Question for the Timing / Fuel Experts....

[Sacbill]

Was working on setting up the timing and Air/Fuel mix on a buddy's car today and had some interesting results I wanted to run by those more knowledgable on SF. Drivetrain in question is a 69 351W, Summit 750 4V carb, Duraspark ignition, and Hooker Comp headers. All the internals are stock for an original 2V 351W. Automatic Trans.

The main problem with the car was a rough stumble at idle that included some slight backfire in the right bank (passenger side). We set the timing at 10 degrees at curb idle of 600 (factory spec calls for 550). The first thing that I found odd was that the A/F screws were fully seated. :wtf I would've thought that the car would be stumbling and gasping for fuel at such a setting, but it wasn't. The best explanation I can come up with is that the carb is slightly big for the motor and so, even with the screws all the way in, is still getting enough fuel to keep running. Does this make sense? Or another reason?

At this point, we backed out the A/F screws to 1.5 turns and reset the curb idle. Vacuum is pulling about 18#. So, we turn in by 1/2...vacuum pulls 19#. So, we turn in by another half, and another half. Vacuum continues to pull, showing no signs of let up, to 20+#, but now the A/F screws are fully seated again! Our final compromise was .75 turns out, with about 18.5#vacuum. Car runs super smooth at idle, gets it when test driven, and no more backfire symptoms. All in all, a winning combination.

I'm still stumped though as to why the carb would continue to pull vacuum and not falter before bottoming out the A/F screws. (As indicated in this recently referred site.) Any ideas?

My own limited experience has been with my '68 302 with a mild cam, so it might just be some differences I'm not used to.

 

[BobV]

If the idle screws are fully seated and it's still running, then you have an internal fuel leak somewhere in the carb - float level too high, ruptured power valve, etc. At idle, all fuel reaching the motor should be coming thru the idle circuit - until that happens, you'll be chasing "issues". The carb size is fine.

 

[tarafied1]

If the idle screws are fully seated and it's still running, then you have an internal fuel leak somewhere in the carb - float level too high, ruptured power valve, etc. At idle, all fuel reaching the motor should be coming thru the idle circuit - until that happens, you'll be chasing "issues". The carb size is fine.

I would agree, how do the plugs look? soot in the tail pipes?

 

[SAC69]

Bill,

Thanks for posting this, it was my car Bill was nice enough to take the time to tune. "WTF" on the air/fuel screws is a correct term as I could swear I had them 1 1/2 out after installing the carb 1 1/2 years (!) ago.

I would agree, how do the plugs look? soot in the tail pipes?

Here's a pic, #1 plug on the right, #5 plug on the left, Autolite 45, they are about 1 year old. The last time I pulled them, the insulator was fairly white. There is soot on the inside ends of the tailpipes.

 

[tarafied1]

so I would still have to agree that fuel is getting into the engine from somewhere other than the idle circuit. The plugs don't look "wet" but it does look rich. They should be more brownish white and the tail pipes shouldn't have too much soot. Some is typical with a carb.
If it is not flooding, it must be internal requiring vacuum to pull the fuel in the engine otherwise it would be really hard to start.

 

[tarafied1]

I'm not familiar with the Summit carb but on a Holley, the metering plate where the jets are, have ports that begin to feed fuel into the venturi based on air flow. I suspect all carbs work off the same principle. Is it possible the butterflies on either the primary or secondary are not closed all the way the engine is idling on the main circuit?

 

[SAC69]

I'm not familiar with the Summit carb but on a Holley, the metering plate where the jets are, have ports that begin to feed fuel into the venturi based on air flow. I suspect all carbs work off the same principle. Is it possible the butterflies on either the primary or secondary are not closed all the way the engine is idling on the main circuit?

How would I go about checking this? Does the carb need to come off the car?

On a possibly-related note, the carb came as an electric choke model but I removed that assembly and converted it to a manual choke control, wired all the way open. I know when the choke is closed it opens up the butterfly a bit to allow more fuel in, but when the choke is open, it should default to the idle screw setting, correct?

 

[tarafied1]

The choke being open shouldn't be the issue. It does have a fast idle setting you might check that the fast idle cam hasn't made the butterflies open. I should think you can check the butterflies with a match I mean a flash (sorry) light to see if they close all the way.

 

[Mach1Rider]

Most likely a float level problem of being too high if it is not internally leaking inside the carb.
Pull the #4 & 8 plugs and see if they are darker than the 1-5 plugs, Yes... the rear float level is too high.
As you are able to raise and lower the idle speed with the set screw and the idle screws were closed it has to be getting fuel from the overflow port As it doesn't take much to idle and you original complaint was a stumble lean condition.

Yes with choke in open position, the idle speed is determined by the set screw on the drivers side if the chokes linkage has fully released the throttle plate.

 

[Sluggo]

There is some handy info about this carb on the dvd that summit sells. I'm looking at it now. They have idle feed restrictors and base nozzles that look pretty much like jets. Both are located in the discharge nozzle body.

Smaller idle feed restrictors will help with the idle and maybe take care of the stumble. I noticed the same thing with my summit 750 carb. the idle screws are cranked way in.

Now that I have this dvd in hand I'm going to order the smaller idle restrictors. This should slow down the amount of fuel passing the restrictor and allow opening the mixture screws to a more normal position.

In your case, I'd probably jet it down two maybe even three steps in the primaries and at least two in the secondaries also. This carb is a bit much for my engine so I'm sure it's even more so on a mostly stock 351.

I bumped mine down two sizes in the front and and left the backs alone.

 

[Sluggo]

Here's some reading that might help.

 

[Sacbill]

Here's some reading that might help.

Thanks for the kickass info on the restrictors! I'm going to have to dig up my Summit carb setup DVD and have another look at it!

 

[TommyK]

I don't know a lot about this style carb but a .052 IFR size is GIGANTIC compared to a typical holley style carb where the normal range is .028-.036.

 

[SAC69]

Here's some reading that might help.

Thanks for the info. I'll watch the dvd again but do remember them taking about the overall adjustability of this carb.

 

[SAC69]

For those interested, I sent an inquiry to Summit's tech line about my initial carb problems.

My question is followed by their response:

Question: Hello,

I have a Summit 750 vacuum secondary carb (which I converted to manual choke) in use for 1 1/2 years. A recent check to diagnose a poorly-running engine revealed both air/fuel screws turned in all the way yet the car still ran. How is this possible? Is there another way for fuel to leak past the needle/seat? The car had 18.5 lbs vacuum, timing sent at 10 degrees advanced.

I've adjusted the needles to 3/4 of the way out and the car runs good, but I'm still worried about the fundamental issue of how the engine ran with the a/f all the way in.

Thanks for any help.

Year: 1969
Make: Ford
Model: Mustang
Engine: 351 windsor
Transmission: FMX (automatic)

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Response Via Email (Kevin) 09/16/2011 01:12 PM
It can still pull air through the throttle blades enough to run just as a Holley would. Usually this means that the throttle blades are open to the top or past the transition slot in the baseplate. For the best idle quality, and efficiency, we recommend closing the throttle blades as much as possible and bringing the idle up with the mixture screws to the highest idle speed, and then screw them back in 1/4 turn, and then set the curb idle screw last. Hope this helps.

Thanks, Kevin

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Today I tried their advice but first, I screwed the a/f all the way in and screwed the curb idle all the way out. The car still ran, but barely idled, because the idle was so low. The question I have is, why would the car even run with the a/f all the way in and the cub idle all the way out so it's not engaging the throttle plate. Shouldn't the engine have stalled?

 

[Mach1Rider]

Yes it should have stalled out.
It will take removing the carb to check but I would bet the secondary throttle plate is out of adjustment.
Flip the carb over and the rear plate should be closed.

 

[SAC69]

Yes it should have stalled out.
It will take removing the carb to check but I would bet the secondary throttle plate is out of adjustment.
Flip the carb over and the rear plate should be closed.

Thanks for the info.

I have no problem taking the carb off but is this something I can do with it on the car? I looked down the throat and can see the primary cracked slightly and the secondary appears to be closed, or at least not cracked open like the primary.

If I take the carb off, how is the secondary plate adjusted?

Thanks.

 

[6t6red]

I haven't read the whole thread but I thought I would throw this out there. It may be possible that your throttle linkage is holding the plates open if it is too long. Try removing the linkage from the carb and check to see if the plates are closed. It is a long shot idea but who knows, it might be the problem.

 

[Mach1Rider]

:stu... 6t6red has a valid point, but I was assuming the linkage and spring were adjusted porperly.
The rear throttle blade adjustment is a small screw on the underside of the base plate on the chokes side.

 

[SAC69]

Guys, thanks for the tips. I'll check the secondary adj and throttle linkage. It's a cable and I was simply eyeballing the curb idle when it would "move" the linkage.