Yamaha TRX850 Enthusiasts Forum

A lot of you will have seen my other post regurding trouble I am having with my newly aquired TRX. I took it for a dyno test and this is my mock up of the fuel chart (couldnt get my scanner to work)

Well at least you will be able to see it if my first attempt at getting a picture uploaded to this site works !!



The chap at the garage said that it isnt too bad, but that it should really be running at a AFR of 13 ?? so it looks like i'm running pretty rich at the top end.

My concern is the apparent 'peaks' - any ideas what this indicates ?

My bike was registered in 99 and as far as i can tell it has stock carbs in place - since getting it and noticing it was running rich (plugs were black) i put a K&N air filter in and also put a few extra holes in the air intake box.

All TRX`s run rich as stock,at least that`s the considered fact on here.
Those `peaks`,they could indeed be the slides `hanging-up` a little in the guides/carb body.
I would try this:

1) Unscrew the two philips screws that hold the black plastic carb tops on.
2) CAREFULLY remove the carb tops(one at a time)and be careful not to loose the small O ring.
3) Withdraw the diaphragm/slide/spring and needle assemblies again one at a time(never mix carb components)and inspect them.
4) The slides will be black plastic and you should feel along their edges for any roughness or visible scratching,if any is found use a metal polish or similar light abrasive to dress them smooth again.
5) Inspect the diapragms by carefully pulling them in various directions to see if any holes or splits are evident.You can make a running repair to a small hole or split by using a cyanoacrolate(super)glue but I recommend you order some replacements ASAP.

Once it`s all checked out,cleaned,polished or repaired etc then use a silicon spray on reassembly to lightly dress the slides in their guides.
Go for a run and see if there`s a difference.

Oooo nearly forgot,these folks are really good for carb diaphragms:

http://www.nrp-carbs.com/

And here,I`ve found the float height/adjustment page:

http://www.factorypro.com/tech_tuning_p ... edure.html

I'm not an expert, but I can tell you that it's absolutely standard TRX fuelling overall - very rich in the powerband, i.e. 4000rpm +.

In my completely inexpert understanding, there is always fluctuation in AFR. What an experienced tuner with a dyno should be able to do is get the fuelling into the "correct" band (which can vary) and keep it there, and they should do part throttle work as well (unless it's a drag bike - I assume this just is the afr from a 4th gear full throttle roll on for the power run/s you had done, and it might look completely different at 1/4 throttle).

Even in a "diesel", where scavenging is less of an issue, you will get pressure effects in intake as well as exhaust tract, which can be accentuated or attenuated by airflow and fuelling. Notice for example the peaks and troughs above 5k seem to follow a sine curve around 12:1. So opening up the airbox and putting the K&N filter in, although they may have increased total available airflow, may have made the AFR less consistent. One reason, aside from noise and water ingestion, why bikes don't just run straight K&N pod filters and why manufacturers do things like fancy airboxes with multi-stage flapper valves.

I'll leave expert guidance and interpretation to some of our more technical members though...

Norfolk Boy wrote:A lot of you will have seen my other post regurding trouble I am having with my newly aquired TRX. I took it for a dyno test and this is my mock up of the fuel chart (couldnt get my scanner to work)

Well at least you will be able to see it if my first attempt at getting a picture uploaded to this site works !!



The chap at the garage said that it isnt too bad, but that it should really be running at a AFR of 13 ?? so it looks like i'm running pretty rich at the top end.

My concern is the apparent 'peaks' - any ideas what this indicates ?

My bike was registered in 99 and as far as i can tell it has stock carbs in place - since getting it and noticing it was running rich (plugs were black) i put a K&N air filter in and also put a few extra holes in the air intake box.