Misc: Carburettors 389 Monoblock questions

[vibrac]

I am coming back to Monoblocks after a long time away with TT and GP carbs
I have 4 or 5 monoblock carbs stamped 389 followed by another number. 2 of them have good bodies and are 1-1/8" which is what I am looking for, however they have no jet blocks, the other 389's are either bigger or smaller bores and of course they have jet blocks. do all 389 have the same jet block? my suspicion is they do since the slides seem to interchange.
Any specific instructions for removal of jet blocks? I seem to remember hot water came into the procedure.
new blocks are not available new jet block base gaskets are
There is also talk of 'stay up' floats are these the floats designed to work on the so called modern petrol?

 

[oexing]

Did you look up the Amal pages ? You could have new carbs, so no hassles about worn bodies and all. At least these are half the money than the pre-monobloc types.

Vic

Monobloc

 

[vibrac]

My castings are unmarked I think they are NOS certainly I think I should try before forking out £500+ I want Monoblocks because they work well on Vincents if I have to buy new its more likely to be Mikuni

 

[fogrider]

I found that passing a suitable size rod through the upper holes in the jet block is essential, so the solid part at the bottom of the main choke can be used to knock it down. Whatever you do, don't knock on the top, it will bend the sides.
I have made a puller in the past to use a slide hammer in the main jet holder threads. It works on some, but I gave in before wrecking threads.
Some hot water won't go amiss. Some are very tight, some slack. Steady as she goes is the way.

 

[roy the mechanic]

The stay up floats are intended to work with modern fuels. Also there is a modern float valve which works well. Don't forget the little spacer on the float spindle on assembly.

 

[LoneStar]

The bore being present through the jet block, they do differ with the carb's bore size:

 

[vibrac]

The bore being present through the jet block, they do differ with the carb's bore size:

Of course they do ( ....: Doh!) that's a show stopper I am not about to bore any out and they dont sell jet blocks (Now I know why)

 

[highbury731]

I am coming back to Monoblocks after a long time away with TT and GP carbs
I have 4 or 5 monoblock carbs stamped 389 followed by another number. 2 of them have good bodies and are 1-1/8" which is what I am looking for, however they have no jet blocks, the other 389's are either bigger or smaller bores and of course they have jet blocks. do all 389 have the same jet block? my suspicion is they do since the slides seem to interchange.
Any specific instructions for removal of jet blocks? I seem to remember hot water came into the procedure.
new blocks are not available new jet block base gaskets are
There is also talk of 'stay up' floats are these the floats designed to work on the so called modern petrol?

When I want to remove a Monobloc jet block, I choose a very tatty jet holder. Screw that most of the way in for good thread engagement, and use it to tap the jet block out.

 

[royrobertson]

Hi Tim, Why not bore them out. It's not difficult. Make yourself a dummy mounting that will hold the whole carb in the lathe on the screw holes and just very gently pass a boring bar through until the bores match. You can even bore out a 389 to 1 3/16inch this way. just clean it very carefully afterward and make sure the tiny bleed hole is clear. I have done quite a few over the years this way.
Roy

 

[royrobertson]

Hi again Tim. Don't forget to remove the protruding brass spray tube assembly before boring and secure the jet block.
Cheers
Roy