Topic: Engine hesitation
in Forum: C3 Engines
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My '75, according to the engine casting number, has a '72 307 engine in it. It seems to have plenty of power (although more is always nicer), but it always acts cold-natured when stepping on the gas quickly. It acts like the accelerator pump is not working, but it does squirt a good flow of gas when stepped on.
I had my mechanic look at it, and he adjusted the float levels, but it is still hesitates badly when stepping down on it quickly.
The carb is a Holly double-pumper -- could it simply be too much carb for this engine? Any suggestions? (I really can't afford a new carb. . .)
Thanks for all of the help you guys have provided!
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Bill, no Holley expert here but hesitation is usually the accelerator pump or the secondaries opening too soon. Since you mention the acc pump seems to be working, I would suggest the secondaries. Does your Holley have vacuum or mechanical secondaries ?? If vacuum, it can be adjusted by changing the internal spring in the diaphragm. not sure about mechanical secondary adjustment, check out the Holley website, they've got some good technical resources there. There are also a coupe adjustments for the accelerator pump to consider, the pumps should have adjustment screws plus the nozzles can be changed to larger dia. holes to deliver more fuel, again check out the Holey website for more info. Hope this helps, let us know what you find.
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There are different cams available for the Holley accelerator pump also. Since the pump seems functional then I agree that either there's too big a shot initially on tip-in or the secondaries are opening too fast. Vacuum secondaries can be adjusted by changing out the diaphragm spring. Not sure about mechanicals. If it's a 307 still, then good chance that it is over carburetored.
Above is why I like Q-jets.
Good news about Holleys is there are a lot of adjustments. The bad news about Holleys is there are a lot of adjustments.
|UPDATED|12/9/2016 1:59:45 PM (AZT)|/UPDATED|
Above is why I like Q-jets.
Good news about Holleys is there are a lot of adjustments. The bad news about Holleys is there are a lot of adjustments.
|UPDATED|12/9/2016 1:59:45 PM (AZT)|/UPDATED|
1973 L-82 4 spd
No idea on the cfm of the carb but just the fact that it is a double pumper I would think it's way too much carb for that engine.
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DanT
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Vette(s): 1973 Orange Metallic Coupe (orig owner), L82, 4 spd (WR), PS, (A/C & PW (I installed from wrecked 73)), leather, AM/FM Stereo, ran with '65 FI unit earlier & will again some day.
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Too bad it's not a spreadbore Holley, then you could just swap it out for a Q-jet. Problem solved. 
If you ease into to it a little before you mash it, does it still bog?
If you ease into to it a little before you mash it, does it still bog?
1973 L-82 4 spd
Vman73 said: Bill, no Holley expert here but hesitation is usually the accelerator pump or the secondaries opening too soon. Since you mention the acc pump seems to be working, I would suggest the secondaries. Does your Holley have vacuum or mechanical secondaries ?? If vacuum, it can be adjusted by changing the internal spring in the diaphragm. not sure about mechanical secondary adjustment, check out the Holley website, they've got some good technical resources there. There are also a coupe adjustments for the accelerator pump to consider, the pumps should have adjustment screws plus the nozzles can be changed to larger dia. holes to deliver more fuel, again check out the Holey website for more info. Hope this helps, let us know what you find.
I thought we were on to something with the secondaries opening too soon. Looking at them there was a bit of play where they could open with little resistance, so I put a light spring on them to hold them shut tighter. Problem solved, I thought... but when I took it out again -- same issue.
The engine has good steady vacuum (27 inches) so it looks like the intake valves and idle adjustments are okay.
I looked at a Holley forum and got some good pointers there, but nothing seems to point to a resolution.
Thanks again!
73shark said: Too bad it's not a spreadbore Holley, then you could just swap it out for a Q-jet. Problem solved.
If you ease into to it a little before you mash it, does it still bog?
If you ease into to it a little before you mash it, does it still bog?
If I ease into it, it is fine. But that's no fun!
I've worked on a lot of Q-jets, but never a Holley.
|UPDATED|12/11/2016 9:21:37 AM (AZT)|/UPDATED|
danascar said: No idea on the cfm of the carb but just the fact that it is a double pumper I would think it's way too much carb for that engine.
Actually, I stated that this is a "double pumper" but while it has float bowls front and back, there is only an accelerator pump on the front section. There are castings on the back (secondary) side for the accelerator pieces, but they are not there. I don't know much about Holleys, but I thought that maybe this is a cheaper version of one.
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I think the double-pumpers are for cammed-up big engines.
You'll just need to play with the pump stroke and rate which is fine with cams and springs. It's been about ten years since I worked on a friend's Holley on a 427/425 and it had vacuum secondaries. Which could be tuned using various springs. If playing with the pump doesn't help and the engine is still a 307 then you might want to get a spread bore manifold and a Q-jet.
You'll just need to play with the pump stroke and rate which is fine with cams and springs. It's been about ten years since I worked on a friend's Holley on a 427/425 and it had vacuum secondaries. Which could be tuned using various springs. If playing with the pump doesn't help and the engine is still a 307 then you might want to get a spread bore manifold and a Q-jet.
1973 L-82 4 spd
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Depending on the cam in the 307, a four barrel may STILL be to much for the digestion of fuel.
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