Yamaha TRX850 Enthusiasts Forum

G'day all. I am about to set the front of the fuel tank 3-5mm higher than standard, this will give more air space under the tank, and allow the air box to suck from a greater supply. I know this makes a difference, as i have done it on previous bikes, and have read here and there that others have done the same, with notable gains. Question; the foam that is glued around the outside of the airbox, is there to slow down the rush of air entering from up and under the front of the tank, and possibley drawing air out of the box,ie;venturi effect. I am wondering if i should remove the foam from the front of the box, and add some to the rear, to create a dam effect, or, put a more open cell type foam at the front, to allow extra air to flow through the foam, but not enough to have a negative effect on the snorkle area. I am running std carbs, with a 'Factory Pro' kit, Titanium needles, jetted to suit, Pipemaster open exhaust, K&N Hi flow filter.
Regards; Trxster

about the airbox...mmmm...ditched mine and the carbs...problem solved... heard you got pipemaster exhaust system tho.... be interested to know a little more on that tho.. 2 into 1 large bore or twin system

yep - raise the airbox, rip off the foam and modify / remove the snorkel will help flow more air.

However, it still restricts airflow but can give decent gains with properly jetted carbs. I chased this route and it cost me over £200 in dyno time (plus jets and my time) for about 5bhp.

If you want more power, I'd suggest putting that money towards some FCR41s (I wish I'd been given that advice at the time)

Cobba, I would rather keep carbs, they certainly help with getting fuel into the cylinders the Factory Pro kit made a huge difference, extremely crisp on the throttle, no flat spots, awsome. The twin Pipemasters where on the bike when purchased, being stainless steel, they are a bit on the weighty side, then again I weigh 72kg, F/glass single seat cowel, and unwanted tubing has been removed from the sub frame, what was gained on the "slippery dip" was lost on the "swing", I re packed them a year ago, which made them sound a tad sweeter. The guys are in Brisvegas and are great to deal with, I had to replace an outer skin, Don't ask , hinterland backroads , they fabricated one for me, and had it ready in a couple of days.

Thank's Phuk, The FCRs are on the wish list, it's also a case of finding a set, when found, I will be doing my best to get hold of them. In the mean time I will tinker with what I've got, I like your way of tinkering, ie; if it's creating any sort of restriction, get rid of it. I will give this a go, and spend a bit time getting the tune right again.

No carbs for me - I've gone to evaporative fuelling for maximum airflow.

trxster wrote:Cobba, I would rather keep carbs, they certainly help with getting fuel into the cylinders

phuk72 wrote: (I wish I'd been given that advice at the time)

I thought you were....

I got good gains by lifting the front of the tank and removing the foam.... in the end I had to drill the funk out of the front of the airbox (with FCRs) Ultimately I thought junking the airbox would get me more, but it didn't really... I think the restriction is in the head (ie my porting could be better...)

I'm off to see the wizard.. the wonderful wizard of pipemasters..Today...wooo hooo to get some big bore 2 into 1 stainless headers fitted with a megacycle muffler i had rolled up on the end.. I'm sure this will compliment the fcr's n bellmouths just nice... Start savin your biccies for some fcr's you'll love em when you get em.. and the airbox will make a great pot for growing your herbs in..

An industrius day was had today, the foam has been removed from around the airbox, and also up inside the tank. Drilled approx 20 x 6mm holes above the vertical tube that leads into the airbox, kept away from the front of the snorkel as i am concerned about disrupting the "down draught" into the a/box. I have packed the fuel tank up 5mm, which, with the removal of the foam pad that sat between the top of the a/box and the fuel tank, gives a space approx 15mm over the top of the a/box. I hope to be able to get a ride tomorrow, RDO , if not, it will be on the way to work on Tuesday. Will be able to give seat of the pants feedback then.

And...

What was the difference?

Sorry 'bout the delay with feedback, we have had quite a bit of rain since the mods have been done, but have finally got a couple of dry runs, all be it with easter traffic .Well, for a modification that cost nothing but time, It was well worth it. More responsive all over the rev range, it feels like another 5 or so HP,or should that be 10Kn of TORQUE? it really does pull better than before the mod. Idle jumped up a couple hundred revs, but easily adjusted back down.I run 16/41 gearing, 95 octane fuel, '99 model bike.

Shame you didn't dyno before and after - you'd know the true gains then...as long as you use the same dyno as these can vary too!

Amusingly An R1 riding pal of mine thought his bike was running a bit rough. Had it dyno'd and only found 85bhp at the back wheel! Power Commander and some dyno time now sees it putting out a rude 150bhp! The dyno never lies...so they say!

Corners

I suppose so, then again, i did say i would give a seat of the pants review . I totally enjoy tinkering with bikes and the results that can be gained with little $$ outlay, and more attention to details ie; suspension setup including lubed pivot links, wheel alignment, chain tension and lube, properly adjusted and lubed cables, regular cleaning of the inside of the fuel tank (you may be by the crap that gets into it after a couple of tank fills), when was the last time some of you cleaned the little filter that sits inside the 'T' junction in the main fuel line ? I put Wynns spitfire carb & injector cleaner into fuel every 2 months or so, tyre pressures, etc. The TRX is a Gem of a bike as we all know , I know that with big financial outlay i could end up big HP gains, then again i could by used R6 or CBR 600, and would get totally pissed off when a TRX blows me to the weeds from a standing start, or mid range roll-ons.