Need your help w tuning Mikuni 45mm carb please

[johnle712]

Hi all,
My dog came with Mikuni 45mm carb. She run really good, strong and smooth through all gear. No hesitation. Just that she spitting and cutting when idling, maybe a little cutting and low rpm and also sound like back fire on decel. My thought is she running to rich and need adjustment on air/fuel ration.

This is my first try with Mikuni, So to all Carb guru specially one familiar with Mikuni carb, do i turn clockwise or counter clockwise to add more air flow?
Thank you.

 

[HMAN]

Everything you need to know about a Mikuni

After @Cruz Dog posted about tuning his Mikuni on his newly purchased bike, I figured I'll make a thread about getting a Mikuni dialed in perfectly. I'm not an expert on certain areas of these bikes. There are quite a few members here that know much more than I do. That's why I love this forum...

www.bigdogbiker.com

 

[johnle712]

Everything you need to know about a Mikuni

After @Cruz Dog posted about tuning his Mikuni on his newly purchased bike, I figured I'll make a thread about getting a Mikuni dialed in perfectly. I'm not an expert on certain areas of these bikes. There are quite a few members here that know much more than I do. That's why I love this forum...

www.bigdogbiker.com

Thank you.

 

[mleach72]

Hi all,
My dog came with Mikuni 45mm carb. She run really good, strong and smooth through all gear. No hesitation. Just that she spitting and cutting when idling, maybe a little cutting and low rpm and also sound like back fire on decel. My thought is she running to rich and need adjustment on air/fuel ration.

This is my first try with Mikuni, So to all Carb guru specially one familiar with Mikuni carb, do i turn clockwise or counter clockwise to add more air flow?
Thank you.
View attachment 124467View attachment 124468

Yeah, sounds like it needs a pilot jet and/or idle mixture adjustment. The stock 25 pilot jet is probably a bit small for a 117. I run a 27.5 in my bike. The idle mixture screw controls air instead of fuel like on an S&S carb, but the procedure is the same. Turn the screw in until the engine stumbles. Then turn it out until it stumbles. Set it half way between.

 

[johnle712]

Yeah, sounds like it needs a pilot jet and/or idle mixture adjustment. The stock 25 pilot jet is probably a bit small for a 117. I run a 27.5 in my bike. The idle mixture screw controls air instead of fuel like on an S&S carb, but the procedure is the same. Turn the screw in until the engine stumbles. Then turn it out until it stumbles. Set it half way between.

Early this evening, i went a little pas quater turn both direction from where it sit and still the same issue. So i think it might be the pilot jet. When i turn counter clockwise, the exhaust pop (backfire) louder with a little shooting flame coming out and seems like it choke a little faster at idle. When i turn clockwise direction, seem like it take a little longer to choke but the flame coming out of the exhaust is crazy and backfire but not as loud. Does it sound right the way engine response? Right to more mixture( less air) left to allow more air. Or i am just seening thing thats not there

 

[mleach72]

Early this evening, i went a little pas quater turn both direction from where it sit and still the same issue. So i think it might be the pilot jet. When i turn counter clockwise, the exhaust pop (backfire) louder with a little shooting flame coming out and seems like it choke a little faster at idle. When i turn clockwise direction, seem like it take a little longer to choke but the flame coming out of the exhaust is crazy and backfire but not as loud. Does it sound right the way engine response? Right to more mixture( less air) left to allow more air. Or i am just seening thing thats not there

Yes, on the mikuni, clockwise is richer. Counterclockwise is leaner. See what pilot jet you have, and maybe try moving up a size.

 

[mleach72]

Early this evening, i went a little pas quater turn both direction from where it sit and still the same issue. So i think it might be the pilot jet. When i turn counter clockwise, the exhaust pop (backfire) louder with a little shooting flame coming out and seems like it choke a little faster at idle. When i turn clockwise direction, seem like it take a little longer to choke but the flame coming out of the exhaust is crazy and backfire but not as loud. Does it sound right the way engine response? Right to more mixture( less air) left to allow more air. Or i am just seening thing thats not there

Also, I can tell that your mikuni is mounted using a rubber boot adapter to the G manifold. That's the cheap and easy way to do it, but it isn't really ideal. You are much more likely to get intake leaks around that adapter. It also results in a mismatch between the carb and manifold resulting in a loss of air velocity. With the engine idling, spray wd40 around that adapter. If the idle changes, you have an intake leak. The ideal way to mount the carb is by using an S&S spigot manifold designed specifically for the 45mm. S&S part #160-1736. You will also need the carb seal and head seals.

 

[johnle712]

Yes, on the mikuni, clockwise is richer. Counterclockwise is leaner. See what pilot jet you have, and maybe try moving up a size.

Thank you.

 

[johnle712]

Also, I can tell that your mikuni is mounted using a rubber boot adapter to the G manifold. That's the cheap and easy way to do it, but it isn't really ideal. You are much more likely to get intake leaks around that adapter. It also results in a mismatch between the carb and manifold resulting in a loss of air velocity. With the engine idling, spray wd40 around that adapter. If the idle changes, you have an intake leak. The ideal way to mount the carb is by using an S&S spigot manifold designed specifically for the 45mm. S&S part #160-1736. You will also need the carb seal and head seals.

That is a great info. Thank you.

 

[johnle712]

yeah my pilot jet is 25. Will get 27.5 tomorrow. Thanks

 

[mleach72]

yeah my pilot jet is 25. Will get 27.5 tomorrow. Thanks View attachment 124744

While you have the bowl off, see what size your main jet is. The stock size on the 45mm is a 175. On a 117, you should be up around a 180 or 182.5.

 

[johnle712]

While you have the bowl off, see what size your main jet is. The stock size on the 45mm is a 175. On a 117, you should be up around a 180 or 182.5.

i will check that out too.

Thank you so much Mleach72.

 

[johnle712]

Will pilot jet 30 be too big Mleach72?

 

[mleach72]

Will 30 be too big Mleach72?

A 30 would probably be ok. The mikunis are pretty forgiving when it comes to jet sizes. The pilot jet supplies a good portion of the fuel in the cruising range, so it's important to get the right size for best fuel mileage. You shouldn't have any problems finding a 27.5. I get all of my mikuni parts from Niche Cycle.

Niche Cycle Supply - NCS

Niche Cycle Supply - We stock what we sell!

www.nichecycle.com

 

[johnle712]

A 30 would probably be ok. The mikunis are pretty forgiving when it comes to jet sizes. The pilot jet supplies a good portion of the fuel in the cruising range, so it's important to get the right size for best fuel mileage. You shouldn't have any problems finding a 27.5. I get all of my mikuni parts from Niche Cycle.

Niche Cycle Supply - NCS

Niche Cycle Supply - We stock what we sell!

www.nichecycle.com

Thank you.

 

[john sachs]

Also, be sure you have a STOCK Mikuni choke assembly. Particularly the inner brass plunger assy. If someone used a Harley choke (CV Carb) inner brass plunger, it will run extremely rich across the board.
John

 

[johnle712]

You are right again Mleach72!
My main yet is 172.5.

 

[johnle712]

Also, be sure you have a STOCK Mikuni choke assembly. Particularly the inner brass plunger assy. If someone used a Harley choke (CV Carb) inner brass plunger, it will run extremely rich across the board.
John

Could you show me how to check that? Do i have to take the carb apart? Here is what it look like. I have not have to use choke when i start the bike.

 

[mleach72]

You are right again Mleach72!
My main yet is 172.5.View attachment 124755

A 172.5 seems a bit small for a 117, but every bike is different. I would get every jet from 177.5-185. Main jets are cheap and easily changed by removing the bowl drain plug. The correct jet should definitely be in that range.

 

[johnle712]

A 172.5 seems a bit small for a 117, but every bike is different. I would get every jet from 177.5-185. Main jets are cheap and easily changed by removing the bowl drain plug. The correct jet should definitely be in that range.

Or could mine be 43 instead of 45?