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Mikuni power jet adjustment

23 May 2020 22:51
ITTY_250350_KS
Mini Enduro
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Mikuni power jet adjustment #1
Hello all,

I have a Mikuni power jet kit installed on my TM38 flatslide carb and Mikuni recommends that original main jet should be reduced by 25% and the reduction will become the size of the power jet.

So in my case my bike is currently using a #380 main jet so i trimmed it back by 25% which now #95 will become the size of the power jet however this is a conversion from VM36 roundslide to TM flatslide with power jet kit so i started off in a richer side by installing a #320 main and a 97.5 power jet then go from there..

If the engine ever shows a rich/lean symptom which most likely it's gonna happen..should i be making the adjustment on the main jet first and leave the power jet alone until i have find a suitable main jet for it or should i be making changes on both simultaneously using the aforementioned Mikuni's formula? and for the future changing condition, should i only make changes on the main jet, power jet or both?

Best Regards,
Jack

Someday you'll own a YAMAHA!!

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24 May 2020 18:35 24 May 2020 19:15 by RT325.
RT325
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Mikuni power jet adjustment #2
Don't know the answer but i recall an IT175 i raced new in 1980 had a power jet carb. Never altered it--but-- i see looking at Babbits online that the "80 IT175 gas a 200 210 220 main & 82.5 power jet. Then the "81 with no power jet still has a 200 210 220 so i don't know sorry. Keen to hear your outcome though. What are you running it on?. I measured my carb off my 1990 RMX250 recently & is a 38mm flat slide, [TM38 think it is]. No power jet though. looks huge & could suck in low flying ducks.

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24 May 2020 19:15
MarkT
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Mikuni power jet adjustment #3
It's been a long time... If I remember correctly the power jet affects things more near full throttle...

So say you're lean from 1/2 to 3/4 but then fine from 3/4 to full you would go up on the main and down on the power jet the same amount you went up on the main... if that makes sense.

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

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24 May 2020 23:32
ITTY_250350_KS
Mini Enduro
Mini Enduro
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Mikuni power jet adjustment #4
That makes sense to me. I will adjust the powerjet accordingly.

Right now i have a piece tape with markings for the throttle position so i can see which part needs adjusting.

It's for my IT250K since i have a brand new unused TM38 that i bought a while back with the power jet kit fitted and when it fitted like an OEM carburetor when i test fitted it on the machine so i thought to myself why not give it a try :).

Someday you'll own a YAMAHA!!
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