Engine won't run with choke flap fully closed

[megamax42]

Specs
- 350 2 bolt main
- I believe .480" lift COMP cam
- "Z28" valve springs
- Holley 600 cfm four barrel (Model 4160)

Background
Hello everybody, was hoping I could pick your brains about a problem I've been having. I'll start off by saying I rebuilt the engine about 3 years ago, but foolishly used an old Holley carb without rebuilding it. The carb came with a project I bought and was supposedly very lightly used. Then it sat for about 5 years before getting thrown on the 350.

I've always had trouble starting the engine on cold mornings, the method I have adopted is pull the choke cable about 1.5" out (full extension is maybe 4 inches) and keep it at around 1500 rpms with the gas pedal, doing this for maybe 30-45 seconds. Then I let it idle (~900 rpm) for a minute, and finally I take the choke off and drive away.

Problem

My problem is that if I try to close the choke flap all the way (choke fully engaged), the engine bogs down and dies, whether it's cold or warm. I apologize for my naivete, however I was wondering if this was normal? I tried googling the issue and all the results were people with the exact opposite problem, their engine would run with the choke on, but would die when they took it off. My instincts tell me that this isn't normal and is probably related to the state of the carb, however I wanted to run this by the pro's first. I should also note that I have ordered a rebuild kit for the carb regardless. Thanks for taking the time to read this and any help is greatly appreciated.

[rspears]

With a manual choke you have to mimic the action of the thermostatic choke control as the engine warms. If you fully close the choke you will get no air through the carburetor, and yes it is normal for the engine to die - no air, no fuel, nothing to burn. An automatic choke is going to be adjusted to be cracked open about 1/8" to 3/16" when fully closed (a drill bit of that size will fit between the choke plate and the carb housing, front & back) to compensate for cold fuel & air (less atomization), plus it has a choke pulloff link that will open the choke plate at WOT - that's why you can "floor it" to clear a flooded condition with an automatic choke. As the engine warms the choke will open proportionally with temperature so that it is fully open as your water temp gauge is getting to mid-range. With a manual choke you have to find that "sweet spot" where your engine is getting just enough air to start, but not too much so that it's too lean to fire with fuel that's too cold to atomize fully. You never want to close the choke plate 100%.

[megamax42]

Ahh, interesting, I really appreciate the thorough response. Well that's definitely a relief to hear. So there's less atomization because the gas is cooler and therefore denser?

Perhaps I should start a new thread for this, but I got a vacuum gauge to do some carb tuning and I found the intake manifold vacuum (at idle) is very low, approximately 6". And what really worries me is the lack of fluctuation upon accelerating/decelerating.

I'm thinking it's most likely a vacuum leak, however I am also considering a timing issue. I'll evaluate the situation more when I get the carb rebuilt though. Thanks again for the awesome and quick response!

[astroracer]

You need to find the cam numbers. High overlap cams will cause a low vacuum signal such as what you are seeing. Although .480 isn't a lot of lift you may have a cam that doesn't generate a lot of vacuum. You need to figure this out to take the cam out of the picture.
Mark

[rspears]

You need to find the cam numbers. High overlap cams will cause a low vacuum signal such as what you are seeing. Although .480 isn't a lot of lift you may have a cam that doesn't generate a lot of vacuum. You need to figure this out to take the cam out of the picture.
Mark

I hadn't considered the cam and was about to have you chasing vacuum leaks or doing compression checks but Mark's right - eliminate the cam from the picture first. Might make any other checks unnecessary.

[36 sedan]

I may be wrong, but my experience with big cams while they do produce less vacuum at idle they also cause the vacuum to fluctuate slightly and when throttle is given the gauge still reacts immediately. And, my experience with vacuum leaks is they tend to like the choke (depending on how large the leak is) as it helps to compensates for the leaner mix caused by the vacuum leak.

Just a thought, by chance is your gauge plugged into the timed vacuum port? And, have checked the gauge for its accuracy?

You may be casing your tail on this, as your question was “can I close the choke completely”, for which Roger answered you correctly, “no, you can not close the choke completely”, with a further description of how to check and adjust the choke.

However, if you are having tuning issues, well that’s a different story and perhaps describing all of the symptoms will enable us to help you better.

[34_40]

I'll jump in and say that RSpears is mostly correct, I'd add that before setting the choke, you should push the throttle pedal about halfway down and hold it there, then pull the choke out fully. This will let the cold idle cam come into play and hold you're idle at a higher rate of rpm..

Once the choke is fully set, crank the engine and once it starts push in the choke handle to allow air into the engine, again you'll need to find that sweet spot RSpears talked about, it's just that now you're engine should be at a higher rpm allowing it to warm up faster.

Once the engine starts and you've begun pushing in the choke handle, the engine will rev high until you step onto the throttle which will allow the high idle cam to come off.

And of course the caveat is this will all work as long as no-one has backed off the adjusting screw for the high idle! HTH!