more info on 750dp problem on 390

[rcodecj]

Here's a link to the old post:
http://fepower.net/simplemachinesforum/index.php?topic=92.0

The gist of it is that the engine goes rich on takeoff, leaves lazy, trying to clear itself out, no bog as if it were lean.
Today I drove it with a 3.5 power valve as suggested and it didn't help.
The carb runs fine on my other car, but it's a 460 in a bit lighter car.
I disconnected the secondaries and it takes off fine.
I have come to two conclusions:

The carb is too big for the engine/car combo, but I can't explain why it goes rich and not lean.

or it is related to the low vacuum that the car has and that is affecting it, I have looked for vacuum leaks and found none.
A previous poster suggested a more thorough way to test for a vacuum leak and I guess I'll have to try that.

The car starts and runs fine except this problem, and I know if I put a vacuum secondary carb on it, I would not have a problem.
I did put my Edelbrock 800 on it and it ran fine. I want to make this carb work as it looks more correct and feels like it makes more power on the top end than the Edelbrock.

[jayb]

If you disconnected the mechanical secondaries and it runs fine, then the mix of the additional air from the mechanical secondaries and the additional fuel from the secondary squirters is not right.  You're sure its rich when it stumbles?  You have a wideband on it to check for this, or see black smoke on acceleration, or something like that?  Or is this an educated guess based on how the engine responds?

Assuming it is rich, then you need to dial back the secondary pump shot, either by playing with pump cam on the secondaries, or reducing the shooter size.  You could confirm that this will solve the problem by adjusting the plunger on the throttle linkage that contacts the accelerator pump arm.  Just tighten up that screw until you have more clearance between the arm and the plunger; this should give you less total fuel from the pump.  You may be able to tighten it up so much that almost no fuel comes out of the shooter when the throttle snaps open, but I can't remember for sure; I haven't got a carb in front of me at the moment.  You could also take off the pump cam altogether and run without any pump shot.

If you do back off the pump fuel and it doesn't solve the problem, I think you may be stumbling because you are lean, not rich.  On many double pumper carbs I have had to add the 50cc REO accelerator pump kit on the primaries to get enough fuel to the engine when you stab the throttle from an idle.  In fact, I can't ever recall having to lean out the pump shot; it always seems to need more fuel, not less.  Your engine is strong enough that this may be the problem; I've had to do that on my 428CJ with stock heads and a 236@.050" cam.  When all four barrels snap open at once, the engine needs a bunch of fuel to go along with it.

[rcodecj]

From a dead stop  initially it does not studder,pop, or stumble, it is a second or so after that.
I believe now that it is using up all the accelerator pump shot.
The carb has .040 nozzles front and back and the pink 330 cams front and back as purchased.
I switched to .045 in the rear and no help, I also switched to .035 in the rear and no help.
I do not see black smoke when it is leaving lazy so I now assume it is lean.
I have never had a dp do this, they always studder/stumble/pop right away and larger nozzles and especially bumping the initial timing up has always helped.
I completely agree with you and I too have never seen a problem with dp's that they were too rich on take off, always a bit lean or worked ok.
I think what had me going was that it does not initially act lean, it is a second or so later and I never noticed that.
I guess I need to buy an accelerator pump cam kit and possibly the 50cc pump for the rear.
I do have a brown #336 cam, but it came off a 50cc pump setup and it looks radically different.
The Holley chart shows a brown cam as being the next up from the pink, Are there two different brown cams?
The Holley chart also shows the pink cam as having a bit more volume on the #2 position.
If I remember right most cams in the #2 position put out more but later, anybody know for sure?
Suggestions and comments welcome and thanks for your input.

This looks like the cam I have, same number but the color is a bit different, I would still call it brown though and the Holley book doesn't list a tan one:
http://www.allcarbs.com/detail.php?pid=420&mfgn=1&ctgn=41&stt=0&gb=2

[rcodecj]

Today I changed the rear pink #330 cam to the #2 position and it does look like it has more stroke compared to the front.
I also changed it back to the original .040 nozzle. I'm just trying some stuff with what I have before I buy any parts.
Tomorrow morning, I'll test drive again.  If it seems to help, but not cure it, I will switch the front cam to #2 position also.
If that cures it, I will put the front cam back to the #1 postion, and try a more agressive cam on the rear.

[rcodecj]

There seemed to be an improvement in switching the pink cam to the #2 position on the rear accelerator pump.
On first take off from a dead stop it spun the tires and ran good through the gears.  
The second time however it seemed to get better traction and that was when it started missing after take off.  
I don't know if the first run was a fluke as it spun which it never has been able to do from a dead stop, on the other hand since it spun it did not put a load on the engine.
I'm sure it would blubber at the track as traction would be so much better.
I'm now debating whether to change the front, which I think would help, maybe cure it or...
change the rear to a more agressive cam like the blue one.
I don't like the idea of changing the front because it ususally makes the throttle stiffer to change to the more agressive cams, if it was only on the rear I wouldn't notice it in regular driving. My throttle linkage right now seems a bit jerky and hard to accelerate smoothly, it's a new cable, but I'm going to have to check it out some more, I have a fairly light spring on it, so that's not it.

[jayb]

Sounds like you are making progress.  I would continue to work the cam issue as much as possible.  You could also try going down in shooter size, because that will make the pump shot last longer after you open the throttle.  Last resort would be the REO pump setup, but as mentioned earlier I've had to do that myself on any number of carbs, so don't be afraid of that option if you need to use it.

I think you are on the right track with this - Jay

[machoneman]

It's possible that a binding throttle cable could be partially responsible.  Any mechanical hesitation on take off can impact how a Holley (or any carb) dumps its accelerator pump shot. Take off the carb's cable end and ensure that the carb's throttle plates open smoothly as you work them.  If so, I'd do all I could to loosen up that 'new' cable or get a replacement. My experience with Morse-type cables on street cars, dragsters and boats is that they work smoothly right from the get go or.....they never will! JMO. 

For that matter, sometimes the accelerator pump lever ends (that touch the cam side) are bent and drag on the primary/secondary throttles.  Straighten them out (off the carb! do not attempt to bend them on carb!) and be sure they are set to hit the center of the plastic cam and don't touch any metal through their travel.     

[rcodecj]

Sounds like you are making progress.  I would continue to work the cam issue as much as possible.  You could also try going down in shooter size, because that will make the pump shot last longer after you open the throttle.  Last resort would be the REO pump setup, but as mentioned earlier I've had to do that myself on any number of carbs, so don't be afraid of that option if you need to use it.

I think you are on the right track with this - Jay

When I tried going down in shooter size it then stumbled from the beginning.
I will work the cam issue as you mentioned.

[rcodecj]

It's possible that a binding throttle cable could be partially responsible.  Any mechanical hesitation on take off can impact how a Holley (or any carb) dumps its accelerator pump shot. Take off the carb's cable end and ensure that the carb's throttle plates open smoothly as you work them.  If so, I'd do all I could to loosen up that 'new' cable or get a replacement. My experience with Morse-type cables on street cars, dragsters and boats is that they work smoothly right from the get go or.....they never will! JMO. 

For that matter, sometimes the accelerator pump lever ends (that touch the cam side) are bent and drag on the primary/secondary throttles.  Straighten them out (off the carb! do not attempt to bend them on carb!) and be sure they are set to hit the center of the plastic cam and don't touch any metal through their travel.     

I'm going to check everything out today, thanks for the tips.

[rcodecj]

This repro accel cable is junk compared to the 40 year old original. The cable just doesn't run in and out as smooth, but there are no catches it's just a rougher finish. The only way I can see to lube it is to take it completley out and get it vertical and spray some silicone down it, as you can't pull the cable out because of it being crimped on the end. I did change where it snapped on to the carb throttle lever, to a hole that is farther out on the lever, which did act smoother and was more controlable. Now to drive it. If it's still goofy, I'll have to find a good used cable, as I don't have any faith that any of the repros are as good as the original, not to mention they don't even look the same.

[machoneman]

I swear....I would not do any other carb stuff until the cable is replaced, based upon your further description.

Had the same issue long ago with my bud's 70 'Vette where he somehow buggered up the OEM cable. He resisted due to the extremely expensive replacement cost....about $12 if I remember right!

Hey, it was in 1976 dollars! Afterwards....smooth as silk.  

One other thing: with the engine off, have someone mat the pedal inside the car while you look into the carb to ensure 100% throttle opening.  Both primary and secondary plates should be straight up at WOT. 

[rcodecj]

I swear....I would not do any other carb stuff until the cable is replaced, based upon your further description.

Had the same issue long ago with my bud's 70 'Vette where he somehow buggered up the OEM cable. He resisted due to the extremely expensive replacement cost....about $12 if I remember right!

Hey, it was in 1976 dollars! Afterwards....smooth as silk.  

One other thing: with the engine off, have someone mat the pedal inside the car while you look into the carb to ensure 100% throttle opening.  Both primary and secondary plates should be straight up at WOT. 

It appears they've gone up a bit!   

http://www.ebay.com/itm/1969-MUSTANG-ACCELERATOR-CABLE-C9ZZ-9A758-A-NOS-FORD-/200619390580?pt=Vintage_Car_Truck_Parts_Accessories&vxp=mtr&hash=item2eb5d8f674

[machoneman]

How about $35? 

http://www.mustangsunlimited.com/search_sorted.asp?CatKey=emustang&Keywords=throttle+cable+&x=11&y=10

[rcodecj]

How about $35? 

http://www.mustangsunlimited.com/search_sorted.asp?CatKey=emustang&Keywords=throttle+cable+&x=11&y=10

That looks like the same one I got from NPD. I'm afraid all the repros are going to be the same, but you never know.
I'm going to watch Ebay for a nice used one. In the mean time I haven't driven the car yet, but I think there will be quite an improvement with the ball stud in the better position. I checked the NPD one, it isn't actually sticking, it just doesn't have the quality look of the original Ford unit. The repros use thick plastic instead of steel where it mounts to the firewall, they even require a longer screw, and are noticable immediately that they aren't original. Thanks.

[rcodecj]

I changed the front accel pump cam to the #2 hole on the cam and now it runs good.
The BFG 275/60/15's are no match for the mighty FE, even when shifting into 2nd gear at 5200 rpm's. 
Just for fun I started in 2nd gear and it still misses, but that's to be expected with that kind of load on it I guess.
To reiterate, I now have pink #330 cams front and back in the #2 cam hole, and .040 nozzles front and back.

Also the change to the further out hole on the carb throttle lever drastically changed the throttle feel.
No more herky-jerky. I can modulate the gas much easier now and it's easier to push down.