Yet another jet question

[Con Rod]

I have now dropped the main to 137.5.

A looks good on that, as best I can tell I am getting about an air/fuel ratio of 13.5. I say as best as I can tell because it is impossible to find somewhere within 150km of Sydney where I can run it for any meaningful amount of time at WOT. My reading for the 140 jet of 11.1 (rich) was up near Barrington tops. If you are familiar with the area you will know it is a good spot to check you main jet

Hope to get a weekend away again sometime in February where I run flat out for a time to confirm the best main, but based on the fact that the 140 was rich I should be closer on the 137.5.

Fuel screw at 3 turns out is giving me 13.3 at tickover. At 2.5 turns out (stock) I was getting about 12.2 which is unnecessarily rich given the choke riches further for starting.

On the needle I am getting between 12.0 and 13.7. I am on the bottom clip (leanest / farthest from the point) with a 0.6mm washer.

This matters. I am seeing meaningful differences in the air fuel ratio with just 0.1mm difference in washer thickness. I have a selection of washers I have been using ranging from 0.15mm thick to 0.7mm thick in .1mm increments. I had been running the same clip position with a 0.5 washer, but while I was getting good air fuel ratio numbers at low speed it was lean surging. This would be down to the distribution of the charge in the combustion chamber I reckon and is not surprising given the size of the pistons / cylinders.

Of course without being certain on the main it could all change,

Hope to get a weekend away again sometime in February where I run flat out for a time to confirm the best main.

[pertrx850]

So, finally I manage to go for a short test run even though it should still be winter up here (+12C in March is remarkable!). The bike seems to be running well with richer main jetting (#147.5) but I'm not sure I got it perfect. Might be a little bit rich on top rev/full throttle but there's still time for more tests and adjustments. Though it feels a lot better at low/mid range revs after I also switched the larger pilot jet (#83) for original (#75). Needle still in highest position + shim approximatly 0.6 mm.

Since the air is still quite cool (of course with higher density and more oxygen/m3), yesterday's run might not be representative for summer conditions. I'll try to do a test run with an CO meter. We'll see how it turns out but at least now it runs quite good and clean.

Surf's up!

[pertrx850]

So, back again after a long nice summer and quite a few miles on the bike It's been running well even though it's pretty obvious that you guys a re right -it got a bit too rich! After changing back to stock main jet (#142.5), keeping stock pilot jet (#75) and needle in top position + shim it runs better. Probably as good as I'm going to get it along with KN airfilter and open slip-ons. Not even the cooler nordic climate could compensate for the #147.5 jet

Per

[Ever-frost]

Greetings from Finland,

This is s very old topic, but borrowing it for my question.

I have a 1997 TRX 850 with Full Over Racing Exhaust and Ignitech unit. I have also modified the filter case and updated the carbs with dynojet stg 1

Does anyone have any idea, what kind of setup i should use with the jets and needles?

I have installed one size bigger pilot jet (in the dynojet there are 3 different) and put the needles in the middle point. It feels, that the bike does run on rich and the full throttle acceleretion wount work as good as it should. In the middle range works OK.

What would be good main jet / needle point / air mixture screw set for a start?

Terv. Markus

[cobbadiggabuddyblooo]

What do you mean by " modified the filter case " ???

[Ever-frost]

I have cut out the horn/snorkel and made some holes to front end of the box. I guess they call it the "Air box mod".

[cobbadiggabuddyblooo]

Why????
The airbox is designed to resonate at a frequency that will give a neutral pressure at your valve overlap and tuned by the snorkel length and width to the volume of the air box to resonate at the overlap RPM range.
You've only robbed yourself of power with this action.
Don't be surprised if you've lost 4 or 5 HP in the mid range on top of the losses in exhaust pulse velocity in the lower RPM range from the Over system.
That's just the nature of the beast once you start playing with tuning.
Cam duration and overlap dictate more on your air box design.

Personally, I would glue up the holes and place the snorkel back on.

[Ever-frost]

If you look at many TRX topics they say that the Air filter is one of the most important thing to be modified. I have tried K/N filter, original filter, some oval foam filters instead of original panel at OEM airbox. In many sites I have seen the mod that they saw the horn part off. I dont know what is the perfect solution, but personally I dont believe on getting rid of they whole Air box and using aftermarket filters at street.

If somebody knows what is the best solution to improve original intake, please share it with the others.

And what comes to the modified box. It can be easilly renovated.

[Mincehead]

READ COBBADIGGABUDDYBLOO`S POSTS AND ALL WILL BE REVEALED.
Basically stock CV carbs work best in still air conditions, gulping their needs from as larger volume airbox as the bike`s design allows.
Now, you CAN modify the airbox, raise the tank on it`s mountings etc but unless you have a tuned motor requiring more air you`ll only get so far, FCR`s OR TDM`s are the way to go for a `better` experience and more power.

[Rod.s]

It's no use modifying anything on a stock bike unless you have; a genuine starting point; a Dyno reading to start with and know where you are heading...there is LOTS of TALK on modifying TRX's and most of these are just hacks that have no engineering knowledge or no real understanding of what they are changing. All you have done is change the airflow/pulse of the airbox without any reference point. Just a waist of time and a los of HP.

[Ever-frost]

Im suprised, that the answers are quite simular to Finnish forums. If you ask for help, youll easilly get answers that'll inform that you dont understand anything from the subject your talking about... As I wrote before I've tried many different filters with the original box.

I know that best solution would be swap the carbs to pair of Keihins etc. but thats not and option for me because of the cost and other issues. I thought that dynojet set (+ignitech improved ignition) would need more air to get the best out of the carbs. Maybe I'll get back to original airbox system.

If I'll get back to the original question, can somebody give me a hint which kind of settings I could start with the dynojet at original carbs?!

[cobbadiggabuddyblooo]

First question...
Where abouts in the rev range are you trying to improve or more to the point .
Start this way in your frame of thinking....
Ultimately where in the rev range do you want to tune your motor ????
If your choice of exhaust is an Over system, then your looking at 5500rpm to 9000rpm purely due to the velocity of the exhaust pulse due to a 39mm I.D. Of the Over headers.
You'll loose velocity especially under 4000rpm and your back to square 1 from from 4000 to 5000rpm and then the Over system with free flowing cans really comes into its own to peak HP at 7500rpm.
You won't make any further HP gains above 8000rpm but the exhaust will allow the motor to rev especially if you've lowered the gearing (16/41).
It's a robbing Peter to pay Paul thing.
Your limited due to the fact that your not changing any of the motor internal characteristics and you've changed the exhaust pulse charateristics.
I have no idea what the needle taper charateristics are on a stage 1 dynojet or the main jet you have fitted and environment conditions as this all plays a part.
Now don't be surprised if you need to find a little less fuel down low with the std needle set up.
If you have the std needle that's in good condition just compare it to the dynojet needle to see what they are trying to achieve ...,

Don't worry Ever I was asking the same types of questions myself 11 yrs ago on the forum...

That's Sir Boffin now thanks Mince

[Ever-frost]

Thanks, cobbadiggabuddyblooo, many good opinions.

I love the strong mid range on TRX, it's the reason why I love the bike so much. But I dont like the fact that there's really no use to rev up the engine. Im looking for little better response and stronger pull at the +5000rpm. No the engine is kind of "sleepy" at the high revs.


I have swapped the gearing little shorter ratio.

The Over exhaust has been always on my bike, one option would change it to custom made 2-1 exhaust.

Does someone else think that dynojet is a waste of time?!

BR

Markus

[Mincehead]

You`re limited by the stock carbs, but setting them up PROPERLY does help a lot, as does some time on a dyno.

Sir Boffin eh?
Sir Monkey Spanker more like!

[Rod.s]

Thanks, cobbadiggabuddyblooo, many good opinions.

I love the strong mid range on TRX, it's the reason why I love the bike so much. But I dont like the fact that there's really no use to rev up the engine. Im looking for little better response and stronger pull at the +5000rpm. No the engine is kind of "sleepy" at the high revs.


I have swapped the gearing little shorter ratio.

The Over exhaust has been always on my bike, one option would change it to custom made 2-1 exhaust.

Does someone else think that dynojet is a waste of time?!

BR

Markus

Are you running an OVER exhaust or just the OVER mufflers?