Relation between bad spark connection and lower compression? Options

[john_g]

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?

[flash914]

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

[john_g]

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.

[jcd914]

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

[john_g]

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?

#1 - 120
#2 - 125
#3 - 115
#4 - 140!!!!!!!

25 lb diff between lowest and highest ain't too good, is it? To me, that's makes it dodgy, as the Brits say.


Jim

[jcd914]

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

#1 - 120
#2 - 125
#3 - 115
#4 - 140!!!!!!!

25 lb diff between lowest and highest ain't too good, is it? To me, that's makes it dodgy, as the Brits say.

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

[r_towle]

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

[john_g]

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

#1 - 120
#2 - 125
#3 - 115
#4 - 140!!!!!!!

25 lb diff between lowest and highest ain't too good, is it? To me, that's makes it dodgy, as the Brits say.

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

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.