Setting up a pre Monobloc carb on a G3 1940 WO

Tommy RE · Post by **Tommy RE** » Tue May 17, 2011 9:33 pm

Whilst my tank was being re painted, I took the opportunity of taking the carb off and giving it a good clean. Since I have owned the bike, it has run mechanically quietly when not under load when fully advanced, but when under load such as climbing a hill there has been the very distinctive 'pinking' sound. When the ignition lever was retarded slightly this 'pinking' ceased.

I have since found from reading the riders instructions of the G3 1940 WO and various chastising's from the readership that in normal operation, I should be running on full advance as a retarded spark (not to dissimilar the author) is not good in the long term for the health of the engine. Since I got the tank back I have re set the carb as per vague directions in the rider instructions, but it is not very precise with the settings for the pilot jet, it does not state whether winding the pilot jet air screw in weakens the mixture, or vice versa

Then engine accelerates smoothly and without misfire when the throttle is rolled on, but hesitates and dies when opened suddenly. It takes quite a few prods to start from cold, but starts easily when hot.

Overall, the engine runs and pulls well throughout the rev range, but the exhaust is quite black as is the plug. any advice on the final tweaking to get optimum carburation correct would be gratefully received.

itma · Post by **itma** » Tue May 17, 2011 9:38 pm

you will never get it to run well untill the ignition advance is correct.
set it to 7/16 or even 3/8 on full advance, THEN adjust the carb.

1608 · Post by **1608** » Tue May 17, 2011 9:51 pm

Even when the ign is set correctly you may find the bike 'pinks' or 'knocks' when under load such as hill climbing. This is quite normal and you just need to retard the ign manually a little during this period. Both too retarded or advanced are bad for the engine.

SPRIDDLER · Post by **SPRIDDLER** » Tue May 17, 2011 10:26 pm

quote:
for the pilot jet, it does not state whether winding the pilot jet air screw in weakens the mixture, or vice versa

Tommy, the pilot adjustment screw is actually regulating the air through a range from throttle closed to 1/4 open.
'Unwinding' it (anticlock) allows more air thus weakening the mixture, screwing it in therefore richens the mixture.

The principles of tuning are on the link below (it's for a monobloc but the procedure is the same for the pre-monobloc):

http://www.hitchcocksmotorcycles.com/mo ... to-tune-up

But time it accurately first (with A/R lever set to full advance) as already mentioned.

en7jos · Post by **en7jos** » Wed May 18, 2011 2:49 am

There is a separate guide for pre-monoblock carb's on the Hitchcocks website too:

http://hitchcocks.sincordia.co.uk/premonobloc-sheet1

Tommy RE · Post by **Tommy RE** » Wed May 18, 2011 10:05 am

quote:
you will never get it to run well untill the ignition advance is correct.
set it to 7/16 or even 3/8 on full advance, THEN adjust the carb.

I have set the ignition as follows:

Advance ignition lever fully
Remove spark plug
Remove Magneto drive cover
Remove Magneto bottom drive sprocket
Insert piece of graseproof paper between points
Piston at T.D.C
Both valves closed and checking that both pushrods can be rotated
Insert rod and mark with permanent marker at suitable datum point
Remove rod measure 7/16 inch further up rod from original mark and mark second point as previous
Re-insert rod and rotate engine backwards until second mark aligns with pre determined datum
Rotate Magneto anti clockwise until greaseproof paper can be just felt to move between points
Re attatch Magneto bottom sprocket without moving drive chain & tighten
Re check to ensure correct adjustment

Is this correct Ken?Edited by - Tommy RE on 18 May 2011 09:05:47 AM

Tommy RE · Post by **Tommy RE** » Wed May 18, 2011 10:14 am

Thank you Itma, Spriddler & en7jos,

I shall re check ignition timing & read the information on the threads provided & report back.

zwarts · Post by **zwarts** » Wed May 18, 2011 11:49 am

Neil, your proceedure for setting the ign. timing if fine except for one minor detail. Before doing anything else, check that the contact breaker points gap is set at the correct 0.010" to 0.012".
Bob.

1608 · Post by **1608** » Wed May 18, 2011 12:08 pm

Being pedantic, one should also turn engine back beyond the 7/16" and then forward to the 7/16" mark this allows for backlash in the timing gear.

Tommy RE · Post by **Tommy RE** » Fri May 20, 2011 11:13 am

Thank you Gentlemen for all your assistance in tuning up the old girl, it worked a treat and I can start her on the first kick