75 corvette 350. Mechanical fuel pump, new, no vacuum leaks. no special modifications. When driving will randomly loose throttle response and just idle, as if the throttle cable isn't attached. Can pull over and shut vehicle off and restart and drive like nothing happened. Drives, starts, idles and accelerates normally 99% of the time. The trips are about a 10 mile run to work and home about two times a week.

 

accel pump jumps to mind if I understand your description.
so,
low fuel level?
worn, stiff cups?
linkage weirdness?
check-valve dirty sticking?

or ignition

 

Thanks, i was thinking a fuel issue the way it acted. Tank is always full and I put a new fuel pump on it. It hasn't died on me I've always had to shut it down, so the ignition i think is ok. But looks like the carburetor needs some attention. I appreciate the advice!

 

Shutting the car off and turning it back on should have no effect on the carburetor unless you have an electronic feed back carb. What year we talking about?
Short blurb on CC carb.
https://www.motortrend.com/how-to/17...b-in-corvette/

 

My first thought would be a fuel problem, but that doesn't usually align with (temporarily) fixing the problem by restarting, that part of it sounds more like an ignition problem.

Are you sure the "new" pump is any good? I don't have any personal experience with them, but from what I've heard the Chinese made Carter replacement pumps have proven to be hit or miss, quality wise. Did you have this problem before replacing the pump? Was this the reason you replaced it, or did this issue only develop after replacing the pump? If it's a new problem since you replaced the pump, I'd be looking right at the new pump.

Do you still have the original Q-Jet on the car, and has it ever been rebuilt? Even if the carb is not the problem, it couldn't hurt to have a 48 year old carb rebuilt by a professional rebuilder. In the meantime you might want to run a can of carb cleaner additive like Sta-Bil Fast Fix or Amsoil Quickshot through the system, and give the butterfly's and linkage a good cleaning with Gumout spray.

If you're sure the pump and carb aren't the the problem, then I think I would be looking at the coil and/or ignition module in the HEI distributor, and maybe the vacuum advance. I only have a basic, working knowledge of the GM HEI distributor, but with a points system, a bad coil will over heat and cause the ignition to shut down, and restart after it's had a chance to cool off for a minute or two. Is it possible that the HEI puts out enough energy that it will overcome a bad coil just enough to keep running at reduced power, I don't know, but I guess it's possible? From my experience when the modules go bad the car dies and won't restart, but like I said, I'm no expert on GM HEI's.

Other then the HEI electronic ignition, everything else on a 75 Corvette is 100 year old technology, so they're usually pretty easy to trouble shoot and fix.

Welcome to the Forum!

 

sounds like you have a fuel issue inside your carb but when you floor it it should bog with no fuel and too much air. Thats a very strange description....

so when the throttle doesnt work, does it just idle smooth or does it gaga and buck when you floor it?
If its runnign but very sluggish and boggy then its probably an ignition issue which may clear up after a restart. THe only thing a restart would fix with a carburetor is a air bubble, a vacuum issue, a clog that is being pushed by fuel pressure and moves when you shut it off....

a bewtter description is needed. did it do this before the new fuel pump. Why does it have a new fuel pump?

 

Your new pump you bought. Is it a two-port or three port?
And if a three-port, is the vapor canister still involved?

 

Yes, it happened before the fuel pump, I was hoping the pump would do the trick but the issue still there ,the pump has 3 fittings As far as what it does when it acts up it looses power and just idles like I'm in park, no sputtering or any kind of symptoms So far ,when it happens, haven't ever been in a safe place to get out and piddle with it. And before you know it, it's running normal again . The carb I'm assuming is original. The car did set idle for several years, I had to replace the hei ignition system and clean out the gas tank, lines up to and including. the filter on the carb., radiator. And re do all the brake systems before I took it out to drive.

 

This makes no sense what so ever.....steady idle with throttle movement would mean the throttle was disconnected some how becasue wide open throttle with now fuel would give you a giant lean spike

 

If you ever get to pull over while it is happening get a video and move the throttle lever by hand so we can see what is going on in the video

 

81 or 82?

 

His original post says it is a 75.

 

Maybe choke messed up and not releasing the butterflies to open?

 

You're right it does. Lol
well,.. pretty weird then.

 

Sounds like possibly a clogged fuel filter. even if you replaced the filter it may be full of dirt again if you cleaned out your tank and didn't get all the crud out. fuel filters are inexpensive.

Pat

 

Then why does it idle smoothly and doesnt bog when he floors it while itsw idling. It would be a bad fly by wire throttle if it were something new.....

 

Couldn't tell you. But i've had 2 vehicles - not corvettes - that had his exact symptoms and it was the fuel filter. they get 90+% clogged and you lose all power on the highway, pull over or let it coast for a few seconds and the float bowls fill up again and then it drives normal again until the bowls run out of fuel again but it never stalls and will idle all day long. never said it was the filter just something to check and it doesn't cost a lot to put a filter in it.

Pat

 

hopefully its something that simple. If its fuel related then it could be the pump lobe on the cam is shot and thats why a new pump didnt solve it

 

Bdunn,

Looking for an update on your weird issue.
I also reread post #8 stating that the gas tank was cleaned out. Often that procedure stirs up a lot of corrosion / crud and plugs-up a carb filter again & again.
Sometimes you have to clean or replace the carb filter 3x

pspicci response in post #17 is spot-on. A carb will idle all day long on a tablespoon supply of gas. But won't make it ten miles on a half a fuel bowl of gas.
Another issue is some of that stirred up gas tank sediment may have gotten past the carb filter. In which case the Needle & Seat is not opening fully. (Its usually the other way around, flooding)

If this was a Holley, a quick inspection of the "fuel-level sight plug" would verify the bowl is indeed low / empty. On a Quadra, different story for inspection.

 

I cannot grasp the OP's statement that it loses throttle reponse and will only idle.

If it was a fuel problem and he stepped on the throttle, wouldn't it die?

Is possible the throttle linkage slipping on the butterfly shaft?