Author Topic: rich-lean issues  (Read 2914 times)

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fryedaddy

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rich-lean issues
« on: April 02, 2019, 11:40:14 AM »
i have a 850 dp carb.it has #80 jets front and back.problem is its running rich at low speeds and at wot it leans out due to the gas going to back of the carb at wot.i ordered a jet kit and ox.senser.im thinking if i install a set of extentions on the secondaries and block off the rear power valve it may help.how many sizes would i need to go up for the extentions and power valve block.i heard to go up 4 sizes for the extentions.and 4-8 for the power valve block.i think i will try 76s in the front with a pv,and 92s on the rear with no pv with extentions,and then go from there.any thoughts
1966 comet caliente 428 4 speed owned since 1983                                                 1973 f250 ranger xlt 360 4 speed papaw bought new

fryedaddy

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Re: rich-lean issues
« Reply #1 on: April 03, 2019, 03:59:50 PM »
made the changes,drove for 30 min and hopped out and checked 4 or 5 plugs.they looked great,no fouling,no lean burn.think im good for now,i also ran through the gears 2 or 3 times with no stumble.i should have put ext. on the rear jets a long time ago
1966 comet caliente 428 4 speed owned since 1983                                                 1973 f250 ranger xlt 360 4 speed papaw bought new

My427stang

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Re: rich-lean issues
« Reply #2 on: April 03, 2019, 05:07:32 PM »
Glad to hear it worked, FYI, standard jetting to start with a 4781 with no rear PV is 80 primary/88 secondary if you decide to experiment
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Ross
Bullock's Power Service, LLC
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, Erson SFT cam, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 461 cid FE, headers, Victor Pro-flo EFI, Comp Custom HFT cam, 3.50 9 inch

fryedaddy

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Re: rich-lean issues
« Reply #3 on: April 03, 2019, 06:02:28 PM »
FYI, standard jetting to start with a 4781 with no rear PV is 80 primary/88 secondary if you decide to experiment
would that be with or without jet extensions on the secondaries.i bought a set of them.
1966 comet caliente 428 4 speed owned since 1983                                                 1973 f250 ranger xlt 360 4 speed papaw bought new

My427stang

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Re: rich-lean issues
« Reply #4 on: April 03, 2019, 08:25:41 PM »
I wouldn't change jetting just for extensions. 80/88 assumes a good fuel source
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Ross
Bullock's Power Service, LLC
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, Erson SFT cam, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 461 cid FE, headers, Victor Pro-flo EFI, Comp Custom HFT cam, 3.50 9 inch

Jim Comet

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Re: rich-lean issues
« Reply #5 on: April 03, 2019, 10:16:26 PM »
I pulled the jet extension out of my carb while on Jays dyno and it seemed to not make a difference. I am going to put them back in for strip testing later this spring. Jim

fryedaddy

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Re: rich-lean issues
« Reply #6 on: April 03, 2019, 11:48:16 PM »
i cant remember where i heard to go up 4 sizes when using extensions.a dyno dont slam all the gas to the rear of the carb like in a car.i just dont want it to lean out because of starvation.if most drag or street drag racers don't fool with them,i can leave them off.i haven't much experience with them.
1966 comet caliente 428 4 speed owned since 1983                                                 1973 f250 ranger xlt 360 4 speed papaw bought new

My427stang

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Re: rich-lean issues
« Reply #7 on: April 04, 2019, 05:50:57 AM »
I think you are mistaking the extensions for when you take a PV out.  80/88 is a good start, it won't be lean, if anything, it will be slightly rich depending on the size of the original power valve restriction.

I am not sure what exactly you are asking now though but to go through my entire deal

1 - 80/88 with a PV in the front, a PV plug in the back
2 - Make sure the PV plug doesn't hit the main body when you assemble, some need a little ridge ground off because they hit the main body
3 - I think removing the rear PV is good, certainly not a must, but it gives a consistent fuel flow when the engine is working the hardest.
4 - The jet extensions are good if you need them, but you generally need a new float to make sure the float can travel fully
5 - The jet extensions should not make it run leaner, in fact, it should get the jet deeper in the fuel
6 - Finally, remember, float level is a tuning tool, you can go up and down a little to change head pressure at every circuit. It doesn't do a lot, but it is an adjustment in itself.
7 - Make sure you have good fuel flow to the carb, pressure doesn't need to be high, but good filters, properly size lines, etc are critical if you are running hard

Hope this helps 

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Ross
Bullock's Power Service, LLC
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, Erson SFT cam, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 461 cid FE, headers, Victor Pro-flo EFI, Comp Custom HFT cam, 3.50 9 inch

Barry_R

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Re: rich-lean issues
« Reply #8 on: April 04, 2019, 06:42:43 AM »
The above is generally good guidance.

But I should point out that adding jet to replace a removed power valve is not really a "use this many more" sort of deal.  Its an area calculation based on the diameter of the PVCR - Power Valve Channel Restriction - that you are making up for.  The fuel flow is determined by the flow capability of the main jets plus the capability of the PVCR restrictions whenever the power valve is open.  On most carbs the PVCR is modest in size (.030s...) but they are really large in some applications (some Dominators were .090+) and would take a ton more jet to replace.

Changing PVCR is a very useful tuning tool on street applications, but pretty much nobody does it because they can be a real pita to alter and change for somebody just getting started on carb stuff.  How many times do you hear something about a car being too rich driving around but going kind of lean at WOT - or the opposite?  Altering the PVCR relationship can address that issue.

Falcon67

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Re: rich-lean issues
« Reply #9 on: April 04, 2019, 09:05:28 AM »
i cant remember where i heard to go up 4 sizes when using extensions.a dyno dont slam all the gas to the rear of the carb like in a car.i just dont want it to lean out because of starvation.if most drag or street drag racers don't fool with them,i can leave them off.i haven't much experience with them.

#1 - never heard of - and have never done it - using more jet just because you added extensions.  Not required IMHO.

Every carb I have uses extensions and is jetted based purely on MPH at the track.  If it goes on a race or street/strip car, the back end gets a notched float, extensions and as-built jet until testing says different.  Typically I end up going up 2 jets in the back, maybe one in the front but not always.  I have messed a bit with air bleeds on main bodies that allow changes, usually just to lean the idle/off idle a bit.  Once I get the MPH dialed in, I pretty much donna-care about "street manners".  4000+ stall and 4.56 or so gears makes that a so what.  If it's a street/street car, then it gets an Edelbrock 1406 with electric choke, a dual plane intake and we're all done.

Side note - if you gave me an 850, I'd sell it and buy something else.  Never met one that worked all that well without a lot of fiddling.  750 or go up to a 950 HP.  Last one I messed with belonged to a guy in the car club.  We tinkered a bit, then I gave him my spare 750 DP.  Car picked up 1 full second in the 1/8 and 5 MPH.  He sold the 850 and I liked to never got my 750 back LOL.
« Last Edit: April 04, 2019, 09:08:05 AM by Falcon67 »

fryedaddy

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Re: rich-lean issues
« Reply #10 on: April 04, 2019, 09:21:30 AM »
i bought a 950 off a guy,he had blocked both powervalves and it had mid 70s size jetts in it too.it idled good but every time i hit the gas it starved and stumbled.i had  it apart 6 times before i got rid of the stumble. think i ended up with 82s in the front with pv and 90s in the rear with no pv. thanks for the replys.i guess you cant believe every thing you hear.it seems like it was a summit tech where i heard to go up 2-4 sizes to make up for the ex.i have a o2 censer in the mail.i will get it straightened out.  Ross,i will go by the steps you gave me,it looks like sound advice.im no carb man,but i can usually get by.
« Last Edit: April 04, 2019, 11:52:58 AM by fryedaddy »
1966 comet caliente 428 4 speed owned since 1983                                                 1973 f250 ranger xlt 360 4 speed papaw bought new

My427stang

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Re: rich-lean issues
« Reply #11 on: April 04, 2019, 01:58:55 PM »
The above is generally good guidance.

But I should point out that adding jet to replace a removed power valve is not really a "use this many more" sort of deal.  Its an area calculation based on the diameter of the PVCR - Power Valve Channel Restriction - that you are making up for.  The fuel flow is determined by the flow capability of the main jets plus the capability of the PVCR restrictions whenever the power valve is open.  On most carbs the PVCR is modest in size (.030s...) but they are really large in some applications (some Dominators were .090+) and would take a ton more jet to replace.

Changing PVCR is a very useful tuning tool on street applications, but pretty much nobody does it because they can be a real pita to alter and change for somebody just getting started on carb stuff.  How many times do you hear something about a car being too rich driving around but going kind of lean at WOT - or the opposite?  Altering the PVCR relationship can address that issue.

Absolutely, but 88 gets you pretty close to most of the 4781's PVCR from the factory. I believe some even came that way, but I don't have my book with me at work.  Unless of course it's been "improved"

A PV on the secondaries can be odd anyways, some run good with them, but odd to think someone would ever be on cruise with the secondaries open. 
« Last Edit: April 05, 2019, 02:19:08 PM by My427stang »
---------------------------------
Ross
Bullock's Power Service, LLC
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, Erson SFT cam, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 461 cid FE, headers, Victor Pro-flo EFI, Comp Custom HFT cam, 3.50 9 inch

Falcon67

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Re: rich-lean issues
« Reply #12 on: April 05, 2019, 08:29:50 AM »
The only carb I have here that doesn't use at least a front PV is the methanol carb on the dragster.  And I paid for that setup and don't mess with it.  Have no idea what jet is in it, don't care.  It runs perfect and I just keep it clean.  Everything else runs a front PV.  No reason not to IMHO.  And that includes drag racing. 

fryedaddy

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Re: rich-lean issues
« Reply #13 on: April 05, 2019, 01:49:45 PM »
i bought a 650dp one time and i never touched it,ever. i sold the car 8 years later.i dont fix stuff that ain't broke.
1966 comet caliente 428 4 speed owned since 1983                                                 1973 f250 ranger xlt 360 4 speed papaw bought new