missing help?

mb911 · mb911's Garage

81 sc engine misses no matter what is done.. The distributor does NOT have hardly any play. The wires are new the plugs are new the cap is new rotor is new a ll fuel presures are good the msd ignition works properly. good compresion.. Do you think the green sensor wire could cause this to miss?? the wire is kind of poor looking. It was cut on the end when I bought the motor and I spliced it to work with the msd iginition. What do you think??

It is a small miss but enough to annoy me

mb911 · mb911's Garage

nobody??

jester911 · jester911's Garage

What kind of plugs did you use?
Some don't mix well with these cars.

Paulporsche · 06-01-2005, 08:12 AM

Could be that wire, although a proper splice should still be OK.

I have found that mixture can cause miss symptoms, especially @ idle. Too lean or too rich can both do this.

mb911 · mb911's Garage

Ngk bp7es plugs with msd ignition. I have set the mixture and it still does it. Now here is an interesting thought . I re did the top end about 2500 miles ago and it dawned on me that the valves when seated would cause for a tighter tolerance with the rockers causing for an open valve. This could be as I only experience the miss when warm.. What do you think??

real550A · real550A's Garage

Not likely if it's the first valve job on these heads, and they are adjusted properly.

buster73 · buster73's Garage

If you rebuilt the top end, are you sure the cam timing is correct. Also have you checked for air leaks (inlet manifold, gaskets...) ?

TerryH · TerryH's Garage

Sounds like you've been everywhere else. Ignition timing, vacuum advance on dist., other vacuum leaks, and fuel injectors are possibilities.

mb911 · mb911's Garage

Checked for vacum leaks every where there are none Rest pressure is good so I am thinking the injectors are still good what do you think?? Is my distributor just no good?? The valve thing was just a shot in the dark pretty sure thats not it (rebuilt over 300 engines in my life thus far but that does not make me an expert ) The miss is most noticable at idle but If I am outside the car running up the throttle by hand I can hear it just slightly at most rpm ranges. Cam timing is perfect! any thing else guys and gals?? I am sure it is something simple but what and how expensive

Eric Coffey · 06-02-2005, 06:43 AM

I'd find another CD box to swap/test. It can be the culprit, even while appearing to be work just fine.

mb911 · mb911's Garage

even with the msd box? that why I had thought the signal wire going to the cd box was bad

Eric Coffey · 06-02-2005, 07:31 AM

Sorry, meant you might try swapping the MSD box. They can have a mind of their own sometimes. The switchover circuit can go TU and foul things up. Also, it's probably a good idea to use an MSD coil as well if you aren't already. I have heard that there can be compatibility issues when using a non-MSD coil. Make sure the MSD is hooked up properly as well (correct polarity). If it is reversed, I think you will get a bunch of advance. If all else fails, I would revert back to the tried-and-true stock Bosch ignition.

JBO · 06-02-2005, 07:49 AM

I have a similar problem with same yr car, recent rebuild, have not yet resolved it. Mine is not at idle or during accel, but at steady state cruising. Maybe mine is the rebuilt distributor my wrench had to buy from Motor Meister, as they were the only source he could find one ($600!). I'm taking it back to him next week for valve adjust and a look at this issue.

Eric Coffey · 06-02-2005, 07:58 AM

Ha, yeah, I would suspect anything from MM. That does bring up an interesting point, and remind me: Jim C. (Superman) had the same ignition troubles a while back and I believe it turned out to be a broken hall sender wire in the distributor.

mb911 · mb911's Garage

Eric are we talking the green wire coming from the distributor?? This was my original thought

mb911 · mb911's Garage

I think I fixed it at lunch! Retimed about 10 degrees at idle 1000Rpms and it has no miss along witha slite tweak of mixture We will see when I go home

Early_S_Man · 06-02-2005, 11:37 AM

Ben,

I believe the old, darkened/cracked green signal coax can/will be an issue from now on! You may have re-established contact of an intermittent connection when you readjusted the timing. All of the stress is concentrated at the point where the cable plugs into the side of the distributor, and the coax doesn't seem to be rated for the high temps in a 911 engine compartment!

I suggest replacing the cable to preclude a future return of the 'missing' adventure. I am sure that you have already had more than $30 worth of aggravation, so far!