QuestionGT6 1971, sat in the garage for 25 years, now much restored. When I picked it up from the restorer the engine was a bit rough (and noisy because the exhaust pipe contacted the chassis) but it ran. But when I thought I'd got it reasonable it ran OK for a few miles then wouldn't idle.
Since then it's had new coil, ballast resistor, I've stripped down the carbs, reground the valves (cause of the roughness I think) and it still won't idle below 2000 - 3000 rpm, much popping in the exhaust (sounds like weak mixture to me) - except that after some particularly alarming bangs in the exhaust it started to run perfectly. Idled in the garage happily for ?an hour? but 1/2 a mile up the road it went back to 2500 rpm min.
Compressions are now about 130-140 psi, they were 125ish before I did the valves and it ran then.
Baffled. Can you help?
AnswerHi Dick,
Your symptoms don't pin point the problem. When I was working in dealerships and shops I learned not to target any particular part when I received a car to work on like yours. I used a systematic method of diagnosis which never failed to find a problem and most of the time I was serprised what it was.
I always start with the basics, Compression, fire and fuel is all an engine needs to run with conditions on each, Enough compression (you got that out of the way, 130 to 140 should run fine, Was that done with throttle wide open? If not do it again.) Fuel needs to be available to the combustion chamber and roughly in the correct % with air.
Fire, (Ignition) needs to be close to factory specs and be strong at the plug (strong = wide blue spark) (if you don't have a scope)
Since compression is not an issue Fire and Fuel are. A firm rule is to do all testing while it is in it's bad made. Testing a car while it is running good is like asking your doctor. "Hay!Doc. my arm use to hurt, it feels fine now, what was wrong with it?"
Checking fire is not hard even if you don't have a scope. When it is in it's bad running mode take a pair of insulated pliers and pull one plug wire off while it is idling and let the spark jump to the plug (not in the sun light) the spark should easily jump a 1/4 inch and be blue in color and be thick (pensile lead size) This just is a rough test of the power of the spark. Now if that is ok set the timing to factory specs. Most specs are set static anyway. This may have to be done when it runs good because you can't set the timing at 2000 RPM. (if you are using a timing light) Now, put a timing light on it and rev it to 2000 RPM and note the amount of advance. Get it into it's failing mode and look at the timing with the timing light. if it is about the same as it was when it was running correctly proceed to Fuel.
For any engine to go from below 1000 RPM up to 2000 to 3000 RPM it MUST receive air from somewhere. If the mixture were to go rich when at idle it would slow down and stumble. So get it in it's failed mode and start pinching off all vacuum hoses (don't forget the vacuum brake booster) Also lift up and push down on the carbs them selves and while it is idling fast. Then cup your hand over the intake of each carb one at a time to try to isolate the problem to one or the other carb.
When testing for fuel you just need to know if it is lean or rich. There are several ways to test this. When it is idling too fast spray some WD-40 into each carb and note the change. If it slows the engine down you have a problem in the carb. If it speeds up and maybe even smoothes out a little it is a vacuum leak.
Run these tests and let me know,
Howard
