18MM Mikuni Carb

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Local time
5:52 PM
Mar 16, 2008
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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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...
I love that you can control the choke with your left hand while your right hand is on the gas. That's the way it should have been from the get go.
If I have a 67cc Heunsheung Chinese HT engine, do I need an 18mm Mikuni? I've seen other sizes on ebay such as 24 and 32mm.

Where might I find a carb like this?
I have read posts stating mikuni 22mm works good. I believe that would be very hard to jet. I am satisified with the 18mm, maybe 16mm would be better and easier to jet, but I got a deal and it mounted the way I wanted..

I got mine off Ebay......
I agree, I think going bigger than 18mm on a 67cc engine would not net any more power, and will result in unacceptable low-mid range even after you dial in the high speed jetting. The new 24 and 32mm carbs on eBay are meant for 150-250cc 4 stroke motorcycles and scooters. My little 110cc 4 stroke motorcycle has a 20mm carb.
Wow!!! Thanks for the info on these Mikuni carbs guys.. I'm gonna have to go snag one off ebay for my own bike now. These look to be the carb the chinese duplicated for the HT engine.

Once you got your carb and started tuning it, where did you get the different jets from?

I tell ya what.. high end carb, sick shifter kit.. these bikes have come a long way in just a few months.
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