Yamaha TRX850 Enthusiasts Forum

Hi All,
Just recently started having problem where spark plug cap on right hand cylinder keeps jumping off plug. Cap is original standard type. Will do it partway through ride, and then when I stop to push it back on, will do it again every time I fire up again and get a few yards. End up limping home on left cylinder. Can't see anything wrong with it, no different to left hand one. Rubber upper grommet in good condition. No mechanical action causing it such as moving cable or similar. When it does come off it is up about 2 cms off where its supposed to sit before I push it down and seat it properly again.
Anyone had same problem and therefore got a fix?

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.

Hi Guys,
Thanks for the replies. I will try swapping the plug leads over and see if there is any difference. I checked the breather hole beside the left plug well and it is clear. Would there be the same kind of breather hole between the left and right plug wells?

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 .

OK, so I haven't done the vacuum cleaner thing yet, but I unscrewed and swapped over the plug caps, pulling out the internals first to make sure they were in good condition and the same. I also checked the right hand plug and it is not loose. I took it for a ride and it did the same thing again after about 30 minutes. (Forgot to mention that it doesn't do it from a cold start, only after it warms up). This time when it happened, I moved the top rubber grommet up on the plug cap so that when I seated the plug cap back onto to the top of the plug the rubber grommet didn't seal onto the top of the head/tappet cover. It then didn't do it again, which makes me think it is some sort of pressure issue that is acting against the rubber grommet when it is seated properly. However, the pressure is not arising until the engine warms up. Will try the vacuum cleaner thing next and let you know the result.

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.

OK so I have just done the vacuum cleaner thing, removed both plug caps, fed the vacuum nozzle into the left plug well on reverse (blow) and there was plenty of air coming up and out of the right hand plug well. So the breather between left and right plug wells must be clear. Therefore, if something is pressurizing in the right plug well, it should escape thru the breather/s and out the left hand side? Anyway, now I have just cut about 1/8 out of the grommet on the right hand lead, and seated it all back down properly into place. This npw leaves a gap in the grommet where upwards air can escape if necessary. I will head out again tomorrow maybe and see what happens.

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

http://www.trx850.com/phpBB_forum/viewt ... 11#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