Hello all. Last year I bought a 1969 Corvette 427/390hp. The car has 52k miles on it but it was restored several years ago that included a motor rebuild. I have spent the last year trying to fix the sins of the previous owner. My friend owns a speed shop and he has been doing most of the work.

My problem is that the motor chokes and stumbles when accelerating somewhere over 4000 rpm. It will not smoothly accelerate. We have cleaned the fuel lines and replaced the fuel pump, installed the correct rebuilt quadrajet carb, replaced the intake manifold, installed an electronic ignition module, new plug wires, smoke tested the vacuum system, compression test, and a multitude of other checks...but it still chokes.

The only abnormality we can find is that if the plugs are pulled immediately after a test run the #1 plug (drivers side front cylinder) is a little darker than the other plugs. We are out of ideas.

Any thoughts??

 

Have you tried I different test carb … sometimes those quadrajet carbs can be finicky.. sounds like your not getting enough flow …did you check if the choke is sticking and the carb is opening full

 

Have you checked the fuel pickup sock in the fuel tank. You can see it if you take the gas cap off and look down inside with a light. Mine came apart and blocked the flow of fuel.

 

Sounds like classic fuel starvation. A 427 is gonna want a lot of fuel at WOT.
it could be as simple as float level is too low, or inlet valve orifice for the float is too small, an inadequate fuel pump, etc. If the carb was rebuilt but not modified to supply a high performance engine such as drilling the float inlet office, I would start there.

 

Do you have a spare ignition coil that you could try? I have a 1968 C3 with it's 427 and I have had several coils fail in the past decade. The ignition coils are not what they "used to be" and their quality leaves much to be desired.

A faulty ignition coil would do exactly what you are describing. I have seen the very same issue on my C3 with a coil that was on it's way "Out". They start and run fine but when you start cruising down the road and you start to wind the engine up the coils starts to misfire. The ignition coils also can fail due to heat as the coils are exposed to plenty sitting on top of a HOT engine. I have been using MSD ignition parts for decades and had no issue until I started buying coils over and over. MSD suggests that we not use the round canister type coil and switch to a square epoxy ignition coil which are much more expensive than the canister type.

Heat and engine vibrations can break down an ignition coil and make it fail. MSD suggests that we mount our ignition coils on the firewall and not on the engine.

How much fuel pressure are you getting at the carburetor at 4-5000 rpm? Have you watched it while the car is accelerating? Let another drive while you watch carefully. Maybe the fuel pressure overcomes the needle and seat and starts to flood the engine at higher rpm's due to the fuel pressure being above the normal settings but not at slower speeds.

How much timing are you giving the 427? I run 36*-38* total on my high compression 427. Be absolutely sure that your vacuum hose on the distributor is working and attached to the right vacuum source. If the timing is not advancing enough the engine will not like it. This might lead to misfiring at engine speeds. When the vacuum hose broke going to the vacuum advance, my engine started running much hotter than normal due to less than full timing at higher speeds.

Is your distributor the original or has it been changed? An HEI distributor does what you are describing when their module has gone bad. I love the HEI distributor as it is easy to blame them for all sorts of ill running issues. I have one in my C4 so I have a spare module, Coil, cap & rotor and more in my spares shelf. I do NOT trust the HEI system but tolerate it on my C4. I take spares when I travel just in case...

Did your C3 come with the Factory Transistorized Ignition? This system uses a special coil and is different from the factory points style ignition. My C3 came with the K66 transistorized ignition, I had the parts rebuilt and then I stashed them away. I have all MSD Ignition on the engine now.

 

Where did the rebuilt Quadrajet come from?? If you have read the Forum more than a couple of times, you will recall all the horror stories surrounding rebuilt carburetors, or even the NEW Chinese knockoffs. If all the other good suggestions don’t help, maybe it’s a carburetor issue. Good Luck

 

Like those above ask, who rebuilt this carb? These q-jets often get butchered or assembled incorrectly or with wrong parts since the internals vary so much model to model and year to year. Even if they did a good job, and it ran fine on their test engine (if that was even done) then that is no guarantee it’ll run perfectly out of the box on your engine.

I agree with those above me that it sounds like fuel starvation. Verify the float level spec for your year and model of engine (they change year to year), and adjust the float height if needed. Accelerating at 4000rpm and up your secondaries should be open, perhaps your secondary air valves don’t have the right spring tension. Look up how to adjust this, it’s very easy.

If not fuel starvation then it may be timing hurting you at high rpm. Verify that your vacuum advance can is actuating and not leaking. Confirm that centrifugal advance weights are not gummed up and slide nicely.

 

If your friend owns a speed shop he’s gotta have access to a dyno. Throw it on the dyno with an exhaust sniffer and get the AFR.
it’ll tell you right away if it’s a fuel issue.
is AFR is good then its ignition related. Bad timing curve or spark is breaking up at high RPM.
while the pull is going on use a timing light and validate your timing curve.
hook up a vac gauge and watch that too while the pull is going on.
i guess what I’m getting at is gather data and then go from there vice parts cannon.

 

 

Thank you for all of the great suggestions.

The distributor was spun and checked. We upgraded the points to an ignition module with new coil and new plug wires. The timing was checked several times but I don't know what it is set at currently.
The fuel lines were drained and cleaned and the pump and filter was replaced. For a time it was pressurizing the fuel tank, but I've replaced the gas cap.
The quadrojet carb was purchased from a shop that re-builds carbs. The car came with a quadrojet for a 350 not a 427.
They smoke tested the vacuum system and several vacuum issues were fixed.
I always thought it was electrical but we've eliminated that. Maybe it's starving for fuel. To me, it's running like the timing is advancing too far under load or the vacuum secondaries aren't working properly.
At this point, we are out of ideas.
In April, we have an appointment to put it on the dyno.

 

I would find out what the timing is actually set to. It’s one of the greatest variables that can impact how an engine performs.

 

Your carb is too small you need a 750 cfm carb else it will lean out in the high rpm’s

 

All Quadrajets are at least 750 CFM. Plus they came on the car from the factory generally without issues at high RPMs.

 

That's likely the problem. The carb is simply not set up correctly. Are you running the factory in-line fuel filter? I'll bet the expert shop put a fuel filter in the carb as well, not understanding that you cannot run two filters in series, so you have a high-rpm fuel starvation problem, too. It's also likely that they "rebuilt" the carb with a commercially available choke pulloff, which will not correctly control the secondaries. It's also probable that the secondary airvalve is not adjusted right.

The Q-Jet doesn't have vacuum secondaries. They are mechanical. But they can still be set up incorrectly, which I bet they are.

All Chevy 4MV Q-Jets (as used in 1969) are 750 cfm. An undersized carb will go rich at high rpm. There is no problem using a small block carb on a big block if you set it up right.


Lars

 

Lars is one of the very few people who can rebuild and test your Q-Jet and ensure it is done correctly.
Contact him direct, off line, by e-mail. Sometimes by just posting a clear carb picture here he can provide professional guidance, if he has time.V8FastCars@msn.com

 

its either the carb or the fuel pump...Im leaning toward the fuel pump....if its a carter, pull the pump and look and see if the pump arm pivot pin hasnt come out of the casting....

 

for what it's worth,I'd take a look at the rubber s hose that goes from the tank line to the fuel pump,

 

Thank you everyone for your input. I will pass it along and discuss these options with my shop.

Last Summer, we did find crud in the fuel lines. They were flushed. The inline filter, fuel pump, and gas cap was replaced. The carb does not have the filter installed in it.

When accelerating under load (highway on ramp) and the rpm goes over approx 4,000 the engine chokes out completely. It stutters and bucks. It even blew-out the left rear muffler.

To me this is right about the time when the secondaries are opening or the timing is advancing.

 

I discussed your suggestions with my mechanic. He is pretty adamant that he either checked or replaced most of the things on your suggestions...but we still don't know what is wrong.

I talked to him specifically about my rebuilt quadrajet. He knows a local guy who only works on carbs. However, I was able to contact Lars at V8FastCars in Colorado. He is suspicious about the rebuilt carb and he wants pictures of both of my quadrajets. For 'piece of mind' and for the process of elimination, I'm gonna spend the money and send the rebuilt quadrajet to him. I want the carb back before my April dyno appointment.

I will let you all know what we find out. Thanks so much!