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Update, I did re check the dist weights, they are moving properly. I wired the exhaust heat riser up, test drove and no improvement. I removed the wire when hot and it looks like the riser is opening by itself. The riser doesn’t move to 90* open but fairly close by itself(not wired in place).
lm moving on to the fuel pump next? Not sure how best to check flow?
Update, I did re check the dist weights, they are moving properly. I wired the exhaust heat riser up, test drove and no improvement. I removed the wire when hot and it looks like the riser is opening by itself. The riser doesn’t move to 90* open but fairly close by itself(not wired in place).
lm moving on to the fuel pump next? Not sure how best to check flow?
Best way is to use an isolated fuel pressure gauge to be sure you have proper pressure under all driving conditions.
If your pump is weak, possibly due to ethanol attacking the diapragm, it won't keep up.
If the fuel pressure nose dives at any rpm, and the engine performance falls off soon after, you have found the weak pump.
It been a busy schedule but I’m finally back to troubleshoot this rig. How quickly should the secondaries open when transitioning from idle to WOT? I did a quick throttle movement to wide open throttle and didn’t notice any movement on the secondary “butterfly’s”.
For continuity please read the main parts of this thread, it may help from repeating every question.
Regards,Kevin
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
The secondary butterflies you cannot see with out openning the air flap manually and looking down into the carb throat. IF your choke is engaged they will not open either
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
the air flap will only open with the engine under load. if you drive around with the hood off or put a camera in there you can see it open. It has a spring and a set screw that controls how much air flow/ engine vacuum signal it takes to open it which will also control how fast it opens. If you stomp on it and it flips open fast you get the quadra-bog that was coined in the 70's. It was just poor carb tuning. If it doesnt open at all under heavy acceleration then someone may have adjusted it to tight, you have to loosen the set screw which is an allen head then use a flat head screwdriver to adjust the spring....there was n old thread around here on how to do it..
If Lars worked his magic on the carb, you should not have to worry about it. Unless someone else has changed any settings since.
The engine dyno'd well, so it's not a cam problem.
I think that points to low fuel psi.
That one you need to check while driving. Like this:
This NCRS judged 454 ran very flat past 2500-3000. This is how the guys checked the fuel psi going down the road. It dropped to 1 psi.
Last edited by leigh1322; Jul 14, 2024 at 11:20 PM.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
bEnt support rod for the gas pedal too.
The zip tie on the Throttle cable is always a good indicator of achieving wide open throttle or having someone else mat the pedal while the mechanic is looking down the throat, then twisting the Throttle more to see if there is anymore Throttle too be had
Last edited by Rescue Rogers; Jul 17, 2024 at 07:59 PM.
Update:
I purchased a fuel pressure testing kit and only getting 1 psi at idle and maybe 1.5 psi at 3000 rpm. Should I do a road test while the equipment is hooked up? I don’t know if much will be learned for the time.
From my research it sounds like I should be at 5 to 6 psi for the quadrajet, do most “stock” or OEM type pumps do this without a regulator?
Any suggestions for a quality OEM type of mechanical pump? I’m pretty excited to get this on the road with the new pump and see how it performs.
KC
Well you certainly found your problem!
While you are in there I would check to see if the fuel pump cam lobe is wiped out, it happens.
The other possible cause is the fuel pump itself.
All the available replacement ones all seem of lesser quality than they were back in the day.
Either the diaphragms fail, they do not like ethanol type gas, or the pivot pins come loose.
If you need correct get what is hopefully a quality replacement.
If you need a really correct one, hopefully yours is an original AC. I would not throw that away, in fact they can be rebuilt, but not easily.
A more reliable choice would be an aftermarket Holley or Edelbrock mechanical pump, but they do not look very original.
But they are better built and the diaphragms can be replaced.
Hhmmm, I replaced my mechanical fuel pump today and was getting ready for the “victory lap”. I put some grease on the rod and everything looked like it seated correct. I’m not getting fuel pressure and the engine only starts with a bit of prime in the carb. I still have the fuel pressure gauge hooked up and no pressure with the starter engaged.
1. Fuel is in the tank and clean.
2. I pulled the supply line to the fuel pump and good flow from tank.
3. I did try to prime at the carb.
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