MJ/MAJ Jetting

Is there a way to use top speed in 5th gear (with high gearing) to determine the best MJ and MAJ? If so, how?



you mean "why do i need tall gearing"?

so you can tell what is happening! your senses need time to tell the difference. it takes more power, and power higher up the rev scale for a bike to top out on tall gearing. the differences in settings are multiplied.

run it on the road on a shagged out old tyre for these tests. bumps and jumps wreck revs especially at high speeds.

i don't know why you didn't just ask for me.

there is a full MAJ/MJ test in jetting Qs and it was last up about 20 days ago.


Thanks Taffy,

I was going to ask directly for you, but didn't want to exclude anyone in case they had the answer I was looking for. I read through the Jetting Qs, but still have some questions... My long term plan is to first get my jetting correct on top with my current EKP needle, then try out the EMM needle and smaller pilot jets.

My current setup:

2001 DRZ400S (with Keihin FCR39 carb)

EKPc3 160 main, 48pj, 100paj, 200maj

Big Gun Race Exhaust, ride 700-1500'

So for now I'm just concerned with the MJ and MAJ I think. From what I've read, and some playing with the JD Jetting Guide I might be better off with a 162MJ. Once I do that change I will need to dial in the MAJ right? Can I use top speed to see if different MAJ are better than others ie (200MAJ=90mph, 160MAJ=93mph), or do I have to use a more seat of the pants kind of testing and see if one revs slower/faster etc.?

Once I get the MJ/MAJ thing sorted out I was thinking about EMMc4, 38PJ/60PAJ. Does this sound ok?



well the tests are fresh in my mind and are still there. the one to look for is the day i went from a #180 to a #160 and felt my head nod forward as i hit the rev limiter!

i think i was running 15/48 and that i found the most perfect gentle slope to ride up on the road to newmarket from a village by the name of six mile bottom!!!

by fitting the lower MAJ you will be making the max power slightly richer because you are feeding less air into the mixture. on the yam that extra richness at max revs was desperately needed and thus the "slam into the redline".

but on dropping the MJ the top revs sharpened up. this will show itself in faster revs or "zing".

if you end up going down on the MJ (i did by 10) you then have to lift the needle. if you lift the needle twice you need a smaller PJ.

good luck and please drop by with any results. mark cantrell has done a good bit of testing and perhaps you should PM him.



I'm running PAJ tests now but haven't done any MAJ tests yet.


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