Yamaha TRX850 Enthusiasts Forum

Hi all - another instalment in the continuing saga

Been having a problem since the ported head and different barrels were fitted with the cooling system pressurising and losing coolant.

I put it possibly down to a pattern head gasket - Athena, not that cheap

When third gear went and I decided to tear it down early in the year I decided to go for a copper head gasket, courtesy of Brian (Cobba). I carefully checked the head gasket I removed, but could see no sign of it blowing, or any sign of the head being faulty????

So I rebuilt the engine and all seemed well, however the pressurisation problem did re-appear. I fitted a lower temp fan sensor, wondering if it was heat from the tuned engine overwhelming the cooling system. I also checked the stat and bingo it was opening late and not opening fully. I replaced the stat and rode the bike to work - unfortunately when I got off the motorway there was water pouring out of one of the little carb warming hoses - maybe it had partially loosened when fitting the stat??? I pulled over, re-fitted the hose and filled it with water, most of the water was gone. The bike started up after a good few chugs on the starter and I got it home.

Started it again - (more difficult to start) and noticed water pooling on the floor - it was coming from the exhaust - bugger - blown head gasket, or maybe the 2nd hand ported head was faulty???

Anyway taken the head off again - No sign of blowing on the head gasket??

No sign of anything wrong with the head???

Anyway I have cleaned, de-coked and re-seated all the valves on my original head and replaced the valve seals and I was going to bolt this head on in place of the ported 2nd hand one as all my problems started since the head was fitted.

However it has occurred to me that I also fitted some 2nd hand barrels I had that I had never run on the bike so were unknown quantity



See photo of discolouration between barrels on liner - its the only visible thing I can see



Re-fitting springs to old head after seating all valves

So question is head or barrels - I don't want to do the wrong thing here!!

anyway - does anyone have any suggestions as I don't think my marriage will survive another engine teardown + I want to ride the bloody thing not continuously work on it - feels like I have an old Brit bike - or maybe I should just bin it and buy a speed triple?

Gidday Simon,
Just remember to re anneal the copperhead gasket before you re-use it.. I fell into this trap.
The couple of dry runs I did with the rebuild checking piston/valve clearances was enough to work harden the gasket so mine leaked .
Head off and re annealed the gasket and problem solved..
Run it up and do 1 heat cycle , let it cool overnite and retention.
Loosen 1 head bolt about 1/8th turn then retention and move to the next till all retentioned.

Have you tried another rad cap??? Maybe its not releasing pressure to the re mmm whats the name again tank beside the battery. ???
I always start the bike 1st up with the rad cap off and run it for a while to release any air that may be trapped.
Any chance you can just bolt the head and barrels together and pressurise it on the bench to the rad cap release pressure???

Cap is new high pressure item, so not that.

Hmm. did think of trying to pressurise the head, but got no compressor + only way I can think of doing it is to make up blanking plugs for the cylinders and pressurise through spark plugs - too much work + all I would see was a pressure loss could be head could be barrels.

Have annealed and cleaned up the head gasket - I think I will have to try my old head which I know is OK if problem persists change barrels - fingers crossed it head - then I just have a shit head that I spent money on getting skimmed and gas flowed.

If its the barrels silver lining is expensive head is good

Well had another thinking session - came to the conclusion, that after trying new stat and seeing how much it opened at what temperature, that my original problem was a faulty stat in failing to cope with extra heat from tuned engine.

I fixed that but unfortunately a loose hose worked its way off causing, the coolant to boil and blowing the gasket. When I checked pressure had deformed the gasket, I annealed it, hammered it flat and lapped it on a sheet of glass. I have reassembled the engine and ran it up today until the fan kicked in. When I released the pressure. It was your standard hiss + coolant level was unchanged!!!!

Fingers well and truly crossed - will do shakedown run before a big or bigish journey - learned my lesson there!!!

No need to hammer the gasket after annealing Simon.
That will only work harden it again.
Fingers crossed all will be good this time.
Compression of the head being tensioned with the gasket annealed should be enough to work itself into any imperfections with the Hylomar on there.
I bet your down to Olimpic contention times on stripping the bike and rad to get to the cams and head bolts by now..
That 1 reason I changed the manifold clamp bolts too to a M4 x 0.7 pitch cap head.
Makes it easier while tuning the carby too.

Make sure you replace the o ring on the head where to water pipe bolts to it.
It will be the next weakest link if you've taken it off and on a few times.
I learnt from that mistake too

I reused a new o ring 3 or 4 times on this rebuild and when I finally got it all sorted, it started to leak.
I had another new one there and didn't listen to my gut.
Make sure you only tension the fixing bolt to 10Nm as a few of late have mentioned about the bolt snapping off in the head.

Cheers Cobba

you must be psychic as that is exactly what happened on first start up, water pissed out of pipe on top of head LOL. However quickly fixed with new O ring from kit - phew

Fingers crossed for some good weather + having to wine and dine the wife to butter her up