Setting up Amal carbs

[a101960]

Having spent what seems like forever trying to set up the Amal 389 on my RGS I seem to be making very little progress. As some of you might remember I had the cylinder head rebuilt. I was advised that due to the work that had been carried out that I would need to weaken the carb. Well, initially the carb did indeed seem rich on the standard settings. (420 main jet .106 needle second notch from the top, and No 3 slide) this set gave me black sooty plugs. So, I fitted a 3 1/2 slide and the engine would not pull. On inspection the plugs were white verifying that it was indeed running week. I then went back to the factory specified No3 slide and the engine was much more lively. The problem however is that although the bike seems to run well and the richness for what ever reason seems to have been eliminated, the plugs are still not that ideal coffee colour. The problem is of course that rich running will cause bore wash, and lean running will melt or hole the pistons. The snag is that because the engine has been rebuilt and has done only 600 miles I am very wary of reving it to hard. The bike starts easily and reliably and although it accelerates O.K. it feels as though it is holding back and not reving as freely as it should. and of course I am not sure if this is caburration or the engine still being tight. At the moment I am not keen to use the bike to much in case i damage the engine due the carb problems and the effect that might have on the life of the engine. One other thing there is an occasional spit back through the carb. Not persistent or regular, but it happens sometimes when opening the throttle from low revs. Does anyone have any ideas about what might be the problem?

[Brian]

Spitting back through the carby is usually a indication of leaness [weakness], you could try lifting your needle one notch. The plug colour is very dependant on what fuel you are using. If you are using a modern unleaded fuel it is very difficult to "read" the plugs. With unleaded you wont get the tan or coffee colour. I have found the plugs are usually very dark around the outer and very light, almost white on the electrode. I use a 98 octane unleaded fuel that is available from the bowser here in Australia. If your carby is set up as per specification for your engine you may have to do some fine tuning but overall you can ride the bike without fear of a serious problem. Try and get some miles on the motor and work on the tuning as you go. Remember, only change one thing at a time and small changes so you can tell what is happening with each change. Hope it all goes well.    Brian.

[groily]

Agree with Brian.  I'd certainly try the needle on the middle notch, which will richen it a bit from 1/4 to 3/4 throttle. Worth also pulling the needle out and rolling it on a flat surface to make absolutely sure it isn't bent. Also agree re plugs and unleaded - almost impossible to tell these days. Not sure what the pilot air jet should be but you could try screwing it in a tiny bit more, if the spitting happens from really low revs. Lastly, I assume you are confident that the ignition timing is spot on? Too much retard would give symptoms of lack of urgency. . .  However, as you say, a stiff motor newly rebuilt may just need a bit of time . . . In my opinion, it's always better to run a little rich than a little weak - the bores won't be awash in fuel unless it's badly out, and the engine will run cooler. Good luck!