craigblock
02-03-2007, 10:19:51 AM
I've been slowly chasing the poor idle problem that I talked about in another thread. I found that the brake booster could not hold vacuum, so I blocked it off to keep it out of the equation. If only it were that simple though.
The engine has a misfire at any rpm, and at low rpm, it's bad enough for it to not even run, which is "poor idle quality" if you ask me. What I'd like to get is some ideas on tracking it down. Here is as much detail as I can think to provide:
If I have the engine revving from 1000-2000rpm, the exhaust sounds pretty normal, but every two seconds, maybe slightly longer, I hear a crackle out the exhaust. It's not loud to the point of making someone jump, it's something you have to listen for. Each time the exhaust makes that crackle, the vacuum gauge drops by about 1.5-2in-hg. The engine then runs normally for a couple seconds, and the process repeats, over and over. If I turn the revs down to below 1000 (the problem zone) the crackles are more frequent, and the engine doesn't "survive" them, making it stall. That explains why I've never had a chance at setting the idle screws correctly.
To find out what I was hearing out the exhaust, I purposely retarded the timing a fair bit, and of course, it crackles nonstop until it either stalls or I turn the timing back up. It is definitely the same noise that it makes when it misfires. I have also tries excessively advancing the timing just to see if I can keep it from happening, and it does not work.
The ignition system is new from battery to spark plug, so things like ignition problems can be ruled out. When I think back, the crackling I hear has been happening even when I was running different ignition components, but I had an exhaust on at the time with such a deep drone that I ignored it, thinking it was just something else.
So, assuming that my ignition system is fine, (which it should be, although I _will_ substitute parts to check), what would cause an _intermittent_ misfire at any rpm. I'm thinking cam or a sticking or broken valve spring. I'm looking for ideas on internals; the ignition system will be easy to isolate if that is the real problem. I still have my doubts about the carb or fueling system as well, given that it has been happening with at least 3 carbs and a brand new pump and fuel lines (and a gas tank you could eat out of).
The engine has a misfire at any rpm, and at low rpm, it's bad enough for it to not even run, which is "poor idle quality" if you ask me. What I'd like to get is some ideas on tracking it down. Here is as much detail as I can think to provide:
If I have the engine revving from 1000-2000rpm, the exhaust sounds pretty normal, but every two seconds, maybe slightly longer, I hear a crackle out the exhaust. It's not loud to the point of making someone jump, it's something you have to listen for. Each time the exhaust makes that crackle, the vacuum gauge drops by about 1.5-2in-hg. The engine then runs normally for a couple seconds, and the process repeats, over and over. If I turn the revs down to below 1000 (the problem zone) the crackles are more frequent, and the engine doesn't "survive" them, making it stall. That explains why I've never had a chance at setting the idle screws correctly.
To find out what I was hearing out the exhaust, I purposely retarded the timing a fair bit, and of course, it crackles nonstop until it either stalls or I turn the timing back up. It is definitely the same noise that it makes when it misfires. I have also tries excessively advancing the timing just to see if I can keep it from happening, and it does not work.
The ignition system is new from battery to spark plug, so things like ignition problems can be ruled out. When I think back, the crackling I hear has been happening even when I was running different ignition components, but I had an exhaust on at the time with such a deep drone that I ignored it, thinking it was just something else.
So, assuming that my ignition system is fine, (which it should be, although I _will_ substitute parts to check), what would cause an _intermittent_ misfire at any rpm. I'm thinking cam or a sticking or broken valve spring. I'm looking for ideas on internals; the ignition system will be easy to isolate if that is the real problem. I still have my doubts about the carb or fueling system as well, given that it has been happening with at least 3 carbs and a brand new pump and fuel lines (and a gas tank you could eat out of).