Quadrajet runs better without accelerator pump.

[NathansCorvette]

Hello all,

1974 Corvette L-82 stock quadrajet. I had been struggling with tracking down the cause of an issue for a long time until recently. The issue was this: Car started right up when cold, idled and sounded great. If I was to rev the engine it was snappy and responsive. Once the engine got up to temp, the throttle response greatly diminished. This resulted in a bogging sensation that could nearly kill the car if repeatedly rev'd. It made 1st gear starts awkward.

I wrote an exhaustive list of every suggestion that was made on CorvetteForum and went through checking/re-adjusting each item. This included timing, idle mixture, re-torquing carb/intake bolts, isolating all vacuum lines, verifying the vac advance works, verifying choke works, checking for vapor lock/heat soak, etc... No change to the issue at all.

Any time there is a post about throttle hesitation the first thing everybody says is "Your accelerator pump is weak and not giving enough gas! There's a momentary lean condition!" Back when I rebuilt this carb, I found the old accelerator pump completely broken and replaced it with the new one in the Cliff's High Performance rebuild kit. Keep this in mind that before the issue started, the accelerator pump was giving no fuel shot.

At the very end of my list was to see if messing with the accelerator pump changed anything. First, I moved the pump rod to the outer hole on the pump arm. Immediately, I noticed that the issue improved, but was still present. Eagerly, I removed the pump arm to completely prevent the accelerator pump from actuating...

The throttle hesitation disappeared! Perfect snappy revs even at full temp now!

But this is weird. There must be some super rich condition where the pump was making it even more rich.

Yes. The idle mixture screws are set correctly. If I move them in any more, it will drop in idle speed and idle poorly. And yes the timing is spot on... I set it according to Lars' paper. And yes I am fully confident in my carb rebuild. I was very methodical and followed a guide by Lars and used Cliff's kit.

My two theories:
Too strong of a power piston spring, causing the power piston to not be fully shut at idle and/or snap wide open when throttle is touched. This should be a very simply fix, if the case. How can I test for this?
The rebuild kit from Cliff's included what is listed on the website as the "HP Accelerator Pump" which "...will deliver more fuel to the engine than a standard pump.". Also available on the site, but not included in the rebuild kits is the "Standard Accelerator Pump". Which should deliver less gas. But remember even with the pump arm in the outer hole (less pump shot) the issue was still half-there.

Linked below is a video I made that shows the issue, shows the acceptor pump fully hooked up sending a full shot of gas at idle, and showing the revs with the pump disconnected.

https://youtu.be/l7fC5txC7a8

Any advice would help, thanks!

Nathan

[Kenth]

Need more info.
Float setting?
Inlet needle clip in correct location?
Idle mixture needles position?
Airhorn gasket correctly mounted for power piston movement?

Ignition system type used?
If points, dwell setting?
Correct voltage to the coil for ignition system used?
Initial timing setting (w/o vakuum advance)?
Distributor weights and springs not stuck?

[NathansCorvette]

Float setting: recently set it to the height specified in my 1974 gm shop manual. I don't remember that exact number. It was set lower before I changed it. Correcting the height made no difference to the issue.

Needle Clip: in the correct location on the float arm.

Idle mixture screws: set with a tachometer and vacuum gauge. I have ran the screws all the way in and all the way out and everywhere between and it made no change to the issue.

Airhorn gasket: it is in correct position and allows power piston to move.

Ignition system: uses points. all new points, cap, rotor, condenser, and coil, put on in attempt to isolate the issue, no change.

Dwell setting: set with a dwell meter to the setting specified on the inspection sticker. No change.

Voltage to coil: Not sure... it's a new coil and changing it from the old one made no difference. It's an MSD blaster II, old one was an AC delco.

Initial timing: 8* without vac advance. Like the idle mixture, I turned it all the way up, down, and in between and no change in the issue. I found it runs best at 8* initial. Yes, my vac advance can works perfectly.

Distributor weights: I had cleaned them as they were a little sticky. No change to issue.

Remember that the ONLY thing that affected the issue was changing/removing the accelerator pump. Or when the pump was still in, the issue would ONLY start once the car warmed up. Something temp-dependent... Not even changing the idle mixture screws affected the issue. I have been running the car for a month now with no accelerator pump and it's running better than ever. But there must be some underlying cause for this to be the case...

[Pav8427]

Hi Nathan.
 I followed your thread. Sounds like a tough one.
Not sure if this helps, but a couple things to check.
Keep in mind, me and electrical stuff is like me trying to skin a elephant. I could get it done, but would take a lot longer if I had to do it myself.
A couple things I have read on other forums that may or may not cause what you have going on.
I have read that some of the aftermarket distributer caps can cause the advance weights to hang up.
The web on the bottom of the cap actually hitting the weights and keeping from advancing or retarding.
Easy check. Easy fix with a dremel.
Another was a thread where someone set dwell with a dwell meter on their dial back timing light and double checking the point gap was way off of the .018-.020? gap.
They went back and set gap and dwell meter was way off the 30ish*
Easyish check if you have another dwell meter to double check with.
As far as it being a carb issue, I cant think of anything and I am following so I can learn from those who skin elephants more often than I.

[Kenth]

The original ignition coil has 1.8-2.0 Ohm primary resistance and about 1.3 Ohm added from the resitor wire with engine running.

MSD Blaster 2 coil has 0.7 Ohm primary windings and is aimed for electronic ignition systems. Running this coil with points requires a special resistor to get 3.1-3.5 ohms with engine running or the points will burn out and degrade performance.

Using the correct ignition coil could be a good start when troubleshooting.

[73ss]

X2 on the coil resistance.

Have you tried removing the main vacuum hose that feeds the vacuum system and plugging the port? Lots of places for a vacuum leak on a C3, headlight doors & vacuum relays, wiper system, etc.

Maybe you have a hidden vacuum leak and have the idle screw too high to compensate the leak and have too much transfer slot exposed causing a rich condition?

edit: Watched the vid again and it looks like you have it capped off.

[quadrajam]

......Tough one. Usually the other way around. How bout metering rods too short or hanging
too high.... PP spring way too strong.... No vacuum under power piston...

Reach in through the vent tube with a tiny screwdriver and check that you can push on
the hanger and move the PP up and down. Then check to see that its down at idle.

QJ

[NathansCorvette]

X2 on the coil resistance.

Have you tried removing the main vacuum hose that feeds the vacuum system and plugging the port? Lots of places for a vacuum leak on a C3, headlight doors & vacuum relays, wiper system, etc.

Maybe you have a hidden vacuum leak and have the idle screw too high to compensate the leak and have too much transfer slot exposed causing a rich condition?

edit: Watched the vid again and it looks like you have it capped off.

Yes, I tried capping every single vacuum port off... no change in the issue. I have also put on new intake manifold gaskets recently in an effort to solve the issue. No change.

[NathansCorvette]

The original ignition coil has 1.8-2.0 Ohm primary resistance and about 1.3 Ohm added from the resitor wire with engine running.

MSD Blaster 2 coil has 0.7 Ohm primary windings and is aimed for electronic ignition systems. Running this coil with points requires a special resistor to get 3.1-3.5 ohms with engine running or the points will burn out and degrade performance.

Using the correct ignition coil could be a good start when troubleshooting.

Again, there was no difference at all when changing from the delco coil to this new one, not in the engine performance or in the issue. Perhaps its some other problem in its own, not that I've noticed anything, but I'm confident it is not what's causing a momentary rich condition when the accelerator pump is actuated with a hot engine... But I will look into the resistance to make sure I'm not frying my points.

[NathansCorvette]

......Tough one. Usually the other way around. How bout metering rods too short or hanging
too high.... PP spring way too strong.... No vacuum under power piston...

Reach in through the vent tube with a tiny screwdriver and check that you can push on
the hanger and move the PP up and down. Then check to see that its down at idle.

QJ

It is the other way around! In my searching, I've found a thousand closed cases of an off-idle bog solved by a new accelerator pump... I can't find ANYTHING about my issue online.

Now you're talking more about the things I'm starting to suspect. I will try moving the power piston like you say.

I don't know anything about metering rod sizes so I'm unsure what type I have.

[Mudsport96]

What is your vacuum at idle?
And I'm thinking you need to go through your rod and jets sizes on the primary side. I had a similar problem and ended up going down in jet size and giving it a little more timing that brought my vacuum up. My rods were raising to early and dumping fuel as soon as I hit the throttle.
And if you don't know what rods you have, it could be anything. Is this a factory carb? Or is it a remanufactured carb?

[NathansCorvette]

What is your vacuum at idle?
And I'm thinking you need to go through your rod and jets sizes on the primary side. I had a similar problem and ended up going down in jet size and giving it a little more timing that brought my vacuum up. My rods were raising to early and dumping fuel as soon as I hit the throttle.
And if you don't know what rods you have, it could be anything. Is this a factory carb? Or is it a remanufactured carb?

My vacuum at idle is around 16". That sounds about right. As soon as I touch the throttle fuel seems to pour out. With the accelerator pump, it was even more. I will disassemble any look at the rods when I get the chance.

It is not the original qjet to the car, but it is not reman'd. I did rebuild it myself. See my post below.

[NathansCorvette]

I should note that my Corvette has a non-original qjet on it. According to the service log I have for it, when it was first barn-found, somebody put a Holley carb on and the original qjet got lost to time. The fellow I bought it from had bought a used 1974 qjet online, I believe he said from a 1974 Camaro, and installed it.

By the time I bought the car, 15 years had past and I rebuilt this non-matching carb with Cliff's kit and by Lars' guide. By looking at the internals and the state of the finish on the outside, I am confident that it is not a remanufactured unit. Mind you, it was never in bad shape, and is super clean after my rebuild.

The carb stamping is 7044502BM. This means its from a 1974 auto trans chevy with California emissions. Could it be that it being a cali emissions carb on a non-cali engine that it could be causing issues? Also my Corvette is a manual, but I doubt that would affect anything...

[Kenth]

According to my notes, there are not big enough differences between 7044502 and for example 7044211 to cause the symptoms you are experiencing.

A too large inlet needle valve seat combined with too high fuel pressure will raise fuel level in bowl and start fuel delivery sooner than intended.