Yamaha TRX850 Enthusiasts Forum

I had the bike on a dyno today. Dyno bloke tells me the engine is healthy and pulls well, but there is more power available.

The bike is running way lean through the rev range - like A/F=20.

Can't understand this as I'm running 148 main jets. Is this likely to be due to fuel level?

youngy wrote:I had the bike on a dyno today. Dyno bloke tells me the engine is healthy and pulls well, but there is more power available.

The bike is running way lean through the rev range - like A/F=20.

Can't understand this as I'm running 148 main jets. Is this likely to be due to fuel level?

Is that on WOT so only mainjet or did you do 1/8ths on the gas?
I must say i´ve not dug deep on the standard carbs but it sounds a tad strange....would be interesting to see if it runs lean on all throttle positions if you did not try that....

You DID chuck your airbox didn´t you??

Oooooh I did chuck the airbox but then put it back on. I'm wondering.............

I had the K&Ns on, with the 170 main jets that Gerben has, and it ran really bad. Felt really lean and I thought it was mains too small. Soooooooooooo took all that off and put the airbox back on with 148 mains. It feels ok but is lean all the way through.

From 4000 it just falls off, small steady throttle is ok but then it's lean soon as you open it up.

Maybe it's fuel level causing trouble on both setups? Good job for tomorrow!

youngy wrote:Oooooh I did chuck the airbox but then put it back on. I'm wondering.............

I had the K&Ns on, with the 170 main jets that Gerben has, and it ran really bad. Felt really lean and I thought it was mains too small. Soooooooooooo took all that off and put the airbox back on with 148 mains. It feels ok but is lean all the way through.

From 4000 it just falls off, small steady throttle is ok but then it's lean soon as you open it up.

Maybe it's fuel level causing trouble on both setups? Good job for tomorrow!

Not sipping air thru intake rubbers? I think mine actually did a bit, bike runs better after changing to new ones...or placebo?!

Could we have a peak at the figures??

...also, he did a DJ stage 3 mod didn´t he? If i remember correctly the stage 3 involves some drilling in the carbs don´t it!?

No figures yet cos they're shit.....

I'll check the inlet rubbers too - carb to head? thanks for the idea.

Once I've got this sorted I'll put it back on the dyno, then post some figures!

please forgive my 4 stroke stupidity but will a leaner mixture not give you slightly more power then being too rich? (like on a two stroke) even tho you risk melting the engine.

kookimonsta wrote:please forgive my 4 stroke stupidity but will a leaner mixture not give you slightly more power then being too rich? (like on a two stroke) even tho you risk melting the engine.

I guess a slightly leaner would but this seems WAY leaner

youngy wrote:No figures yet cos they're shit.....

I'll check the inlet rubbers too - carb to head? thanks for the idea.

I was thinking that someone might get an AHA moment if saw the figures....i don´t think anyone expects big numbers with that kinda A/R values....

Yes, rubbers carb to head....mine where tired in general and the clamps had moved from it´s locking positions..

KW - don't have the graph with me. Waiting for it to be emailed. I'll post it when I have it.

youngy you have a pm,,, when i gave my carbs a service i found the std float height to be miles out ,,they were set up very rich ,,one side was 12mm and the other was 11mm ,so i contacted factory pro ,he said he had seen this lots of times and it will make the bike run rich ,,so i reset the floats to fp recommendations at 14mm ,,i also fitted new std needles and new needle jets ,and holders ,i also polished the scratches out of the black slides as they were not moving smoothly in the carbs , gave them a proper clean and fitted them back on the bike ,,it now runs very smooth all the snatchyness has gone it also goes at lot further on a tank of petrol,,i also changed the rubber carb to head connectors as they were full of tiny cracks ,,hope this helps

cheers Steve. much appreciated. Had the carbs off today and checked the fuel level to the Yam manual, and it is within spec. I'll rip it all apart again this afternoon and check the float itself. Still scratching my head over this. Couldn't find anything amiss.

Made a great discovery - fuel tap doesn't shut off any more. Got a bit wet.

fuel tap ,,you may get away with a good clean ,,mine did the same you probably know but for the benefit of others ,,there's a diaphragm and a demand valve inside the tap ,,as the engine sucks on one side of the diaphragm it opens the demand valve, which is spring loaded ,,if there is any rubbish (shite) on the demand valve it holds it open,,,this has the potential of being very dangerous if the carb float valve stays open as well ,,,it fills your engine with petrol this happened to v&m racing in rochdale ,,they started this bike up after it had been stood over night and it burnt there shop down ........O JUST CLEAN YOUR TAP.....iam goin to pub

took your advice Steve - went to the pub. when I got back I stripped the tap, looked ok, put it back together and it's fine. cheers