Here's what probably should be a really basic question... do you turn the idle mixture screw in for a richer mix or out? on a '78 xs 400, with the stock mikuni carbs.

thanks

n8

To add to the above, I've been fiddling with the mixture screw for what seems like hours, and it doesn't seem to make much difference how I adjust it. In fact, I can completely remove it and the bike keeps running with a slight increase in RPMs. I can bottom it out and it keeps running about the same as 1 or 2 turns backed out from the bottom. I might add it runs very rich, and can foul the spark plugs in only a couple of miles.

n8

Time for a full overhaul of the fuel and induction system. If fiddling makes little difference then something else is wrong.

To add to the above, I've been fiddling with the mixture screw for what seems like hours, and it doesn't seem to make much difference how I adjust it. In fact, I can completely remove it and the bike keeps running with a slight increase in RPMs. I can bottom it out and it keeps running about the same as 1 or 2 turns backed out from the bottom. I might add it runs very rich, and can foul the spark plugs in only a couple of miles.

n8

A common problem was hamfisted peeps overtightening the mixture screw in, then the fine brass tip snaps off in the carb body prventing any change in the low speed mixture.

my carb had both of these and got an engineering friend to "push" them through. It opened up the hole slightly but I just screw in the new screw more.

Thanks for the info, I'll pull the carbs back off and check. I've never used a screwdriver to tighten them, could it break off by finger tightening? And again, will tightening the mixture screw richen the mix or lean it?

n8

Thanks for the info, I'll pull the carbs back off and check. I've never used a screwdriver to tighten them, could it break off by finger tightening? And again, will tightening the mixture screw richen the mix or lean it?

n8

If youv'e had the bike from brand new and you can say you have never had a screw driver on the pilot, you can eliminate that then.

if you have the carbs off, take out the pilot screw and feel under neath for a little "pin" if its there its snapped off!!

Its an air screw, in for rich out for lean.

I pulled the carbs off and no pin pieces, in fact, I can hold it up to the light and see light thru the hole. I don't know yet if I'm relieved or disappointed. I guess the next question is where does the air come from before the mixture? does it just filter down thru the threads of the mixture screw? or is there a port inside the tube the screw screws into?

thanks, drewpy, you are a fount of knowledge.

n8

I pulled the carbs off and no pin pieces, in fact, I can hold it up to the light and see light thru the hole. I don't know yet if I'm relieved or disappointed.

Ans. Scary or what. That is an air mixture orifice and I'm not certain that light should be there.

I guess the next question is where does the air come from before the mixture? does it just filter down thru the threads of the mixture screw? or is there a port inside the tube the screw screws into?

Ans. There are internal drilled ports used for adjusting air /fuel ratios that are carefully sealed on manufacture. failue of the interigty will cause loss of mixture control.

I can see the light if I take out the mix screw and look down the screw hole to where the pin goes, makes sense to me to see light thru there.

The drilled ports - since the bike sat for a long time (a couple of years), could they be filled with varnish? maybe a good soak in carb cleaner?

I can see the light if I take out the mix screw and look down the screw hole to where the pin goes, makes sense to me to see light thru there.

The drilled ports - since the bike sat for a long time (a couple of years), could they be filled with varnish? maybe a good soak in carb cleaner?

just get them ultrasonically cleaned, you can buy cheap ones, but big enough for your carb and they also clean your dentures!!!

I love it when a solution has multiple uses!!

thanks