hesitation on hard acceleration - Team Camaro Tech
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post #1 of 9 (permalink) Old Oct 29th, 09, 04:52 PM Thread Starter
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Ron
 
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hesitation on hard acceleration

This problem seems to be getting worse and not sure where to start first. When I accelerate hard the motor loses power/hesitates but if I go easy there is no problem. Even when I'm not moving, just idling and hit the pedal hard there is a noticeable hesitation. I have new wires, plugs, accel ignition coil, holley fuel pump, fuel filter, points, cap & rotor; around 100 miles on these parts. This spring installed 268 cam, lifters, 2.02 summit heads, timing chain, edelbrock performer mani & used quadrajet which appears to be in good shape but I am now wondering. My timing is set at 12 degrees, starts fine. After a few days of sitting I have to pump the pedal quite a bit, but assume that's from the carb gas evaporating. I've pulled some of the plugs and they have some carbon build up on them but don't think that's the problem. My first instinct tells me it's not getting enough gas. What do you guys think and how and where should I start first. Oh yeah, I also replaced the entire fuel line from tank to carb. All advice welcome, thanks.
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post #2 of 9 (permalink) Old Oct 29th, 09, 08:41 PM
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Re: hesitation on hard acceleration

Timing and carb setup are the main issues you need to fine tune. A vacuum gauge is a valuable tool for both. Vaccum advance is desirable. Vacuum leaks can cause it and might explain the "seems to be getting worse" aspect of it.

Go to the home page of this site and check out the tech reference link. There are some carberator tuning tips burried in there. I'd check the tension on the secondary air valve clock spring. If it's too loose, the secondaries can open too quickly and essentially slow the air flow past the venturis to the point that they can't deliver fuel until the RPMs wind up.

Your problem is common and there are at least 50 things to check. Hopefully the tech reference will get you started.

Dave
========================
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #3 of 9 (permalink) Old Oct 30th, 09, 01:09 AM
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Jess
 
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Re: hesitation on hard acceleration

Get the vacuum gauge. I would say you have a gasket leak around the carb or vacuum leak somewhere. Make sure all your vacuum hoses are in good shape, not cracked or have split ends around the fittings. Or that none have came loose, "it seems to be getting worse" could be explanation. You could also check your fuel filter, is it in line or in the carb. Simplest and quickest things I check first. Hope this helps.
Jess

Last edited by Jess67; Oct 30th, 09 at 01:34 AM.
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post #4 of 9 (permalink) Old Oct 30th, 09, 06:17 AM Thread Starter
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Ron
 
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Re: hesitation on hard acceleration

Fuel filter is one of those inline Summit aluminum filters, it's new but I guess I'll pull it off and see if I can blow any air through it, easy to do. I have one vacuum hose which comes from the distributor and that is new, I plugged all the other fittings on the carb. A clogged pcv valve wouldn't cause this would it? I did notice the valve cover where I have the pcv valve was warm and the other cover where I had a breather was cool. I'm thinking the pcv valve does not allow as much air out so that is why the cover is warm but...
I know there are several issues it could be that's why I put it out here, thinking if I got enough of the same thoughts than I would start there. I'll take a look at the tech reference and see if I can nail this down.
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post #5 of 9 (permalink) Old Oct 30th, 09, 01:52 PM
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Re: hesitation on hard acceleration

Sounds like the dreaded Quadra-Bog, usually caused by a too loose secondary air flap.
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post #6 of 9 (permalink) Old Nov 3rd, 09, 04:47 PM
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Re: hesitation on hard acceleration

The Quadrajet is a wonderful carb when set up properly. I used to race-tweak these things for a lot of people and they were consistently amazed that a "Quadrajunk" could easily perform as well as a Holley.

I have not worked on one for about 25 years, but the very first thing I would check is your accelerator pump shot. With engine off, remove air cleaner and hold choke plate open. Then go to WOT (wide open throttle) position using your hand (or a buddy inside the car). You should see a healthy squirt of fuel on each side of the primaries - two healthy squirts total.

The Quadrajet carb came in a wide variety of flavors, but one mistake I often saw was that people would "rebuild" them and they'd lose a check ball (or two), without even noticing. So it's quite possible you're missing a check ball, which would allow the pump shot system to lose its' prime, and thus it would hesitate.

There are quite a few little tricks, but if you spend some time and do some research, you can get that carb performing very well. Here's a site with plenty of links to various Q-Jet articles: http://www.florida4x4.com/tech/quadrajet/index.php

It's important to tune the secondary flap to the right tension. Also, there is a vacuum dashpot there that holds it closed, check to make sure that is operable.

Good luck!

Dave F. in Rhode Island
'68 Camaro SS Accel DFI 454 (SOLD - I'm a boring Corvette owner now)
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post #7 of 9 (permalink) Old Nov 4th, 09, 03:03 AM
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Re: hesitation on hard acceleration

As said, timing and secondary air valve tension adjustment.
Too loose of tension, secondary air valve opens too soon, engine bogs.
Too little initial advance, engine backfires.

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post #8 of 9 (permalink) Old Nov 4th, 09, 05:42 AM
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Re: hesitation on hard acceleration

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Originally Posted by rjg67rs View Post
This problem seems to be getting worse and not sure where to start first. When I accelerate hard the motor loses power/hesitates but if I go easy there is no problem. Even when I'm not moving, just idling and hit the pedal hard there is a noticeable hesitation. I have new wires, plugs, accel ignition coil, holley fuel pump, fuel filter, points, cap & rotor; around 100 miles on these parts. This spring installed 268 cam, lifters, 2.02 summit heads, timing chain, edelbrock performer mani & used quadrajet which appears to be in good shape but I am now wondering. My timing is set at 12 degrees, starts fine. After a few days of sitting I have to pump the pedal quite a bit, but assume that's from the carb gas evaporating. I've pulled some of the plugs and they have some carbon build up on them but don't think that's the problem. My first instinct tells me it's not getting enough gas. What do you guys think and how and where should I start first. Oh yeah, I also replaced the entire fuel line from tank to carb. All advice welcome, thanks.
The early Q-Jets(pre 75) have a bad issue with the well plugs leaking. They leak fuel directly into the intake. This is 99% the reason for you having to pump the pedal after it sitting and also bogging issue at idle. What happens is fuel leaks from the fuel bowl into the intake and causes a rich condition. To solve this issue, you need to remove the carb and pull off the bottom plate. The part that has the throttle plates in it. You will see two large brass cups that sit directly under the discs that are pressed into the main body that act as the secondary jets. The brass cups have nearly a 100% leak ratio. To correct the issue you need to remove the existing plugs, tap the casting, get yourself some Marine-Tex, make yourself some new plugs(7/16) out of an old bolt, coat the threads in the main body, not the plug and screw them in. Then you can dab some epoxy over the top of the plugs. Allow to dry and this will PERMANENTLY solve the issue. You can dab epoxy over the brass cups, but this is only a TEMPORARY fix. The leaking plugs cause driveability issues and idle issues. PLEASE, do not jump what everyone is going to say and ditch the Q-Jet. The Q-Jet is a great carb. It would be easy to install a Holley, but there is no need to. I run a Q-Jet and it runs great. I have a later model(78) and they don't have the leaking plug issue, but yours can be fixed. It just takes a little effort. I would also look at the secondary air valve. The easiest method for adjustment, since you already have a bog, is to tighten the spring in 1/16 turn increments until the bog is gone. There is also a vacuum that controls how fast the air valve opens. Usually this is only an issue if the valve doesn't open fast enough and you are able to feel the transition when the secondaries come online. This transition should be seemless. If there is a bog when the secondary comes online, it's usually not the vacuum break.

Charlie Knudsen 67 Camaro RS/SS 350 TKO 600, 4.10's, AFR 195 Street Heads, Comp 270H, Rhoads Lifters, Crane Ignition, Pertronix Dizzy, Performer RPM Q-Jet, Heavily Modified Q-Jet, Hooker Comp Headers, MagnaFlow 2 1/2", 144 Decibles(Alpine CDA-7998 Head Unit, RF X7 and RF 750X Amps, MB Quart Mid's & High's, Kicker L7 12" Solo Barics)
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post #9 of 9 (permalink) Old Nov 4th, 09, 03:47 PM Thread Starter
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Ron
 
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Re: hesitation on hard acceleration

Thanks for all your advice guys, it seems like a carb issue and I may tackle rebuilding it over the winter, just not sure how difficult that will be. I would rather fix it before replacing it, cheaper I think too therefore keeping the wife happy.
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