Yamaha TRX850 Enthusiasts Forum

Hi just started to service a second hand set of carbs as my original carbs had a seized pilot adjuster the second hand carbs have a thin tapered needle and have 140 mains but they look worn so I was going to use my std parts that look good ..the main jet is a 142.5 and I am using separate k&n filters ( it came like this ) I was expecting a bigger main jet than std ..I have fitted new plugs and checking them after running the bike they look the right colour and the bike runs smooth My point is do you think the std jet size is ok or should I be upping the size ? any thoughts would be appreciated cheers.

Trx runs rich from factory and nothing unusual to see a drop of 2 sizes in mains. All depends what you want from the motor. 12-13:1 A/F gives best power 14's give best economy. Thinner taper on the needle will open to more fuel plus you should keep matching tubes with needles. Some are made from different metals so corrosion comes into play due to mismatches on a galvanic scale. Example like dynajet titanium needle and brass tubes are at opposite ends of the galvanic scale and excessive needle corrosion occurs until it finally just snaps.A few members over the years have fallen into this trap.

142 with K&N sounds pretty much perfect, I'm running 140 and it doesn't take long for the temperature needle to start climbing far too high if I spend any length of time in town/traffic. Side winds also lean the mixture, 140 is quite marginal when this happens.

Also beware of emulsion tube wear, some people try to counteract the resulting richness by going with smaller jets! Mine came with 136's, after I replaced the tubes I was getting an easy 70MPG on a run! It barely ran in crosswinds though and was quite weak all the way through the rev range.

Thanks for your comments much appreciated .