Edelbrock Electric Choke Issue

[Timothy B.]

Re: Edelbrock Electric Choke Issue

Paul,

On the piston type accelerator pumps like the Carter/Edelbrock, the piston height controls the volume of fuel. The squirter diameter controls the timing of the shot, (how fast it's delivered).

In Rogers situation, a bit more fuel earlier in the pump shot may cover this stumble without making a rich situation when warm.

There is a real art to carburetor tuning and when you get away from a stock set up (like Roger) it's important to understand how these carburetor circuits work and overlap each other.. Remember, timing first then carburetor.

[Roger P.]

Re: Edelbrock Electric Choke Issue

Michael, John, Tim, Paul, and Stu,
I wanted to thank you guys for your support and assistance as I try to solve my stumble issue. Things in South Florida are different from other parts of the country; it's hard to find people who take pride in their work and are willing to spend time to solve a problem. And that's when you're lucky to find someone with the necessary skills. In my business as an architect, I have to deal with complex issues on a daily basis. Like tuning an old engine, it takes time and patience. Thankfully, I have a lot of patience... but not always a lot of time with only 24 hours in a day . With all the advice that I have gathered from you guys and everything I have read online, I feel confident that I will resolve this issue. I believe that leaning the carb by changing the metering rods was a good start. My car was getting very poor gas mileage (about 11 MPG); and that was with a decent amount of highway driving. Tomorrow, I will purchase an Edelbrock accelerator pump nozzle set (there are 3 different sizes). Obviously, I have one of the 3 on the carb now, so I hope that one of the other 2 will fix what's left of the hesitation/stumble. I understand that it is almost as easy as changing the metering rods & springs. If I am able to eliminate, or at least reduce, the stumble and perhaps improve the gas mileage, I will be very happy with my investment of $ 90.00 in parts (it's too bad I can't get paid for my time ). I will complete the project this weekend and will report back with the conclusion to my saga. Yes, I am being positive as failure is not an option!

Roger (50141)

[Stuart F.]

Re: Edelbrock Electric Choke Issue

Roger;

I took my 63 out yesterday in the colder weather and it started up and warmed up so fine - it made me think of you. It's funny too, when you have had such problems in the past, i.e. stumble and hesitation specially when cold, you tend to drive as if you were still expecting it by slipping the clutch and giving it too much throttle. I had to remind myself to let the car run for itself and drive it like it was normal (even with my wife with me). My reminder came after I ran through a stop sign (not wanting to stop) in a shopping center parking lot. The wife reminded me real quick; "Hey! Hey! you just went through a stop sign!!"

Architect huh? I wanted to be one too when I was in High School (when we used pencils, triangles and T squares). In the Army training school, I beat out all the graduate Architects and Engineers finishing first in class merely by giving them what they were looking for; technique and accuracy.

Stu Fox

[John G.]

Re: Edelbrock Electric Choke Issue

I could suggest a good place to start.........

Not to take away from Roger's thread:
Along the lines of fuel injected cars having been equipped with the FI spacer the goal in this instance is an 'experiment' of sorts to see if the FI spacer will actually work well in place of the heat riser valve. It doesn't get so cold here that climate could play a major role so that figured into the mix, as well.

In mapping the distributor advance curve there might be some timing related issue related to the problem I'm having (too much advance, too early). That needs to be looked at, also. The engine seems to run a bit rough in the lower rpm range. 'Lighter' advance weight springs were installed.

Outside of initial engine break-in my car really isn't drivable, just yet, as the mechanicals have been set aside in place of some panel replacement and body work going on at this point. Getting closer to finishing up that part up so the carb issue can be addressed.

Roger - it will be interesting to see what comes up with the fix. Might learn something to help out in my situation, too!

[Roger P.]

Re: Edelbrock Electric Choke Issue

Hey Guys,
Took the "lady in red" out of work week hibernation late yesterday afternoon to see a client in the neighborhood who owns a '70 convertible. I love my old C2 girl, but have to admit I have thought about cheating and taking a "romp on the track" with a sexy C3!! It was a short drive and I didn't give her any time to warm up, so I did experience stumble/bog just like before. However, she seems to be warming up and shaking the stumble much faster now. All I did so far was replace the choke and change the metering rods for leaner rods and lighter springs. By chance, an airplane mechanic friend passed by when I returned home and told me that a friend with a '67 had a similar problem. The guys replaced his spark plugs, rotor, and distributor cap and his stumble issue disappeared. I have put just over 3,000 miles on the spark plugs in 2-1/2 years. Any thoughts on the plugs, cap, and rotor before I replace the accelerator pump nozzles? The cap and rotor could be as old as the 20+ year old engine (same with the spark plug wires)!

John, you're not hijacking my thread at all. I appreciate your input and hope that we can resolve our respective problems by brainstorming together along with the guys. This is definitely a tricky problem; similar to tracking down a rattle. It requires patience and the willpower not to become obsessed or (too) frustrated. Stu, I completely understand your conditioned response to a cold engine. I just hope to be like you and be conditioned to juice it due to an "old" problem. BTW, we do our plans on the computer these days, but my formal education from high school through college was all pencil & pen drafting... as you put it, technique and accuracy. That training gave me the skills to make my computer drawings look like high quality hand drawings. Like my Vette, I take a lot of pride in my work... and I can chat on the TDB and work on my projects from the same computer screen!

I'll keep you guys posted,
Roger (50141)

[Timothy B.]

Re: Edelbrock Electric Choke Issue

Roger,

The mechanic that is working on the car should be able to tell you if the secondary parts of the ignition are in good condition. From pictures you posted it appears the distributor is a HEI which is a great ignition, better than points by far..

Look at a few spark plugs to see how much the electrode is wore, how clean and what color they are. My Guess is that everything is fine but all I can do is guess from looking at the pics. Just try one thing at a time so you can go back if not happy with the change.

I have measured .028 diameter on Carter 3721SB accelerator nozzles, you need a small drill bit set to measure these. I don't know what size the 750 edelbrock is but they may be stamped on the top. It's a easy change provided you don't drop any screws down the carburetor so be careful. If you determine the size, go up one or two sizes and see if this cures your stumble. If your results are positive then you can reduce volume of the pump shot a little at a time so there is just enough gas to allow smooth overlap of the carburetor circuits, too much is as bad as not enough.

[Stuart F.]

Re: Edelbrock Electric Choke Issue

Tim;

My notes show the 3721SB with a #264 pump nozzle (.0265"). My 3720SA had the #284 pump nozzle (.028"), which is what I put into my 3721SB and achieved a significant improvement. You are right about the numbered drill set being necessary for measuring jets, air bleeds and nozzles, etc. I don't know where I'd be without them.

I'm sure Roger, given the time, will figure out his problem or at least improve on it within the limits of today's gas. He seems to have the basic mechanical knack and common sense necessary. He also knows now he can not count on a mechanic to pick a sure cure out of the sky. Even you or I would have to spend days on such a car as his to figure out what's going on and what to do about it, many times by trial and error.

Stu Fox

[Roger P.]

Re: Edelbrock Electric Choke Issue

Tim and Stu,
Time for my end of the weekend update. Being that the parts are cheap, I replaced my distributor cap, rotor, and spark plugs on the ignition side. The plugs didn't look too bad, but they were definitely somewhere between black & dark brown - I assume a bit rich. Again since the parts were reasonable, I decided to remove the carb from the intake in order to replace the base gasket and then remove the air horn to change the accelerator pump & nozzle. Edelbrock sells a nozzle set with a .024, .033, and .042. The one that was already installed was the .033, so I changed that for the .042. The float level was off a bit, so I adjusted that floats, installed the new pump, put on a new gasket, and put it back together. Once I got the carb back on the engine, she fired right up and I went for a test drive while the engine was cold. I still have some off-idle hesitation, but not as bad as before. It feels like there is more power, and the side pipes sound quieter in the driveway and on the highway. Maybe that's just my imagination?? I am glad that the tune-up was done, but I hope that I can still resolve the remaining stumble issue. I left the accelerator pump linkage closest to the carb for max. pump shot, but I am going to try moving it to the other two holes to see if that makes a difference. If not, I was going to try the .024 nozzle since I have read on a few websites that this problem can be due to too much fuel and not always too little fuel. Also, when I adjusted the idle screws, the driver side screw adjusted properly, but the passenger side screw didn't change anything even when it was fully closed. In fact, when I opened it past 1-1/2 turns, I think the idle got a little worse. I left it open at about 1-1/4 turns. Any thoughts?

Thanks for your patience and ideas,
Roger (50141)

[Timothy B.]

Re: Edelbrock Electric Choke Issue

Roger,

I think it's a good idea to try the smaller shooter size. If the pump shot is used by the engine before the main circuit starts flowing there could be stumble also.

The A/F emulsion screws should be adjusted on a warmed engine and both sides approx the same turnes out from seated.

Where did you end up setting the choke coil on the index scale?

Something else to consider, if you loose the adjustability on the A/F emulsion screws this usually means the primary throttle blades are set to far open and the motor is pulling harder on the transfer slots. If you close the throttle blades to where there is approx .020 (little squares) of the transfer slot showing at idle your A/F screws should allow you to clean the idle up. If idle is to low at that point you may want to get more timing at idle (full manifold vacuum advance). What RPM does your engine idle when warmed up?

[Stuart F.]

Re: Edelbrock Electric Choke Issue

Roger;

Just off hand, what rear axle ratio do you have? Reason I ask is I have a 3.36 with a close ratio transmission. As such, sometimes it feels like I'm starting out in second gear and, under certain ambient conditions and points in my warm up cycle, my engine will stumble a bit. But, with the Duntov cam my engine has, it is what could be called the "nature of the beast". Perhaps you are spoiled and expect it to run too smoothly for a carburated engine. Sometimes I have to "feather" mine just a bit to get a smooth start.

I just did a "Williams/Hinkley" valve adjustment on my engine this past weekend during which I barred the engine over about 8 times (do it once and check it twice). When I first took the car out (down off the ramps) and backed out into the driveway, it stalled and I had trouble getting it started - having to pump it several times. Now, I don't know exaxctly why because it had never done that before and it continued to run a little erratic until I got it out on a central Florida back road and ran it trough the gears (6500 in 1st, 2nd, and 3rd). After that, it cleared up and ran like a top. Perhaps the barring loaded it up, or maybe now I have more "cam" with the tighter and more accurate valve settings, I don't know. My point is, sometimes these old Vettes can be temperamental. Next time I take it out it may be something else or just fine. We'll see as next week i'll drive it to the winter meet in Kissimmee.

Stu Fox

[Roger P.]

Re: Edelbrock Electric Choke Issue

Tim and Stu,
I will first try setting the accelerator pump linkage to one of the other two holes to see if that makes a difference. If not, then I will change the shooter to the .024 (smallest) nozzle. I set the A/F emulsion screws when the engine was at operating temperature, but the passenger side did very little to nothing when all the way in or 2 turns out. I will remove the screw, blow out the hole, and spray some carb cleaner in case it is clogged. If necessary, how would I adjust the primary throttle blades? Do I have to remove the top of the carb? If so, I will make the adjustment when I replace the pump nozzle. My engine idles nice & smooth at 850-900 RPM. I have a 3.55 rear which is a nice middle-of-the-road gear ratio, but it would be great to have a 5th gear on the highway (turns 3,500 RPM at 70+ MPH). The performance cam in my engine might not allow me to get rid of all the cold off-idle stumble, but it is already much improved thanks to your help. If these last few ideas don't completely fix the issue, I will have to live with its "imperfection".

Roger (50141)

[Timothy B.]

Re: Edelbrock Electric Choke Issue

Roger,

When looking at the bottom of the carburetor, the primary blades are adjusted when there is approx .020 of the transfer slot (little squares) showing below the throttle blades. Doing this will allow the emulsion screws to adjust the idle for max vacuum.

If your car needs 850-900 RPM to idle this may not be possible unless you speed the idle by more timing or air. I don't know if this has anything to do with the stumble.

I noticed in one of your pictures there is a well for a divorced choke coil on the passenger side of the intake manifold, you may want to see if linkage is available for this because the opening of the choke will be tied to temperature in the manifold and not electric timed opening.

[Stuart F.]

Re: Edelbrock Electric Choke Issue

Roger;

FWIW, Regarding "Bog, Hesitation & Stumble"; If a bog exists from an idle, and not when accelerating from a constant speed, the idle adjusment may be at fault. Carburetor theorists tell us that most modern carbs are designed to run best with around .020" clearance between the center of the throttle plate edges and the throttle body (as measured at a point equidistant from the throttle shaft bearing areas). This clearance allows for the maximum velocity of idle air past the idle ports. Setting the idle for the highest vacuum idle reading may result in too little clearance at the throttle plates which forces too much idle mixture through the lower idle ports and too little through the idle transfer slots. This will cause what they refer to as "puddling", which is when little droplets of fuel adhere to the intake manifold runners. Then, when the throttle is opened, there is sufficient velocity of air to "sweep" these droplets into the cylinders, creating a "too-rich to burn" mixture which causes a bog. As soon as they overrich mixture is pumped out into the exhaust, and a normal mixture is ingested into the cylinders, the bog disappears.

If a bog exists from a cruising speed, it is more likely to be caused by a defective ignition advance or accelerator pump.

Stu Fox

[Roger P.]

Re: Edelbrock Electric Choke Issue

Tim and Stu,
OMG, carburetors are more complicated than women... and I didn't think that was possible!! Just joking ladies . If I need to adjust the throttle plates, what is the procedure? Do I have to remove the top part of the carb, or the entire carb from the intake manifold? Is there an adjustment screw? Will my newly installed gaskets still be OK to use again? I thought that 850-900 RPM is the correct idle for a performance cam and a manual transmission. I can adjust the idle with the idle speed screw, but as I said, the passenger side idle A/F emulsion screw doesn't seem to affect the idle speed. The driver side emulsion screw works properly. Can one side not working be due to the throttle plates needing adjustment?

Roger (50141)

[Timothy B.]

Re: Edelbrock Electric Choke Issue

Roger,

Adjusting for the .020 transfer slot exposure is just a matter or lowering the idle screw to close the primary blades. You don't need to take anything apart..

The more transfer slot that's exposed to manifold vacuum at idle the less adjustability of the curb idle A/F emulsion screws. The idle vacuum will pull hard on the slots and not as much from the curb idle ports.