Amal Pre-Monobloc Fuel Level Adjustment Options

[Iron Head]

Apologies if this is an old chestnut that has done the rounds many times before. The 89N/1ED on my '54 G80CS which has the correct 405 NOS body and slide only seems to want to fire the bike up hot or cold when tickled. Similarly I can only get good starting, tickover, pick-up and no spitting-back with the pilot air screw fully home(?) Don't use or need the choke. Valve clearances, timing and points gap all good. Seems to me like the fuel level is too low albeit that, like all Pre-Monobloc's, one small tickle until the body air holes 'weep' then results in a drip until the engine runs.

I understand the options to adjust fuel height are:

Try one of three mixing chamber nut thicknesses (mine I believe is correct). Not sure how these work with not perhaps hitting/obstructing the main jet when everything is screwed up.

Varying fibre washer thicknesses between float bowl and body.

Possibly file a higher groove on the float pin to my brass float whilst avoiding tickler contact at rest. I think worth a go. The nylon float needle with two grooves might give me what I need.

Swing float chamber through the arc that space allows - tried doesn't seem to have made difference. Assume furthest back from the engine gives biggest 'head' of fuel height?

All thoughts welcome as always, thanks.

Iron Head

[peter ranson]

Hi , have you checked the pilot jet drilling in the jet block ? it sounds like this could be partially blocked.

[Iron Head]

Hi Peter, good suggestion and my first port of call, but all as clean as a whistle, checked carefully with my tiny jewellers drill set too just to make sure. I guess I can live with tickling each time and if it runs fine with the air screw fully in then, perhaps, so be it?!

Iron Head

[ajscomboman]

For the same symptoms your pre monobloc is showing I got fed up with mine and swapped it for a monobloc of the same size and all my issues went away and I ended up with clean carburation and an easy starter.

[Iron Head]

Thanks Rob, although I'm doggedly (probably stupidly?) trying to stick with originality, it might be the way to go and, perhaps an early Christmas present idea to myself! Which exact model did you go for?


Iron Head

[ajscomboman]

I tried twice with the pre-monobloc, an original in perfect condition bore wise as it was NOS. New jets etc lapped in needle, brass float you name it. The bugger absolutely refused to remain consistent. Starting was a swine if stalled. A new 376 1" bore monobloc jetted exactly the same and BINGO!

[Iron Head]

Thanks for the information Rob.

Iron Head