jrs_dodge_diesel

I have a moped (48 cc 2 stroke motor) with a Mikuni carb that it starting to irritate me. I had to take the bowl off and clean the passages and float area. It runs again now, kinda.

It will start and idle, although it idles a little low. Out on the road, it will bog down from about 1/8th to 1/2 throttle, but if it twist the handle past 1/2 the revs come back up and it accelerates like it should. From what I see it has two main adjustments :



I had thought that the #2 screw was the idle screw, but it is not. There are 2 marks where the screw lined up with, but I cannot remember how far in or out that screw was.

If anyone could give me some pointers I would appreciate it.

vzdude

#1 is definately the idle screw. #2 is the air bleed/mixture screw. Make sure you have good fuel flow. Last one I did, the #2 screw was just flush with the housing. HTH

jrs_dodge_diesel

Thank you for the quick response vzdude.

OK, so #1 is the idle. Which directions increase and decrease the idle?

#2, which way is rich/lean? And how far should I turn it as I test it (1/4 turn increments)?

vzdude

#1 - in = idle up
#2 in should richen the mixture.

Try using some carb cleaner spray around the intake and see if it picks up. If you can't get it to accelerate, IIRC the tube going into the bowl has some passages that tend to clog as well.

Lary Ellis (Top)

Been a few years since I worked on Carbs but seems like we would turn the mixture screw in until it just began idle rough then back it out half a turn...

wurkenman

On a Mikuni turn the air mixture screw in until it LIGHTLY seats. If you apply too much pressure you will seat the tip of the screw in the carb body and break off the tip. Back out the air mixture screw 1.5 turns. This will serve as a good starting point. Then like top said turn it in and out slowly until you get a smooth idle.

Totallyrad

And was it running correctly before you pulled the bowl? If so, take it back off and recheck the internals. Something may have moved during assembly.

jrs_dodge_diesel

The passages should be good now, theay all got a good healthy dose of carb cleaner and some pipe cleaner action.

Thank you all for the advice. This gives me a great place to start when I tinker with it again tomorrow.

Just wish this carb was like the one on my generator, that one is a lot easier. It has the normal butterfly valve, idle screw and mixture arrangement. This Mikuni just seems backwards to me.

Nope, didn't run at all since the passages were gummed up. Before that I had adjusted the idle speed, or so I thought. I was turning the wrong screw before.

vzdude

Basically they are the same. The slide up and down is the same as the butterfly in the other type carb. Everything else is just bowl vent, and vacuum to the fuel valve.

jrs_dodge_diesel

Yeah, that slide/gate valve really threw me off the first time I saw it. This is the first carb I've seen without a butterfly.

rattlerbob5.9

The slide controls the volume of air that the engine receives and your number one screw is an adjustable slide stop - screw in lifts the slide. In the center of the slide their will be a needle which drops down into the main jet when the throttle is closed and reduces the fuel flow from the main jet. This is the main fuel system and from your description it is working correctly . Inside the float bowl to the side their should be a small pipe usually with some small holes in the side of it this pipe feeds up to your number 2 screw and is likely the source of your problem run a piece of wire through it and make sure it is clear.
What i see most of the time is a yellow waxy deposit which reduces the fuel flow caused by our overpriced garbage they call gas these days . Carb cleaner in the spray cans does not cut it you need to mechanically remove it you may also have a layer of it on the main jet or needle i use dental instruments to scrape the junk off.
You may also have a small screw in jet in the top of the float bowl that would be the pilot jet and it controls the fuel flow when you come off idle and before the main jet/needle and slide take over completly so this would also be the source of your trouble (the bog or hesitation) on light throttle. It does not take much crud to cause a problem in these little carbs blow all the passages out before you reassemble .

Pentastar

Rule number one when working with faulty carbs is to seat the adjustment needles gently counting the turns to to bottom so that you can verify your repairs and give you an educated starting point to set them back up again.
There are many websites these days that can give you just about any info you might need plus sell you the parts if you like.Mikuni is a very popular carb to be found in a lot of asian engines and Hitachi also made a lot of carbs as well.Be very careful on disassembly that you don't lose any springs,***** etc from the various ports and it always helps to have a parts list and diagram to make sure the o rings etc get installed in the right order.The floats and metering needles need special attention these days because of the additives in the fuel that create varnish and crystals on the internal parts when it dries which causes the floats to hang until they are cleaned properly.Get some documentation if you can,it will pay off in the end.Ron G

irocpractice

"JR" you have no other options.You need to pick up a 1970/74 Suzuki 500 or,a 1972/76 Suzuki 750 water Buffalo.All the Mikuni carb experience you can handle.(trust me)For parts/kits try Sudco.

Dodgezilla

Man! The lengths people go to just to save a few bucks on fuel!!
Keep that thing off the highway!!!

vzdude

I did find out that the mixture is adjusted by moving the clip on the needle, not the air screw. The air screw should be about 1.5 turns out like said before.