Yamaha TRX850 Enthusiasts Forum

Problem.

Did an hour or so of 42 degree heat riding a few months back and after a while TRX ran real bad, cough and splutter down low RPM, wont pull, big lag on throttle, backfiring.
Runs sort of OK when cold but still shyte when at operating temperature.
I figured it was a carb failure of sorts and chucked them, replace with FCRs. The original carbs were a bit tired looking anyway, had no issues before the hot day though.
Now have the FCRs on (un-tuned of course, just to make things really tricky) and still the same or really similar issue, SORT of OK when cold, unridable when warm.
Did the TPS bypass mod (only on 2 prongs, not the third, I was hoping this prong marked SS was the sidestand bypass, please advise if wrong)

Hmmmmmm

could this be valve clearance issue?

I have re-built the top end and not re-shimmed since, maybe 5000k, did the shims on the head before torquing down.

Any thoughts or similar experience??

Thanks all

Excluding setup issues on the FCR's, definitely sounds like a valve clearance problem. Tell you what, we can put my fully setup carbs on your bike to eliminate that possibility. I'll need your FCR's in the meantime though.

I had the clearances close up on my replacement engine after about 1K, I expect the same has happened. Did you replace valves when you rebuilt the top end?

didnt replace but ground and re-seated.
Looks like in we go then, pass me the scalpel

So in we went.
there was a tight one, only a thou clear when cold
"Ahaa" said I -that will be the culprit. Re-shimmed and put it back together.
Duh, forgot to plug the radiator fan back in and burnt the connector to smithereeens on the header pipe whilst warming it up for the first trial. forf uck sake.
fix it all up again, my bad. Piled on a lap of town and came back saddened by the exact same symptoms as before.
Expert mate says "gotta be valve clearances- set 'em all at the loose end of the scale, its real cold here in Oz, remember, they will tighten up heaps when your up to temperature.
"Ahaa" I say, you expert you, what do I know. Back to the freezing cold shed again, grovelling about on the floor with a flashlight betwixt my rattling teeth.
So doggedly I pull it down AGAIN and re-shim to maximum recommended cam gap.
They were all perfectly on the low end of clear due to my expert skills of prior installation.
Duh, Pull a cam cap thread too hard and the cam wont turn freely, scrap it and sand it till it rotates OK again. My bad.
Duh again,Drop a 1/4 drive 10mm socket down the coolant gallery and have to buy a magnetic snake thingy to pull the carnt out. My bad again.
Loose maybe a pint of perfectly good blood as I slipped whilst tightening a pathetic little hose clamp for the radiator when putting it back together again. Grrrrrr

Warms up smooth and clean, I pile on and do a lap.
EXACTLY the same symptoms again. This time its a lot more rattly though, with all that valve clearance. Fo rfuc ksake again.
A small dint remains in the shed wall where the TRX keys hit it at great speed.

So I change tack, mebbe its electrical. Doesnt seem electrical, (starts fine,idles fine, runs fine cold, misses under load up till 3500 rpm then comes good)
Replace the plugs -HEY! SLIGHT IMPROVEMENT! This could be the right track were on!
Smartarse sparky neighbor says "dude - check the resistors in the plug caps" (there is something in the plug caps? -news to me)
So I did, checked them into the rubbish bin and replaced them with some 1/8 brass screws, cut down to the correct length.
To hell with peoples A.M. radio as I ride past, me thinks.
And what do you know, all better. Pulls like a dream hot or cold, a little jetting will have it perfect.

2 resistors, total value maybe $0.20c.
Now I know how frustrated NASA was when the Challenger blew up because of an O ring.

roll on summer!

bagmaker wrote:So in we went.
there was a tight one, only a thou clear when cold
"Ahaa" said I -that will be the culprit. Re-shimmed and put it back together.
Duh, forgot to plug the radiator fan back in and burnt the connector to smithereeens on the header pipe whilst warming it up for the first trial. forf uck sake.
fix it all up again, my bad. Piled on a lap of town and came back saddened by the exact same symptoms as before.
Expert mate says "gotta be valve clearances- set 'em all at the loose end of the scale, its real cold here in Oz, remember, they will tighten up heaps when your up to temperature.
"Ahaa" I say, you expert you, what do I know. Back to the freezing cold shed again, grovelling about on the floor with a flashlight betwixt my rattling teeth.
So doggedly I pull it down AGAIN and re-shim to maximum recommended cam gap.
They were all perfectly on the low end of clear due to my expert skills of prior installation.
Duh, Pull a cam cap thread too hard and the cam wont turn freely, scrap it and sand it till it rotates OK again. My bad.
Duh again,Drop a 1/4 drive 10mm socket down the coolant gallery and have to buy a magnetic snake thingy to pull the carnt out. My bad again.
Loose maybe a pint of perfectly good blood as I slipped whilst tightening a pathetic little hose clamp for the radiator when putting it back together again. Grrrrrr

Warms up smooth and clean, I pile on and do a lap.
EXACTLY the same symptoms again. This time its a lot more rattly though, with all that valve clearance. Fo rfuc ksake again.
A small dint remains in the shed wall where the TRX keys hit it at great speed.

So I change tack, mebbe its electrical. Doesnt seem electrical, (starts fine,idles fine, runs fine cold, misses under load up till 3500 rpm then comes good)
Replace the plugs -HEY! SLIGHT IMPROVEMENT! This could be the right track were on!
Smartarse sparky neighbor says "dude - check the resistors in the plug caps" (there is something in the plug caps? -news to me)
So I did, checked them into the rubbish bin and replaced them with some 1/8 brass screws, cut down to the correct length.
To hell with peoples A.M. radio as I ride past, me thinks.
And what do you know, all better. Pulls like a dream hot or cold, a little jetting will have it perfect.

2 resistors, total value maybe $0.20c.
Now I know how frustrated NASA was when the Challenger blew up because of an O ring.

roll on summer!

The normal reference is to `suppressor plug cap`, usually a carbon suppressor built into them with a resistance value of 5 to 10 K-ohms.
The stock plugs also have built in resistors too ( DPR8EA-9) so I strongly suspect your leads were breaking down at the plug caps.
I would recommend some new NGK long caps , I`m not sure I completely understand how you`ve attached your leads to the plugs but believe me you really don`t want any stray sparks kicking around.
You`ll also suffer in the wet with no insulation around your plugs, buy some new plug caps you know it makes sense.
A useful resource here:

http://www.z1enterprises.com/reference/ ... guide.aspx

Basically, it's shagged and wants to die.....























I'll buy your FCR's

Hi Mince, thanks for that

the plug caps have a little brass screw thing that attaches to the actual plug, it has some wires through it to give some "grip" on the plug top thread. You need to pull the cap OFF the plug and look up inside it.
If you unscrew this brass thing with a flathead screwdriver you will find (note- I found these, meaning I expect everyone will but in fact it may be an Australian design rules thing for all I know) a resistor pushing against the brass thing with a spring behind it. The resistor looks like a small fuse.
This resistor is about 3 or 4mm diameter and maybe 15mm long. Replace it with solid brass, problem fixed.

The neighbor is in fact an old trail riding buddy and he reminded me how much greif these resistors caused us in the early days of their fitment.
They did NOT mix well with cold water. When they first came out we had lots of trouble with wet days trailriding or after river crossings.

But hey, the radio worked clear and crisp!

I havent given it a really good test, will this weekend maybe. I would expect to keep chasing along the electrical route if there is further issues.


Bagmaker

gix, dude!
1- a TRX never "wants" to die
2- NEVER be beaten by a machine, there are no failures in physics

Ahhh, now I understand.
You`re describing the ferrule with the wire clip that catches on the plugs thread as you push the cap down onto the plug?
Yes it sits up inside the plug cap but really the caps are not meant to be taken apart and I`m not sure you can even buy those 5 to 10 K-ohm resistors to replace them.
Well, if you`re not creating RFI whenever you run your bike (radio frequency interference) and your brass scew `mod` is working see how you go on with it.

3 years later and I still have the problem.
Just replaced the stator and ignition pickup, thats it, nothing more to replace but bits of wire.
I have tried a different CDI
I have tried different coils
I have replaced HT leads
I have replaced the plugs

Idles fine, pulls like a train when cold.
Misses, farts back through the carbs under 3500 rpm when hot, still idles fine. Gets over 3500 and runs on both cyls again
NOT just if the motor is hot, summink in the engine AREA. For example today I went 10kms @25 degrees ambient, no problem.
Temp gauge comes up to running spec about 3kms. Cooled off (washed, lubed bike all excited thinking its fixed) then road maybe 20kms before it starts to break down.
If riding on a cool day open roads its OK, if riding in traffic with no air flow its a nightmare.

I am so down on this fukka
Reward given for fix

Wild stab in the dark here,electrical problems are not my forte,how's the fit of the magneto rotor on the shaft and woodruff key? could there be a possibility of some movement messing up the timing as it warms up?

I fully accept I could be talking complete nonsense here

Electrical frame earth..coil mount earth ...
You might have more than one gremlin in there as well....
I'd also pull rubber tubes as well ..one by one, block and suck for integrity...the tubes I mean.
Basically eliminate all possible one by one, borrow items from a good donor bike and just go through it from top to bottom.
Maybe buy a completely new set of hoses and sand paper those earths...you will get it eventually.

alextrx850 wrote:Electrical frame earth..coil mount earth ...
You might have more than one gremlin in there as well....
I'd also pull rubber tubes as well ..one by one, block and suck for integrity...the tubes I mean.
Basically eliminate all possible one by one, borrow items from a good donor bike and just go through it from top to bottom.
Maybe buy a completely new set of hoses and sand paper those earths...you will get it eventually.

Thanks Alex,
I went over the coil earth area's pretty well when I changed them, including sanding where they touch the frame.
The rubber tubing is minimised with the inclusion of FCRs, same issue before FCRs, same issue after.
Significantly, I cannot see issues like these changing much between 20 degrees ambient and 25.
Also, I cannot see how the issue would go away completely for a clean idle and clean running above 4000 rpm at any temperature, but not allow the engine to fire on one cylinder between those revs.

"Wild stab in the dark here,electrical problems are not my forte,how's the fit of the magneto rotor on the shaft and woodruff key? could there be a possibility of some movement messing up the timing as it warms up?" - probably closer to the mark, some wierd thing like this. I will check it too, cheers mate.

Cheers also to Paul for a tip to try it with the tacho disconnected, interesting theory which I will try as soon as it warms up again.

You may well have already tried putting a strobe on it to check ignition timing plus advance/retard working OK with motor hot and cold? May reveal something wierd?

Seems key that it is so temperature/airflow related (If riding on a cool day open roads its OK, if riding in traffic with no air flow its a nightmare). Does hot motor always correlate with poor/annoying performance?

How hot does it get on the gauge when it messes you about?

Seems really unlikely as a cause but I would be checking the cooling system is all good; sounds like you have tried everything else electrical and changing carbs has done nothing. Run with no stat perhaps and see if that changes anything.

Good luck with it; I will keep thinking of other ridiculous possibilities!

Sounds like a coil fault I had a few years ago, would only break down when hot...

Or the timing could be out. It's got to be something simple...