Low compression tuning?

[Hwy 49er]

I have a #2 cylinder compression down about 10%. Lots of missing. Can the carb be tuned to make the cylinder run better in this condition? Like a smaller jet or pilot adjustment? I assume it's quite a polluter in this condition. I've riden it this way for 15K. Four valve adjusts. Four syncs. SISF jets, overflow tubes and foam in the intake. I guess i'm going to keep riding it this way a while. I don't have enough money for another bike and this one is worthless. Sell for parts? Part it out? Used engine? Ghost ride it off a cliff? I haven't been able to find a engine.

[Steve in Sunny Fla]

no, you don't select - tune a cylinder that way, as it's compression that's different, not displacement. actually, 10% isn't that bad, i'd keep riding it til it pukes, which may be a long time - steve

[T Cro ®]

Depends on the reasons why it is down; which BTW 10 percent is really not a lot.... If your down because of non sealing valves then there is little to be done by way of carb tuning to minimize the missing as your passing combustion gases either into the carb or into the exhaust when it should not be there. When was the last time the carbs were properly cleaned and tuned? Syncs do not count.

[Hwy 49er]

Depends on the reasons why it is down; which BTW 10 percent is really not a lot.... If your down because of non sealing valves then there is little to be done by way of carb tuning to minimize the missing as your passing combustion gases either into the carb or into the exhaust when it should not be there. When was the last time the carbs were properly cleaned and tuned? Syncs do not count.

10% is enough to make it miss a lot and smell rich. I guess it's not rings by the results of the dry vs wet compression readings. Steve had the carbs in February. That's about 5K ago.

[T Cro ®]

10% is enough to make it miss a lot and smell rich. I guess it's not rings by the results of the dry vs wet compression readings. Steve had the carbs in February. That's about 5K ago.

You could also be dealing with a non or poor firing cylinder due to an ignition problem by way of a fowled spark plug, rotten high voltage wire, bad plug boot. As commented by Steve 10 % is not a lot of loss if you have all your other ducks in a row.

[kreaky]

You could also be dealing with a non or poor firing cylinder due to an ignition problem by way of a fowled spark plug, rotten high voltage wire, bad plug boot. As commented by Steve 10 % is not a lot of loss if you have all your other ducks in a row.

Good thought. Might be a good idea to check the boots & plug wires.

[Steve in Sunny Fla]

10% is not enough to make it run rich and to miss. That' sounds way more like either a carb issue or an ignition issue. Since if it were a carb issue it would have to be pouring fuel into the cylinder to cause it to miss, my vote is as T's, it's an ignition issue. Steve

[George R. Young]

Sounds like you have stock ignition so the same coil drives cylinders 2 and 3..

You could interchange the ignition wiring for cylinders #2 and #3 and see if the problem moves or goes away.

[Hwy 49er]

You could also be dealing with a non or poor firing cylinder due to an ignition problem by way of a fowled spark plug, rotten high voltage wire, bad plug boot. As commented by Steve 10 % is not a lot of loss if you have all your other ducks in a row.

I've had two different sets of wires on the bike. The wires and boots that were on the bike and a new set from Murph's, and three sets of plugs in there. The last set went in there about 3K ago at Steve's suggestion. I've checked the wires as recent as 1K ago when #1 boot came loose. I also found and replaced a vac cap on the #3 carb at that time.

[Hwy 49er]

Sounds like you have stock ignition so the same coil drives cylinders 2 and 3..

You could interchange the ignition wiring for cylinders #2 and #3 and see if the problem moves or goes away.

I'll try that and report back.

[Hwy 49er]

Instead of  interchanging the ignition wiring for cylinders #2 and #3, I put on stick coils and the problem is still the same.

[jim snyder]

Have you tried going to the hotter range plug (DR7EA) Not saying that will cure it but its one more idea to throw in the mix.

[jworth]

Just curious.  How do you know for sure it's just the #3 cylinder that's missing?

[Hwy 49er]

Have you tried going to the hotter range plug (DR7EA) Not saying that will cure it but its one more idea to throw in the mix.

No. I'll give it a try if it can't hurt anything.

[Hwy 49er]

Just curious.  How do you know for sure it's just the #3 cylinder that's missing?

I read my posts and don't see where I said #3 is missing.

[jworth]

Ok, smarta**, #2.  Question still stands.

[Mettler1]

  Who's the smarta$$??

[Hwy 49er]

Have you tried going to the hotter range plug (DR7EA) Not saying that will cure it but its one more idea to throw in the mix.

Could you explain why?

Are you suggesting just running the DR7EA in the #2? Because of the lower compression (cooler temperature)? Does lower compression=lower temperatures? I would guess that the DR8EA would show some dark carbon if the plug is too cold. The #2 and #1,3,4 plugs were golden brown last time I changed plugs about 4000 miles ago after installing SISF 2 minute mod w/ power jetting.

Or, are you saying to put them in all four because of other factors like low-speed riding, or rich mixture contributing to cooler temps?

[jim snyder]

If the plugs are golden brown then leave them alone that means real close on jetting. I was only suggesting the hotter plug if you might have been experiencing any plug fowling issues in the lower compression cylinder. If you could do a leak down test on all four cylinders that would tell you alot. If a little carbon is stuck on the valve seat it could cause your one cylinder to have lower compression.
I would have bet it was a tight valve but you mentioned that you adjusted the valves already.

[jklhill]

You say "lots of missing" but you don't give any details. Is it all the time and throughout the entire rpm range? Or is it just at idle? Does it change as the engine warms up? What about at wide open throttle?