376 Monoblock - Jet Block

I have recently had to pass a tap through the jet block to allow the fitting of a jet holder from an otherwise matching carb. I think the thread tollerance has been changed  or something similar. the jet blocks can be a tight fit in the body due to distorsion of the body from overtight  fixings and the effect of heat cyckling over the years. I will check what tap i used , i think it was BSF.

Paul

  I recently bought a ES2 with a 376/3 monoblock fitted. It is in poor condition and the bike would only run (for a while) on full choke.    I contacted Burlen Fuel Systems (01722 412500) and spoke to their Technical guy who was very helpful. He told me my carb was designed for an Ariel350 and I should be using an Amal 376/17 for my ES2.   It might be worth giving them a ring?

I think the tap I used  was  7/16 BSF  Bottoming  type as its a blind hole in the block . Both carbs would strip and assemble easily  but  parts were not interchangeable between carbs. Could not see anything wrong with threads but they became interchangeable after a pass with the tap. Very puzzling.

... a jet holder and it's 19 tpi and 0.435" diameter which I can't find in my Machinery thread book. 19 tpi is usually a pipe or gas thread count but the diameter doesn't correspond to any of those.

Hi Ian, I Have another beat up mono which I have knocked the jet block out of . After this bit of abuse the jet holder was reluctant to re- enter the block   A pass thro the jet block with the 7/16 BSF  2nd tap  (i don't appear to have a bottom tap )  and the jet holder fits perfectly by hand. It seems that the action of driving a tightish block out of the body is enough to distort the thread a tad.

.... I've tried the 18tpi thread gauge (7/16 BSF) and it "almost" fits whereas the 19 tpi one is much closer. Of course my jet holder is well past its youth so may have distorted over the years. If the BSF tap works then fine....

Note I said jet block in my previous post where I meant jet holder - now corrected. Female threads are very tricky to measure accurately.

Like you Ian I am a bit mystified  by the threads  which do appear to run off the gauge slightly.. I'm sure i looked at bsp taps  but they did not work either.  Perhaps a real expert will step up soon !!.

I notice that  BSTP  (taper) threads around that size are 19 tpi  ,  with the same thread form , but 7/16 seems to be a non std size ??. Will we ever know ?.  Does it matter  anyway !!

... as Paul says the 7/16 BSF does the job then that's good enough for me.

I looked at the Monobloc data pages thoughtfully provided on Jampot Archives:

http://archives.jampot.dk/technical/Carburation/Amal_Monobloc__numbers_type_and_use.pdf

It lists the 376/3 as being 1" bore, for a BSA B31, whereas the ES2 body is stamped 376/17 and has a 1 1/16" carburetor.  As long as the body you are looking at is the right bore size, and is not excessively worn, it will do.  The /?? number is just a build specification.  Change one small detail, and it got a different build number at Amal.  Make sure the settings are correct for the bike you intend to fit the carb to, and it will be fine - unless your objective is points in a concours competition.  Then you will need the 'correct' carburetor body.

If the bike will not run unless the choke is on, it's probably dirty.  Strip it and give it a good clean.  There may be obstructions in obscure places.  One of those is a compensator hole aft of the spray tube.  I clean that with a strand from a bicycle control cable, and a magnifying glass so I can see what I am doing.  Same goes for the cross drillings in the pilot jet.