Mikuni Carb Adjustments

I've been tinkering with the jetting on Salty the past couple days, and have had a couple old Honda enthusiasts show interest in how this larger 24mm Mikuni will work on a small 50cc Honda. I'm curious too, and have only begun to start messing with things.

First, I wanted to link everyone to this page: http://www.iwt.com.au/mikunicarb.htm

This is a fantastic primer on how a typical Mikuni motorcycle carb functions.

The first two times I've run Salty, it was very boggy at lower rpm. This was compounded by the fact I have it geared very tall, using a 16-tooth front sprocket, a stock 40-tooth rear sprocket, and the taller 2.75"x17" rear tire. I'm also running a 1/4-turn throttle mechanism, which reacts much quicker than a typical slide throttle.

Pulling away from a stop wasn't easy. It would bog very badly unless I gave it a little Fred Flinstone action to get rolling. It also ran very poorly at low rpm in second gear, when trying to pull away from slower turns.

So I started by adjusting the pilot circuit, as this has the biggest effect on low speed, and low throttle opening running. Usually this screw is set at 1.5 to 2 turns out from all the way in. In my case, it was set at 1.5 turns out. After getting the bike warmed up, and with a decent idle, you simply turn the adjustment screw in or out, and you'll be able to hear/feel a change in idle speed. In my case, it was rather dramatic, as turning the screw out (making it leaner) raised the rpm quite a bit. If you go too far, the rpm will start to lower. So you get a feel for where to set it based on rpm. In my case, I settled on 2.5 turns out. I then adjusted the idle speed back to where I wanted it.

It immediately improved throttle response. Even with the bike just sitting in neutral at an idle, it was very boggy before. Now it just revs clean with no hesitation. I then went for a short ride. It was dramatically improved, especially pulling away from slow turns in 2nd gear at lower rpm.

I now need to turn my attention to full throttle use, as it isn't quite right. I have a feeling I'm quite rich overall, especially since I needed to lean it out so much even at small throttle openings. When I'm cruising at mid to upper rpm in second gear, it pulls very nicely as I open the throttle. But I notice the last 10-15% of the throttle opening makes zero difference to acceleration. If anything, it feels like it stumbles a bit. A check of the plug indicates it is running rich. Going from 3/4 to full throttle opening is right in the transition range between the needle and main jet circuits.

I don't have any spare main jets, so I'll need to order some. In the meantime, I thought I would play with the needle jet position (1/4 to 3/4 throttle mixture) by adjusting the needle clip position leaner. I'm currently set in the middle of 5 clip positions. so I'll move it up a clip and see what that feels like.

If it isn't too cold tomorrow, I might go for a short ride.

C'