Misfire

[DaveT]

Need some help. My 78 (stock carb) always ran fine - starts, idles and runs well. Last week on a steady run @ 60-65(about 3000-3500 rpm) I noticed a slight misfire if I tried to accelerate a bit more. Once off the highway, the car ran fine. When putting it away, I sat and revved it to 3000-4500 and it was fine except for an occasional skip. Took it out today and was @ 3000 rpm and really put my foot into it and WOW, the misfire came back with a vengeance. Didn't,t think I'd make it home - consistent stumbling/ misfire. Would come down to idle no problem, could begin to accelerate from a stoplight and would be ok until around 2500rpm and misfire would come back. Got home and checked fuel filter. Looked Ok but will replace tomorrow anyway. Pulled distributor cap - cap, rotor& points look clean but One set was way out of gap @ .021". Set that to The correct gap, started it up and revved all thru the rpm range and it seemed fine. Will take it for a drive tomorrow to see how it goes. Could that one set of points being that far out of gap cause this? Plugs & wires are fairly new. Thanks in advance

[azruss]

sounds like a fuel delivery issue under load. dirty filter or dying fuel pump.

[So Cal Mark]

the points could definitely cause that problem

[DaveT]

Well, the new fuel filter and the reset points didn't help. It still starts right up and idles smoothly but once it's warmed up it'll begin to stumble at around 2500rpm and continue to do so thru the rpm range. I'm gonna pull the plugs to check those but I don't think they're the culprit. I'll also check the timing. Could it be a bad condenser(s)? If it's a fuel pump, what's the best way to check its operation? Or are ther other things to look at before that? Anyone have a checklist?

[124ADDHE]

I once had a chunk of carbon break free and lodge itself in the spark plug, causing a missfire.

[JConnerly]

To add my two cents worth, I had a misfire that was only noticeable at idle since buying the car a year and have been throwing money at the whole time. New cap, rotor, plugs, wires. Checked the timing. Tracked down possible vacuum leaks, checked the compression, adjusted the idle speed. Finally had the valves adjusted last week and it runs and idles as smoothe as silk. Better mileage too. It turned out the number one exhaust valve was off. Just one valve! Something to consider if they haven't been checked in recent memory.

[DaveT]

JConnerly,
Thanks. Valves were adjusted last summer. I'm going to just replace the points and condensers and start from there.

[majicwrench]

Dave, in one of your earlier posts you state "...Once it'swarmed up it begins to stumble...."
Does it not have an issue until it warms up?? That would be an interesting bit of info..
Timing is not going to cause you this type of problem.
If it idles well and will accel a bit probably no condensor, for the most part they either work (car runs) or don't work (car won't run)
Best way to check fuel pump is to "T" a gauge in right before the carb, and run enough hose that you can get gauge up under wiper or something like that. Now go for drive, watch gauge when it stumbles ( can you make it stumble in driveway?)
Gauge should NOT be going down at any point when engine running.
Keith

[DaveT]

Keith,
Thanks! Yeah, when engine is warmed up it begins to stumble. Checked the points again and noticed that the contact points on one set of points doesn't line up exactly. So, I ll just replace them.
Regarding checking the fuel pump, at the risk of sounding dumb, what gauge am I hooking up to the 'T"

[DaveT]

Well, after replacing the points and condensers, it ran perfectly fine... until yesterday. Started misfiring @ around 2800 - 3000rpm and got worse. Pulled off the highway and the points were out of gap - one @ .018 and the other @ .012. i reset them and headed home. ran ok for about 15-20 miles and started misfiring again but i was able to chug (literally) home. l started it tonite and it ran really rough but i had no time to get into to things. My plan is to re-check the points. i did notice that one of the small springs on the dizzy weights looked a little 'stretched' (for lack of a better word)- could that make the points go out of gap? i'm also going to pull the plugs and check fuel pump pressure. After that, i'm looking for a check list. Any help that anyone can give is appreciated.

Thanks

[So Cal Mark]

you might have a coil that is breaking down when it gets hot

[DaveT]

Thanks, Mark. i used my multimeter for the first time to check the coil resistance. Here's what i found (bear w/ me because i'm new w/ the multimeter). With the ohms setting on 200, i measured the primary resistance @ 3.7 - 3.8 ohms, higher than my manuals said it should be. w/ the ohms setting on 20K, i measured secondary resistance and that measured 7.58. I assume i have to extend the decimal point on this value so i think it's actually 7580 ohms. so i bought a new coilt at the local parts store and it measured 1.7-1.8 ohms and 8.68ohms. i installed that and rechecked point gap and they were good. Pulled plugs and they were a bit carbon fouled but were still gapped correctly. Cleaned them up & reinstalled. Tried starting the car and it was tough to start and once started it barely ran, constantly stumbling.. at this point, am i looking at a fuel issue, and if so, do i start at the pump? can i effecively measure fuel pressure if i can't get the car to run somewhat smoothly?
what's 'crazy' is that two weeks it passed inspection and the emission test this year was better than the last one done two years ago!

[majicwrench]

So is it worse now that you have been changing things?? Change em back. And find your fuel line into carb, buy a "T" that will go in there, buy a hunk of fuel hose right size, and a cheapy vacuum pressure gauge from any parts store. Hook it all up and see what fuel pressure is.
So is it sitting in driveway barely running or can you get it on the road??
Keith

[DaveT]

Keith,
All i replaced was the coil (which looked to be very old) because the resistance readings were out of spec from what my fiat maunals said. Thats the only new part. tonite i'm gonna measure the fuel pump pressure. it's not driveble now beause once it starts it just sputters and stumbles.

[DaveT]

Just tested fuel pump pressure and it's reading 5 -5.5 psi. Is that high for the mechanical pump? I think I read somewhere that it should be 3-4 psi