Strange issue - black transfers only on front cylinder

[Jcary85]

Hey all,
I'm still going through everything trying to find the cause here but curious if anyone has ever seen this. Ski has been running kinda crappy. feels like it loads up. If i keep it cleared out it makes decent power. Pulled head and all transfer ports on front cylinder are almost black. I assume its exhaust blowback but not sure. Rear cylinder transfers are clean aluminum. Went through carbs and everything looks good. Front plug is whiter vs rear cylinder. Motor has even compression. Reeds are all intact. Piston doesn't have a hole in it (checked with bore scope). I'm stumped.

This is a TPE 1200 with SE carbs and MSD ignition. I suspect ignition issue but who knows.

Curious if any 2 stroke experts know what causes this? Might help narrow my search.

 

[bkspeedwerks]

check crank phase. i wonder if the crank spun?

 

[Jcary85]

check crank phase. i wonder if the crank spun?

Visually it looks dead on. I’m pretty confident it didn’t move - at least not enough to cause this. Good thought though

 

[OCD Solutions]

Weak spark in just the one hole? Have you run a different set of plugs yet or swapped coils?

 

[Jcary85]

Weak spark in just the one hole? Have you run a different set of plugs yet or swapped coils?

swapped plugs yes. Have not swapped coils. visually spark looks identical on crank test but that doesn't mean much. ugh i hate diagnosing ignition problems. Maybe this is the excuse i need to switch my 1200 ski to my new ignition

 

[wikstromm]

Reeds?

 

[Jcary85]

Reeds?

Reeds are all fine. That was my first thought and would make sense

 

[Storbeck]

Power valve stuck, not opening, so it doesn't have enough blowdown and exhaust is blowing into transfer on that cylinder maybe, just throwing out ideas.

 

[Jcary85]

Power valve stuck, not opening, so it doesn't have enough blowdown and exhaust is blowing into transfer on that cylinder maybe, just throwing out ideas.

It’s not suck but good idea. I’ll at least rev it and check

 

[Storbeck]

Could be the little passage to the bellows plugged too I suppose.

 

[Pro-pulsion]

I'm with bkspeedwerx on crank phase. Don't go with just a visual appearance, it only takes a few degrees of delayed timing to send a flame front down the transfer ports. Advanced phase yes, you are correct and detonation will occur melting a hole in the piston. But without a spark plug hole mounted dial indicator or pulling the crank out entirely to 100% verify the phase, going by what it looks like through the plug hole is really not a good method, it's like trying to verify if something is out by 0.001" or 0.0055" with a dull flashlight in the dark.

 

[Jcary85]

I'm with bkspeedwerx on crank phase. Don't go with just a visual appearance, it only takes a few degrees of delayed timing to send a flame front down the transfer ports. Advanced phase yes, you are correct and detonation will occur melting a hole in the piston. But without a spark plug hole mounted dial indicator or pulling the crank out entirely to 100% verify the phase, going by what it looks like through the plug hole is really not a good method, it's like trying to verify if something is out by 0.001" or 0.0055" with a dull flashlight in the dark.

Ok ok I’ll check it with an indicator in a bit

 

[Jcary85]

Was kinda hoping that was it but crank is not out of phase. Checked with dial indicator.

 

[Storbeck]

Did you figure it out?

 

[Jcary85]

Did you figure it out?

It’s running great again but I didn’t find an issue. Cleaned carbs, pressure checked motor, pressure checked cooling system, fixed one semi crusty wiring connection. It was either they connection or some junk in the carbs I just didn’t see.