Yamaha TRX850 Enthusiasts Forum

Hello all,

I've had Yamaha TRX's for decades and have experienced many of their little problems that come with age and wear. Ignition coils with internal breakage, worn needles/emulsion tubes, deteriorated carb membranes and one time big-end failure as a result of the previous owner not topping up the oil. It happened when I drove the TRX home after buying it privately.

Anyway, after having one TRX standing for nearly seven years (the other one has done over 150.000 now) I can't get it running properly now. First, I just let the old fuel out and filled it up with fresh. It started and ran idle, but started misfiring when I gave it some revs. Just with a tiny bit of throttle, I could get it home. It was a gamble anyway, as I had expected the jets needing to be cleaned anyway, which I did.

The bike then ran okay, but still had a misfire at full throttle. After some miles, I couldn't get it over half throttle, and it would stall when I tried to let it idle. It had started leaking too, as the 10% ethanol in the new fuel probably proved too much for the old rubbers. The tap needed new rubbers as it wouldn't close anymore. I checked the pump, which seemed to do its job still fine, in that it would still let petrol through, although I don't understand how the thing could actually pump. Seems more like an extra vacuum operated valve to me.

The biggest problem was the bit of petrol tube with the knee bend in it. That was ripped and it proved impossible to find a bit of tube with a sharp bend in it. That, and one end is wider than the other. I 'solved' it by using a really long piece of tube that now makes a 270 degrees turn.

Before trying again I had a look at the spark plugs, and they were blacker than black with sooth, which I found strange, as fuel starvation seemed the most logical explanation for the stuttering at higher revs.

I put in new plugs and all was fine... but after ten minutes it started misfiring at full throttle again, and it wouldn't idle anymore.

Before I take everything apart once again (my knuckles really start to hate getting to the plugs right now, and I'm fed up with draining the coolant every time), I thought it wise to seek some advice on this forum. Who knows, somebody might have had the same problems before me.

So, if anyone could offer me some advice, that would be greatly appreciated.

P.s. I did check the throttle sensor and tried re-setting it by means of the rev counter, but it was alright to begin with.

Kind regards,

Tim
(Netherlands)

Hi Teedee!

When I hear such a symptom my thoughts spontaneously go to kinked vacuum hose(s).

As you know, the fuel tap and the fuel pump work with a vacuum (negative pressure) run through hoses connected to the inlet ports between the carbs and the cylinder.
If these hoses are kinked they cause the symptom you described.
And the get kinked easily when mounting the fuel tank back on.

By the way:
With your longer fuel hose replacing the (way too) short 90 degree one you can easily pivot the fuel tank around its rear mounting point (with the front two side screws removed of course) and don't have to take it off to have access to the air filter, carbs and so on.
Just keep in mind that the tank should not be too full - otherwise fuel will leak out through the breather openings.

"Hope this helps" - greets from Munich!

Cheers HolerTogni,

The symptoms indeed look like fuel starvation and kinked hoses would be logical indeed. However, I wish it was that simple, but I've checked.

It's weird combined problem that has started after standing for years. Before that, the bike ran perfect.

First: there's the fuel starvation at full throttle (I've cleaned the jets).

Second: it runs perfectly at lower to middle revs, but won't idle when the engine's warmed up. When hot, it won't start again.

Third: the plugs foul badly (which doesn't really makes sense with a poor mixture)

When cold, the bike starts and idles normally again.

P.s. I like your track bike!

Hi Tedee!

The black spark plugs clearly indicate a (too) rich mixture - did you check the needles and the tubes* for the needles (* emulsion tubes, iirc)?

The tubes really wear and do cause such symptoms.

If you can't find original ones look at Microtec - their needles have a slightly different shape and (according to my experience) are better. Ah, and they are made from aluminium not from steel, so they do not wear the tubes that much.

"Successful tinkering" - greets from Munich!

PS:
Thanks!
I love riding her on the track as I love riding my Road-TRiXie on the streets.
Due to the Daytona fairing parts she looks like the stepsister of the racebike of my wife!

Thanks again. I've had needles and tubes worn before and that was indicated by stuttering around 5.000 rpm. It runs perfectly in that range, just stutters at high revs. The Microtec tip is welcome, thanks. Though original parts can still be found.

I was contemplating last night, that the stuttering at high revs indicates fuel starvation. Which means I'm gonna put the carbs in the ultrasonic cleaner once more as I'm going to dismantle them again. I might opt for a revision kit, including new membranes. If these are still available. I had one membrane hardened and torn and used a spare one from an old carb. However, new ones might be the solution, as I can imagine that the old and hardened membranes might prevent the needles from going fully up.

It's just a thought, nothing is proven yet.

Then the black plugs: although the air filter seems alright, it's a red K&N one, which makes it difficult to see if it's clean enough. Maybe the years of standing still have dusted it up and the oil has gone thick. So, I'll clean that one too. It starts good when cold, just as when using a choke. It's when warm when the problems start. And as a clogged up air filter makes for a very rich mixture, that might blacken the plugs, causing them to fail. And that might explain the failing to idle after riding.

I'm not a mechanic, so I could be completely wrong with everything.

Hi Tedee!

First I have to apologise for a weired mistake:
The needles I meant are from Micron Systems - and not "Microtec". Sorry for the confusion.

Then I have a remedy for hardened membranes - not tried by myself yet, but apparently working for some guys at the "German" TRX-Forum:
Let the rubber of the membrane "soak" in petrol (rubberside down in a glass of petrol) and it becomes soft again - and if shrunk it even grows back to its original size.

"Hope your measures taken will lead to success" - greets from Munich!

my 2 cents after a long time not visiting the forum:

I've experienced something similar with my TDM (4TX) after an ultrasonic cleaning and rebuild of the carbs.
Turned out the choke needlevalve (or whatever it's called) wasn't fully closing. It's a black plastic needlevalve (just follow the cable down from the choke knob) It kept adding extra fuel to the mix so it ran way too rich. After lubricating them with some engine oil they closed again and the problem was gone.
To see the part I mean:
https://www.cmsnl.com/nl-us/yamaha-trx8 ... en1410a01/