Pilot jet on a 376 Monobloc

I would say its worth an experiment.   It may be that the new carb is being manufactured to closer tollerances  which is creating more "pull"  than  before, making the old settings less relevant.

Being a new carb it is possible that dust or even some swaff are blocking the jets and/or passageways. Take out all the external screws and jets and blow them through either with compressed air or something like WD 40.  I have not used a 376 for years but would have thought that 2 turns out was close to correct.

Attachments might be of some help.

Concentric is best between 1 and 2 turns with a default starting point of 1.5 turns. The tuning guide for the Mono is silent on the default starting position for the air screw so 2 turns could be the right figure or not.

I used an old ColourTune to set pilot screw.  It's quite a good application because you want it just nearly at a rich position so as not to bog down when opening the throttle quickly.

Bringing the pilot jet down a size is what I would do.

Hi Tony,

Quite often,  hunting at tickover can be due to varying ignition advance - can be quite common with mechanical advance/retard units. Ignore this if you have electronic ignition.

Also, I seem to remember you mentioning that you have a triumph as well? If so, and it starts and idles properly, have you tried fitting the/one of the carbs from that and see how it goes?

Regards, George 

Thats disapointing,   Have you checked the fuel level?  , might be worth dropping it a mm , If the needle valve body sits on a fibre washer ,try leaving it out.

Make 2  , I could use one myself !!

That would be  very nice,and in keeping with the helpfull attitude we are trying to maintain in this great  club.  My workshop bench  at the moment is half the top of a washing machine (if the wife allows) ,   I think that new monos  now come with that fibre washer under the seating that raises the fuel level, never needed one before so something has changed ?, try without.  

No need for a perspex cover, drill and tap a spare bottom main jet cover for a threaded pipe and fit a plastic tube onto the pipe. Open the fuel tap and with the plastic pipe held vertical against the float chamber cover compare the fuel level to the cast pip on the float chamber cover. You want as close to the pip as possible. Burlen now make shims to adjust the float needle housing for fuel height adjustment.

Remember the perspex cover doesn't need to be pretty. Mine is only roughly square and well oversize. Only the three holes need to be reasonably accurate, and it's easy to mark them out through the clear plastic, and easy to drill if the cover is oversize.  Took a couple of minutes to make (but of course you need the perspex...mine was from a garden shed window I think).

I checked the Jampot Archive on Monobloc tuning.  The data can be found here:

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

If you scroll down, you will find that the Monobloc carburetor fitted to Model 77 and 99 Nortons was the 376/67

1 1/16" bore, 25 pilot jet, 3 cut-away, 250 main jet, .106 needle jet, needle on 3rd clip

The standard starting position for the pilot screw on Monoblocs is 1 1/4 turns out. In most cases you do not need to go beyond the range 1 turn out to 1.5 turns out. The pilot jet on Amal carburetors (except TT) meters fuel, so if you have to wind your pilot needle out 2 turns, it is too far out and still not allowing enough fuel through.  That suggests you need a LARGER pilot jet, not a smaller one.  If 25 is too lean, try a 30.

Should you decide that the Monobloc body on your bike is too worn for further service, just find a body of the right bore size.  Service it, and fit jets and other minor components to the spec you need for your bike.  The /?? number just denotes the build spec and does not matter at all (unless you are building a garage queen which needs to look perfect but does not need to work).

Paul

Hi Paul,  you might want to think again about your paragraph on pilot jet adjustment. It would be correct if you were talking about a Dellorto carb ,some of which work the opposite to an Amal.

I think that is very important to make parts new buy new needle and Needle Jet ,(needlo jet is durig ears oval)

pilot jet

.trottle must be without clearence in all positions

on my monoblocks I have 2 on 650ss I must complete dismaint all and take 3 times to ultrasound

after that runs much better but without little slips i think will never be. its old think and old conception.

i found big clearence on trotle screvs i make new. still some fight with this carburettors :-)

good luck to you

I have been following the above discussion with great interest and have learned a lot so thanks everyone.

But one small comment Tony have you checked for air leaks around the carb manifold and heat isolation piece, also when fitting my new monoblock to my Navigator I was a bit concerned that the cap on the top of the carb (cable entry point) had a good seal as I think if it had a poor fitting it could give tick over issues.

Steven