Misfires at constant speed while tachometer jumps around

[124JOE]

well one in the trunk
but check both ends of it

[124JOE]

pull all the spark plugs and look at the top of the pistons
that might show you witch cylinder its having trouble with



i agree it sounds like an air leak

[SouthSwedeSpider]

Head and cylinders are just fine, I installed a new head earlier this year, but that had no effect either way on the rough idle (but the oil consumption went down dramatically ). And I know it sounds like an air leak, I just can't figure out where it is (if there is any, which I doubt). Is there any good way to detect air leaks?

[124JOE]

spraying carb cleaner around the intake top and bottom while running
and listen for a stumble so go slow

also useing a long small hose as a stehascope
poke around and listen for leaks



hope this helps

[Sunracer]

Cables inside distributor are OK, but I discovered that the pickup air gap was 0.6 mm instead of the specified 0.3-0.4 mm. After adjustment it runs a little smoother, still not perfect but good enough for the time being. I will continue to overhaul the ground points, alredy done most of'em in the engine bay, but still have the ones in the trunk to fix (or is it only one back there?)

jWell that's a start-look for the threads on adjusting modifying your air flow meter-yours is probably old and would benefit.

[forzaphil]

I agree with Manoa Matt. It sounds like it might be the ignition control module behind the coil. It seems like sometimes they will start to fail only after heating up/ being driven for a long time. Your misfiring occurs only after being driven for a while right? In the US there are parts stores like autozone where you can take your ignition module to have them test it if you don't feel like guessing on it and after buying a new one finding out it wasn't the problem. Maybe there is some place you can take it to near where you're at in Sweden? Doubt there are any autozones close . But anyway, long story short, it is possible for ignition modules to fail only when heated.

[pro4art]

Missing at light throttle, with tach flucuations usually means the internal wires in the pick up unit are broken. You can't see that, unless you cut out the wires and split them open. Too test, assuming your vacuum unit actually works, disconnect the vac unit and drive it. The vac unit will constantly move the plate that has the pickup mounted on it. The normal running position at light throttle, is were the wires spend most of their time. The copper wires inside the heavy soft outer covering, will fatique over time, break, and then give give the tach an off and on signal.
Don't know if a blown vac unit can give enough air leak at light throttle to make a miss, but shouldn't bother the tach. Use a piece of rubber hose to suck on the vac unit to see if it moves. It also should hold poistion with vacuum.