Main jet fine tuning trick.... WOT and back off a little?

[idr5c]

I'm having a brain fart regarding a tuning technique that I have used for years  , hoping someone on the board can set me straight. Not sure how I managed to forget this rule of thumb... Lyn told me about this a decade or so ago and I've used it ever since.

Once the bike will pull cleanly on the MJ (no blubbering), I will go to WOT in top gear and hold it there. As the speed levels out, I'll back off to 7/8 throttle or so and see if the engine picks up a bit.

If the engine picks up a bit, does that indicate the MJ is a little rich or lean?

[Dvsrd]

I'd say lean, from experience with my RD350, oem emulsionntypr needle jet setup, 30 years ago...

[m in sc]

could also be fuel feed issue. had same problem with my lc, had to go to 3.5mm? needle seats at wide open. only did it by the time i was wot at top of 5th gear, 6th would stall out. took forever to diagnose..

always err on the side of going richer on the main for tests like this. IF it gets worse, then you know its too fat. however, it DOES sound lean. but dont disregard how fast the bowl fills vs how fast its being used.  what size are your mains? if over 300, then, look at needle seat and verify hole size.

[idr5c]

could also be fuel feed issue. had same problem with my lc, had to go to 3.5mm? needle seats at wide open. only did it by the time i was wot at top of 5th gear, 6th would stall out. took forever to diagnose..

always err on the side of going richer on the main for tests like this. IF it gets worse, then you know its too fat. however, it DOES sound lean. but dont disregard how fast the bowl fills vs how fast its being used.  what size are your mains? if over 300, then, look at needle seat and verify hole size.

Not asking about issues with a particular bike, just the tuning technique in general.

Does anybody actually have experience with this tuning technique?

[m in sc]

YES. LEAN.

[idr5c]

YES. LEAN.

Now THAT is a clear answer!!  Many thanks.

[m in sc]

i mean i did say:

however, it DOES sound lean.

 

[Yamaha 179]

It has been my experience that the needle reduces he flow of gas more than the slide reduces the air flow therefore leaning out the mixture.  Less gas but almost the same amount of air = seizure.
Lyn Garland
PS: I didn't say 7/8 throttle, I said close the throttle just a bit; just ease off a tad.

[KANDY]

"the needle reduces he flow of gas more than the slide reduces the air flow therefore leaning out the mixture"

wont get a better explanation than this. 

[quocle603]

Trust me, no shame in relearning the basics. I always have to second guess myself.

[idr5c]

It has been my experience that the needle reduces he flow of gas more than the slide reduces the air flow therefore leaning out the mixture.  Less gas but almost the same amount of air = seizure.
Lyn Garland
PS: I didn't say 7/8 throttle, I said close the throttle just a bit; just ease off a tad.

Bingo, that's the info I needed. Lyn's right and I used the technique this weekend at the track. My main jet was still a little rich after working down from blubbery, and when I'd ease back on the throttle from WOT the bike would pull a little harder. Went down a size and all was well. Thanks Lyn!

i mean i did say:

however, it DOES sound lean.

Yep, you sure did.

[Czakky]

Mine acts very similar...