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Hej guys
im Sorry if this have been ask before, but I feel like I have searched the web thin for an answer.
when I’m driving my vette at cruising no problem, but when I’m flooring it then it sometimes accelerates as normal, but sometimes it completely stops mid acceleration and then the rpm’s just won’t go higher than were they stop
i have put in a new ignition coil, and vacuum advancer (the old vacuum was broken) why I thought I found the problem, but it still does the same. Do the new one need to be adjusted before it works correctly?
note: in the video I have pressed the throttle to the floor and do not lift it at any time.
Sorry for long text and bad English, hope you can help so this summer won’t be missed 😅
To answer your question we’ll need a little more information on your car. What year? Stock or aftermarket carb? Have you changed your fuel filter recently? And what is your timing set to?
It’s a 1977 with what I know of stock engine with the quad jet carb
I replaced the fuel filter in the carb some months ago
I haven’t checked timing yet, but I just got my hands on a timing gun, so I’m doing that as soon as I can, any experience with what it best to time it to? Or just follow the instruction book I have?
I would email Lars at V8fastcars@msn.com and ask for a copy of his timing papers. In the meantime setting it to 12 degrees with the vacuum advance unplugged then putting the vacuum advance on a manifold vacuum source should get you pretty close.
That looks like a nice carb, with some shiny choke tubes. I would check to make sure that choke pulloff holds vacuum, and is hooked up.
Last edited by Piersonpie; Apr 2, 2025 at 03:21 PM.
The distributors vacuum canister is not involved at full throttle. The Vac Can requires vacuum to activate & change timing.
Full throttle has zero vacuum. Canister only works from idle to mid throttle.
Some of the aftermarket units are fully adjustable. The adjustment is usually done with an allen wrench thru the (removed) hose port.
You want about 10 - 12 degrees of Vac Adv added, as seen with a timing light at idle.
But you have something else going on here. Fuel starvation issue. IGN issue. Who knows.
Even a plugged-up exhaust system can act like this.
I would email Lars at V8fastcars@msn.com and ask for a copy of his timing papers. In the meantime setting it to 12 degrees with the vacuum advance unplugged then putting the vacuum advance on a manifold vacuum source should get you pretty close.
That looks like a nice carb, with some shiny choke tubes. I would check to make sure that choke pulloff holds vacuum, and is hooked up.
thanks you very much, I will try that as soon as I can, I’ll make an update when I have tried it 👍🏼
The distributors vacuum canister is not involved at full throttle. The Vac Can requires vacuum to activate & change timing.
Full throttle has zero vacuum. Canister only works from idle to mid throttle.
Some of the aftermarket units are fully adjustable. The adjustment is usually done with an allen wrench thru the (removed) hose port.
You want about 10 - 12 degrees of Vac Adv added, as seen with a timing light at idle.
But you have something else going on here. Fuel starvation issue. IGN issue. Who knows.
Even a plugged-up exhaust system can act like this.
Ah okay, there I learned something new, and I have to ask I’m from Denmark so I don’t know what IGN stands for 😅
oh yeah that’s makes so much sense 😅
and thank you for your input, I will try and check the timing, and after that I will check the exhaust to see if it is plugged up😊
I agree with the check to make sure that there is a vacuum hose connected from carburetor to the vacuum break (choke pulloff).
Also check that the choke pulloff holds a vacuum.
If the pulloff is bad I have some part numbers that are correct for your 77, let me know.
From: At my Bar drinking and wrenching in Lafayette Colorado
Your choke pulloff is disconnected, which causes 2 significant problems:
You have a vacuum leak
You have no control over your secondary airvalves when the secondaries open, which will cause your car to fall flat on its face when you go into the secondaries
If the pulloff is leaking or seized, you need to replace it. Be aware that almost all pulloffs sold by the various vendors are defective: They do not have the correct bleed-off restrictor installed in them, causing the airvalves to open too quickly. This will cause the problem you are having. Make sure that the pulloff you install extends fully in about 1.5 - 2 seconds upon loss of vacuum. Also make sure that your secondary airvalve spring is adjusted correctly to 3/4 turn windup.
The only vendor that I am aware of that is selling correctly calibrated pulloffs is Cliff Ruggles.
Lars
It sounds like the best answer is from Lars, He is an "expert" and very few people know the Quadra Jet like he does. I would follow his advice and it will get you and your C3 back on the road so you can enjoy it this spring!
IF the problem persists and your C3 still has a catalytic converter on it, then I would start looking at the catalytic converter. They do plug up and when they are nearing their end the plug might be making changes inside the catalytic converter. A plugged exhaust or specifically the a plugged catalytic converter will not allow a engine to rev very much at all. The first time it happened to me is when I left my car for a couple months while I finished a trip. When I returned, the car would start but not rev past about 1200-1500 rpm before falling flat on its face. I noticed a exhaust port under the hood that had a rubber cap on it (for checking/sampling the exhaust) where the rubber cap would balloon and turn into a giant ball before deflating when the engine returned to idle. This told me right away that I had a plugged catalytic converter.
If you do have a Catalytic and it is plugged then I would suggest replacing it with a newer style higher flowing catalytic as they have really improved since the 1970's. If you remove the catalytic converter completely it is possible that you might have a bad drone afterwards.
It was luckily a small and easy problem, one of my fuel lines to the pump were broken in the reinforcement metal wires so instead of a soft bent it was almost 90 degrees
but now it works again, thank you all very much for helping me with this 😊
C3 won’t take full throttle
