Pilot screw?

Hutzpah · Posted: January 5, 2009, 5:07 am Post subject: Pilot screw?

Could someone please explain what the pilot screw is
and what its purpose is.
Its a very small adjusting screw on the side of the carb.

I have had the timing light on my bike and balanced the carbs with a newly
made manometer.
It starting real good when cold and running sweet on the road yet abit hard to start when hot any ideas?

Just starting to get my head into the mechanics of this bike.
Your help would be much appreciated.

Thanks

Hutzpah

yamaman · Posted: January 5, 2009, 7:59 am Post subject: Re: Pilot screw?

Some useful reading here, cheers

www.mikuni.com/fs-tuning_guide.html

Retiredgentleman · Posted: January 5, 2009, 12:51 pm Post subject: Re: Pilot screw?

The pilot screw (also called mixture screw and idle screw) is used to set the air/fuel ratio for engine operation from idle to about 1/4 throttle.

650garage.com has an excellent "Carb Guide", which you should read carefully. To adjust the pilot screw, you use something called the "dead cylinder method". It is explained very well in the carb guide. When you want to run the engine on just 1 cylinder for this adjustment, its important to make sure that the disconnected spark plug lead is grounded to a good ground on the bike. This prevents damage to your ignition sytem.

www.amckayltd.com/carbguide.pdf

www.iwt.com.au/mikunicarb.htm

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

The pilot screws on the original CV carbs are true mixture screws - they regulate fuel flow not air. Opening them more will richen your idle circuit, closing them (screwing them in) will lean it.

The popular aftermarket Mikuni slide carb that many use on these bikes has an air screw - it meters air flow to the pilot circuit. It functions totally opposite from the fuel regulating screw on the CV carbs. Turning it out adds air and leans your idle circuit, turning it in cuts the air supply and richens the idle circuit.

jpowell · Posted: January 5, 2009, 10:44 pm Post subject: Re: Pilot screw?

RG, I atemped this last year, but instead of pulling the wire off the plug, I pulled one wire at a time where the pig tail comes off the points. this way you're preventing one cylinder from firing, but leaving the plug attached so no damage can come. This probably is only possible with the stock points.

Retiredgentleman · Posted: January 6, 2009, 12:49 am Post subject: Re: Pilot screw?

jpowell;
Yes, that would work fine when you have 2 individual coils. Lots of guys don't say what year bike they have when they ask qestions. Single coil dual output ignitions such as 80 to 83 must ground the unused spark plug/lead, so I simply generalize to be safe.

Hutzpah · Posted: January 6, 2009, 3:06 am Post subject: Re: Pilot screw?

"Lots of guys don't say what year bike they have when they ask questions."

Your dead right Retiredgentleman my bikes a 1979 xs650 Special.

A big thanks one and all looks like I've got a fair bit of reading to do.

Thanks

Hutzpah

jayel · Posted: January 6, 2009, 9:48 am Post subject: Re: Pilot screw?

it's real easy guys, over on the left side is a box titled "Current Member Info" click on "My Account" under "My Account Tools" click on "My profile information" scroll down a bit to "My Location" fill in where you're at and what you got....

650skull · Support Staff Joined: Jul 19, 2007 Posts: 1186

I'm not sure that is what Hutzpah meant jayal..............it is a good idea to post what bike or bikes you have with your avatar but that is no indication exactly the make up of the bike.............The information that is recieved on this site has helped and saved people thousands of dollars i am sure, (how many mechanics now days could work on these and get it write without learning themselves and charging for the experiance), that's why it is important to know the the bike and the make up of it. If not then the answers given may be write for the question asked but wrong for the aplication..........As we all know the PO's may have mixed and matched any part of these bikes so saying what year the bike is may not help either..........SKULL

xsleo · Posted: January 7, 2009, 7:00 am Post subject: Re: Pilot screw?

yes mixed and matched parts are often found, giving a year and model of bike is a start. any info is better than no info.

650skull · Support Staff Joined: Jul 19, 2007 Posts: 1186

xsleo! any info is better than no info. Shocked

xsleo · Posted: January 8, 2009, 5:23 am Post subject: Re: Pilot screw?

yes, any info such as year and model is better than no info at all. any info a person can give is helpful to us, so we can better help answer thier question.

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