Hey, guys, I'm curious about an interesting coincidence. My latest 914 is relatively new to me. When I purchased it, I noticed that it missed significantly at higher revs, especially in the lower gears. The owner said that it there was a rev limiter in the electronic ignition, but I knew that was BS because I removed the dizzie cap and saw that it had the normal rotor w/o the rev limiter, and also because I never ran it even close to the red line. The other issue I noted was that the #3 cylinder's compression was significantly lower than the other cylinders, but of course I assumed these two issues were unrelated.

When I got it home I found that the spark wire connection at the distributor cap for the #3 cylinder was very poor. As soon as I replaced the spark wires the problem of the engine missing stopped. This has got me thinking. Obviously, from what the PO said, the missing (and the bad spark connection at #3) had been going on for a while. Is it possible that running the car for a long time w/ poor spark at one cylinder could lower compression, through incomplete combustion, unburned gas, etc? And ... if the cylinder and valves are gummed up from this, could I expect that the compression could increase after some lengthy drives and cleaner combustion?

Yes and No. The term is ring wash. If the unburnt gas end up diluting the oil on the cylinders it becomes metal to metal and the rings/ cylinders go away. A miss like this or a over rich carb can also cause this. Give it some time and it may come back around or? Gordon

Thanks. Actually, that's what my first thought was - that the rich mixture would dilute the oil and cause wear - which isn't good and isn't reversible w/o a rebuild. Nevertheless, I'm going to monitor it, run it, change oil and adjust the valves more frequently in the hopes that I might clean things out and have some improvement. The compression in #3 is currently 115.

WOW, 115 is pretty good compression (for "significantly lower").
If that is low what are the others?


Jim

Yes I would agree that 25 psi between highest & lowest is a bit dodgy.

But #4 at 140 compared to the others would concern me as much as #3 at 115.
The average compression is 125 psi and #4 at 140 is off the average by 15 psi where #3 at 115 is only off by 10 psi.

It seems to me that something has caused #4 to go up in compression (carbon build up) or all the 3 others are low on compression.

Have you had a leak down test done? That would tell you how all 4 cylinders are holding up and if you have compression going past rings or valves.

Jim

It's not terribly hard to re-ring these motors...

No, I haven't had a leak-down test. Might be a good idea. Re: compression, my understanding is that w/ the 2.0 engine, 150 lbs is the compression when new/rebuilt.