Yamaha TRX850 Enthusiasts Forum

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weird

Make sure the plug is tight and mechanically OK, sounds like it maybe bypassing cylinder gas and pushing the cap off.

There is a small hole on the left side of the head..it's the breather for the plug wells, make sure it is clear.

the caps and leads just unscrew same at the coil. change them around, and if continues to do this ,I'd bet you have this area being pressurized either past the plug or gasket.

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Place a vacuum cleaner hose over one spark plug well and feel is you have suction from the other. Plug the breather hole with a bit of tape beforehand. Suck them both clean before you do anything incase you need to remove a plug.. Cleanliness is next to TRX850 ness .

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Not saying anything new...but as air heats expands and perhaps popping the plugcap which seals nicely ...giving credo to the blocked bleed hole theory.
Hmmm.

How about a new plug....it has a wire down the middle for the spark into the bang dept.

If the plug is leaking on compression it can have a grey stain running up the ceramic part. This can be confused with general grime though.

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You have modded your exhaust haven't you Torino and it runs down the RHS by the head/barrells???
Can this excess heat from the headers be be causing something???

Maybe you need a hole in the right hand side torino?

But you would think with the pressure build up in the RHS spark plug well , it would bleed ok through the centre joining breather hole and relieve any build up out the hole on the LHS of the head.
It has been discussed before about the RH cylinder being a little hotter due to the way the water jacket is designed and the cam chain galley being on the RHS too.
Maybe the hole is just a little too restrictive and can not pass the excess pressure due to added heat on the RHS with the exhaust there too

viewtopic.php?f=31&t=9838&p=100411#p100411

I stand corrected after chasing down the photo of the exhaust,I knew it was different but not like a tiger exhaust with both down one side

This doesn't make sense to me, the breather hole is only going to vent initial build up of expanded air between cold engine and running-temp engine. Once the engine is up to temp, there is little CHANGE in temp- that would cause the trapped gas to expand.
The OP has stated once the cap jumps off, it jumps off repeatedly afterwards.
This can only be due to a leak of reasonable flow (its gotta overcome the breather hole flow rate as well) being injected into the plug cavity.


like I said, weird