Yamaha TRX850 Enthusiasts Forum

Hi. I’m new to the forum and first time I’ve used one, so if I’m covering old ground I do apologise. If not posted in right area let me know. I’m rebuilding a TRX850 motor, and to be mounted in the back of Piranha off-road buggy so am chasing extra performance, around 100HP would be nice. Not really sure of the ins and outs of it though, so here are my plans:

• Kent YAM9 changeover cams based on TDM850, specs are: (do I need heavier springs, if so which ones will work?)
Cam Lift Cam Lift Duration Duration lobe centre lobe centre Valve Valve Valve lift Valve lift
at 1 mm at 1 mm angle angle Clearance Clearance at TDC at TDC
8.43mm 8.43mm 240 240 105 109 0.15mm 0.15mm 2.05mm 1.65mm

• TTS-Performance 878cc Big Bore kit: (is there a better one or bigger kit with barrels included?)
• 41mm flat sides: (not sure which ones are best, also the throttle position sensor on old carbs has me stumped as I believe it retards/advances ignition?)
• Using the flat slides, what ignition/CDI/ECU do I then use?
• A RM Stator for bigger spark, refer http://www.rmstator.com/en/motorcycles/ ... 93/365.htm
(not even sure if fits my motor as says for TDM850 92-93 and I have TRX850 and not sure of year?)
• Head port/flow work
• Clean up the intakes
• Raise compression to 11.1 or thereabouts
• Exhaust – race mufflers: (must a balance pipe be used between the exhaust outlets, and would a 2 into 1 be better?)

Really stuck on getting the bigger spark and correct ECU/CDI to use or if the old system will work with carbs without throttle position sensor.

Will standard clutch hold out, or should I go for a heavy duty one, and if so which one? Any other ideas or advice would be greatly appreciated.

Thanks very much in advance for any help.

If you have a quick search you will find details of the tuning on my bike - pretty much exactly what you suggest above and the motor is making just over 103bhp.

One piece of advice though, make sure you have a free flowing exhaust - not just end cans.

However, this ain't gonna be cheap. Assuming you can do all of the work yourself so theer are no labour costs:

FCR 41s - £500 (if you can get them second hand, otherwise £7-800 new)
Pistons and rebore - £400
Cams - £200
Gas flowing / porting - £200
Oil / gaskets etc - £100

Wouldn't it be cheaper to buy a different engine? Having mounts made / having subframe modded to accept a Mille engine that makes and easy 120bhp would surely be cheaper? Or if you want to retain carbs, a Firestorm lump?

forgot to mention -

proper TRX FCR41 carbs come with TPS. Otherwise, if you have a search you will find a mod which advances the TPS and removes warning codes

phuk72 wrote:
Gas flowing / porting - £200

Where please and how good are they?

Kenny wrote:

phuk72 wrote:
Gas flowing / porting - £200

Where please and how good are they?

Bloke who did mine is no longer trading but there are plenty of places who will do it for you (TTS at Silverstone being one)

IMHO not worth it wihtout cams as well (and if you are going to tear down the engine, you may as well put new pistons in )

phuk72 wrote: Wouldn't it be cheaper to buy a different engine? Having mounts made / having subframe modded to accept a Mille engine that makes and easy 120bhp would surely be cheaper? Or if you want to retain carbs, a Firestorm lump?

I'd have to agree with Phuk72 here, surely getting an engine with more power already would be cheaper possibly even more reliable?

Plenty of crashed TL1000R/S's around from when the rotary damper failed, 130odd HP would be a lot more fun in a buggy

Thanks for your replies. Yeah a bigger motor would be nice with more HP standard, but motor already dummy mounted and dont want to spend more on another motor, plus i think uni's and transaxles are only rated to about 110HP, anymore and I have to upgrade.

Transmisson will take more punishment with a heavier vehicle. Clutch and transmisson is apt for what it's desgined for, but i suspect a heavier clutch wouldn't hurt...

yeah, with the extra weight, id be looking at some heavy duty plates and springs.