Monobloc 389 B 32 on an Gold flash iron head

[Guy Wilson]

I've been reading through some old posts re carb and jetting etc.. My SA '55 Gold Flash has been my regular bike for the last year or so. Its always run rich, although I use it mainly on short runs so I was sort okay with that and relaxed about cleaning plugs on a regular basis.  It was dismantled when it came to me originally and had a 389 B 32 carb (made in Spain) in the box which has a larger bore than the iron head inlet.. Its always run well with this carb, ticks over and is easy to start etc. Fuel economy is questionable although I haven't done a really solid test yet..
Question: is it always going to run rich with this size of slide and bore? Should I look for a 376 which I think is what it would have had on originally?

[duTch]

 I have a 389 (forget which size) into a BV Ally head, and it runs fine now I think I've sorted the jetting- had to go to a 105 needle jet,  otherwise stock stuff....except a custom inlet tract.
 I believe it's not good to have a step in the transition, maybe you could fabricate a spacer to match each side (Carb/Inlet)

[RDfella]

Larger bore carb into a smaller bore manifold is never good. Other way around can be acceptable up to a limit. Larger into smaller will gave flat spots which would have to be corrected by an over-rich mixture. Maybe that's the problem.

[Triton Thrasher]

The first thing to suspect is a worn needle jet.  It’s not expensive: buy a new one.

Richness (and weakness) is remedied by following the carburettor tuning procedure.  It’s in the service sheets and most manuals.

It’s fair to say that an oversized carburettor is harder to tune.

[Guy Wilson]

thank you, the needle jet sounds like the next thing to consider or I find a carb that matches the inlet. I have a concentric that would match. I had a Mukuni on my Triumph before I broke it - the Triumph that is.. I should have mentioned i'm at 6200 ft above sea level (1800m) I currently have the needle on the lowest setting (the top notch) and I have jetted teh main jet down to 180 today, so I'll see how that translates this next week or so.
Thank you again, all good advice and things to try,
Guy

[Triton Thrasher]

Reducing the main jet is correct if it’s rich at full throttle.  Have you been riding at full throttle?

[ironhead]

thank you, the needle jet sounds like the next thing to consider or I find a carb that matches the inlet. I have a concentric that would match. I had a Mukuni on my Triumph before I broke it - the Triumph that is.. I should have mentioned i'm at 6200 ft above sea level (1800m) I currently have the needle on the lowest setting (the top notch) and I have jetted teh main jet down to 180 today, so I'll see how that translates this next week or so.
Thank you again, all good advice and things to try,
Guy

At 6200ft above sea level the std jetting will definitely need to be leaned off. I would think a 105 needle jet would be a must. Pilot jet also as well as the main.

[Guy Wilson]

I've not been on a long run for a while so I think a lot of the running is the short run use... I'll look around around for a 105 and carry it with and swap it out the return of the next long trip I do. I'll probably be dropping 2000ft in altitudes so no guarantee I'll get it right..whichever way, always a good excuse for a ride!
Many thanks
Guy

[atkin]

 The needle holder when worn can cause it run rich try lower the adjustable needle to begin .

[RDfella]

I think we're in danger here of trying to correct a mis-fitting issue. The first post tells us the carb is too big for the inlet tract. trying to correct that by changing jets is a fool's errand - it may be better, but it'll never be right.

[chaterlea25]

Hi All,
Guy the Spainish 389 is quite possibly a 2 stroke carb ? they were on some Bultaco's and such
the 2 stroke needle jet does not have a cross drilling and will have something like 106T stamped on it
the spray tube that sticks up into the inlet tract is also different on a 2 stroke

John

[Guy Wilson]

RD and Chaterlea, thank you. The bike originated ( I think )  in Southern Rhodesia, now Zimbabwe and during UDI all kinds of 'needs must' to keep things running existed and its very possible the Spanish Amal is off a two-stroke.. I'm intending to do a two or three hundred mile run in the next month or so (work permitting ) and see what the plugs say and the MPG say. In the mean time, its just running around town - never the best for getting up to temperature or giving accurate indications of mixture settings.. Above all I enjoy riding it and thats the driving force!
All the best
Guy

[Guy Wilson]

A friend of mine gave me a Monobloc 376 / 5 which is probably better suited to my gold flash and most likely off a B33 500.. it doesn't seem to have an idle or tick over screw though. The hole is there but appears not to be drilled...is this common/normal ? should the tick over be adjusted on the cable only..
Thank you,
Guy

[muskrat]

G'day Guy.
That carb was for a 350cc AJS ( https://tinyurl.com/y33ftcvs ). the screw should be there as it's not a great idea to use the cable adjustment to get the right tickover especially on a road machine. It may have come off a racer as they usually aren't set to idle (more engine braking into corners).
Cheers

[Guy Wilson]

Thanks Muskrat...I'll have a closer look at the idle screw hole..