Yamaha TRX850 Enthusiasts Forum

Hello you guys!
I have finally received my Mikuni TDMR40 carbs from Germany - can't wait to fit them...

So I have been doing some polishing of the intake spigots, while I was waiting for the order to come through:
http://3.bp.blogspot.com/_OwjX-5jX_P8/S ... /after.jpg
(mine looks almost as nice as this)
But when I fit the new carbs, I realize this:
The inlet rubbers are 38 mm inside diameter, the inlet spigots are 38 mm inside diameter, the Mikunis are 40 mm inside diameter!
What do I do?
Should I get a hold of some inlet rubbers from a Yam R1 2000 (uses 40 mm carbs) and grind another 1 mm off the inlet spigots, or am I just being hysterical?
But what is the point of polishing the spigots for hours if the inlet rubber "lip" protrudes 1 mm; wouldn't that disturb the linear flow?

How have you guys dealt with this when you fitted FCR41's to your bikes?

Thomas, Denmark

You can open out the standard inlet rubbers to the same diameter as the carb mouth, a die grinder or Dremel can be used or just run a boring bar through them on a lathe to match internal diameters which is what I did.
Alternatively speak to Allens, they`ll sort you out. :

http://www.allensperformance.co.uk/prod ... folds.html

By the way, don`t `over polish` the inside of those inlet tracts, it`s not necessary and if overdone can actually create the type of air/mixture flow that will loose you power.

http://www.motorcycleproject.com/motorc ... rting.html

thch wrote:Hello you guys!
I have finally received my Mikuni TDMR40 carbs

How have you guys dealt with this when you fitted FCR41's to your bike?

Most people buy the Fcr41's as a model specific kit complete with the correct inlet adapters & TPS mount to save all of of your mentioned questions - otherwise they buy random 41's & ask for help on here

Mincehead is right - concentrate more on matching the inlet SIZES rather than porting at this stage, if you want decent gas flow concentrate more on the cylinder head for better results (although both are benificial)

Just out of interest, why Mikuni's rather than FCR's?

I think I remember reading somewhere that he was told the Mikuni`s were the better, higher quality carbs than the Keihins.
Personally I prefer the FCR`s as do most of the racers I know who use them.

I agrre, they look far too polished. I checked if i had any pics of mine, but unfortunately not. But they are much rougher than yours. I have also modified the inlets in both ends, but that involved adding more material, and that wasn't easy. The inlets are made of cheese... But I'd suggest at least opening up the inlets on the carbside, then roughen up the surface a bit, and finally match the rubbers to carbs/inlets.

Well, I get wiser every day
Thank you for a most informative article Mincehead, this was an eyeopener! I suppose its back to sandpaper again

I think I'll just stick to the inlet rubbers and take off 1 mm.

The reason I bought the Mikuni's?
Asked for a professional opinion from Allen's and he recommended them wholeheartedly - they come complete with cables and tps, ready to bolt on - that was good enough for me!

Hi guys.
Finally got the Mikuni's fitted and Dyno'ed: 87.5 HP at the rear wheel.
This is done using K&N cone airfilters, Shark oval (_very_ open) cans, standard headers.
Just love the sound (clatter clatter clatter)

thch wrote:Hi guys.
Finally got the Mikuni's fitted and Dyno'ed: 87.5 HP at the rear wheel.
This is done using K&N cone airfilters, Shark oval (_very_ open) cans, standard headers.
Just love the sound (clatter clatter clatter)

I would have expected more than that, I've 83hp with well sorted standard carbs & K&N filter... Different dyno's obviously, but...

Did you do a before & after run?

I'm only getting 87 at the rear wheel with the FCR's and open exhaust but 100,000k's on the clock and the heads never come off the bike. Still running std 150 main jet just lifted the needle a notch and changed the pilot air I think to accomidate the bellmouths. I'm sure if I changed the headers and rebuilt the head I'd get close to that magical 90

cobbadiggabuddyblooo wrote:I'm only getting 87 at the rear wheel with the FCR's and open exhaust but 100,000k's on the clock and the heads never come off the bike. Still running std 150 main jet just lifted the needle a notch and changed the pilot air I think to accomidate the bellmouths. I'm sure if I changed the headers and rebuilt the head I'd get close to that magical 90

That's interesting, maybe the dyno I used was a bit optomistic... I went from 78 to 83hp by setting up the carbs (132.5 main BTW) & fitting a K&N filter... Same dyno, but different days.

My engine is of unknown km from the wreckers too...

At 70,000k's I 1st put it on the dyno with std carbs and std headers and staintunes with 70 hp but after a carb rebuild and rejett back to std I found 76hp which was factory std for the TRX but not bad for the old dog back then. I later fitted a K&N filter and felt a little difference down low but no major jump in HP.. But FCR's are the go bolt on 10HP

Last autumn I got the bike Dyno'ed (same Dyno).
The old bitch delivered 80 HP at the rear wheel, which I find quite ok when the specs say 83 HP at the crank. A 7.5 HP increase using the new Mikuni flatslides is all right, I suppose - I can certainly feel a difference in performance (acceleration and top speed)
The Dyno guy (Lars MC - nice bloke) says that it's running perfect.
I don't think it's worth doing the complete overhaul with new cams and bigger bores to get the last 10-15 HP on the rear - propably cheaper to buy another bike

I've the same results; Before 80 bhp, after ~88 bhp.
Running TDMR's with bellmouths, sock air filter (RAMAIR) and Akra headers.

Hi tarwetijger.
Looks like we almost have the same setup, only the headers/airfilters differ.
I was wondering if the Over exhausts could add a few HP ?
I have seen them at http://www.cmsnl.com/over-racing-exhaus ... odel14833/
Although they are nearly 1000 euros, they have an _absolute_ ace sound.
Since you're located in Holland, do you have any knowledge of CMS?
Does anyone know if it's possible to get a set second hand? I've tried Ebay for ages without luck...

Regards Thomas.

I did, yet to fit the system though, luckily for me it was a member here that was selling it. The cans were `legal` spec, unable to be doctored too so I`ve had some titanium shorties made up with carbon caps.
They do RARELY come up for sale so keep looking.
That CMS outfit seems to specialise in pattern parts, the Over system they do is in stainless, the REAL Over system is in titanium and of a larger bore in places too.
Bl**dy expensive copy / compromise and made under licence to Over I reckon.