Yamaha TRX850 Enthusiasts Forum

Well, since I've owned my TRX,(my second) I've had big problems getting the jetting dialed in.
I replaced the worn emulsion tubes with Factory Pro tubes. I couldn't get it to run at all with the Factory Pro needles so refitted the Yamaha needles. This got it running OK. I had it dyno'd, fitting a smaller 137.5 main jet, and its producing 73hp at WOT, according to the dyno.

The problem is that it starts out running ok, but after a while its farts and runs poorly from 3000 rpm to 5000 rpm.

This occurs after the engine has been running constantly for a minute or so. If I stop at a red light, it runs ok for a while and then starts playing up again. However, if I pull the choke out, it runs better from 3k to 5k, but dies if the revs drop below 3000.

The temperature gauge never gets above 70 degrees.

I tested the fuel pump, and it didn't seem to be pumping enough fuel, so I fitted a low pressure electric fuel pump, thinking it may have been a fuel level issue.

This hads got it running a bit better at low revs, but the problem between 3000 to 5000 persists.

I'm pretty sure the problem is its too lean in this rev range, but I'm not sure how to adjust the mixture in this rev range.

The Factory Pro website lists steps to take to adjust mixture up to 3000rpm, and from 5000 rpm to 7000 rpm, but doesn't mention what to do for 3000 rpm to 5000 rpm.

Any ideas????

back to my settings :
std pump and tap arrangement.
using all the factory pro bits. i.e. Ti needles in the middle clip, and their tubes.
std size jets.
std float heights.
mixtures at 3 turns out.
K&N filter.
std airbox.
2 into 1 stepped at the collector and to 52mm inlet 450mm long can with db restricter.
without the restricter, it dropped into a hole if you cracked the throttle open between 3000 and 4000.
with the restricter, no problems.
cheers
fung

have you thought it may be fuel starvation issues instead of jets, if the rough spot is further up the rev range i.e. When it needs to be pumping more fuel?
Possible blocked breather, or vacuum in the tank, or split membrane in the pump, or dirt in a fuel line?

i had another read of factorypro's data, and it seems that if between 3 and 4 thou the throttle is 'WET' or 'SOGGY' and gets worse as the engine heats up, try lowering the float level by 1mm.
if the engine is 'DRY' then raise the float level by 1mm.
the 'MAIN' jet must be within one or two sizes of correct for this to gel.
so its main jet first for 7500-redline; needle clip for 5k-7k; then float height.
hope that helps some.
cheers
fung

Yep, I'd be playing with float height.

I fitted Factory Pro needlejets too and on the dyno we went down to 132mains and got 76bhp and I could never get the thing to run properly in the lower rpm no matter what I did. I put it down to worn carbs on mine and bought a set of FCR41's and never looked back they carburate almost perfectly now and much much more power and better fuel economy too compared to the std worn carbs.

at 76,000 with std carbs I replaced the neddles with factory std , std main jets , and dynoed up at 76hp and was running fine with staintunes. But opted for the FCR's myself

might be worth checking to see that your TPS is working right. Section 6.6 I think of the workshop manual

edit ignore that, more likely to be the float height

Thanks for the help guys.

I spoke to the service manager at Ric Andrews, he's seen the bike and reckons the DR650 forks on the bike has tilted the engine back a bit, and so I need to raise the float level.


Going to give it a go tomorrow, after I fix the flat tyre I got yesterday.

Homer wrote:Thanks for the help guys.

I spoke to the service manager at Ric Andrews, he's seen the bike and reckons the DR650 forks on the bike has tilted the engine back a bit, and so I need to raise the float level.


Going to give it a go tomorrow, after I fix the flat tyre I got yesterday.

The man is wise...very rational suggestion...luck

Thinking back on it, when I got the bike the float level was set at 9mm instead of 14mm, which would raise the fuel level, compensating for the angle of the engine and carbs.

FINALLY, jetting sorted.

As suggested, I tried dropping the floats by 3mm to 11mm, effectively raising the fuel level in the float bowls by 3mm. Surging between 3000 to 5000rpm completely gone.

Huge thanks Fung, MV, Dav, Conrod etc for helping me work through this.

Don't mention it mate, just glad you got sorted out

thats why we're all here homer.
if two heads are better than one, then surely 1900ish heads must be friggin fantastic ! !
cheers
fung

once spent 3 days trying to sort a carby out on my old trailbike after fitting new piston to it.
Turned out to be a wasps nest in the fuel tank breather