TM Flatslide jetting?

[83atc200]

I am currently working on a 300r build and purchased a 38mm flatslide carb. its a T38 85a mikuni and I have no idea on where to start my jetting on this thing. anyone have any experience with this. Also, do the jet holes match between kehin and mikuni jets? for example, is a 152 main jet made by mikuni the same as a 152 main jet made by kehin? any help would be much appreciated.

[barnett468]

Hello 83atc200;


Ok, I looked this up on the net and came up with the following.

Those carbs are jetted for a 125 with a 230 main, 22.5 pilot 389 Q-2 jet needle, 6fj41 needle.

If you do not get any info from someone that is using one I would start with the following.

Pilot - 45

Main – 350

Needle - 6fj40

Jet needle – 389 Q-6

Fuel mix screw – 1 1/2 turns out from full in.


Mikuni and Keihin jets use different numbers.

Jet conversion chart

http://www.jetsrus.com/FAQs/FAQ_mikuni_vs_dynojet_vs_keihin_sizes.htm

[83atc200]

Barnett 468,
Thanks a lot for the info, I think I found the same info as you did on the jetting for the 125 last night but I wasn't sure where to go with it from there.. Are the sizes that you suggested to me just richened up a bit from the 125 jetting? I was just curious how you got to those sizes. Also I know the stock main jet for the 250r keihin carb is 152, I was just wondering why is it that the size jumps all the way up to a 350 when using a mikuni, I am just trying to understand a little better from someone who does. I will definatly use your suggestion as a starting point and hoped you could teach me how you got there. thanks a lot,
jake

also, the one thing I don't understand about the conversion chart is that when I look at it, it looks to me that the mikuni jets are smaller than the keihin jets in relation to size. for example it says a 92 keihin would be a 82 mikuni, which confuses me because I thought it would be the other way around being that I am going to be starting with a 350 main when like I mentioned earlier the stock keihin main jet is 152 in a 250r 27mm carb. unless when upsizing carbs, do the jets have no relation to each other when comparing to jets in a smaller or different model carb? sorry for all the questions! thanks again
jake

[DohcBikes]

The numbers are just numbers, for reference, they don't mean any particular size, its honestly more like a part number.

Don't get too wrapped up on the differences in the numbers, bigger is bigger, smaller is smaller.

[barnett468]

hi;

you're welcome.

the jets i suggested are richer than a 125

i got them by looking after looking at several sites regarding jetting it for a 250 using a 38 tm mikuni. he ranges were 40 - 50 pilot, 300 - 310 main, 389 Q-6 needle jet and 40 needle. since you have a 300, I "guestimated" that a 350 main "should" be a reasonable place to start. just don't run it hard in case it is too lean.

if it burbles/sputters/gesticulates when opening the throttle QUICKLY from a STEADY 1/2 throttle position, then the main is likely too big. if it just fails to accelerate like it is not getting gas, then the main is likely too small.

if you open the throttle quickly to 1/2 only from a slow roll and it burbles, lower the needle all the way and try again, if it seems like it is out of gas raise it all the way.

changing the needle will only have a small affect.



the jet size conversion is a bit screwy, it is not a straight fixed percent like 10% etc., so as DohcBikes implied, don't worry about it. its kinda like a hole that is 1 mm. 1 mm = .03937". the hole is the same size but the method of describing its size is different.

carbs are pretty complicated and i don't want to discourage you from learning about them so if you want, i will post some links to some info on them, but be aware that the more you learn about them, the more you realize you don't know. i could write 76000 characters of text on how this does that and how elevation, barometric pressure, humidity and the phase of the moon affect their function but i'd get yelled at, lol.

the basics are pretty easy though, burble = rich, hesitation/failure to accelerate = lean.

:beer

[83atc200]

Barnett 468,
You gave me all the info I should need to get started, very much appreciated!!

[barnett468]

sure no prob, if you do the testing on a slight uphill it might be better but doing it on flat ground will get you extremely close. i would start with a new plug. the plug color is a good indicator of jetting also however it might take around 30 minutes - 1 hour to build up enough color that reading it will helo.

use the recimmemnded heat range plug.

white/gray = lean and should have hesitation when accelerating.

lite tan = slightly lean

medium tan = ideal

dark tan = slightly rich

dry black = rich and it should burble upon acceration.