Carb jetting and float hight?

[crackerjac]

Mikuni (clone) VM26 30mm. Main jet 115, pilot 30. What should the float height be set too? I ported and opened up the intake opening on the head to match the carb opening size.

Also, the air fuel mixture screw? Thoughts on a good starting point to set it at?

[Megadan]

I think a better starting point would be the bike/engine?

On my OHV 167fmm I have found the 30 pilot on the VM26 to be too big, and was just a bit past half a turn out on the mixture screw before I ever got a steady idle. I run a 27.5 with a bit over 1 turn out and the bike runs a lot better.

Obviously, a different motor may act differently, so it's apples to oranges. A 30 is a good starting point at least.

Mixture screw. General rule of thumb is always start 1 1/2 turns out from lightly seated. Set the idle speed on a warmed up bike to a little higher than standard, adjust the mixture screw a quarter turn at a time and pay attention to the idle speed as it changes. You are looking for the fastest point in the idle. Re-adjust the idle speed, and fine tune it further making 1/8th turns, and you can even go into finer tuning until you are happy. Then set the idle speed back to where it should be.

If it takes less than 1 turn out to get the best idle, the pilot is too big. If it takes more than 3 turns, it's too small. Ideally, you want a pilot jet that sits between 1 and 2 turns out on the mixture screw, while also exhibiting the best light throttle behavior. Example. A 30 was too big so I installed a 25. The mixture settings with a 25 were just a bit past 1 1/2 turns, but it displayed the hesitation, popping, and jerky throttle of being too lean. Thus I stepped up to a 27.5 pilot.

If this is an aircooled motor, I would suggest having a good fan pointed at the engine to help keep it cool while it runs.

[crackerjac]

Originally Posted by Megadan

I think a better starting point would be the bike/engine?

On my OHV 167fmm I have found the 30 pilot on the VM26 to be too big, and was just a bit past half a turn out on the mixture screw before I ever got a steady idle. I run a 27.5 with a bit over 1 turn out and the bike runs a lot better.

Obviously, a different motor may act differently, so it's apples to oranges. A 30 is a good starting point at least.

Mixture screw. General rule of thumb is always start 1 1/2 turns out from lightly seated. Set the idle speed on a warmed up bike to a little higher than standard, adjust the mixture screw a quarter turn at a time and pay attention to the idle speed as it changes. You are looking for the fastest point in the idle. Re-adjust the idle speed, and fine tune it further making 1/8th turns, and you can even go into finer tuning until you are happy. Then set the idle speed back to where it should be.

If it takes less than 1 turn out to get the best idle, the pilot is too big. If it takes more than 3 turns, it's too small. Ideally, you want a pilot jet that sits between 1 and 2 turns out on the mixture screw, while also exhibiting the best light throttle behavior. Example. A 30 was too big so I installed a 25. The mixture settings with a 25 were just a bit past 1 1/2 turns, but it displayed the hesitation, popping, and jerky throttle of being too lean. Thus I stepped up to a 27.5 pilot.

If this is an aircooled motor, I would suggest having a good fan pointed at the engine to help keep it cool while it runs.

Thanks for the reply. It's a Zongshen air cooled Honda crf230 clone. Labeled as a ZS169FMM but when I rebuilt it it ended up being 65.5mm bore x 66mm stroke or whatever the crf230 is which is 223cc.

I was having a no spark issue but have worked through a few issues and now have spark. Not sure if it's too weak or not. Could still be causing issues. :(

Anyway, still won't start, but when I put my hand over the carb inlet and crank the motor over with a wrench via the hole in the stator cover (kill switch on of course) as fast as I can I get a bit of fuel splashed back at one point in the cycle from I think one or both of the holes on the back of the carb on the bottom where the air cleaner mounts. Normal? Seems like a lot of gas. Thoughts?

I have the mixture screw 1.5 out, I guess I'll try it 0.5 and go from there.

Off topic, it's a real pain getting to the mixture screw in my setup and I was thinking of getting one of those mixture screw extentions. A lot of the suppliers say the mixture screw on these Mikuni VM26 clones are the same as used in the Honda CRf250, 450 etc. Can you confirm??

[Megadan]

When you get that fuel splash back, does the intake ever feel like there is any sort of reverse airflow or pressure buildup?

Also, since I forgot the answer this before. With the carburetor inverted (bowl side up) the float height for a genuine VM26 carburetor, from the bowl flange to the bottom of the float (will be the top since it is upside down) is 21mm.

[crackerjac]

Originally Posted by Megadan

When you get that fuel splash back, does the intake ever feel like there is any sort of reverse airflow or pressure buildup?

Also, since I forgot the answer this before. With the carburetor inverted (bowl side up) the float height for a genuine VM26 carburetor, from the bowl flange to the bottom of the float (will be the top since it is upside down) is 21mm.

Hmm, I'll double check tomorrow when I can look at it again but there may have been a bit of pressure. What does that mean?

[Megadan]

Well, if you are cranking it over by hand and you feel air coming OUT of the intake, then either the valve lash setting is wrong, or the cam timing is off. In either case, this would cause the bike to not start or run properly, if at all.

[crackerjac]

Originally Posted by Megadan

Well, if you are cranking it over by hand and you feel air coming OUT of the intake, then either the valve lash setting is wrong, or the cam timing is off. In either case, this would cause the bike to not start or run properly, if at all.

Pretty sure I had both set right. Do have any links that show setting the cam up right? I thought crank at TDC the cam lobs should be pionted down and the two lines on the cam gear level with the head surface?

[pete]

Originally Posted by crackerjac

Pretty sure I had both set right. Do have any links that show setting the cam up right? I thought crank at TDC the cam lobs should be pionted down and the two lines on the cam gear level with the head surface?

that should work.... if it has rocker arms
but wheather the lobes are up or down depends
wheather it has rocker arms or buckets...
ie.. buckets the lobes face up...


..




.

[crackerjac]

Originally Posted by pete

that should work.... if it has rocker arms
but wheather the lobes are up or down depends
wheather it has rocker arms or buckets...
ie.. buckets the lobes face up...
..



.

Its a OHC so the rocker arms ride right off the cam.


I'm guessing with a 30 pilot jet and 1.5 out on the air/fuel screw its flooding out the motor more than I think. I'll try 0.5 out and see if anything changes. I read somewhere that a 30m mikuni float should be set 23~25mm???

[Megadan]

Originally Posted by crackerjac

Its a OHC so the rocker arms ride right off the cam.


I'm guessing with a 30 pilot jet and 1.5 out on the air/fuel screw its flooding out the motor more than I think. I'll try 0.5 out and see if anything changes. I read somewhere that a 30m mikuni float should be set 23~25mm???

The 21mm spec is for a genuine Mikuni VM26 carburetor from the float bowl flange surface. As far as the other specs you say, I can't verify those or not. I do know that the VM22 and VM24 carburetors are generally set within 22-24mm, and these 30mm "VM26's" are essentially larger versions of those.

I do know that if you set the floats to where the seam on the floats are level with the flange surface, this puts the Mikuni clone carbs within spec a majority of the time.

[crackerjac]

What do you have, or what is most running, the needle clip at? (from the top number of notches down on the needle).

[HawkTwoFifty]

I also have a VM26 Mikuni-clone and was wondering if anyone knew which screw on the carb is used to increase the idle speed?

I did not receive any tuning guide or manual on this carb when I bought it and am finding it rather difficult to find this answer anywhere online since this carb is a Mikuni clone.

I need to increase my idle speed on this carb but do not know which screw on the carb handles this adjustment. Anyone know?

Thanks.

[Azhule]

Usually, it's on the "right side" of the carb/bike there is a "Phillips head" looking screw on these mikuni clones, that will be the one that will raise/lower the idle speed

Look for the "Blue/Purple marker" in the pic below, if this is the same carb you have, that is the screw to look for