dead cylinder

[TurboKeith]

A buddy of mine just did a powderlites install on his Apex and after a couple of trips across my yard(less than 75 of full throttle), the #1 cylinder crapped out. All we can get now is the occasional misfire. It doesnt leak antifreeze from the head shim, it doesnt have antifreeze or oil on the spark plug. No steam or oil are coming from the breather tube.

We did a compression test and it came out nearly identical to the #2 cylinder (less than 1 psi difference).

I dont have the tool for a leak down test so we put compressed air in the cylinder and listened for leaks from the breather tube, intake, and exhaust. no airleak to be heard.

removed and reinstalled the throttle body boots so there's no leaks.

swapped the coil from #1 to #2 (yes the coil wires stayed on the right cylinder)

layed the plug and coil on the frame and could visually see a strong spark when cranking.

changed spark plugs.

with throttle body wide open, made a visual check to see that fuel is being sprayed from the injector.

swapped fuel injectors.

swapped cdi's with my sled

unplugged all of the rb3 so it's back to stock.

checked all of the fuses

checked all of the wire connectors that could be found

made sure it has a half tank of gas.

dripped gas into the #1 throttle body with the sled running.

Any other ideas on why one cylinder would drop?

 

[Skydog]

So you have spark,fuel and compression. The only thing left is to have them all meet at the right time...

 

[TurboKeith]

Ya, I was thinking the same thing. You'd think the cdi would take care of that though.

 

[Skydog]

Yea you would think.....mmmmmm

 

[T-Nytro]

Guy had a rapid bike on a stock nytro in the mountains this weekend. It would not fire on one cylinder. We dissconnected the Rapid bike and all was well. He rode the rest of the day without troubles. Could be the ignition part of the rapid bike took a dump.

 

[Skydog]

Very true buddy but he did say,((unplugged all of the rb3 so it's back to stock.))

Skydog

 

[mbarryracing]

Since it appears you have already eliminated a mechanical issue, sounds like a coil harness lead to that cylinder, maybe you tweeked the connector pin or wire / terminal crimp when installing the kit and now have an intermittent signal giving you fits (the "wiggle wire" syndrome).
Process of elimination, it's the only thing remaining, minus a rag in the exhaust runner.
- You removed the RB3, problem remains...
- I take it you are positive it's that cylinder that is not firing.
- You have good compression and the valves don't leak...
- You appear to have adaquate fuel (I take it the plug is wet when you take it out)...
- It's not the plug (swapped and also new)...
- It's not the coil (swapped).
- Just because you have a spark in the air with the plug out doesn't mean it's strong enough to do the same under compression.
Start looking at the dead cylinders coil harness, wiggle wires while it's running to see if you can get it to change. Then pin out / ohm the connector thru to the other end of the harness with a multi-meter while wiggling the wires around to see if the reading fluctuates.

PS: did you disassemble the head / valves during the install?

 

[TurboKeith]

He took the valve cover off today to double check all of the valves (the intakes were re-shimmed during the install but the exhaust was left alone). So far it looks like one of the exhaust valves is a bit on the tight side, kinda odd since they weren't touched. Might be loose enough to seal for compression when the motors off but not loose enough to close all the way when its running. I would have thought if one of the shims got misaligned it wouldnt close off at all. He's going to pull the cam out tomorrow and take a look under the bucket and see whats up. I'm keeping my fingers crossed and hoping thats all it is.

I thought there were a few issues last year with some of the boondocker guys having a problem with the shims either falling out or getting misaligned, anyone remember?

 

[lakercr]

He took the valve cover off today to double check all of the valves (the intakes were re-shimmed during the install but the exhaust was left alone). So far it looks like one of the exhaust valves is a bit on the tight side, kinda odd since they weren't touched. Might be loose enough to seal for compression when the motors off but not loose enough to close all the way when its running. I would have thought if one of the shims got misaligned it wouldnt close off at all. He's going to pull the cam out tomorrow and take a look under the bucket and see whats up. I'm keeping my fingers crossed and hoping thats all it is.

I thought there were a few issues last year with some of the boondocker guys having a problem with the shims either falling out or getting misaligned, anyone remember?

I mention shims in the build post above and there a photo of the right/wrong way to install them. Might be helpful for your buddy to take a look?

 

[TurboKeith]

All fixed... Turns out that the one exhaust valve that was tight had an issue with the shim. No idea how it came loose??? especially considering the first couple rips across the yard it was running on all four cylinders. Kinda crazy that the valve could close enough to get compression but not enough to get it to fire. My speculation on this is that the backpressure from the turbo was just enough to keep the valve from closing all the way when the motor was running. Then when there was no back pressure the valve could close enough to show compression. Another lesson learned.

 

[mbarryracing]

PS: did you disassemble the head / valves during the install?

Good find! Lucky it wasn't a broken valve spring or a gremlin electrical issue.

 

[Skydog]

WOW yea GOOD FIND!! That could have been VERY VERY hard to find!!!

Skydog