I am having trouble with my 1969 Buick Skylark. She is misfiring when I am in drive, stopped at a light or stop sign and sometimes it goes away and other times it gets rough and when I then step on the gas, she tries to die and sometimes succeeds. I have done these things and still no results: new spark plugs gaped the same thirty thousands of an inch, new plug wires, checked distributor cap, its good and clean. Checked vacuum reads 18inches. Timing is 2 degrees advanced, if I advance anymore gets very annoying pinging. lol Dwell is at 30 degrees. Rebuilt carburetor Rochester Quadrajet adjusted fuel screws and idle screw. I noticed this misfire happening after when voltage regulator went out and burned up the wire from regulator to alternator, did not get all new wire but cut off burned end and clamped new ring and cleaned it up. put a voltage meter on it while car was running and I had it connected to battery and was reading 14. something volts so a friend that was helping said that was good, he told me that it means alternator is charging the battery. I noticed now the engine shakes more than usual, motor mounts are good so not sure what could cause that. so with all of that, she still misses and I am over my head on this and have no idea. Some more information, recently changed timing chain and cam gear. Rebuilt oil pump, newer fuel pump. Everything else is original. Odometer reads 90,000 original miles. If anyone has a suggestion or another way to diagnose this problem I would appreciate the input. Thank you.
I'd start with a cylinder-balance test. Short out the spark to each cylinder in turn, watch vacuum gauge to see if each cylinder is contributing the same amount of power. I have a cylinder-balance tester; but a 12-volt test light or even a small screwdriver and a long ground-strap can be used. Ground the test light or screwdriver, slide the point of the test light or screwdriver BETWEEN the plug wire and the distributor cap boot, so the spark to that cylinder is grounded. A bit of silicone grease on the probe helps it to slide between the boot and the wire. DO NOT PIERCE the boot or wire. Grounding the spark to one cylinder will reduce the vacuum. Assure that all cylinders reduce the vacuum about the same amount. If the spark is grounded, but the vacuum doesn't drop--that cylinder is lazy and not contributing. If you find a weak cylinder, or cylinders, you'll be doing a cranking compression test. If the compression is reasonably even among all cylinders, you'd start looking for vacuum leaks or other causes of uneven mixture distribution.
"Alrighty, I understand what your saying. I will have to try that on my next day off. lol Ill let you know the results. I do have a question, if I had a vacuum leak, wouldnt my vacuum already be lower than 18inch? and How would I be able to dertermine the air fuel mix is percise. I understand the concept of adjusting and I have, but I just adjusted the fuel screws evenly till the engine was not bogging while I steped on the gas when driving. As for the air/idle screw,( was told two different names for that screw) I adjusted till engine was a decent idle and when I go to trun the key off, engine would stop and not try to keep running. First car, only car, still learning. lol but I love my car. And Thank you for your help. ---------- Post added at 09:16 PM ---------- Previous post was at 09:04 PM ---------- She has the original engine. When I was dealing with the cam gear I had both valve covers off to replace gaskets and looked at each head and could not find any damage or cracks or broken anything, just some sludge and little oil. once I kinda cleaned around still looked fine, no signs of extreme wear or damage. But that work was a while ago, like 3 months ago. How would I tell if a rocker retainer did break off? Would it just be a big mess in there with pieces and what not? But I'll double check that and let you know what I find. Thank you.
He's not talking about adjusting the vacuum,he's saying,use the vacuum reading as a reference when you ground out 1 plug at a time. If there isn't a change in the vacuum reading when grounding out a plug(or just pulling 1 plug wire at a time when its running,but be careful not to get shocked) THAT is the bad cylinder that is misfiring. So when disconnecting one plug at a time,the vacuum SHOULD go down when you look at the vacuum gauge to know that cylinder IS firing. Could be a bad plug wire,a bad plug,or a bad cap if you're lucky. If not,you need to do a compression test to make sure the heads or rings are still good.GL Derek
