Yamaha TRX850 Enthusiasts Forum

Hi folks, I'm new around here but have a few questions I hope you can help with.

I have searched the site, and got such a huge variety of hits that it is difficult to sort out the salient info'. Has made for some great reading though and you have some cracking bikes and projects on the go.
It's actually a TDM850 project. I'm a member on the Carpe also, which is another great forum (I see a few familiar names cropping up on both), but the different focus of the TDM means less of the guys are into serious engine tuning, certainly not some of the extreme mods I've seen on here.

I'm not after ultimate power, but a good useable real world circa 90+ bhp is the target. Reliability is also key for me, I want more power, but also a really strong motor. This will be my daily ride when it's done.

I have accumulated various parts over a number of years (yes this has been going on for ages), including a set of FCR41's, Akrapovic exhaust and two top ends - a JE878 and a TDM900. The original intent was to combine a flowed head with a set of kent cams and the FCR's. Big bore was the original idea (hence the JE psitons) but then a 900 top end came up and that seemed a better solution.

What I have gleaned so far is that the JE pistons are good, but need careful set-up/balancing because they are heavy. The 900 top end really appeals with it's coated bores. (No more oil consumption issues). But I gather this takes quite a bit of work to get the right compression ratios/squish clearances. Also, big bore/high comp puts strain on the motor, and cranks are known to go with LHS big end being the primary culprit.

So to the real questions;
1) In your opinion which is the better bet, JE878 or 900?
2) If 900 top end, am I better with a TDM900 crank also and is this a straight fit? This is designed to go with the top end obviously.
3) Maybe I'm better with a complete TDM900 lump and fit the 850 head? I want carbs for simplicity, but would appreciate the better 900 gearbox. By the time I source a crank, it may be easier to get an entire 900 lump. Might also benefit in terms of sourcing spares in the future.
4) Assuming 900, is it worth investing in high comp pistons and/or stronger conrods (I've seen a couple of references to Wiseco pistons for the 900).

Any info gratefully received

Alan

I have a full TDM900 engine install with wiring ecu fuel injection, basically a TDM900 in a TRX chassis.
its actually ever so slightly quicker than my father in laws TRX with a TDM900 engine, and FCR41's

there are a few issues with fitting the 900 lump in its entirety, the main ones being the chain alignment and the lack of a mounting lug on the left side of the head.
Both easily resolved with the right application of know how.

Personally if I had to do it again and having experienced both flavours I would still go for the fuel injection over and above the FCR41's, but its a fair bit more work than the carb route.

On the plus side by the time you have finished you will be a machine at soldering!

Cheers, appreciate the response. Yours was one of the projects that came up a lot in the searches, it looks real interesting.

I do appreciate the benefits of the injection, particularly the flexibility of applying fueling changes by re-programming, rather than having to strip a set of carbs to alter float heights, jetting etc.

I can't explain it really, but just really want to retain the carbs. I guess I'm a bit old fashioned (or maybe just old!!). Set up aside, I prefer the fact that they are essentially a mechanical device that I can strip and service, and the fact that I don't get into serious mods on the loom to make it work. I also have a TDM900 with the usual filter/pipe/PCIII mods, and just prefer the way the carb fed TDM responds to the throttle inputs once it's set up correctly. I can solder OK, but deciphering the wiring diagrams isn't high up my list of abilities, so hats off to you guys that have the skills and patience to get that right.

A.

Hi from the Carpe camp too

I had a lightly tuned 900 TDM with Kent cams, higher compression, Akra silencers and Power Commander. Throttle response was sharp and enough low end grunt to hoike up the wheel with ease. Putting out 69 bhp @ 5000 and around 85 max at the wheel, it was a lovely motor. My stock 11,000 mile TRX is tame in comparison. I'm now going down the big valve/TDMR40/big pipe route to try to get the kind of performance I previously enjoyed.

My 900 had Carrillo rods installed when I got it but I personally think they are a waste of money, unless you're going racing/track days.

You will need to ream out the stock small end eyes to accommodate the bigger 900 gudgeon pins if you go down the 900 barrels on an 850 route.

With hindsight, I might have been better to keep my 900 motor and put it into the TRX (now that coxxxxxxy has given us the instruction manual ) and broken the 900 remains for spares......................................

Look forward to hearing your report

Alan

if i ever find that bike of yours Al, I'll be buying it!!

He's on Carpe - under the name Celticbiker - Welsh variety of Celt..........................

Thanks guys, appreciate the advice

I have two 850 4TX motors, one really nice clean 5000 mile unit (in the bike) and one spare with the usual crank issues. So I had intended to rebuild the spare with a 900 top end and then transplant, giving me a complete spare standard engine. However given I will have to recondition the crank anyway, now I'm thinking I might be better to sell the good 850 unit, purchase a 900 motor and use the spare 850 head. I can probably sell the 900 barrels etc and the JE kit and end up spending the same or even less than re-conditioning the 850 unit. Plus I get the 900 bottom end which seems stronger (never heard of a 900 with a knackered crank) and 6 speed box. Hmmm we may have a plan.

Alternatively of course, I could just buy a Husqvarna Nuda and save all the hassle !!

But that would just be boring

The TRX is a great project bike, there's so much you can do with it. I have a spare 900 head in my garage that I may just have ported and fitted with bigger valves and possibly cams. In the future though, for now if I want more power I have an aprilia mille track bike sat next to the TRX in the garage. Interestingly I have done 5 track days this year and of that I spent 1 x 20 minute session on the aprilia and the rest was on the TRX. It's just so much fun.

Alan White wrote:Thanks guys, appreciate the advice


Alternatively of course, I could just buy a Husqvarna Nuda and save all the hassle !!

Funny that I was goin to suggest just that only 2 hrs earlier and use the TRX cosmetics on a 900 Nuda but I thought you may not see the humour...

Intent is a supermoto style machine. Originally based around the tdm, not trx, I use a lot of bumpy rural roads, so a more upright/compliant bike is definitely the way to go for me. Plus I have the donor machine, a set of KTM Duke forks, and Metmechex swingarm and uprated shock for a TDM, so it's the obvious choice to start from.

Mind you the TRX chassis does look really good, and I've seen a couple of really nice cafe racer projects, hmmmm. (Too many ideas, so little time - stick to the plan Alan, stick to the plan!!!! )

So to get back to the engine piece. I need to decide whether to sell the 850 engine and go with a 900, or pursue the build on the 850.

So, has anyone put 900 barrels on the 850 bottom end - if so what issues do I need to be aware of?

Has anyone used a TDM900 crank in the 850 cases?

Thanks

A

For what is worth if you are looking at sticking with the 850 block the easiest route is to get a set of 878 pistons and take the capacity up through that route. It's tried and tested

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…

oof you'll get a bit stick for that rod

thats the first time I have heard anyone say the 41FCRs are a waste of money

coxylaad wrote:oof you'll get a bit stick for that rod

thats the first time I have heard anyone say the 41FCRs are a waste of money

They are great for top end but don't give to much down low where these engines really love it, a well tuned 39FCR will hold it's own with a 41FCR up till 7000Rpm and be mind blowing compared to stock.

In reality it all about how hard you think you can go and how hard you wake up in the morning. Spend all that $ and it's still a 15 year old bike.

As for the Nuda, amazing, fun and sexy bike, but even fitted with the comfort seat a real trial to live with and lets not get into the Fuel injection issues!

My stock bike still keeps up with Cobba up to about 150K's and he has a hard time shaking me through the twists and turns, and I'm not that good, in fact I'm very average