Yamaha TRX850 Enthusiasts Forum

Hi,

I have rebuilt my TRX with new pistons and a top end overhaul. She was running fine before the rebuild, all be it a bit down on power and a huge oil consumption problem.
I have rebuilt her and she fired up fine and idles with not real issues. The problem I have (which I did not have before) is she pulls so much better under acceleration but if I try to cruise (just of idle) she tends to surge as if I am running out of petrol and when I come to a stop she will stall. She starts up ok with a bit of throttle.
I am a bit puzzled as I did not touch the fuel system at all.

Any ideas would be appreciated.

Cheers
Ron

Cracked inlet manifold between carbs & cylinder head? Spray some brake cleaner on them whilst engine idling & listen for any changes..
The new top end will behave differently to the old one so carbs setup/balancing wouldn't do any harm I guess.

One the gurus here will be able to give you more precise advice

You might want to check this as well,( air cut valve cover,) right side carb... the plastic tends to crack and you get a lot of the symptoms you describe.
This little bugger
http://www.kedopartsaustralia.com/brands/Mikuni.html
click for better view.

also try the choke mechanism, as they tend to stick on and cause these exact symptoms

Thanks Guys,

The choke seems to working ok, not sticking.
Will check for leaks.
Never heard of or seen an air cut valve cover before. Obviously will need to check.

Right side of the bike( oil filler side,)
check the carb up near the tank, you will see a white (probably a little faded now) round plastic thingy. If it has cracks in it, that could be the cause of your problem.
If it is cracked, It seems to let air into the carb and upsets it.
Easy fix, new cover or a bit of 200 mph tape works.

I'm having similar issues and my cover is looking pretty aged. Does anyone know where I could find a new one in Australia? Will Yamaha stock it?

Yamaha do not have a part number for the cover, only the internal valve parts, but it is only the cover that causes the problem.
Kedo Australia list the part but currently show out of stock.
http://www.kedopartsaustralia.com/air-c ... st38-carb/
It is listed as in stock on the German Kedo site (kedo.de) item 28211. You can set the site to English, makes it easier to navigate if you don't read German.
The same carbs (Mikuni BDST38) are used on the TDM850 and some early 90's Ducati (750SS, 900SS) so searching for those may give more options as these were sold in the USA.
You could simply epoxy over the existing cover, it's been known to work

Look at exhaust pulse acting upon inlet vacuum may be causing the problem too.
Fuel pump and fuel tap are both vaccum operated.
Another mod becoming popular as the trx ages is to replace the vaccum operated fuel tap with a old school passive on/off/reserve style fuel tap and then block the vaccum outlet from the corresponding inlet manifold.
As you mentioned all was running good before and surging is usually a fuel issue and your bound to have noticable changes in inlet/exhaust pressures now with the motor refreshed.