[Duke94]

First post need help with bucking on accel.-UPDATE

This is my first post on this Forum, came over from one of the Corvette Forums. Hopefully we can help each other out. I posted this in the other Forum and so far have not got to root cause.
My 78' 4 speed has started bucking during acceleration. Worst at part throttle. Lars just rebuilt the Q-Jet (it did it before he rebuilt it too). I have installed the Lars recommended vacuum advance and set timing per his paper. Still does it. I removed my MSD distributor and box and installed the bone stock 78' HEI. Still does it. It idles fine but seems to misfire during off idle light load. I have not touched the carb since Lars rebuilt it (though it is tempting). Compression is 160-170 on all cylinders. Installed new plugs also. Tried retarting timing, no vacuum advance=no improvement. It has good power once it gets past the bucking. Where do I go from here?

Anyone have any ideas?


MY UPDATE IS AT THE END OF THE THREAD.:cheers:

 

[WIT911]

Welcome to DC! We have a lot of C3 experts here, so I'm sure one of them will be able to get to the bottom of your problem. :thumbsup:

 

[lsejlowe]

really does sound like a fuel issue, like it's starving for some gas. How long have you owned the car? Any chance a previous owner put an extra filter in the fuel line somewhere?

 

[Duke94]

really does sound like a fuel issue, like it's starving for some gas. How long have you owned the car? Any chance a previous owner put an extra filter in the fuel line somewhere?

I have owned the car for 10 years and have rebuilt it frome the ground up. Fuel pump is 3 years old as is the sock in the tank. I think I may have had tunnel vision on this problem and "ass" umed it was ignition. I have a electric fuel pressure gage installed and the pressure always stays at 7-8 PSI. Could a bad pump show good pressure, but not deliver the flow? I have another pump I can try, but changing the pump is a PITA.

 

[jbeidl]

I had a problem like this with a 68 coupe (72 350/300 engine). This was over 15 years ago so I don't remember the exact details. There was a missing spring for the top butterfly of the Q'jet secondary. It would open and close by itself when the throttle was opened quickly. Hope this helps, sorry I can't remember the details.

The best I can remember, I could not find a part number for the spring. I found a local carb repair shop that had some Q'jets laying around and I was able to get the spring from one of them. Fixed my problem.

John :cheers:

 

[big2bird]

Well, let's ASSUME for the moment it's not the dizzy/carb. Lars usually tests the carbs on an engine before shipping.
How are the plug wires?
Checked for vacuum leaks?
Are you running an EGR? Carbon can block the valve from closing. Ask me how I know this. Connect hand vacuum pump and apply vacuum. Engine RPM should drop, or valve is leaking.
Fuel pump should pump 1 pint in 30 seconds at idle.
Disconnect the fuel pump inlet. Connect vacuum gauge . Turn over till maximum vacuum reading attained. Minimum 12" or replace.
If it's the 71, the coil wire beneath the point mounting plate can rub/short out at certain advance positions. Ask Easyasone2three.
If it's the 78, the HEI solder joints can be bad. Ask Noonie.
Hope this helps. All I can think of.
TimAT. Where are you?

 

[Duke94]

Thanks for the ideas. I think I have now ruled out the dist since I have had 2 complete different systems on it and both did the same thing. Lars just rebuilt the carb and ran it on an engine so I will eliminate that. I have removed and plugged every vacuum line to no avail. I will attack the fuel pump next.

 

[TimAT]

Just for grins, look at the coil. I know the whole thing has been changed, but hey, it's a shot. I've seen a coil fail and shoot an intermittent to ground INSIDE the coil. Thing will jump and buck, but idle fine, and then run fine, but then without any warning, do it all over again. After that, how old are the plug wires? Take a spray bottle, mix some dish soap and water, and soak the secondary side of the igniton- from the cap to the plugs- including the dizzy.

:cheers:

 

[Fred Mann]

Thanks for the ideas. I think I have now ruled out the dist since I have had 2 complete different systems on it and both did the same thing. Lars just rebuilt the carb and ran it on an engine so I will eliminate that. I have removed and plugged every vacuum line to no avail. I will attack the fuel pump next.

You said it has good power once it gets past the bucking. I would not think the pump wouldn't be working under a light load and then start at higher rpm.

Any back-firing during the bucking? Carb base tight on the manifold? Vapor-lock? Water in the fuel? Other than the carb, have you made any major changes to anything? I dunno, just throwing spaghetti at the wall.

 

[Duke94]

You said it has good power once it gets past the bucking. I would not think the pump wouldn't be working under a light load and then start at higher rpm.

Any back-firing during the bucking? Carb base tight on the manifold? Vapor-lock? Water in the fuel? Other than the carb, have you made any major changes to anything? I dunno, just throwing spaghetti at the wall.

No back firing at all. I have not made any changes other than the Lars carb rebuild. It does still have last falls gas in it. I will try a different coil.

 

[big2bird]

No back firing at all. I have not made any changes other than the Lars carb rebuild. It does still have last falls gas in it. I will try a different coil.

Try adding a pint of alcohol. Will absorb the water and burn .

 

[Duke94]

Try adding a pint of alcohol. Will absorb the water and burn .

Added 42 bucks worth of premium gas (1/2 tank!!) and a jug of alky. Still runs like crap:down: . Tomorrow I will check the plug wires and replace the fuel pump.

 

[standup]

......I have a electric fuel pressure gage installed and the pressure always stays at 7-8 PSI......

Shouldn't the pressure be around 4 PSI ? That's what it is on my BB 800cfm....maybe you're getting too much fuel ?

 

[Duke94]

Shouldn't the pressure be around 4 PSI ? That's what it is on my BB 800cfm....maybe you're getting too much fuel ?

I have no idea what it should be, but I thought 6-7 PSI was a good number. Anyone know what the spec is?

 

[big2bird]

I have no idea what it should be, but I thought 6-7 PSI was a good number. Anyone know what the spec is?

7.5-9

 

[Duke94]

I've dumped the dry gas in the tank and no help. I have a vacuum gage on the dash and noticed that when I give it 1/3 throttle (under load), the vacuum level goes to near zero, from 18"-20" at idle. I never noticed it dropping this low before. Pulled the air cleaner and when I put my hand on top the carb to restrict air flow, the idle smoothes out and increased in speed. I assume this means I have a vacuum leak, right? I disconnected all external vac hoses and there was no change. Sprayed Gumout at the manifold gasket and carb joints and didin't notice any change in idle. Any ideas?

 

[010752]

Check to make sure your secondary air valve is operating smoothly and doesn't open too quickly
Then check to make sure your APT is operating smoothly and not sticking. It may have to be removed and both the barrel and piston polished a little.

I'm sure Lars set both correctly, but with the great gas we get these days, winter storage may have gummed up the apt a little causing it to stick at times. If you end up taking the top off, clean it out with some Injection Throttle Body Cleaner in a spray can.

Some crap in the gas may have gotten in the carb from installation. It is a good idea to run a long hose to a bucket and pump out a pint or so with the sarter before changing carb filters or carb.

Here is a pdf GM downloadable manual for the qjets.

http://www.bdub.net/manuals/Quadrajet_Service_Manual_1981.pdf

Good luck.:thumbsup:

 

[Duke94]

Check to make sure your secondary air valve is operating smoothly and doesn't open too quickly
Then check to make sure your APT is operating smoothly and not sticking. It may have to be removed and both the barrel and piston polished a little.

I'm sure Lars set both correctly, but with the great gas we get these days, winter storage may have gummed up the apt a little causing it to stick at times. If you end up taking the top off, clean it out with some Injection Throttle Body Cleaner in a spray can.

Some crap in the gas may have gotten in the carb from installation. It is a good idea to run a long hose to a bucket and pump out a pint or so with the sarter before changing carb filters or carb.

Here is a pdf GM downloadable manual for the qjets.

http://www.bdub.net/manuals/Quadrajet_Service_Manual_1981.pdf

Good luck.:thumbsup:

You don't think I have a vacuum leak? I just got the carb back from Lars a couple weeks ago. It did this even before he rebuilt the carb. At the risk of sounding stupid, what is "APT"?

 

[gtr1999]

Something to check, probably not your porblem, but I had a problem years ago where the car cut out under load. I was thinking ignition but couldn't find anything. Then I remembered I installed regualr gas hose install of the "S" bent pump hoses and found the hose was crimping under load and shutting off gas to the carb. To test it I put a hose clamp on the line where it was supposed to be form bent and it held it open and problem solved. I actually left that clamp on the line until I stripped the car down for a body on resto 8 years ago!

 

[Duke94]

Do I have a vacuum leak?

I guess I would still like someones opinion on what I posted earlier:

I have a vacuum gage on the dash and noticed that when I give it 1/3 throttle (under load), the vacuum level goes to near zero, from 18"-20" at idle. I never noticed it dropping this low before. Pulled the air cleaner and when I put my hand on top the carb to restrict air flow, the idle smoothes out and increased in speed. I assume this means I have a vacuum leak, right? :huh: I disconnected all external vac hoses and there was no change. Sprayed Gumout at the manifold gasket and carb joints and didin't notice any change in idle. Any ideas?