Mikuni low throttle probs.

breno · Dec 21, 2011

Hi all.
I have a 18mm mikuni the top end is nice now with .70 main jet on the top notch possibly a .65 would b a touch better so I could come down on the needle but it's good enough. Thing is the only wat I can get a nice pickup from low speed is to hold it wot.

I have a 20 pilot jet in with the air screw 2 turns out.

Overall it's running really well with a gr8 midrange and top end.

I would just like to work out the low throttle setup for when I just want to cruise at a lower speed.

Could it be the cutaway in the slide which might explain why I need to open the throtle more at low speed to make it run smooth?
Almost forgot its on a 70cc skyhawk gr5..
Thanx for ur input guys.

vachon644 · Dec 21, 2011

If your engine is 4-stroking/sputtering at 10%-50% throttle then it might aswell be your needle that is too rich or too lean. The needle is what comes down in the jet and which is raised and lowered with throttle position. When you raise the needle, it clears the jet opening earlier on your throttle, when you lower the needle, it blocks the jet until later on the throttle position. I would try raising it just in case you're running lean unless you're sure it's running rich, then lower it. Carb tuning is about trying things.

breno · Dec 24, 2011

Any other ideas people?

Goat Herder · Dec 24, 2011

For me on my Morini bike and the bigger PHBG carbs my low throttle sucked it ran lowsey until I went with a different throttle body needle. I changed a lot of stuff out for my altitude that needed to be done. The last thing I did was that needle and I loved it.

I am not totally familiar with your carb but it seams to share similar construction with the PHBG save yours is a air fuel mixture screw and mine is a fuel mix one at the idle circuit.

What my needle did for me I ran a little larger main jet for it. The needle I switched to was thicker in diameter. This in turn changed dynamics for the better. That's about the best I got for yeah.

Barnfresh · Dec 24, 2011

You can usually tune around the slide cutaway fairly well unless the carby was originally set up to be used on a 4 stroke. Have you determined yet whether your problem is a rich or lean condition? Does it get worse when you raise the needle (lower the clip)? With the engine set as low as possible, if you turn the air screw in a 1/4 turn or out a 1/4 turn from your 2 turn setting does the engine die? Is it better or worse with the choke on, or partialy on? These are all things I would try first to determine where to go next.

rustycase · Dec 24, 2011

I have heard... never tried it... a small square, filed into the downstream side of a mikuni slide, aids in the low - throttle transition/increase.

I don't know if that will help u, but it's the only 2cents I can contribute.

Best
rc

timboellner · Dec 24, 2011

The throttle slide affects engine performance from 1/8 to 1/4 throttle.

The larger the cut-away .. leaner mixture
Smaller cut away... richer mixture
Here's a terrific and easy to follow guide for your Mikuni.
(It's easier than talking you through )

You've got to start with the main jet and work your way through. Follow this guide step by step
Good luck.
There is a small chance that the carb is too big to draft with enough velocity at low rpm's on your engine unless you have mods to your engine that help it breathe.

http://klx250s.suncup.net/Misc Docs/Tuning the Mikuni Carb.pdf

Tell us what works out,
TiM

mdlee1958 · Dec 25, 2011

Breno, on my Mikuni (VM16 roundslide) I found that the pilot jet circuit is REALLY touchy, any guck, grit or whatever in that circuit which includes, the pilot jet itself, the mix screw AND the choke plunger will make it not want to idle properly or irregularly. Once I pulled the pilot jet, mix screw and choke plunger and cleaned the whole circuit with carb cleaner and a pipe cleaner and a fine piece of wire from a multi-strand electric wire for cleaning the pilot jet, most, if not all of my idle problems went away.

breno · Dec 27, 2011

Thanx for all the input guys. Think I'll wait till after the new year and chase up a thicker needle and a spare slide I can play with if needed later on to really dial it in.
in the mean time I cleaned the carby thoroughly and double checked for any leaks (none found) and its starting really well and idles nicely still.

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Mikuni low throttle probs.

breno

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