Thunderbird Mikuni Pillot Screw Air Or Fuel

Is the idle screw on the mikuni carbs a fuel screw or air screw? Its on the Fuel side of the carb, which would indicate it controls fuel but I'm not sure.

Does winding in lean the mixture? And do you have any recommended setting, Haynes manual says to start with one half turns out but is popping and spluting like a good un.

Cheers

 

It’s been a long time since I adjusted a carburettor but from what I remember the IDLE screw only sets the height of the slide to control the idle speed. Hopefully @darkman will be along soon, he has the knowledge on carbs as he rebuilds and sets up lots of carb fed British bikes.

 

Thanks for the reply

 

Actually now I’ve read your post again it looks like you’re referring to the pilot screw which adjust the amount of air going into the engine at low revs. Each bike will be set slightly different, something like one and a quarter turns from fully in, but you’ll need the proper setting to get the bike to run right.

 

Lol, sorry on this one again as i have no experience of Mikuni carbs

 

No worries. I'll sus it out by experiementing it should be obvious once I start to wind in and out.

 

Terry Smith
You really need to know what is in the carbs as jetting and cut of settings. But usually playing is a fair way to see if it is better or not.
Joe.

 

Hi Terry, the one on the fuel side is indeed the mixture screw, if you screw in the mixture screw all the way. (do not tighten the screw as the seat for the screw can be easily damaged), then open the screw 1-1/4 turns, this approximates where the screws are placed when the carb is delivered from the factory. Follow the following sequence
1 Warm the engine to a normal operating temperature.
2 Open the air adjustment screw slowly, turning it counter-clockwise, until there is a rise in RPMs
3 Continue opening the screw slowly until the RPMs peak, level off and then start to drop.
4 Once the RPMs begin to drop, turn the screw the opposite way until you again hit peak RPMs.
5 With the RPMs at peak, enrichen the mixture by turning the screw approximately 1/4 turn.
7 Turn the screw clockwise to enrichen or turn the screw counter- clockwise to enrichen

 

This brings back memories of the Mikuni 33mm smooth bores I had on the Harris Magnum I built in '85. Playing around with main jets, jet needles profiles, needle heights etc. What a faff! The pilot air screws on Mikinis control the idle mixture, screw in for rich, out for lean.

 

I agree, carbs are fine when running right but they can be a right pain when they get worn

 

I agree, carbs are fine when running right but they can be a right pain when they get worn over time

 

Just be grateful you not running Amals

 

All done. Thanks for the tips. Mixture screw set at 2 1/4 to 2 1/2 turns out seems to be about right.