Help with 900 conversion

[cobbadiggabuddyblooo]

I'll second that , for most day to day riding a really well tuned stock bike will hold its own till I'm around 4000rpm on the original Trx motor with FCR's and l/b 2 into 1 exhaust .
I started to find gains again under 4000 rpm but it took the akrapovic system, coils , playing with dwell and catch tank to get things burning better down low with the FCR's with this current stock motor.
Then lighter wheels and suspension to make noticable ground.
But with just the 41mm fcr's and previous exhaust only if there was not that couple of hundred metres between the next twisty section I still find Rod in the mirrors with really little gains .

Money is better spent on getting the fueling right on the stock carbs, ignition upgrade to get the timing right ,suspension, brake upgrades and lighter weight wheels in my opinion as these will give you more useable gains.
The suspension and brake upgrades alone will give you more feedback and confidence and again the lighter wheels will improve handling and again instill more confidence.
Sure shorter dogbones may make a the bike a little more flickable but at the expense of higher speed handling.
Lighter wheels give you that flick ability with the std dogbones without compromising your mid/high speed handling plus you'll find gains in braking acceleration only giving you again more confidence.

One of the biggest mistakes I made with the Trx earlier on was the std TPS mod ,
You loose so much of the useable power down low and midrange punch when first on the throttle it was a joke all for the fact or fiction that you may find 1 or 2 extra HP up top.

If you really want the 900, go for the 900 barrels and 900 conrods into the Trx bottom end and head.
Look at ceramic coating as you can easily loose 2-3 hp throuout the rev range once the bike starts running a little hotter so keep the temp down to start
There are better options out there too than just the JE 878.
I'm in the process of getting 878 pistons made that are more efficient and less weight than the JE so no balance issues on std trx bottom end like the JE.
Think long and hard what you want from your Trx and where you want it and try to make each modification to what you really want from the bike.
More useable midrange and increased torque or just an out n out hp figure to the detriment of so much useable power..

[dandywarhol]

Interesting responses - I've just picked up a pair of almost new TDMR40 Mikunis which I'll initially use on a stock, low miles motor with a modded airbox before I get the 2mm big valve head and cams set up.

Does anybody have experience with the Mikunis for road use? I'd be interested in hearing....................

[Rod.s]

Interesting responses - I've just picked up a pair of almost new TDMR40 Mikunis which I'll initially use on a stock, low miles motor with a modded airbox before I get the 2mm big valve head and cams set up.

Does anybody have experience with the Mikunis for road use? I'd be interested in hearing....................

yep i've had several sets of Mikuni's including CR's etc and fid them a really nice carb and easy to tune.

[cobbadiggabuddyblooo]

I had a pair of tdmr 40's that Rich now has on his bike.
Put the tdmr's beside the FCR's and you can see the tdmr's are a much better overall manufactured carby.
I was in 2 minds to keep the tdmr's and I'll prob kick myself for selling them as I could see them being a happy medium .

[Alan White]

Thanks folks, all really useful info.

Absolutely agree with the "think long and hard about each mod and how it really applies to you" approach, hence the research here before I get into it.

However, I am in the fortunate position that I already have the FCR's (41's not 39's). Picked them up years ago at a good price so the cost is largely irrelevant. True the 39's might be slightly better for my purposes, but unless anyone wants to swap that decision is made by virtue of them being on the shelf. I also have both a set of JE pistons and a set of 900 barrels/pistons/rods, again aquired at various times over the last few years, so have a choice and will probably sell whatever I don't use.

It seems to me that using the 900 top end is wise for a couple of reasons - the ceramic coating is a definite step forward plus I don't have to fork out on a re-bore. I can then focus on careful skimming/porting of the head and probably some re-profiling on the cams which will get the best out of the larger FCR's and concentrate on a careful set up of the fuelling to give the best drivability.

With that in mind, I have heard that getting the right compression and squish with the 850 head on 900 barrels can be tricky, any experience/advice would be welcome. And to return to my earlier question, is there any benefit in using a 900 crank to beef up the bottom end, or is it essentially the same as the 850?

Appreciate the input guys, I know you have probably been round a lot of this before, but I haven't and your hard earned experience is invaluable.

Had to laugh at the "go to the gym and lose a few pounds" comment. Met a guy at a car rally a few years back. He was moaning about having to spend £2000 on carbon fibre panels to get the weight of his car down to make it competitive. He was about 20 stone and some guy behind us whispered "£2000!! jeez he could have saved twice that by cancelling his pie order and saved twice the f=@#ing weight into the bargain!" I nearly choked, spat my coffee all over the big fella. I daresay I could knock a few kilo's off this rider's weight without too much trouble.

Alan

[cobbadiggabuddyblooo]

Running the 850 head is no issue. 1.25mm squish (0.048" ) is a bit of an industry standard just some like to push it to 0.030" to raise compression a tad at the detriment of hot spots and pre ignition of fuel. One of the jobs of your squish band is to cool things.

Nikasil bores are different tho to ceramic coating but ceramic coating of your pistons, chambers, valves and exhaust ports is a worth investment for $300 as heat robs your motor of power.
So less heat that is absorbed by components and just pushed out the exhaust ,the better more consistent power from your motor.
If you want a slightly higher compression, design your pistons to give you what you desire.
Race motors commonly run 0.030" but they have the pleasure of being pulled apart at their own pleasure and expense and timed to a poofteenth of detonation.
You will find the 900 head is flowed for injection too.

[SDN]

Stick to the KISS theory.

• Use the standard engine,
  pop the head off, plastigauge the valve to piston clearance,
  buy copper head gasket to suite a slightly (00.5/1) raised compression
  clean head and valves etc
  fit new timing chain and guides etc
  rebuild standard carbs (41 FCR's are a waist of $, if you want FCR's then fit 39mm FCR's far better for everyday riding)
  fit better Ignition module
  better coils
  2 into one tuned exhaust (your Akrapovic exhaust)
  Dyno tune
  go to the gym and loose some mass

Then ride,

All up about the same as a set of FCR's…

Rod where do you get a copper head gasket? do they have any oil / water leak problems??

[misterdimwiddy]

Stick to the KISS theory.

• Use the standard engine,
  pop the head off, plastigauge the valve to piston clearance,
  buy copper head gasket to suite a slightly (00.5/1) raised compression
  clean head and valves etc
  fit new timing chain and guides etc
  rebuild standard carbs (41 FCR's are a waist of $, if you want FCR's then fit 39mm FCR's far better for everyday riding)
  fit better Ignition module
  better coils
  2 into one tuned exhaust (your Akrapovic exhaust)
  Dyno tune
  go to the gym and loose some mass

Then ride,

All up about the same as a set of FCR's…

Not meaning to be confrontational but I'm puzzled Rod by your "41FCR's are a waste of $..............use 39's".

Is it really possible to distinguish between 41's and 39's riding on the road? I would have thought that dyno plots for each would be pretty close. My arse end is not so sensitive to +/- a few bhp here or there, particularly when I am losing more like 40hp to what is out on track with me.

Do you mean that there is only true benefit from using 41's if combined with other radical engine mods (pistons, valves, cams etc etc)?

Putting 41's on my 878 TRX transformed it from a quite tame road bike into an awesome monster of a bike; huge torque and power increase and revved like a wild thing (to my eventual expense). The difference (on the track) was stunning and worth every penny. The induction noise alone is worth the expense.

I may be missing your point but surely the first TRX performance mod would be 41's combined with a big bore system???

[cobbadiggabuddyblooo]

I found someone local who can produce copper head gaskets at what ever thickness or bore size for about he same price of an OEM Yamaha gasket. Re useable and no need to bore it out to suit oversize piston as you can choose bore size you want to suit . Plus you can rebore it if need be.