Main jet, should I go up?

I have Dave's mod complete, currently running a 55/160 factory keihin jets with ground header inlets, FMF can, and uni filter. It runs awesome off the bottom and mid range, no stumbles or short of torque at all. However when I run wide open it feels like its still lacking fuel 3/4 throttle and above. It doesnt cut out or run bad, but just weak like it could use more fuel. Should I up to a 162-166 or so and try that? Or let it be. I'd rather it be richer for the stupid hot summers here.

By ground header inlets, this is what I mean. 1395406714720.jpg

It sounds like you are too rich. Low power is on both sides of correct jetting, not just lean, and running too rich causes engine damage too, not to mention just blowing more money out the exhaust (more unburned fuel, yes, raw gas out the rear).


If you are tuning for top end power, a marked section of road (distance) and a stopwatch will get you good results. Buy a selection of main jets and spend a day finding which one lets the bike cover the same distance within the least amount of time; you've found your ideal main jet for top power (at least for that elevation, ambient temperature, and humidity).


You could also measure the time in top gear from maybe ~55 mph to a repeatable top speed, maybe 85-90 mph, but that could get expensive if you don't have a very low traffic and isolated road to break the law on.


Is your air filter over-oiled or dirty? Some people get a little carried away with the oil and that will affect maximum power. Just make sure everything else is in the proper operating condition before testing jet sizes, or if you change something after selecting a main jet, it will be off again.

Before pulling out your carburetor multiple times and trying a bunch of different jets, there are simpler ways to find out if it's rich or lean.


I like making repeated runs on a marked section or road, but I've found it good enough to check the speed attained at a certain point.  I would probably mark the end point at wherever I hit 70 mph on my XR.

To see if it's running rich, take off any airbox covers and take the air filter out.  If you get a better run when doing that, then it was rich and adding more air improved things.  That means you need to jet down.

To see if it's running lean, start covering up openings with duct tape.  If it's lean you'll get a better run at some point, before you start making it worse by cutting off too much air.



Make sure you have enough gas before you start making acceleration runs.  The bike will get way worse mileage doing a bunch of full throttle runs.  I made that mistake on a Ducati Monster once and ran it dead dry a couple of miles from home.

Edited by ScottRNelson

Before pulling out your carburetor multiple times and trying a bunch of different jets, there are simpler ways to find out if it's rich or lean.



To see if it's running rich, take off any airbox covers and take the air filter out.  If you get a better run when doing that, then it was rich and adding more air improved things. 




The carburetor doesn't have to be removed to change jets, only the float bowl. It's less than a five minute job once a person gets the hang of it.


Removing the air filter changes the airflow characteristics into the carburetor and in itself changes the degree of fuel/air mixture.  A homogeneous mixture burns the fuel more efficiently.


Changes to the intake or exhaust will obviously have an effect on jetting, the point is to adjust the jetting for the modifications not to modify things to make the jetting work.

I'll have to go time myself now. ;)


I'm pretty sure that it take me significantly longer to get to a main jet.  Maybe I'm not doing it right.  It always seems to be major surgery when I touch a carburetor.

I will have to try the filter removal trick and give it a few runs.



 The first time someone does it, it will definitely take more than five minutes. Just trying to get those JIS screws out can be a PITA. The is definitely no need to even so much as tilt or loosen the carb.


I don't even consider putting them back in, they always get replaced with allen head screws and I use a thumb ratchet with a short extension and allen bit. Super easy.


I also use a 1/4" Gearwrench with a flat bit for the jets.


If a person could get a float bowl like the XR600R has, it would take even less time. I have an XR600R carb on my bike now, so it came with that great float bowl.

Not sure even what bike the op is talking about,,If it's an XR600 I ran 165/65 on my stock carb prior

to replacing it with the FCR..Sea level that jetting with the XL600 bore and CRF muffler..


Whoops,,bit of confusion,,I also have it down as 168/68 but I'll double check that when I get to the storage unit sometime..

Nope cancel the 168/68..It was 165/65..needle one slot down from blunt..

Edited by Horri

You lot really do like to make work for yourselves  - give it a run - turn off engine - read the plug - that will tell you all you need to know - and you can check through any range. 

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