Intermittent Stumble

Hey guys, I have a 1972 Buick Skylark 350 with an 800cfm EDP Q-jet I bought about 2 years ago. Unfortunately due to work and funds being tight my car has sat over the past year as I had to tackle a few big projects (new 200-4r transmission and power disc brake swap)

Finally able to drive my car again I notice things have changed quite a bit. I took the car to the track an went from running consistent 14.4 1/4mi to 15.5. Up top the car was breaking up and sometimes would stumble as well. The more I drove the car I noticed this issue would come and go.

Sometimes the car runs great and pulls right up to 4800 rpm, other times I would get plenty of surging and sometimes almost no power at all. There is no relation to temperature, moisture or amount of time the car has been running.

What could have changed that would be causing these symptoms? I haven't played with the car at all other than to swap the trans and do the disc brake conversion. I've noticed it seems the secondary air doors open a little too easy. A blip of the throttle in park can get the doors to open.

I just don't understand how this has suddenly become an issue and wonder if more is going on. I do know the best way to tackle this is to go out and get a vacuum gauge, vacuum pump and set up a fuel pressure gauge and make sure everything is working as it should be but in the interest of time I would appreciate if you guys had any tips from previous experience.

This is the original fuel system, I have only replaced the fuel pump with a cheapo autozone unit a long time ago. Honestly with the miles and hours I have on the Q-jet I would have no qualms sending it back to Ken to be freshened up as well as upgrading the fuel system. I would much rather replace 40 year old parts than to start throwing band-aids at things.

 

Might be that the sock in the tank has a film or restriction of some sort. 40 years is a long time.

 

Here are a couple of basic things to check/change.
Fuel filter.
Rubber fuel lines for cracks, even return lines
Spark plugs. Sometimes after sitting, a couple of hard starts can gas foul a plug.

I would guess there is some junk in the fuel somewhere based on the description. Fuel filter is easy. If you can set up a fuel pressure gauge before the carb to view while driving, that might be helpful in diagnosis.

Good luck with it.

 

Makes plenty of sense that the issue I have is fuel related. Can anyone recommend a decent street setup? I can spend a little bit of money on this and would rather just replace everything this winter versus spending time diagnosing my current 40 year old setup. I am on the fence on whether I should do an electric or mechanical setup. Not sure what direction I am going with the engine so I am trying to go with the most flexible setup.

 

You can't normally see the secondary air doors open up with the car in park and stationary. There normally has to be a load (being driven) on the engine before the secondary air doors will open. On all the cars I have ever owned with Quadrajets - I have never seen the secondary air doors open while the car is just sitting. There is a tension adjustment for the secondary air doors located on the passenger side of the carb. It consists of a single slotted screw and an allen headed screw and a spring. Once you find it - you'll understand how it works. Look at the end of the secondary air door shaft to find these adjustment screws. A lot of people don't even know about this adjustment. I always adjusted the secondary air doors as tight as I could get it without it binding. Try different adjustments to fit your preference. An early opening of the secondary air doors will cause a bog (too much air too soon). Hope this helps. 66rivigeezer

 

With the exception of surging I've had similar issues with points being out of adjustment. If you are running points, stick a dwell meter on it and make sure you aren't looking for one answer to two different problems.

 

I have a spare set of points and that was the first thing I did was swap the points. I know the dwell was set right because they were the set I removed for replacement when I did my last tune up but I stuck a meter on there to be sure.

I removed the quadrajet over the weekend and found some fuel pooled up on the spacer and the primary throttle shaft and bowl plugs were wet with fuel. Car was sitting for a week prior and I didn't start it before I removed the carb. Could be anything but after 3 years and 40,000 miles I felt it was time for a rebuild anyway so it's getting sent to Ken for a rebuild and to have some dress up done.

I also ordered an aeromotive stealth fuel tank and will be feeding the qjet through -8 line. I want to start from fresh since I intend to keep this a street strip car and originality isn't a major concern. I just want a close to factory appearing look.

Will be sure to post back if this all takes care of the issue

next up is to decide what direction I want to go for an ignition System

 

I would say that the air doors were causing my bog but since the issue is intermittent I think it's more of a fuel starvation. Car always ran fine until I let it sit for a year so I am assuming either ccontaminants settled in the tank and clogged things up or I have a pinhole leak somewhere.

 

how about the choke pulloff since it controls the air valve opening, also if the secondary lockout is still present, it might be sticking, good luck

 

Any update or solution to what your problem was ? Now having the same issue, car sat for 2 months while getting an axle replaced, after repair work done, car wouldn't crank. After numerous attempts, pulled the fuel line to the carb and no gas coming through, guy at shop put an air line in the fuel tank, just a little enough to push the gas through and "presto" gas comes a flowing, car cranks and running but noticed ever since, car from time to time will hesitate and stumble, rough idle. Twice cut completely off (as if out of gas and would not start back for at least 15-20 min.) let it sit, starts and drives as normal, as if nothing happen. Replacing rubber fuel lines today (just installed new rubber lines about 4 years ago when I removed the tank for cleaning and re-installed)

 

I am fairly confident my issue is due to weak valve springs or a worn cam. I have checked everything I can short of doing a compression and leak down test. I had the carb rebuilt. I replaced the entire fuel system and I checked all my wires with a DMM and replaced and regapped the sparkplugs. Changed the points and set the dwell as well. No major variations in dwell at idle but I will change the distributor as there is some play and the vacuum advance sticks slightly

The major change before I noticed all these issues was putting in a new transmission that shifts at 5000rpm. Sometimes it pulls great, sometimes it won't pull at all. So it's most likely the case that the stock cam just doesn't make power after 4500rpm. Couple that with a 40 year old valve train and I'm sure I get a decent amount of valve float.

 

Think i found the problem, under car (rear to front) inspecting fuel lines, and doing all types of trouble shooting, get to front of car with a flash light and thats when i see it, signs of gas all around the back side of the fuel pump and front lower corner of engine. From top side and front of car can't see it, but under neath and there it is. Guessing and pretty sure my fuel pump is leaking around the bowl and shooting gas out at high pressure momments, each time the car has shut off, were after highway runs of 75-80 mph or after i stomp on it, immediately after shuts off as if running out of gas, let sit for 20 minutes and numerous attempts of starting, and "bamm" starts right up and runs as if nothing happen.. leaking fuel pump

 

Just to bring this thread back from the dead as I've been scrolling through some old posts, the issue was a few wiped cam lobes. There were a few cupped lifters as well. My own fault as later on in my ownership of this car I relied on some off the shelf oil with supposedly high levels of zddp instead of using the additive. Needless to say, not enough zinc additive and 5000+ rpm was too much for the stock flat tappet valvetrain.

 

Hate to hear that but at least you have found the problem and know the solution for future reference.

Thanks for closing the loop.

 

Almost a year later I went from planning a simple cam swap to pretty much replacing everything on the car. I shouldn't be allowed to touch things, I always go overboard =\ Hopefully the car will be done by this March.

http://www.v8buick.com/showthread.php?285708-Buick-350-Shortblock-build&highlight=Shortblock+build

 

You can request your name be changed to "Snowball" p