Yet another jet question

[pertrx850]

Hello!

I'm new to the forum and have been a TRX owner for just one season, why I'm in need of some jetting advice. I've seen a few threads on the topic but can't find one that really fits my needs. Sorry if I repeat old topics...

The bike came with stock and open slip-on exhausts. With the slip-ons mounted I've had the feeling that it runs a bit too lean, lacking some punch in my view. It seems it runs slightly better with stock exahusts. What I want to do is to tune the bike for using the slip-ons.

After some examining work I found that the bike at present has the following setup:

-K&N airfilter in standard airbox.
-Main jet #142.5 (standard according to printed specs).
-Starter jet (?) #83.

I wonder:
-Which main jet size should I use?
-Which needle position? (original needles)

It seems from a Dynojet paper I read that with an open exhaust and an airfilter like mine, #148 main jet should be used along with standard needle position (2:nd from the top). Though they (Dynojet) use other needles, which makes me unsure weather to adjust the needle position. They also suggest pilot screw to be 2.5 turns out.

Thanks for your help!

Regards,
Per

[pertrx850]

Forgot to say the bike is a TRX850 1996, if it is of significance

[Con Rod]

Typically the problem with the TRX is rich running rather than lean. Switching the cans and air filter help but don't change the fact that the air box is a bit restrictive and the needle jets wear themselves rich.

I have a k&n filter and straight through cans.

I found the bike slugginsh at wide open throttle and toping out about 210. Going down to a 140 on the main does in fact see the bike pull better at wide wide open throttle and will get me to about 230kmph on standard gearing.

Tuning on on the needle you will need to compensate for wear on the needle jet. I find with a new jet I can run it best at the bottom clip with a .7mm washer but I have to progressively lean it out as the jets wear. They seem to last me about 30, 000 km.

Fuel screw I have at 2.25 turns out.

Tuning by feel will usually see you with a richer fuel air mix than ideal. A richer mix will feel smoother. I have found fine tuning on the needle is best done with plug reading. This can allow you lean it out to improve performance while keeping an eye that you don't go too lean. A camera down the plug holes is also good. Remember that the TRX uses cast pistons.

I am getting about 60mpg (4.5ltr per 100km) when riding out of the city. 55mpg commuting. She pulls clean and strong.

[pertrx850]

Thanks Paul! Interesting. What you say sounds like the opposite of what I thought I needed. Still, the fact that my bike runs better with stock than open cans suggests to me that the mixture might be too lean, or? I will keep your experience in mind when I try my way to a good jet setup!

Regards,
Per

[pertrx850]

But of course, my bike does take a lot more gas than yours, Paul. About 0.7l/10 km. You're probably right about the wear on the jets/needles...

Cheers,
Per

[Con Rod]

take the spark plugs out and have a look

[Con Rod]

......the fact that my bike runs better with stock than open cans suggests to me that the mixture might be too lean, or? .....

This is what I would expect as stock they are certainly rich and the improved air flow from the air filter and cans will lean out the mixture, helping it run better. Your fuel use also indicates rich running

See this:

http://www.factorypro.com/products/Miku ... t_buy.html

and this will help with tuning

http://www.factorypro.com/tech/tech_tun ... gines.html

[pertrx850]

Thanks, I'll have a look!

[pertrx850]

If intersted =) I've now fitted the carbs with what I hope to be a proper jetting setup. Since the pilot jet ("second main") was earlier switched to a larger one (#83), I swithced it back to standard #75. Figured some previous owner had switched it by mistake, beliving it to be the main jet.
Also, I did switch the "real" main jet from standard #142.5 to a larger one #147.5. Along with this I also put in new emulsion tubes and needles (in highest position).

Hopefully it will run better together with the KN airfilter and open cans now. When running it outside the garage it spins fine but the winter in Sweden makes it hard to take it for a real ride... We'll see when the spring comes=) Thanks for your help so far!

[1prodigy]

I have a full OVER Racing system and K&N filter. Standard air box.

Mr Dynojet told me that 137.5 was the main jet for me. Stock needle on middle slot. New emulsion tubes.

77RWHP and near perfect fuelling above 1500RPM.

Why do they put out these bikes setup up so rich from the factory????

[Con Rod]

yes definitely rich from factory, as above I also have a smaller than stack main,

I just connected up an o2 sensor so I am tuning on that at the momment, will post back here with my results

[pertrx850]

It's very interesting that we seem to tune in different directions. Looking forward to hear about your results from the O2, Paul!
I'll be back with a running report too but unfortunatly not until April sometime...

[cobbadiggabuddyblooo]

Air density and altitude all have a part to play thats why. What may be fine in a hot Australian climate close to sea level will be different than someone in a colder climate or higher terrain..
Cooler climate you can cram more air/ fuel molecules down into the chamber and in altitude the air is thinner so all these specs need to be factored in. The std CV carbs deal with this in some respect but the pilot jets may need to be adressed as they would be too rich if constantly travelling at altitude.

[bagmaker]

your not going to seize a stock yamaha because its too lean

[Con Rod]

The main jet is without question too rich. The stock jet is a 142.5, I had swapped it out to a 140 and this weekend with the oxygen sensor I was getting a air/fuel ratio of 11.1 at wide open throttle. So if I am getting that with a 140 the 142.5 is crazy rich

I cant say for sure on mine but I think you are also right about the pilot being rich, but I really need to get the main right before I can get real confidence on the other circuits