Correct Jetting

Just built a polini 130 et3 engine, race crank, 25 Carb and all associated upgrades. My questions are :-

25mil carb came with a 90 main jet, will this be ojk for running in or should I go bigger.

What size of idle jet should I run it on.

There appears to be a central jet adjacent to the main jet/needle housing with a no. 55 on it. Am I correct in thinking this jet is fine unaltered

& finally- Polini suggest 16 BTDC for timing- does anyone think different as they seem to on

thanx [:roll:]

Oh confusion- you say 18 degrees

Sip and polini recommend 16 degree which works out to 1.3mm BTDC.

& then to top it all "Norrie kerr " recommends 2mmBTDC, thanx for the jetting advice though.

Was the 2mm BTDC from Norrie Kerrs book?

This was written when you could still get good old leaded petrol with a higher octane than unleaded so you could run more advanced ignition timing.

To be on the safe side start with the lower figure and see how it goes, you can always advance it later.


hello again, thought id put in my tuppence worth in here. your main will be something like 90 but id go up a couple. really normal jetting practice applies. easy check, put choke on once at working temp, at top end of revs (whilst riding, no good stationary!), if pulls better with choke out go up 2 sizes and try again. this is all supposing your prepared to run your engine under full load without running in?! the jet behind main is pilot and should be fine at 55, atomiser up under the main should be around 262/4/6 and for starters put the needle clip nearest the point. check plug colour VERY regular till run in and happy. run an 7 plug and timing at 18 and adjust to suit. there, thatll keep you bizzy tomorrow. happy jetting.

Very well perceived mate, it was a Mr Kerr quote and I suspected it would be out of date. Someone has just sent me a complete milimeters to degrees conversion and its actually 1.3mm which equals 16 BTDC (1.243) to be precise.

I am now a wiser man

god bless t’internet.

Mike (Liverpool sc)