johnlush

Hi All, I could sure use some suggestions for chasing down a problem. Pretty much stock '76 930 except for SC cams, later exhaust and K27-7200. When I start it, it runs great for the first 5 minutes or less - smooth idle, etc. After that it begins to miss. I can watch this happen with my timing light - when it's first started, the timing light gives very steady pulses of light and after a short period it starts missing flashes and you can hear the corresponding missing in the exhaust note. It does this for any plug lead that the timing light inductive clamp is attached to which says to me that the problem is in the distributor or somewhere "north" of there. I've been through the distributor and replaced the pickup coil and the green coax lead. Temporarily replaced the coil, HT lead and CDI unit with known good pieces with the same result. I'm thinking it's about got to be a bad/marginal connection somewhere but I'm running out of things to check. Any thoughts? Thanks! -John

02fly

Hey John I,ve had the same problem with mine if I let i sit and idle while cold it some times developes a mis but if I don't let it idle and leave my foot on the gas I have no problems my thought was that the car is real rich while warming up and stating to foul plugs. I would think that if we were having the same problem that if it was a connection the problem wouldn't go away just my two cents I'm looking forward to other chiming in.

Stevie 77 930

When was the last time you replaced your spark plugs and perhaps your wires? Chasing down a miss like this always bring us to "Is it ignition or is it fuel" realted. Could be a bad WUR. Just some thoughts.

johnlush

Thanks for the thoughts! I would sure think it's ignition based on the behavior of the timing light. Plugs are new Bosch W3DPO's, Wires are not new but all check out OK on ohm meter. Is there anything in the circuit that intentionally disrupts spark?

nathanUK '81 930 G50

The rotor arm.

senna21

This would be my guess. As your coils and wires are good that pretty much leaves us with the rotor and cap. As the timing lite skips on all six plug wires and not just one, I'd say that eliminates the cap. That leaves the rotor.

johnlush

I will swap it out and see. It's a new Bosch rotor (the kind with the rev-limiter). Beyond that, I'm not sure where else to look except to start looking upstream from there for bad connections, frayed/broken wires, etc. I don't have a great schematic of the ignition system but that may be my next step if the rotor swap doesn't fix it. Again, it's heat/time related - has anyone had a rotor fail in this manner? -John

A930Rocket

Try a rotor w/o the rev limiter. Also, as things warm up, it sounds like something is expanding and losing contact?

nathanUK '81 930 G50

I read about someone who upgraded their ignition system (possibly to a MSD) and it burns out the rev limiting rotor arm.

Good luck.

johnlush

Well.......... I took off the brand new cap and rotor and compared them to the old ones. The only difference I noticed was the center contact on the cap - the old one protruded from the cap further than the new one and seemed to have more spring tension as well. The contact patch where it met the old rotor was larger than the new one - maybe normal wear??? Anyway, the car fires perfectly now, never misses a beat. I haven't driven it but based on how it runs I think I'm okay. Thanks to all who replied - I was victim of "can't see the trees for the forest" (or is it the other way 'round?) and "it must be good because it's new". Sometimes it's good to have a fresh opinion. Now I need to hook up the LM1, get it dialed in and get a few miles on this engine before winter sets in. I should post some pictures in case anyone's interested - like lots of us the 930 has always been my dream car and I'm a huge fan of the early cars despite their shortcomings. Thanks again! -John