This is a long one so bear with me. (see update at bottom)

When I bought my Vette it would "stutter" when I put my foot on the gas at around 2500-3500 RPM. If you eased up the RPM it would run OK to about 4000 RPM. Didn't matter whether the car was hot, cold, had full tank of gas etc. It also occasionally acted like a pig when you acclerated at lights. Wanting to stutter and stall. Most of the time it behaved at low speed until it hit 3000 RPM

Since then I have replaced the spark plugs, spark plug leads, set the dwell, set the timing. Replaced the alternator and battery. I also had the quadrajet rebuilt (twice). I have adjusted the mixtures on the QJET but to no avail.

Since rebuilding the quadrajet (both times) it intermittitently does one of the following:

1. Stutters at 2500 RPM. It then will want to die and stall. Pushing the clutch in and removing the load from the engine lets it run again but it still wants to stall under load. Seemed like it was running out of gas?

2. On one occasion, without me changing anything, it drove like a new car ie threw me in the back of the seat , revved great up to 5500 (sorry engine). This was at 8:30 PM at night on a cool Florida night if that matters.

3. Behaves as per in (1) but backfires as well.

At idle it seems smooth and revs freely to any RPM whilst it is sitting still under load.

I have checked the fuel filter and replaced it but no change. I now have a clear glass filter in line with the QJET and can see plenty of gas but as you rev it their does appear to be air in the filter. It is mounted horizontal right beside the QJET and a friend tells me air is normal if it is mounted horizontally.

I have plenty of advice and I am about to do the following.

a. check/replace fuel sock in fuel tank (and fuel tank for sediment)
update: rusty tank,no sock, new tank here next week!

b. check fuel pump flow rate (a quart in 30 secs)
Good pressure 5-51/2psi and good flow rate

c. timing again
update: once electronic ignition is in

d. check points dwell again
update: ordered Pertonix electronic ignition, arrived, have to install

e. check points advance
update: N/A - electronic ignition, arrived, have to install

f. flexible fuel hose
update: ordered new hoses from Ecklers

g. replace QJET with another fulley rebuilt HOLLEY QJET
update: second QJET next week but not a HOLLEY rebuilt

h. rebuild distributor to remove vertical movement
update: will check when I put in Pertronic kit

i. drink lots of beer and cry
update: not crying so much (well except for the tank ouch!) but beer is so good.....

Any other ideas/advice/experience. After having it run properly once I would like to experience it every day. I think the QJET has made a problem I had
worse rather then being the actual problem. I am getting a completing different rebuilt QJET to remove that variable this week.

Any help appreciated.

Pete :cheers:


[Modified by stingry, 2:46 AM 3/14/2003]

 

Replace the fuel pump first. Sounds exactly like a problem I had due to a marginal and failing fuel pump.

 

I like to measure things first before I start changing everything. I agree it sounds like the fuel pump. I'd fit a fuel pressure guage into the line after the fuel pump and see if the fuel pressure is ok, around 5psi, when these problems occur....I tape the gauge to the outside of the windscreen with the wiper door open to bring the fuel line out. That way you can see the fuel pressure exactly when the stuttering happens.

Another long shot....I once set the timing at 6deg with the vacuum advance still connected.....however the car consistentl misbehaved, not intermittently, pretty dumb huh!

Good luck, Joe

 

If it's a fuel delivery problem then i would check the sock in the tank (had trouble there myself) and check the fuel pressure. If these two are good you should check for manifold leaks. You can do this with a can of WD40. With the engine idling spray a small amount around the edge of the manifold and also around the base of the carb. If there are any leaks you will see a change in idle. If there is no change temporarily disconnect the hose to the manifold vacuum fitting and plug it to see if you have leaks in your hose system. If there is no change the hoses are OK. If there is a change start checking the hoses to see which one is bad. Then move on to the PCV and Evap hoses if you have them. Disconnect each one and plug the port on the carb to see if it makes a difference. If the car runs well with either one plugged then you have to either replace the PCV or you have to service the Evap cannister (replace). If none of this bears fruit I would suggest you have a carb problem. Now I know there are a lot of people here who are proponents of the Q-Jet and God knows Lars is the forum expert in residence but in my experience they're boat anchors. I know you can rebuild them and get them to perform admirable but you have to be an expert at it as they are very complicated (flame suit on) :lol: . My numbers matching original carb never worked right but i attributed it to age. I bought a brand new GM Q-Jet when they still manufactured them and it ran well for a few years and probably only about 10K miles if that. After that it had the same problems as the old one. I recently converted over to a new Edelbrock carb and I can tell you I'll never go back. That's my opinion for what its worth. If you want to stick with your Q-Jet I would see if you can get Lars to rebuild it or go to a recommended company that specializes in them and guarantees their work. I may have my two Q-Jets rebuilt just to keep them on the shelf for the future but I'd be hard pressed to install one over the Edelbrock right now. Hope some of this helps you to diagnose your problem. Good luck.

 

Great advice.

Tested fuel pump tonight. Taped gauge to windshield and observed 5-51/2 psi under all revs (that it wanted to do anyway). Pump didn't drop when engine played up. Also pumps a neat 2 cups in 15 secs (quart in 30 as per manual).

Found sock was missing in gas tank. Tank did have plenty of rust and CHUNKS of rubbish in it though. Ordered a new tank and sock today. Also ordered a PERTEK ignition kit.

Checked the fuel filter and it had plenty of small particles trapped in it from the tank. The QJET did not ahve a filter in it so I am guessing it is full of tank particles from the first two or three days I drove it without an inline filter. I am taking it back to the guy who built it and getting it rebuilt (again) and a filter added. I am guessing that there is junk in the bowl and jets which is adding or causing my problem. Add the possibility that chunks can get sucked through the fuel line causing blockage and erratic flow rate.

Once I get the new fuel tank, sock, clean lines, add electronic ignition, get the QJET back and set the timing I will add an update.

Thanks for you help.

Pete :)

 

Thanks for your help. I will check for vaccumn leaks once I get the other issues sorted. See my update above.

Pete
:cheers:

 

Sounds like a weak spark to me. Coil?

 

I had symptoms like this with my firebird and It was fixed with a distrubutor rebuild.

 

After the tank sock thats what I was thinking. Coils can do some strange things when heated.
John

 

Did you check the PCV valve? My little 350 crusier had a stumble that went away after I replaced my PCV. Just a thought, it seems that you may have dirty fuel issues.