Miss on start up

[spidernut]

Here's one that I've never run into on one of my bone stock 79 Spiders. Any ideas?

Symptom: After the car sits for 3 or more hours regardless of outside or motor temperature, it will fire right up but has a very significant miss as if one cylinder is not firing. The colder the car is, the longer it will miss before smoothing out. The miss seems to be an electronic issue (but could theoretically be a valve sticking open). Fuel delivery has been checked and it is not a fuel issue. The miss will last from 10-30 seconds depending upon how long the car has been sitting. When it stops missing, it is instantly running smoothly (versus a gradual change)...like a switch went off or on or a valve became un-stuck. There is no odd valve tap or other noises...just the miss. I have about 1,000 miles on the rebuilt motor and head.

I've run my checks and can't find anything obvious (wires, plugs, cap, rotor, coil, pick-up, idle solenoid, etc.). I only get one very short amount of time to find the problem since it only misses for less than 30 seconds and only after siting for hours and hours. At this point, I'm theorizing. Anyone run into this before?

I still have some other things to check like the ignition control module, but so far I'm finding nothing that could cause this issue. There's also not enough time to determine if it is one specific cylinder missing or all of them. I'm leaning towards a sticky valve but thought I'd check with others.

[majicwrench]

Sticking valve would clack like crazy.
Keep it simple. I hate guessing, but if I had to, I would guess a plug is fouling, then starts firing. If you have a non-contact temp gun, you could easily find out which ex runner is not getting hot. Ten seconds would be tough, but doable in 30 seconds.

[RoyBatty]

Does this car run the dual points distributor?

If so, maybe the start up set of points or that condenser is bad.

[majicwrench]

Ignition Module almost can't affect just one cylinder. Same for condensor. Point, theoreticly could, but really, no.

If nothing else stuff a set of plugs in it. Easy quick. See if it makes a diff

[azruss]

sounds like a bad or fouling plug. you need to determine if it is the same plug that fouls each time. I would pull the plugs after the car has sat to see if any are coated. I had this issue with a very tired motor. I assumed it was valve seals leaking. It took a bit for all the plugs to clear.

[seabeelt]

I had that hapenn once on our old 78. Ended up being a bad plug. Swapped it out and it ran fine afterwards.
r/

[spidernut]

Electronic ignition - everything seems to check out O.K.
Plugs - no fouling...nice and clean

I've swapped carbs, distributor, magnetic pickups, coils, plug wires, distributor cap, etc., from my other 79s. I'll swap out plugs and see what happens. I've not had that happen with plugs before, but that doesn't mean it may not have happened this time. I'll give that a shot. I felt the same about the valve issue - it would be quite noisy if stuck open but it is quiet as can be and there's no smoke.

I'm hoping to have time this weekend to work on it. I'm going through Spider driving withdrawal. Can't stand it since we have had awesome weather lately too.

[So Cal Mark]

this is common if you have a slight coolant leak into a cylinder. Once the engine is shut off, pressure in the cooling system forces coolant into a cylinder, getting the plug wet. It only takes a few seconds of running to blow the coolant from the cylinder and the plug starts firing again

[spidernut]

Hmmm. I hadn't thought of that one Mark...I'll check the plugs and then check for a leak. I'm not losing any coolant or pressure in the system so I know it isn't a large leak at least.

[spidernut]

Problem solved - bad spark plug though it looked just fine. So much for new plugs. I guess some are bad out of the box.

[stpher]

I have a funny story to tell everyone about a missfire in a BMW 2002 that I had in the shop many years ago. The fellow had a tune up done at a local tune up shop and after a few days it started to missfire. They checked it out and said they could not find anything. Brought it into us at a small dealership that I was working at. Pulled the plugs, looked pretty good, they were just put it. Compression test OK, checked valve clearance and all the ignition side, everything looked good and was within specs. Put the plugs back in and again right away, missfire. Pulled the plugs out again and checked them Looked OK Well as we all check spark plugs by holding them with the electrode upwards so you can read the plug, wasn't until I for some reason, I turned the plug up side down so the electrode was at the bottom enf on theings. Guess what, the damn center electrode was loose and would fall down and hit the gorunding tip and short out. Don't know why I turned it upside down but now after 35 years and countless spark plugs changed, I still turn every plug upside down for a quick check. Never had it happen again BTW it was a Bosch plug. It was a bit of a head scratcher.

Thanks, Rick

[spidernut]

These were also Bosch plugs.

[So Cal Mark]

you can find those defects quickly with a scope