Yama Mikuni SBN 44mm Max jet size?

[Quinc]

I have been tuning my blaster and noticed that going from a 165M jet 2 turns out to a 170M jet 2 turns out made no difference in top speed, rpms, or the way it felt. The plugs also had no change in color. So I am thinking at this point adding larger jets wont make a difference since the main jet hole is only so big.

So my question is how big is the main jet hole on a stock Yama 44mm sbn?

 

[speedysprocket]

First, shouldn't need more than a turn out on the high speed screws. After one turn, you should go to a bigger jet. Second, what engine are you running that you think it needs that much fuel?!

 

[oside-tanner]

A larger jet should flow more fuel, maybe you are maxing out the fuel at your needle and seat, what combination needle, seat and spring you running

 

[WB1994]

A larger jet should flow more fuel, maybe you are maxing out the fuel at your needle and seat, what combination needle, seat and spring you running

The mikuni bible says the smallest N/S flows more fuel than thier carbs can use. Thats why they say in the bible to use the smallest N/S to achieve your popoff and thats all you need to do for N/S size. It doesnt say anywhere in the bible to go up a N/S size to richen your carb.

 

[Vumad]

The mikuni bible says the smallest N/S flows more fuel than thier carbs can use. Thats why they say in the bible to use the smallest N/S to achieve your popoff and thats all you need to do for N/S size. It doesnt say anywhere in the bible to go up a N/S size to richen your carb.

A 1.5. Exuding a 1.3 which isn't listed in the manual. None of us should have a popoff that high however. Couches run that though.

 

[kevbo]

I have been tuning my blaster and noticed that going from a 165M jet 2 turns out to a 170M jet 2 turns out made no difference in top speed, rpms, or the way it felt. The plugs also had no change in color. So I am thinking at this point adding larger jets wont make a difference since the main jet hole is only so big.

So my question is how big is the main jet hole on a stock Yama 44mm sbn?

The main jet hole is huge compared to any jet. I ended up with a 160 and 2 turns out on a stock 44/61X case under a high port 701. You may very well be choking a little at the needle or even at the basket. A 2.0 N/S has more area than all downstream orifices so I always use this size or larger on singles. Twin carbs get down time between pulses so much smaller N/S can be used.

 

[Quinc]

i am running 2.3 n/s and not sure on the spring (65g maybe?) So instead of upping the jet size should I go to the next size down spring, or go up to a 2.5 N/S?

 

[speedysprocket]

You need to read the mikuni manual 6 more times. The pop off doesn't do anything to your top speed. Maybe you're not going any faster because that's all your engine/setup has in it.

 

[kevbo]

He should be able to slow peak RPM by opening the top screw. Otherwise there is a restriction upstream.

 

[Manny]

Want more speed change the impeller picht

 

[Quinc]

I could care less about top speed, it is just a way to measure performance. And regardless of if my engine is maxed out or not I should be able to make it run rich.

 

[Vumad]

Check the compression. Compression too high can make the plugs appear lean.

Check for air leaks. Run the ski on the hose and spray carb cleaner around the base gasket, crack seals, intake, if the motor changes speed you definately need to do a leak down test. If the motor does not change speed, that does not necessarily mean you don't need to do a leak down test.

Make sure all the little venturis on the carb are clean.

Find out what others are jetting for your pipe. You might still need to be richer. Stock superjet is 130, recommendation is 135, I actually needed 140s.

Popoff won't help your wot, but pilot jet size will. If you have hesitation below 1/2 throttle, try a bigger pilot.

 

[Vumad]

First, shouldn't need more than a turn out on the high speed screws. After one turn, you should go to a bigger jet. Second, what engine are you running that you think it needs that much fuel?!

I believe that is incorrect. It's more like 2.0-2.5 turns before you max out that adjustment.

 

[Vumad]

i am running 2.3 n/s and not sure on the spring (65g maybe?) So instead of upping the jet size should I go to the next size down spring, or go up to a 2.5 N/S?

Maybe you should raise your popoff and increase your pilot jet size. Popoff and pilot work together in the end (pilot is more low and pop is more middle), however, pilot jet size will affect fuel in the high end, popoff pressure will not.