Help diagnose performance problems

[bioandy]

My 1970 350ci 4sp has been running great since I purchased it about 6 months ago. The other day it started running very rough, so I thought I would do the routine tuneup stuff, including points, plugs condenser, cap, rotor, igniton wires. While trying to set the ignition timing, I noticed that the vacuum advance was inoperable, and replaced the vacuum canister on the distributer. Found a couple of other vacuum leaks elsewhere and the car is running much better now. I'm left with two symptoms. The car use to idle fine under 1000RPM, now, unless I set the idle higher, 1200-1400 RPM, the RPM's drop very very slowly untill it eventually stalls. The other problem is a break-up and loss of power over 4000 RPM. I know this stuff is probably elementary to some of you, but thought someone might point me in the right direction.

The engine is a stock 350, with a mild cam (I'm told) Stock Rochester quadrajet carb.

Thanks for any help.

Andy

 

[84Vette]

my car would slowly drop in RPM too until it would stall it turned out to be a bad fuel pump. If your is original it might be weak and dying. Just my .02

 

[cntrhub]

Pull a spark plug....anyone. If it's black and sooty, you better check the ignition system again. Sounds like the advance is stuck returning to idle? Maybe you have it too retarded? How are you checking point gap? Are you using a feeler gauge or a dwell meter to adjust the gap?
4000RPM?.....Those are scary revs for this test! Take the timing light, hook it up to a plug wire....any wire. Rev the engine up steady, and watch the timing light flicker. If the engine starts to cut out and the timing light flickers, it's ignition. If the timing light stays steady, and the engine coughs and spits, it's too rich running at high rpm. This is not too accurate of a test, but this should get you started in one direction to look for.

 

[WhalePirot]

Have a vacuum gauge?

You can tell a lot about the engine from the vacuum reading. How much does your car pull at idle? Should be 18 psi or more, unless the cam is radical.

Is it steady, or wavering?

 

[hotrodd]

Sound's a little like a lean idle mixture.Does it backfire thru the carb. when it is at operating temp.on acceleration ? It is probably a little bit of adjustment and a clean fuel filter.Just something wild happened to me on a tune-up,the center electrode insulator on a spark plug was cracked up inside the plug and would slide up and down,point it up to look at it and it looked fine,just happened to start to lay it down and I felt it click sliding down and closing the gap. Funny that it seemed to run ok until you put it under a load and they were brand new A.C plugs so I was real lucky I found it!! Just some of my experience,hope that helps.

 

[bioandy]

Thanks guys, for all the suggestions.

It will be a little while before I try all of your suggestions. Don't have all the time I'd like to tinker with the vette.

In poking around, I noticed a cracked fuel line on the supply side of the fuel pump. Think I'll take care of this before proceeding further. Could possibly be sucking air. Also embarassed to admit that I didn't think of checking the fuel filter - opps.

Plugs look very good. Used dwell meter to set points to 30. No backfires through carb. Haven't put my vacuume guage on it yet.

The loss of power at 4000 RPM was under load, winding out the gears - not in my driveway. Could be related to the fuel issue above - we'll see.

Thanks again guys, I will attack the problems with all your suggestions in mind.

Andy

 

[Evolution1980]

Maybe you have it too retarded?

Sorry...I know it's childish, but that just made me laugh... :L :L
OK...as you were...

 

[bioandy]

Well, got a chance to check vacuume.

Readings were 12 lbs at idle. Had to hold RPMs at 1500 to acheive 18 lbs. Guage held steady. Do these numbers mean anything, considering I have a non stock cam? I tapped into the vacuume advance line with a T fitting for the test.

We all have our moments of being "too retarded"

Andy

 

[cntrhub]

You might want to manually check the vacuum-chamber diaphram. Do you have a vacuum gauge with a value face on it. Perform with engine off. Load the breaker-plate with vacuum and see if it will hold an airtight seal and not lose pressure. Set vacuum at 15 inches. Does it hold pressure? Did the needle on the face go to zero? If all is well, go to next step. Keep the vacuum gauge on for the next test. Hook up the dwell meter.
Check for a worn breaker-plate bushing: Set idle at 1,000 rpm. Increase the vacuum from 0 to 5 inches while watching the dwell meter. If the reading varies more than 2 degrees, wear in the breaker-plate bushing is indicated.
These are tests that should be tested on a Sun distributor machine. Sometimes, doing it this way will give you close to the same results.
Is there a vacuum leak between the carb and manifold? Use a flameless hand held propane can, and move the gas tip close to the carb and manifold. Does the engine increase in RPM? Keep the air cleaner on for this test. You don't need a lot of gas vapor to make this test work. Work in a safe manor.....make this test outside, and away from buildings. Tjink safety!!

 

[vette-dude]

Don't know if this year motor had 'em but if you have an EGR valve on the intake manifold I would certainly check this for a broken diaghram. If the diaghram is broke it is like having a huge vacuum leak in the carb. If it doesn't have an EGR, sorry, just can't remember when GM and The FEDS decided we needed to clean up the air. I would have to agree with everyone else that it appears you have a vacuum leak somewhere. The other source may be the carb at the throttle plates. Where the shaft goes through the base of the carb does wear out and will cause some of the symptoms you are experiencing. What carb are you running? If it is a Holley, the diaphram in the power valve has a tendency to fail if the carb has had a backfire. This will also cause the problems you are having. Simple fix but be sure if this is it to replace the power valve with the same identical part. Hope you get it straightened out. :cool

 

[vette-dude]

I see it was a rochester Q-jet carb. Sorry about that. I suspect you have a worn throttle plate at this point or the carb is internally leaking ( these carbs are kinda famous for both problems). Someone in this group is an expert on Q-jets. Hope he is there for you.