich at WOT. I pulled this carb and sent it back to the builder for rebuild/recalibration.
Next I installed my rebuilt/cleaned out totally stock 850 double pumper that I originally purchased 25 years ago. This carb ran a little lean at part throttle and less rich at WOT. After pulling the secondary PV and changing some jets it ran MUCH better than the first carb ever did. After the changes indicated a few comments ago it is now running 12.7 to 13.0 at WOT, idling at 13.5, cruising at 13 to 1 (this is why I’m interested in dropping the idle jet 1 number), and mid throttle (just before PV opens) at 14.5 to 1. Then 12.2 to 1 when PV opens.
As you can see none of these numbers are lean despite all the small jet sizes.
However, after all this, the motor pulls strong up to 5000 RPM then feels held back. RPM climb slows down and the sensation of acceleration seems to slow down.
I’m thinking the 2” restriction in my side pipes may fully explain this. The cam timing could be a huge part but I’ll know more after I replace the side pipes. New pipes are ordered and will be on the car soon. I also anticipate the new exhaust may let me put the carburetor tune back to normal. I hope.
Basically I’m going to stop chasing my tail until the new less restrictive exhaust is installed.
I just mentioned I’m running non-ethanol fuel because I thought some where thinking the variations in ethanol content was complicating my tune.
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As I think I said above,before doing “jet” changes to correct low speed low load cruising I lean the mixture screws 1/2 turn then test. The mixture screws do have some effect on low throttle angle Cruise and early transition circuit operation. With fairy large engines with strong pull on the mains you could be past the transition and be in the main, conversely you may not start pulling on the main till higher than expected throttle angle and RPM. This light load tuning area is where a vacuum gauge really helps.
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I didn’t mention it but I did what you suggested. Turned the idle screws in 1/2 turn and no difference in how it behaved and nothing noticeable with AFR while driving. The only change was the Poor idle
I ran with my vacuum gauge attached and got a pretty good idea what readings where at different events. When the engine is running super lean at mid throttle it had approx 10” or more vacuum. Idle vacuum = 11. Cruise vacuum is 11-20 depending on load. And I could see the PV open at exactly 6.5.
I checked my timing, it was 10 at 850rpm, 12 at 1000, and 32 at 3000 rpm.
And I checked my valve lash. It was .018 and .022 cold. None needed readjusted.
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That seems like really low timing numbers, especially at 3000...
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How high does one normally run the timing in a SOHC?
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Not enough data to give any sort of hard recommendation, but the 32 number feels about 6 or more degrees low. The 32 is what I might expect for a modern, efficient chamber - the SOHC ain't that...
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29 degrees total is all my iron headed, 10.3 to 1 SOHC engine will tolerate on our premium pump fuel, which is equivalent to 93 octane in the US.
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Where do they usually make max power on the dyno?
Could be too little but too much initial and comes in too quickly if pinging on 29. Seems to me that it's a big open chamber with little obstruction, so shouldn't need a lot of timing, but never worked with the hemi style chambers