Villiers S25 Pilot Jet Initial Setting on 8e

Hi jaygee

None of the Villiers information that I have gives a nominal setting for the pilot air adjusting screw for the S25 or any other Villiers carburettor. The Villiers 4T manual states that the pilot air adjusting screw setting should be within ½ turn of the factory setting which is not a lot of help.

As you state that you need to improve the running are you sure that it is the pilot mixture that is a problem. The pilot mixture only affects tick-over and the transition onto the throttle cut away. You only use this when you are pulling away or at very small throttle openings. When you are riding the bike it will mainly be operating on the needle jet. The factory setting for the throttle needle is 1.95" out from the bottom of the throttle slide but if the needle jet is worn it may require dropping down a bit to weaken the mixture. If the mixture is rich try going to 2.00" then 2.05" which will probably be about the limit because the throttle needle will bottom. If its still rich you would need to replace the centrepiece which contains the needle jet and the throttle needle.

If the problem is tick-over please remember that two stokes do not tick-over as reliably as four strokes. Also any engine wear has more effect at tick-over. If there are crankcase leaks due to worn crank seals or the compression is poor due to worn piston rings or worn cylinder bore it will have a serious effect on starting and tick-over.

If the engine is in good condition and your problem is just poor tick-over. Get the engine hot then have a play with the pilot air adjusting screw. Start by screwing it in a ¼ turn turn at a time and see what effect it has. Give the engine a short run at a higher engine speed between adjustments to clear excess fuel in the crankcase. If screwing in the adjuster makes the tick-over worse try screwing it out ¼ turn at a time. With a two stoke you try to keep the tick-over mixture slightly weak.

The other issue with most Villiers carburettors is that they do not have a throttle stop screw so the tick-over speed is adjusted on the throttle cable. The throttle cable needs to be carefully routed or you will have problems with the tick-over speed varying when you turn the handlebars. A few S25 carburettors have throttle stop screws which allows you to have a small amount of free play in the throttle cable, but I believe that these were originally made for use in micro cars and they are prized by the Trials Boys.

Best of luck.

Regards
Butty Bach