18mm Mikuni Upgrade

misteright1_99

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Mar 21, 2008
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Treasure Coast, Florida
I bought a used 18mm Mikuni carb from a 70's Yamaha 80cc dirtbike and installed it on my 48cc HT engine. The carb was a direct fit right down to the clamp on type of mount to the intake manifold. The throttle cable fit the replacement carb as well with no modifications. I did have to lengthen the fuel line, as the inlet is on the opposite side in the Mikuni. Because of the style bike I decided to make an air filter assy. on a 90 degree turn. It runs much smoother than the stock carb (no surprise), I am however waiting on some smaller pilot jets as the ones in it are way too large. When I get the jets instaled and know the sizes I will post the Pilot, and Main jet sizes, as well as the air screw adjustment. Here are a few pics.......
 

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Dave31

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Mar 1, 2008
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Re: 19mm Mikuni Upgrade

HEY!! That's not fair, that was my ideal...you beat me to it before I could rebuild mine rotfl




Great job misteright (^)
 

misteright1_99

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Mar 21, 2008
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Your looks like a 73 also. Wonder why mine has the compression mount and your has the 2 bolt mount. Never seen one like that I have only seen the compression mount and the "spigot" mount. Well Ill keep u posted on jet sizes although I am only running a 48cc motor...
 

Dave31

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Mine is off a 1975 GTMX....I've had it since 1978....I've done a few mod's over the years and it's not the original carb but it is a Mikuni.. It will bolt right up but waiting for a rebuild kit. I just sent for a pack of five jets for it. Hope it all comes soon.
 

misteright1_99

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Mar 21, 2008
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Well I think I have the correct main jet size already I have an 80 but the pilot is soooo large, a 45 that I cant run wot without turning the air screw in, and cant idle or run low or mid without turning the air screw out. I have soldered and drilled the pilot to 35 but that is the smallest drill i cound find and was able to get closer with the airscrew, but not enough. I have ordered a 15, 20, and 25 pilots, and a 75, 85, and 90 mains.
 

misteright1_99

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Mar 21, 2008
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Well I think I am finally close to having the carb dialed in. It has been raining here forever it seems, and took even longer to get the jets. It runs very smooth, and I think I could go one size larger on the main but here is what I have...

Pilot Jet is a 25 (I believe it could use one size smaller a 20 but I didnt order it lol)
Main Jet is a 80 (Again one larger would probably work, But I didnt order it)
Needle setting I had to groove needle to lean it out I believe because of the 25 pilot jet
Air Screw setting here in Florida 1-1/2 turns out seems best.
I am running a NGK B6HS gapped at .028

I hope this will give anyone that is going to try the Mikuni a baseline to start with...
 

eDJ

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Jul 8, 2008
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Somewhere back in the early 70's my dad bought a Cycle magazine (I believe it was Cycle) that had a special section on Mikuni Carbs. It was a whole section of the magazine printed on brown paper for the feature. It explained the double tapered jet needles and the needle jets they moved up and down in. There was a chart that explained the sizing, and suggested "groove" position at the top of the jet needle that the circlip be set in. It also suggested slides (which have numbers regarding the slanted cut at the front) to be used in the sizing and matching to the cc displacement of the engine.

So, there is much to sizing and matching a Mikuni to an aplication. What makes the biggest difference is the 4 cycle carbs use a straight tapered needle and the 2 cycle's use a dual taper. Adjusting the jet needle upwards or downwards in the slide along with the number of the needle, jet, and slide all come into play. Then the adjustment screws can be tweeked to provide a power curve thru the spectrum that comes alive in the rpm power band. (and leaves the spark plug and piston clean and undamaged)

I've seen guys bolt on a high compression head and try to ride without waiting to get the right jets etc and burn the electrode off the plug. If they didn't get the message the first time........they slammed another spark plug in it and kept riding till a hole melted in the top of the piston.

His bike sat at the dealership till the carb jets got in and they put a new piston & rings in the engine and installed the high compression head again only resetting the cab with the new jetting so it would run properly and not self destruct.

I'm sure somewhere out there....perhaps Mikuni's web site there should be a chart like was in that old magazine.

Just doing a quick Google I found these resources:

Mikuni American Corporation

See the chart at the bottom of this page:

Mikuni carburetor operation and tuning


On the manifold mounting of your Mikuni........was it for a spiggot on the cylinder for port tuning or was the carb mounted on side of the case as it was a rotary valve model ?

But this is what I remember. I've forgotten so much as I was into this stuff in my 20's.

A carb that's too large may have a great top end but be soggy thru the lower spectrum. A carb that's a but too small may have a lightning takeoff and then choke out before it an come up to the power band. (assuming we're talking 2 cycle here)

I've hand ground carb slides for some of my Mikuni's when I couldn't get a half size 2.5 when I wanted one. I purchased 3 slides which were 2.0 and calculated the difference between the 3.0 to make a template to transfer with machinist blueing. But if that's what it takes.

Good luck with your project. When you get one dialed in spot on......it's real magic for the performance.
 
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RLorange

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Jun 21, 2008
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After looking at your thread I am starting to wonder if I need such a small main jet for my 19mm PHBG because my pilot is also very big.

Do you know if the pilots contribute to the fuel delivery at all rev ranges?

I think my PHBG has the wrong 4-stroke style atomiser and/or maybe the pin is tapered too steeply... did your Mikuni come from 2 or 4 stroke motorcycle?
 

misteright1_99

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Mar 21, 2008
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Treasure Coast, Florida
After looking at your thread I am starting to wonder if I need such a small main jet for my 19mm PHBG because my pilot is also very big.

Do you know if the pilots contribute to the fuel delivery at all rev ranges?

I think my PHBG has the wrong 4-stroke style atomiser and/or maybe the pin is tapered too steeply... did your Mikuni come from 2 or 4 stroke motorcycle?
Before I got the Mikuni I bought a 4 stroke carb by mistake and never was able to make it work, jetting, slide cutaway, and needle taper were different.
I am not familiar with the workings of the Dellorto, but with the Mikuni the pilot works from 1/8 to 1/4 throttle, along with the air screw. My carb came off a 80cc 2 stroke Yamaha...
 

DanielMaia

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Jun 24, 2009
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Portugal
I have also a Bing 18mm, with a #80 main jet, and it seems its working nice.

Only it boggs a little when i push too fast the throttle, maybe i should raise the needle one notch..
 

bairdco

a guy who makes cool bikes
Aug 18, 2009
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living the dream in southern california
i have a 16mm mikuni VM round slide flange mount carb, and the stock main is a #80.

i bought it from a swapmeet for 5 bucks new, but didn't have an air filter.

when i got the bike running, it was bogging heavily and i figured i'd need to re-jet it.

as soon as i got an air filter for it it smoothed right out and works great.

i could still probably re-jet to fine tune it to perfection, but i'm pretty happy with it as it is.