Yamaha TRX850 Enthusiasts Forum

Hi,

I’m newish here. I picked up a RED 1996 TRX in Melbourne Aus. The bike seems to have some go fast bits that I am trying to get my head around.

I have noticed that on WOT under 3k rpm the bike bogs and stutters, it won’t pick up unless I slip the clutch or roll off the throttle and roll back on slowly. Above 3k the bike seems to pull quite well its a very smooth pull right through the rest of the rev range.

My question: is This normal for the OVER RACING CDI as I have read that it advances the timing which may have this effect.

Other mods I have discovered so far:
- no air box
- motad venom pipes these were a challenge for the RWC
- missing TPS
- what looks like a blocked vacuum hose plugged into the frame on the left side of the bike

I am hoping that by chance I can get in touch with the previous owner who did the mods. The guy I purchased it off seemed to be in the dark on most things.

Any advice or guidance would be appreciated.

If running the std CV carbies you need the airbox unless he’s messed with the carbies and changed to different stage Dyno kit or equivalent so finding some history would be advantageous.

No airbox and missing TPS seems to be the culprits at a quick glance.

As a sidenote, trying to go WOT below 3000 have never given an astonishing result on any TRX....

Yes JPE No TPS won’t help at all let alone WOT down so low in the RPM.
The Ducati run the same CV carbie but don’t have a tps too.
I sold about 1/2 dozen TDMR 40’s from Ducati’s that need little modification for the trx.
A smaller pilot jet and different routing for the throttle cables but they had no TPS either.
I wonder if it’s one of these or someone else split a bank of 4.

The blocked off vaccum hole into the frame I suspect they are different carbies.

Hi trx850dan!

The missing TPS could be resulting from Keihin flat slide carburettors - if I remember correctly, they don't use a TPS.

Anyway, the type of the carbies in your bike should be identified - original ones or flatslides from Mikuni or Keihin.

The tube plugged into the frame on the left side of the bike lets me think of the tube from the airbox leading oil from it into the frame - in case of a 1996 frame.
However, if there's no airbox ...

From where comes this tube plugged into the frame? Maybe from the carbies and to the same effect as the tube from the airbox?

Guessing greets from Munich!

Wow! Thanks for all the input guys.

I rushed out this morning to do some quick smoothing before heading to work. Didn’t uncover too much to be honest haha.

I did manage to pick up that the carbs are definitely Mikuni they are stamped with the following 4UN 00 K631 google didn’t help too much.

The vacuum hose goes nowhere it’s just plugged into the frame with a bolt in the other end - I have some pictures I will try upload fingers crossed it will work.

And the blocked hose.

Ohhh lots of fuckery going on here!
From 1 to xxx
1. These are CV carbs and require the airbox to be fitted for them to work their best, find or refit/replace the air box and fit a quality air filter of your choice.
2. The vacuum line you're talking about is the water drain line from the airbox, on early bikes this drained any water collected in the airbox to the frame, why you'd want to drain water into the frame tubes is a mystery.
3. The Over ignition unit may be genuine or may just have a sticker....is your bike a JDM import? If so then it may be genuine, there is some conjecture as to their effectiveness, I'm not sure if they just changed the curve and did away with the TPS or not. Brian has a catalog that may shed some light on it.
4. The TPS may have been removed when the Over ignition was fitted. You'll need to make a decision on wether to refit it or not as the Over ignition may not even read the signal fro the TPS.

So don't go crazy with changing things in one hit. Firstly refit the air box and ride it then address the issues as they arise. The lack of TPS and Over ignition may be just fine.

Yes correct Rod, I can see they are std carbies and the TPS has been removed.
CV and no airbox is a recipe for problems and even those who have gone down the route of heavily modifying them to perform better create more problems than it solves in my eyes.
Over Racing TCI still runs the a tps so it’s been removed for some unknown reason.

Is the TCI unit labelled with an Over Racing sticker or was that what he told you???
Peel the Over Racing label back and see if there is another label underneath...

Off to hunt for an air box then I guess...

The sticker on the CDI is well and truely on there no evidence of anything under it. Bike from the available information is an aus delivered bike. The side stand switch is also overridden and had to be doctored up with a micro switch into the handlebar kill switches to get past roadworthy here - helps confirm my theory.

God I hope I done need a tps! Tried tracking one down a few weeks ago and those suckers aren’t cheap.

Thanks again for the insights.

Anyone have a spare air box in their shed?

Air box shouldn't be an issue as many TRX's have had FCR'e etc fitted, ask on the forum and "the other place" As for the TPS, ask on here I'm sure one will turn up, (I'll look tomorrow as I've a vague felling I have a spare ) try to get the screws that hold it on as well, save scrounging for some that may fit. Also download the workshop manual and read the section on the TPS, it's relatively easy to fit and adjust.
These bike went through a faze of being a cheep hack, so plenty were maintained by backyard butchers!! Ask lots of questions as we're here to help.
Cheers
Rod

Cobba I have a peeled back the over racing sticker on the ignition box and confirmed there is nothing under it. I did read on another post here a number of weeks back that the OVER unit bypasses the TPS and the side stand kill switch. I am fairly sure that it is a real one.

Now I'm no expert so please set this young whipper snapper straight where required.


This bike was bought as a passion project (I have had a rough year and needed something to bring me some joy). This is my first road bike - I am not afraid to tinker and have done so on my dirtbikes which oddly feel simpler to me. I am struggling with the prospect of losing the beautiful meaty song the bike has at the moment with a move back to an airbox setup. That being said I am not set on any particular direction yet. My options seem to be as follows:

- dig deep and fork out for some FCR's - im not sure what would be required in addition to bolting these on - will i need a tune etc? ho would this go with an Over CDI?

- is there any value in getting the CV's dyno tuned and jetted by a professional? has anyone gone down this road that can offer any needle/jet settings that would be 80-90% right? how much would this even cost roughly in aus (this one is a biggie had a mechaninc try charge me 1.6k for sprokets, brakes and chain, needless to say i did it myself for $700-ish including tools). I have also noticced the last couple days when i back off and the bike drops to 2k rpm it pops a little - which sounds great but I know this is telling me something lean, rich I don't even know anymore haha. note I'm probably sitting closer to this one at present I have some down time thanks to COVID so I can play around and potentially hit the road dyno

- whack in an airbox and pray this solves the performance issues potentially at the expense of some eargasms


the bike will be more of a weekender and will be going on club plates when it qualifies so i am not too fussed with having it 110% right through the revs and throttle positions - its just that little bit difficult to put around a carpark atm.


I have also noticced the last couple days when i back off and the bike drops to 2k rpm it pops a little - which sounds great but i know this is telling me something lean, rich i dont even know anymore haha.

Carbs:
CV carbs need relatively still air to work correctly, hence the use of the airbox.
Take away the airbox and CV carbs become an absolute prick to try and tune. Even a light crosswind will play havoc with them.
With the airbox and a decent filter they are quite easy to tune and really only lack the immediate punch on twisting the throttle wide open that you get from flat slides with an accelerator pump. Now While I do admit that is a nice thing to have, how often to you actually need it when you are not an the last lap, gaining on first place and about to exit MG corner?
The standard jetting of the TRX CV's is too rich, the mains can usually be reduced to 140'ish. Check Cobba's posts for more details.
CV tuning is not difficult, do it yourself.
The jet needle and needle jet (AKA emulsion tube) in the standard carbs ends up wearing and causing all sorts of issues at low to mid throttle range. You can get replacement tubes in many of the aftermarket carb kits, but beware of using the needles unless you want to spend a lot of time playing around with main jet size and needle clip position.They all have different taper to the stnadard needle. You can keep the original needle and just use the aftermarket tube.
Popping on closed throttle may be due to air leaks on the engine side of the carbs.
The rubber boots on the inlet manifolds may be cracked, or the vacuum ports on the inlet manifolds may not be sealing. There should be two lines, one runs to the fuel tap and the other to the fuel pump.
Check that both hoses are intact and connected, then buy a new R1 5JJ fuel tap from ebay and ditch the original tap and that vacuum line.
Also check that the plastic cover on the air cut valve is not cracked. Search for air cut valve, you'll find pics and everything.
The TPS does serve a function. It was probably disconnected from the OVER CDI because OVER were (are) a race outfit and they tune their bikes for WOT at max RPM.
On a street bike? Get an Ignitech unit and plug it back in. It's better than the OVER unit ever was. You can probably sell it to some idiot on FB and make a profit.
FCR's, yeah they are better than CV's if you are racing, but on the street, in these days of zero tolerance policing... I'm happy to keep the CV's. They are also fkkn expensive.

Ignition: As already mentioned, get an Ignitech and be done with it. You won't find better. (I can't believe that they are not paying me commission yet!)

You can get a lot of the meaty engine note with a pair if Staintune mufflers, if they are still available.