Yamaha TRX850 Enthusiasts Forum

Hi. My TRX isn't starting anymore, after checking the obvious i then checked the valve clearances. The exhaust valves are withing tolerance but the inlet valve are totally closed up, i can't even get the thinnest feeler gauge in there. I've removed the shims to get an idea of what size i need to replace them with but I've been told by Yamaha that they don't make shims as thin as i need. So what to do, can i/should i get the shims ground down to the thickness i need or do i need new valves?
Is it the valves stretching that causes the shim gap to close or is some other part in the engine wearing to cause the gap to close up. Just want to get an idea of what my options are now that i've run out of shim room.

Thanks.

It is fairly well documented that bikes made before '98 stretch their inlet valves, the more aggressive cam timing (and consequent slamming shut) is not kind to the bog standard TDM valves Yamaha used. The part was silently revised in '98 and bikes made after then have no such problem - the best way forward is to fit these later valves. It might even be cheaper to find a later head and swap the whole lot.

Quite what happens if you keep using thinner and thinner shims though, I don't know.. Presumably a valve would eventually stretch to breaking point. Of course I'll sound like I'm talking out of my arse if you have a post '98 bike! Normally it's a '95-97 bike with 50-60K miles on it?

They don't actually stretch at all.

Some inlet valves (often the centre ones) deform "plasticly" at the seat area thus seating deeper and deeper into the head over time. I personally have some in this condition in the workshop. The poor valve condition is immiediately apparent once the carbon is removed by using a rotary wire wheel or scraper. It can also be felt with the thumb nail after cleaning. I would try to post a photo but a standard digital camera wouldn't focus to show the area . The valve seats in the cylinder head remain unaffected.

The only remedy is to replace the affected valves with new or good used items and shim valve clearance as normal.Do not scrap an otherwise perfectly good head suffering from 2 or 3 duff inlet valves. The situation can be fairly easily recovered .


Mark.

Ahhh very interesting, would love a picture though!

Does the seat come out of this situation just fine? Would it need to be re-cut to take a new valve? With a '95 bike at ~30K I find this a very interesting subject

I'm happy to put one in the post to you, if you're happy to post a photo up then pass the valve on if anyone else wants to see it?


Mark.

I only have a normal compact digital camera, nothing fancy.. Might be better off sending it to someone who likes their photography?

I have a digital microscope/camera at work (times 50 magnification) that will give some decent images.

More than happy to take some pics and post these.

I am also interested in this phenomenon of inlet valve clearances closing up though I have not experienced it so far. Need to research the materials used for valve (and seat) manufacture and their properties as it surprises me that they exceed the elastic limit of the material in operation. I always thought that valve clearance changes simply because of wear in the valve train.

Valve deceleration on closing must be pretty high and so I can see that potential forces on the seat are considerable (from F=ma) but there is still a reasonable contact area to distribute this force which I would have thought would reduce the compressive stress below levels that would deform the material.

Another intriguing subject (along with clearly defined causes of oil consumption and LH crank journal failure) that has me scratching my head.

Another point Rod had made me aware of through my engine rebuild with wear on the crank bearings.... How the top shell had hardly any or no wear yet the bottom shell being the one taking all the downward force and how these ones where showing all the wear.

Phil, It's great that you have access to such a "Hi-tech" camera at work. If you are happy to post up photos, then I'll get a valve off to you.

It would be good to dispel the valve stretch myth once and for all .


Mark.

Thanks for the replies everyone, looks like it's replacement valves for me.

Mine had the same problem at under 10,000 miles - minus zero clearance. I got a late 90s TDM head to replace it with but it's still lying in my garage. How different is the cam timing then, seeing as there's only around 2 bhp difference?

dandywarhol wrote:Mine had the same problem at under 10,000 miles - minus zero clearance. I got a late 90s TDM head to replace it with but it's still lying in my garage. How different is the cam timing then, seeing as there's only around 2 bhp difference?

Sorry? What cam timing difference


Mark.

Well, that's the question I've been asking on here for eons - I was always under the impression that the TDM mark2 and TRX had identical engines apart from gear ratios and carbs. I was also under the impression that the free(er) flowing exhaust of the TRX, along with different jetting was the reason for the 2 BHP difference but there's a few folks on here who swear the cams are different too.

I've never heard of a mark 2 TDM engine close up it's inlet valve clearances and the problem TRX engines seemed to be from '95/'96. The TDM came out a year after the TDM so I assumed the valves were changed at that time.

Maybe I've opened up a can of worms here...................................

had a 96 motor from 25.000 to 100.000 kms, not real kind to it either, used it well and serviced it well. used minimal oil...had a third gear whine tho.
this is over real old ground.
Valves streatching or degrading the seats, oil issues......

now with a 98 motor, over 100.000kays.....sama same. quieter third whine.

all ive done is to vary the operation of the motor load and service it well..

minimal oil usage.

I dunno

Photo proof is really needed here.

sanddune51 wrote:Sorry? What cam timing difference

When I was researching TRX's several places mentioned it has slightly sportier cams, but googling it is bringing up nothing concrete. Even worse is that the physical dimensions of the lobes appear to be exactly the same between TRX and TDM Mk2 which sort of suggests they are the same. Maybe it is just more aggressive than what was in the Mk1?

Does anyone have a proper TDM workshop manual they could pull the actual timings from? I can't find one for download anywhere, just service manuals and owners manuals, neither of which have the timing - but the service manual does have the lobe dimensions.