The Vette has a stumbling problem on hard acceleration. It cruises at speed fine with no audible misses or jerking.
I am a novice at tuning but I feel confident that I followed Lars timing/tuning paper to a T.
I have an adjustable timing light and set the timing to 36 per Lars Paper. I set the dwell to 30 per Lars paper.
Lars rebuilt my Q-Jet and he helped me rebuild my Distributor. I have new plug wires and new plugs.
I dont believe I have any vacuum leaks.
What could I have missed? Should I start from scratch?
This is really bumming me out.

some engine info: Bored 30 over, New pistons etc. HE268H Comp Cam, Edelbrock Performer 2101 intake. TH400. 3.08 rear.

Please Help a Rookie.

Stew

 

Did you replace the fuel filter? Do you have access to a fuel pressure gauge?
Is it the same in every gear and at every speed? What RPM?
Sounds like fuel starvation...

Good luck troubleshooting!

Stefan

 

Yep, new fuel filter. No I dont have a pressure guage. The stumble/chugging happens around 2500 rpm's and only when I punch it.


[Modified by Stewart's74, 2:20 PM 4/10/2003]


[Modified by Stewart's74, 2:51 PM 4/10/2003]

 

When I had a small carb (500 cfm) mine would "build up" (stumble) a bit when I punched then it would kick in. I put a larger carb on (650 cfm) and everything's peachy now. :thumbs:

 

:lurk: I'm interested too. Mine comes and goes though. First time I fixed it by rebuilding the carb. Then it came back the next year and I replaced the fuel pump. Then it happened on and off last year but happened more after putting on the sidepipes :confused:

 

This sounds almost like float levels in the fuel bowls (the punch-it part), but since Lars rebuilt your carb- that's VERY unlikely. Maybe your mixture is off? You have done some quite extensive mods to the car and a stock Q-Jet might not be able to supply what's needed. Did you 'read' the spark plugs as per Lars recommendation?
If it would help you I can scan a page from a Bosch book on car electrics where it shows typical spark plug pictures....

Good luck!

Stefan

 

Was the problem there before the timing was adjusted? You might have to tweak the timing a little. For waht it is worth, I had the same problem, but mine also had an off idle stumble. The problem was a air/fuel mixture issue. Increased the size of the filter, dropped a size in jets, problem gone.

 

I have the same problem. I stopped halfway through trying to solve it but found the following which may help. (thanks to those older forum members that put me on the path..).
I haven't sent my QJET to LARS yet but I will once I crack this nut.

FIRST - LOOK, MEASURE, TEST before spending money

1. The filter in your gas tank (also called the green sock). Check to see if it is there. Mine was not. They are ten bucks from http://www.gastanks.com or ecklers.

2. Check the inside of the gastank. Mine was full of rust flakes. New tank $195 - http://www.gastanks.com.

3. Check fuel filter - inside QJET. If its not there put one in $2-3 at any autoshop

4. Add a bigger clear glass filter prior to the QJET to check that the fuel is there and is clean. $10 pepboys. Mine has some air which still concerns me. I think it is a fram filter which has replaceable filters for $5 for 3.

5. Test fuel pressure using an inline vaccumn gauge that Tees of the fuel line.
I did what someone else suggested and tested it in the garage and also under load. Mine measured 5-5/2 PSI. It think this is OK but an old GM manual a buddy of mine has says 6-7 PSI. The chiltons manual says 5PSI. I will be curious to see what you get. This reading was with my old fuel pump, no filter, crappy gas tank etc. Will let you know when I get it going with all new stuff. You can also check the flow rate of the fuel pump by cranking the engine whilst the ouput of the fuel pum goes in to a bucket /jar etc. You should get 1 QUART in 30 seconds (1 PINT in 15 SECS). Mine was spot on.

Oh yeah you tape the gauge to the OUTSIDE of the window when you drive the car, you don;t want pressurised fuel line inside the cockpit......that can be bad!!!!!!!!!!

5. New fuel pump $30-$40 ACDELCO ecklers/Gm/autoshop

What I think was happening was that the acceleration was sucking rustycrap into my fuel intake and slowing fuel flow, and small bits were partially blocking the fuel filter and then starving the carbeurator. Won't know till I finish putting it all back together. ( I am rebuilding my suspension in the middle of this problem).

:cheers:


[Modified by stingry, 9:28 PM 4/10/2003]


[Modified by stingry, 9:31 PM 4/10/2003]


[Modified by stingry, 3:33 PM 4/13/2003]

 

The problem was there before I messed around with the timing. I'm going to get some more help with the timing (Mechanic friend) and if that doesnt work I'll probably get a new fuel pump. The one on there now is a POS from AutoZone. I'll go Delco this time....Anyone have a part #????

 

Stewart -
Try tightening the secondary airvalve windup spring about another 1/8 turn and see if that smooths it out. If you need help with it, drop me an e-mail and I'll coach you through it.

 

Thanks Lars,
I sent you an e-mail.....hope this works. I hate replacing fuel pumps :sad:

 

Sounds like another Lars to the rescue situation. I don't thing enough can be said about what a great guy Lars is. Thanks from all of us.

 

to me it sound like the accelerator pump.
but check out this article. http://www.dapa.org/tech_articles/rfrazier-qjet.htm

 

Sounds like ignition. On later cars with HEI, a stray spark from a bad rotor can damage the module.

Yours is a 74 so it could be bad wires, plugs or a bad condensor.

Over the many years I'm tried to solve carb problems and found out it was ignition.

Sorry to add more confusion, but igition can show up when a car is under hard acceleration and the fuel mixture is harder to ignite.

 

My car was doing something similar. Bogging if I gave too much throttle anytime rpm was less than 2500. After some tips from the pros and reading Lars' paper on Q-jet tuning I found my secondary air valve had no tension whatsoever on it and was flopping around like a fish. It took a little while to find the tension windup screw, but once I did and adjusted it, to Lars' suggested initial settings, the car quit bogging. Now I can punch it at any RPM and it jumps! :jester

 

I agree!!! No cost, easy to try. My '73 was stumbling HARD on acceleration and adjusting the spring cured it.

Slick