Spitting back

[beezermacc]

I had a 376 monobloc on my 1959 Golden Flash. The bike ran well for a number of years until about eighteen months ago when it started running rich. I had a look at the obvious things, float sticking etc but decided the carb was generally fairly badly worn anyway so decided to replace it with a 389 as I thought all the later A10's could run on a 389 as there are no apparent differences in specifications elsewhere. I fitted a new 389 from Amal and initially everything seemed fine. The mixture was weaker but the bike ran well - I noticed I had to use much more choke in the morning. Just recently the bike has started spitting back very badly unless I use full choke for the first few miles, then when the choke is opened there is frequent spitting back if the throttle is blipped and poor running at low revs. All the info suggests weak mixture, particularly the throttle slide, standard is 3 1/2 . I stripped the carb and found the slide to be quite loose and I recall noticing this when I fitted the carb when new. I rummaged in the carb spares box and found an old slide which didn't appear to have seen much use and it was a much better fit than the new one supplied with the carb. I fitted the new slide - massive transformation!. Spitting back now a rare occurrence, but may experiment further with a no. 3 slide. I have spoken to Amal about this and they say that slides are hand selected for best fit before being sent out, but they would be keen to have my carb back to have a look at it. Any views?

[chaterlea25]

Hi,
I have found that "new" Amals seem to run very weak?????
these were all 3 types, 276, mono and concentric ???
I drilled out the pilot bush on the concentric and fitted the separate jet instead
The monobloc needed bigger jets than those specified in the "book"
The 276 would not respond  to adjustment?? I'm assuming the pilot holes in the jet block are too small ???
Its sitting in its box on a shelf!!!!

I heard recently that the ethanol  % affects the specific gravity of the fuel and so alters the bouyancy of the float , altering the fuel level 

Is you monobloc the same bore as original?
Any step in the inlet tract can /will alter the airflow over the pilot and needle jet outlets at low throttle openings causing problems like you describe?

HTH
John

[beezermacc]

Monobloc same as original. interesting information. Will have a closer look at inlet tract. Ta

[Triton Thrasher]

I have spoken to Amal about this and they say that slides are hand selected for best fit before being sent out,

Good stuff! I didn't even know they had a sense of humour.

An Amal carburettor that gets rich, over a few thousand miles, probably has a worn needle jet. Cheap and easy to replace.

A carburettor that starts spitting back, after running for a time without spitting back, usually has developed an air leak into the manifold, via the flange mounting.

[beezermacc]

I have spoken to Amal about this and they say that slides are hand selected for best fit before being sent out,

An Amal carburettor that gets rich, over a few thousand miles, probably has a worn needle jet. Cheap and easy to replace.

A carburettor that starts spitting back, after running for a time without spitting back, usually has developed an air leak into the manifold, via the flange mounting.

Definitely no air leak but will put old slide back just in case disturbing carb cured air leak. With old slide back if problem reoccurs then problem is definitely the slide. Good point about the needle jet.