Hello. I purchased my vette 2 years ago. I have a 69 427/390 stock motor. My local speed shop and myself have gone over the car many times.

I've been chasing this issue for a while now. At higher rpms 4500-5000 the engine coughs and chokes. I had the q-jet carb professionally rebuilt. I put it on the dyno last fall and its was running lean at full throttle. They said it was running out of gas. The technician pinched off the return fuel line and it magically ran better. However; now it's getting worse. At around 70mph on the highway and 3000 rpms it starts to sputter and choke. It has a new fuel filter, timing is right, good compression, good vacuum, and good gas.

The fuel pump was replaced 2 years ago. But, I'll thinking that the leading culprit has to be the fuel pump. Any other thoughts??

 

Do a fuel pressure test and check the float level.
Who is the professional rebuilder?
Another thing you can do is insert a number 40 Holley carburetor jet in the 1/4" rubber fuel return line where it connects to the filter.

 

Have you tried a different fuel filter?

 

Yes. It is the original style factory replacement. Steel canister with a outgoing carb line and a return line.

 

I forget the carb company out of New Jersey that rebuilt it. When I can get it back to the shop we will run another fuel pressure test. I've never heard of the carb jet trick.

Somehow, it is either loosing fuel pressure or volume.

 

I have a 69 small block with same type of filter. I ran competition events with the return line blocked and when coming back to idle it would run rough. I inserted the jet in the return hose, had no top end problems and still had a smooth idle.

 

Does your fuel have alcohol in it?

 

Have you checked the fuel supply line on the car recently? I see people asking about your fuel and whether or not it is Ethanol Laden or not. Since Ethanol I have seen more rusted fuel lines.

I have a 1968 C3 with it's 427 and 4 speed as well. A few years ago the car was running roughly and I found a tiny Pinhole (if that big) in my fuel supply line and the steel line was shooting a small mist of gasoline out the hole which dried up on the ground very quickly. Well if the hole can leak gas then in turn it will pull air into your fuel system when the fuel pump is pulling fuel. The more air that leaks into the fuel line the leaner it could get.

Check to be sure that you are not pulling any air into the system. I saw that happen on my C3 about a decade ago and replaced the fuel lines. The ethanol collects and sits in the lowest spot. I had a similar thing happen to my C4 where a fuel line rusted shut internally and prevented enough flow for the engine to run at speeds above 10-15 mph. It had the proper pressure but not enough volume of gasoline to allow the car to run normally.

The only other thing that could cause symptoms like those would be an Ignition coil starting to fail. As they continue to fail the RPM at which they break up is dropping and getting lower as they age. I have had a rash of bad coils from suppliers like MSD.

Just a couple ideas that had not been brought up before. Let us know what you find and I hope it goes well for you!

 

Who manufactured your fuel pump....If its a carter its probably bad. If you are pinching the return line closed and it suddenly has enough fuel you most likely are losing volume and pressure down that line

the fuel pressure check mentioned above will tell you alot

 

Bad Rad

When you say "good vacuum" what do you mean?

Is your vacuum advance currently plugged into a MANIFOLD vacuum source?

If so, your issue may be ignition timing. That original emissions control distributor would expect your Quadrajet to provide PORTED vacuum source supplying ignition advance at higher rpm.

If builder didn't sort out your distributor for manifold vacuum source, you likely are not getting enough advance at higher rpm, say 3000.

Mapman

 

FWIW I had a similar problem many years ago. An engine that would normally spin 6500 rpm with no problem suddenly started breaking up at 6000, then 5500. Turned out the plugs were breaking down. Fresh set of plugs solved the problem.