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12/8/16 4:48pm - Original Message: 'Engine hesitation'
BillHanna
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Cana, VA - USA

Vette(s):
1975 Stingray

Joined: 7/3/2016
Posts: 63
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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12/8/16 8:24pm - Reply: 'Re: Engine hesitation'
Vman73
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Gig Harbor, WA - USA

Vette(s):
1973 L82 4 Speed Coupe - Very original, well documented driver/survivor. 2016 Bend Regional NCRS Top Flight.

Joined: 10/2/2014
Posts: 353
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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12/9/16 4:59pm - Reply: 'Re: Engine hesitation'
73shark
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Overland Park, KS - USA

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.

Joined: 7/9/2003
Posts: 376
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|


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1973 L-82 4 spd

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12/9/16 5:39pm - Reply: 'Re: Engine hesitation'
danascar
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Germansville, PA - USA

Vette(s):
1974 Coupe 358ci 4 speed Black w/Medium Saddle interior w/79 custom leather seats

Joined: 1/28/2010
Posts: 763
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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12/9/16 5:58pm - Reply: 'Re: Engine hesitation'
73shark
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Overland Park, KS - USA

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.

Joined: 7/9/2003
Posts: 376
Too bad it's not a spreadbore Holley, then you could just swap it out for a Q-jet.  Problem solved.  Wink

If you ease into to it a little before you mash it, does it still bog?


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1973 L-82 4 spd

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12/11/16 12:19pm - Reply: 'Re: Engine hesitation'
BillHanna
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Cana, VA - USA

Vette(s):
1975 Stingray

Joined: 7/3/2016
Posts: 63
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!



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12/11/16 12:21pm - Reply: 'Re: Engine hesitation'
BillHanna
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Cana, VA - USA

Vette(s):
1975 Stingray

Joined: 7/3/2016
Posts: 63
73shark said: Too bad it's not a spreadbore Holley, then you could just swap it out for a Q-jet.  Problem solved.  Wink

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|


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12/11/16 12:25pm - Reply: 'Re: Engine hesitation'
BillHanna
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Cana, VA - USA

Vette(s):
1975 Stingray

Joined: 7/3/2016
Posts: 63
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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12/11/16 4:58pm - Reply: 'Re: Engine hesitation'
73shark
Standard Member
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Overland Park, KS - USA

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.

Joined: 7/9/2003
Posts: 376
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.

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1973 L-82 4 spd

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12/11/16 6:30pm - Reply: 'Re: Engine hesitation'
manchestershark Lifetime Member
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Equinunk, PA - USA

Vette(s):
1972 conv, 4-speed, 350, 200hp, numbers match, rally wheels, war bonnett yellow w/white top. good condition, nice driver.

Joined: 10/31/2007
Posts: 1895
Depending on the cam in the 307, a four barrel may STILL be to much for the digestion of fuel.

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12/11/16 6:30pm - Reply: 'Re: Engine hesitation'
manchestershark Lifetime Member
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Equinunk, PA - USA

Vette(s):
1972 conv, 4-speed, 350, 200hp, numbers match, rally wheels, war bonnett yellow w/white top. good condition, nice driver.

Joined: 10/31/2007
Posts: 1895
Depending on the cam in the 307, a four barrel may STILL be to much for the digestion of fuel.

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12/11/16 8:17pm - Reply: 'Re: Engine hesitation'
Tomtall
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Vette(s):
1973 Coupe -- 454 / 4 speed - Black on Black, Greenwood aftermarket turbo hood, Greenwood GTO style front bumper.American Racing Vector rims.

Joined: 3/5/2016
Posts: 43
Bill - Somthing that I havn't seen discused yet is ignition timming. If your timming is not set correctly it can cause serious hesitation issues. Also don't assume your harmonic balancer timming marks are correct. Dampners go bad and the outer rings can move causing fales readings of timming. I have repaired many a big block and small block for this issue over the last 40 years. You also didn't mention what size of cfm Holley your running. A 307 cu. in. Chevy usally will be quit happy with a 650 cfm carb. on a street driven vehicle. To large a cfm carb. can cause tunning issues.
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12/11/16 8:20pm - Reply: 'Re: Engine hesitation'
Tomtall
Limited Member
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Vette(s):
1973 Coupe -- 454 / 4 speed - Black on Black, Greenwood aftermarket turbo hood, Greenwood GTO style front bumper.American Racing Vector rims.

Joined: 3/5/2016
Posts: 43
Bill - Somthing that I havn't seen discused yet is ignition timming. If your timming is not set correctly it can cause serious hesitation issues. Also don't assume your harmonic balancer timming marks are correct. Dampners go bad and the outer rings can move causing fales readings of timming. I have repaired many a big block and small block for this issue over the last 40 years. You also didn't mention what size of cfm Holley your running. A 307 cu. in. Chevy usally will be quit happy with a 650 cfm carb. on a street driven vehicle. Too large a cfm carb. can cause tunning issues.
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