[DML] Is My Idle OK?

["Todd Nelson" <tan5732@xxxxxxx>]

List:

The idle on my car seems to be a little rough, this could be due to it using a very old mechanical fuel injection system or it could be something else.  As this is the only DeLorean I've ever seen, much less driven, I'm not sure what I should expect from the car as far as idle quality, and startup ease is concerned.  When started, which can sometimes take a little help from the throttle pedal, the idle will fluctuate slightly, according to the in car tachometer it doesn't appear to be too bad, maybe 50 RPM's.  However a short time later the idle will sag maybe 150 RPM then pick itself up again, it will continue in this cycle until I begin to drive it, at which time it may reappear.  Upon inspection of the engine when idling I will notice that it does seem to shake a little more than I would consider normal.  When sitting at a stop light you can really feel the car rumble, I kind of like it, but if it's not normal then I should do something about it.  When I bought the car I noticed that the cap to the mixture screw was gone, assuming the mixture was set incorrectly I used the dwell meter approach connected to the diagnostic plug to watch the readings while the car was at running temp.  Needless to say it was off, not swinging between 45 degrees.  However I am unable to get it to a steady swing, sometimes swinging between 45 and 50, sometimes 35 and 40, sometimes 40 and 50, I will notice this with the screw in the same place.  The mixture screw hole is plugged during these readings, it just seems more erratic than it should.  This leads me to think there is a vacuum leak of some kind in the system.  Although this seems unlikely since I have replaced all of the engine vacuum hoses, intake O-rings, "air horn" paper and rubber O-rings, cold start valve and tube O-rings, fuel distributor to mixture unit O-ring and the paper gasket between the two halves of the mixture unit.

I have used the propane method for finding leaks, but it has proved unsuccessful.  What seems odd is as a test I removed the air cleaner while the engine was idling and "squirted" some propane directly in the intake to see/hear the results, but surprisingly nothing happened, this didn't seem right to me.  I thought it could be the constant idle speed system regulating the idle, but this seems unlikely as I wouldn't suspect it to function that quickly.  At any rate I disabled the system by unplugging the ISM and manually idling the car by using the "brass screws".  I experienced the same results.  The only other running problem I've noticed is when manually working the throttle spool I can just barely hear a quiet pop like a small backfire, just when holding at a constant high RPM.  When driving the car it's completely unnoticeable.  Other than that the car runs fine, acceleration seems good and power seems to be normal.  Then again, I've never driven another DeLorean.  Any thoughts?

Thanks,
Todd Nelson
1561, Vermont
http://www.rit.edu/~tan5732


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