G12 1959 Carb jets

rockinron1941 · Post by **rockinron1941** » Thu Feb 24, 2011 8:50 am

Hi Guys,I have a 1959 G12 Standard first regesterd 4th March 1959 Engine no 59/G12 08643 if I look in the Instuction Book with red cover down loaded from the club site it says that the main jet should be 390 with the pilot jet 20,but when I look at the Work Shop Institution Manuel for 1959 model it says main jet 400 pilot jet 30 ,my bike has a 30 pilot jet in is this right? I know they chaged the jeting for the 1960 Std ,how will the fitting of larger jets (30 insted of 20) effected the running of the bike,the bike seen to be running a bit rich but that might be just the setup.Over to you oh wise ones.Regards Ron

petert120r · Post by **petert120r** » Thu Feb 24, 2011 11:01 am

My workshop manual says 400 main and 30 pilot for '59 650's, as does the F Neill bible. A smaller main would cause it to run slightly leaner at full throttle but the difference would be so marginal as to be unnoticeable. A smaller pilot would make it leaner at idle but have no effect at normal road speeds.

paul knapp · Post by **paul knapp** » Thu Feb 24, 2011 11:09 am

Amal specify for a '59 G12, 1-1/8" bore, a 400 main, 30 pilot, No. 3 slide cutaway, needle 3rd notch from the top!
If you think it's running rich with the standard settings above:

Air filter?
Check that the fuel level in the bowl isn't too high
Make sure the air valve (choke slide) is fully retracting up in the throttle valve (air slide).

Better to run a tad rich than lean.

Rob Harknett · Post by **Rob Harknett** » Thu Feb 24, 2011 11:15 am

If everything was as new, carb engine etc, you may still need a supply of 1959 petrol for it to run correctly. Petrol s/gravity will not give correct level in the float chamber. Don't let perfection stop you enjoying the ride an old bike will give.

MikeM. · Post by **MikeM.** » Thu Feb 24, 2011 11:21 am

My '59 Model 31 (same engine) has a 400 man jet and a 30 pilot, I did try a 25 pilot but it didn't like it.

Merlin · Post by **Merlin** » Thu Feb 24, 2011 12:11 pm

correct level in the float chamber.

I don't understand the reason for this statement surely the volume will be the same but of course the weight will be different for different Specific gravities.Discuss.

rockinron1941 · Post by **rockinron1941** » Thu Feb 24, 2011 3:58 pm

Thanks once again for your comments,I have it in the back of my mind that when a bike was being run in that the petrol mix was set a bit rich ,anybody recall this or is the mind going? Regard Ron

rockinron1941 · Post by **rockinron1941** » Sun Mar 27, 2011 6:53 pm

Fitted all new jets to the carb bike running well thanks for the advise.Ron

Rob Harknett · Post by **Rob Harknett** » Sun Mar 27, 2011 8:55 pm

correct level in the float chamber??
does not the float regulate flow of petrol into the chamber? the specific gravity will control the float level which will control the petrol flow. Result the level in the float chamber will vary. Too high = flooding, too low = stavation.
Or have I got that totaly wrong.

1608 · Post by **1608** » Mon Mar 28, 2011 11:34 pm

I don't think the specific gravity alters that much. However, I posed a similar question a while back and nobody responded, viz 'does a brass float needle hold the float lower in the chamber than a nylon one because its heavier, therfore richening the mixture'.