Yamaha TRX850 Enthusiasts Forum

Evening chaps.
Always had a tiny misfire on the Trx that comes and goes, sometimes bad, sometimes not so bad, but always there. I thought it was electrical at first and have buggerred about with everything, tested everything, and its all good. I did have a fault where the ht lead was squashed, sorted that, and it was great for a couple of weeks. But its back with a vengeance. Its been frustrating because everytime I've changed something like the coils its seemed cured, for a day or so.
Coils, pick up, all good. Big sparks and good fuel supply. Starts on the nail but now refuses to rev at all. I've swapped carbs and its no different. Its getting on me wick now
Valves ?

If it seems better after you faff with the electrics then surely it's the electrics, no? Was it any different after fiddling with fuel in any way?

Can you rev through the misfire and it's fine again or is it constant or what?

I think I've just been kidding myself that its electricery. The fault always got worse the hotter it got, so when I mend anything its ok cold/warm but gets worse again the hotter it gets. It could be ok going to work a couple of miles and a quick blast, then on the way home, nerr nerrr.
It won't rev at all now, well, doesn't want to but I can coax it a bit then it farts and cuts out.

I would not think it is the valves; can't see why a valve related issue would come and go intermittently.

Valve issues also not that likely to start suddenly in my opinion. Gradual change/deterioration maybe. Even if it was the cam timing out, this would not come and go.

Have you got a spare ECU? I have never had probs with my own but if the coils are good, sparks healthy, etc why would it not rev? Assume that ign advance is electronic on the TRX. Have you got a strobe to check the rotor marks line up on tick over and advance functioning? Doubt ECU would be temperature related however.

Years ago my motor always used to fire up on one pot and then after a minute or so the second cylinder decided to do something. Changing the battery seemed to improve this. How's your battery and charging voltage? Doubt however it's either if it fires up OK. Poor chassis/motor earth perhaps? Tricky from a distance to diagnose.

Don't want to freak you out but my old air cooled CB125 years ago ran fine from cold but gradually faded when hot until it would stop. Eventually found that oil screen mesh was blocked and motor was on the point of siezing it was getting so hot!! I was only 20 at the time and didn't know shit about engines.

Check your oil gallery bolt flows oil on tickover maybe.................is it cooking on the temp gauge when it goes flat?

I'll think on and post any other disturbing thoughts.

Good luck

Might be valves, the clearances might be closing up just too much when it's hot? Is it noisy/quiet at the top end? Mine is more than a rustle at mostly max specs, quiet would be very suspicious. Though as Mr Widdy says, it wouldn't be intermittent.

Somehow I doubt it's fuel, it seems you can chuck the stuff in and they keep on running, and if it was lean it should get better the hotter it gets, not worse.. A puzzler for sure!

As an aside a mate of mine had trouble with his VFR400 running on only a couple of cylinders and it seemed to come and go, it turned out to be several poor connections between the coils and the plug caps. If the caps are complicated in any way (i.e. not straight through) then they could perhaps cause a problem like this.. Only thinking aloud though!

When I did the Mille/fighter fettle I was hummin an arrin about getting a top end sorted because of the oil consumption and what I thought was a bit of a tick. I decided to leave it after a chat with another forum member whilst on a ride out who reckons it was a just bit of cam chain rattle, made sense to me, and he knows a damn site more about this shit than me. My intention was to get to winter then sort it. I've had it up on the ramp and can see oil moving around the site tube. Someone behind has commented that is stinks of oil when I boot it.
It has however got noisier and sounds a bit tappety if you get me

Put a new set of plugs in there Yoosey..you will be surprised.
The cause of many intermittent problems.

alextrx850 wrote:Put a new set of plugs in there Yoosey..you will be surprised.
The cause of many intermittent problems.

Done it mate, caps, leads, coils all A1.

Mine was like that, gradually getting worse and would cut out when hot and wouldn't restart till it cooled down. I checked the valve clearances and all but two were ok. I adjusted those two (they had no clearance) and the bike has run sweet ever since. Got to be worth checking......

I had a similar issue. Turned out to be the slider on the choke shaft getting jammed, meaning one pot kept the choke full on. A bit of grease sorted it. Will not hurt to have a look.

Cheers chaps, all good thinking, but tbh I think I've checked everything suggested except the valves.

Yoozy wrote:Cheers chaps, all good thinking, but tbh I think I've checked everything suggested except the valves.

Better check the valves then!

Could be a dodgy earth/connection on something that heats up, hence coming & going? I had a dodgy plugpack that would only play up after the bike was properly warmed up, that was a prick of a thing.

The stator output has been checked when hot? Had a CB250rs single (wet stator) long time ago that would do near precisely what you're describing. From cold it would be ok but about 10 miles and the miss firing /no idling would commence till it just ran out of noise. Let it cool right down and it would start and run again for a bit. Thermal overload in the stator was eventually found to be the cause but the problem wouldn't show till the oil reached full temp.
I am aware you have probably checked this already ...

Gordo-

This has been an intermittent fault that has gradually got worse to the state it won't rev from cold, starts on the button and idles fine but a tad lumpier than normal. Convinced its not electrical though

Well, came home from work with a kill or cure attitude to it. Fired her up, good. Rev, nope. Coaxed it a little and cough fart splutter, coax a little more and gave it a handfull. Boooom blast bang, biiiig cloud of shmoke and has been sitting there purring away for 20 minutes now. Reving cleanly and sounding better than ever