KH500 Carburettor Tuning

[Les Carter]

I've been trying to get my newly restored 1951 KH500 to run properly on the new, correct spec Amal 276 I fitted. With all the book settings it was rich at low throttle openings, sooting the plugs up but lean at mid throttle causing overheating and nipping up. Cured the low throttle probs by fitting a No 4 slide but mid and wot still lean. Could it be that the new exhausts are straight through requiring richer jetting? I have done compression tests on a hot engine with wot and get 10 bar on each cyl, seems a bit high since the new club pistons fitted were 7.5 :1 I think. I have richer main jets and needle jets but not sure which way to go. Any ideas will be welcome, thanks.

[Roger Gwynn]

150 psi is high, indicates a good engine. Compression is affected by valve timing as well as compression ratio. Restriction of silencers will affect the mixture but not by a great deal and not enough to cause problems on an old style engine in a low state of tune. Stick with the standard main and needle jets. the main only affects large throttle openings, unless of course it is way too small. The needle jet is only changed when the needle has been moved to one extreme or another. It is the needle I would move. Usually a new carb with standard settings will work, have you checked the ignition timing on both cylinders assuming that you are using a mag there will be a difference between the cylinders. If it is a large amount, say more than 1/16" you will experience lumpy running etc, usual remedy is to split the difference between the cylinders or fit electronic.