Yamaha TRX850 Enthusiasts Forum

Yes, having destroyed one engine following a rash attempt to do the valve shim replacement myself (always good to have a second opion when doing something like that for the first time), I successfully managed to re-shim the inlet valves on the TRX last year (almost no clearance on most of the valves). Had a lot of trouble getting the timing marks to line up and stay lined up due, I think, to wear in the chain. At any rate, a new chain made the job easier.
This weekend (28 Sept '24) I decided to swap the 140.2 main jet to a 140 (as fitted to the Tedium). But first, I decided to check the valve clearances, as I've put a few hundred miles on it since the re-shim. The inlet valves were still in spec but the exhaust, which were fine last time, are under spec now (0.20 on all of them - should be 0.25 to 0.30). If I'm reading Haynes' chart correctly, that means they require 180s shims to restore correct clearance. I have 1x180, 2x 182s and 1x184, so one tappet gap might be a bit big. Oh well, as a friend says, a slappy tappet is a happy tappet. I hope so.
I'm going to wait till a responsible adult can assist with the re-build, because having two heads on the job can be beneficial even if you have done it before. And it doesn't hurt to have a bit of moral support.
Blimp
P.s. Just realised that with the shims i have available the gaps will be smaller, not larger. Anything over the 180 specified (going on the chart in the Haynes manual) will result in a smaller gap. Anyway, as long as it is 0.25 or over I should be alright; for now.

The shims you require will depend on what shims are currently installed.
You measure the clearances and write them down
Then remove the cams, lift the buckets and remove the shims, measure and write them down
Then calculate what new shims you need based on the existing shim plus (or minus) the clearance.
Rinse, repeat, wipe hands on pants.

The instructions in the Yamaha service manual are very clear.
The example for intake clearance they give is:
Measured clearance 0.27
Spec clearance: 0.15 to 0.20
Installed shim: 175
Required shim: 0.27 is too big, so the required shim must be bigger than the installed shim to close the gap.
Shim increments are 0.05, so going up one size to 180 would result in a clearance of 0.22 (.27 - 0.05), still too big.
Going up two sizes to 185 would give a clearance of 0.17 (0.27 - 0.1). Sweet.
You don't need the lookup table.
I wouldn't use the Haynes manual to wipe things with.

Hi Dicky,
I've added a post script to my post. The installed shim sizes were 3x175, and 1x 176 (these are on exhaust valves). I couldn't get any feeler guage over 0.20 in. I'll try the shims I've got and see what clearances they produce.

A ps from me as well.
I think you'll find the carb main jet is a 142.5, not a 140.2
Going down to 140 or even smaller should help.
This thread may help (or not?)
viewtopic.php?f=5&t=12666

Hi Dicky
Here's a p.p.s
You're right, of course. Standard mains for TRX is 142.5.
I have an unfortunate tendency to mis-remember things I've previously looked up. I now believe I require 4x170 shims to restore clearance. All I've got is 1x180, and a couple of 182s. I've heard it's possible to reduce thickness by rubbing shims gently on fine wet or dry, but would that effect case hardening?
Thanks again
Blimp

I would keep well away from reducing the thickness of existing shims unless you can replace the number with the appropriate new one.
You can buy a full shim kit with a few of each size you'll ever need.
The shims don't wear out, so you put the old ones back in the box.