Yamaha TRX850 Enthusiasts Forum

Today was the first time I've ridden my TRX at a constant high speed (100km/h) and when cruising at a constant speed I felt a loss of power - at times quite violent bucking but other times a gradual loss of speed like riding into a cyclonic wind. A positive adjustment of the throttle would immediately pick the bike up and idle and acceleration are both strong and consistent.

In the last twelve months I've only ridden this bike in inner city traffic commuting and I've encountered some surging, but nothing as dramatic as at 100km/h.

The suggestion I've heard is it could be the emulsion tubes, but I've also read in another thread that another riders bike was too lean and he turned up the idle enrichment. I don't have a huge understanding of carb voodoo (give me fuel injection any day) but I would have thought the difference between pig rich and lean would have been more apparent.

I've ordered two carb rebuild kits and will have a look but is there anything else I should be looking for? It's not pleasant riding with the feeling that the bike could snuff out any moment.

As you can probably tell from the lack of replies, this isn't normal. They do surge a little bit (usually if the chain is too loose) but it's never violent like you describe. I would start by looking for air leaks around the carbs..

I've had similar symptoms with an alloy fuel inlet which didn't have a breather like the original fitment has.
I found that the tank breather hose which is the outlet tube on the right hand side of the tank lower... was in the underneath air flow which had ..as I believe ,..a vacume effect and more evident as the tank was emptied.
I drilled a .5 mm hole in the upper of the alloy fuel cap and then shortened the breather tube.

Also check the OEM fuel filter inside the TEE piece (if its still there), to the fuel inlet line and the carburettors.

As already suggested, start by checking all hoses and carb fitments. If alls good you'll need to check that fuelling is correct. Your post suggests you are familiar with fuel injection. Well, carbs are no different, you just use screw drivers instead of a laptop. A particular engine's fuel requirements are the same regardless of whether it is carb or FI. Visit the Factory Pro website for lots of good info on carb tuning.
http://www.factorypro.com/tech/tech_tun ... gines.html

One other tip, there is a tonne of good info on this forum and lots of help here, but you'll make better of it if you pop into the Introduce Yourself section and introduce yourself to other members.

I would clean out the carbies and as Alex says check the filter in the fuel line. Better still as three of here have done is to remove this t piece fuel filter and place an inline filter.

Just an update:
I rebuilt my carbs a few months ago and the surging seemed to go away.

Unfortunately this morning it seems to be back. Fine at idle, when under load and at full throttle. I'm going to test my TPS as I have a theory that it might be giving funny readings to the ECU when cruising.

Kinked/blocked fuel line.

trixynut wrote:Kinked/blocked fuel line.

Much more likely than TPS

Don't overlook the obvious, dirty fuel.
Check the drain on the float bowls.

If someone local you know with a Trx has a programable ignition, see if you can just bolt that in place and you can check the TPS values and monitor if any changes. Takes 2 minutes.
Calibrate the TPS takes 20 seconds start the motor or go for a quick ride around the block then check the TPS values once again.

Mine use to do something similar. 100 kph there abouts, it would start to lose power, not anything as dramatic as yours sounds, mine was fixed when I fitted one of Brians Programmable ignitions.