1961 G12 jetting mystery

bjork · Post by **bjork** » Wed Sep 14, 2011 8:11 pm

Hi tpeever, and welcome to the site. ref your last post, you are right about the altitude, on both counts. Yes, it will get richer with less air pressure, and yes, the sort of altitudes mentioned shouldn't be a problem either. As for more helpful ideas, sorry I can't think of any more than has already been suggested.
1608's mention of the idle circuit (are you really sure it's clean?) and fuel level would seem to be more important to get right. Both will have an effect all the way up!
Also, don't be misled about thin gaskets, the faces should be plenty good enough for them and a smear of sealant if you like. Another point to check ref air leaks at the manifold-if the heads are bolted down without aligning the inlet manifold faces then air leaks can easily happen. A thick gasket on the carb joint can also lead to a warped carb flange and body-more potential for mixture problems.
Good luck with it and make sure to tell us the answer!

greasemonkey62 · Post by **greasemonkey62** » Fri Sep 16, 2011 4:38 pm

I have a model 31 1962 AJS and it was also running too weak a mixture, I have a 410 (or is it 420 ?) main jet fitted now and all is fine now.

tpeever · Post by **tpeever** » Fri Sep 16, 2011 10:39 pm

quote:
I have a model 31 1962 AJS and it was also running too weak a mixture, I have a 410 (or is it 420 ?) main jet fitted now and all is fine now.

Very interesting!! I am running a 420 in mine now and it has definitely improved dramatically. I am thinking it might even require a larger main jet. What needle position are you running at?

1608 · Post by **1608** » Sat Sep 17, 2011 1:00 am

Tpeever, what do you mean by needle on position 5, I thought there were only 5 positions possible, so 5 would be the highest position !! It sounds to me like you are trying to compensate for a lack of fuel or an air leak somewhere. My 1960 G12 is set up as per the book,ie. needle on the centre notch and runs fine. The larger main jet should,t affect the first three quarters of throttle opening.I have submitted a simple technique for checking the float / fuel level in the Tech Articles under carb. might be worth a try.

tpeever · Post by **tpeever** » Sat Sep 17, 2011 7:29 am

quote:
Tpeever, what do you mean by needle on position 5, I thought there were only 5 positions possible, so 5 would be the highest position !! It sounds to me like you are trying to compensate for a lack of fuel or an air leak somewhere. My 1960 G12 is set up as per the book,ie. needle on the centre notch and runs fine. The larger main jet should,t affect the first three quarters of throttle opening.I have submitted a simple technique for checking the float / fuel level in the Tech Articles under carb. might be worth a try.

Yep, highest position. I have been road testing at all positions starting and the center notch.

Will check for air leaks and check the fuel level next.

tpeever · Post by **tpeever** » Sat Sep 17, 2011 9:23 pm

Checked float level and if seems perfect. Fuel is supposed to be up to the pip on the cover correct? That's exactly where it sits.

Re-assembled intake manifold using some high temp RTV between manifold and head.

Time for road test!

Wavy · Post by **Wavy** » Sun Sep 18, 2011 9:43 am

Hi! You say the carb has been sleaved but sometimes the carb is bored out and the slide is sleaved and turned down to the new bore size! In which case the bore will be slightly larger than the original and affect the mixture leaning on the weak side. Could be the case!

tpeever · Post by **tpeever** » Mon Sep 19, 2011 5:06 am

quote:
Hi! You say the carb has been sleaved but sometimes the carb is bored out and the slide is sleaved and turned down to the new bore size! In which case the bore will be slightly larger than the original and affect the mixture leaning on the weak side. Could be the case!

Excellent point!! Hadn't thought about that one but it is a real possibility. I checked float height and it is perfect. I sealed all joints against air leaks. I am pretty sure pilot circuit is clear. So I am still left with why I need to have needle in 5th position and such a large main jet for decent performance. I did a road test yesterday and bike ran very well. So maybe I just have to accept the fact the fact that my bike will not be happy with factor settings for whatever reason!

Thanks to everyone for all their help!!

Tobin

greasemonkey62 · Post by **greasemonkey62** » Mon Sep 19, 2011 9:07 pm

quote:
quote:
I have a model 31 1962 AJS and it was also running too weak a mixture, I have a 410 (or is it 420 ?) main jet fitted now and all is fine now.

Very interesting!! I am running a 420 in mine now and it has definitely improved dramatically. I am thinking it might even require a larger main jet. What needle position are you running at?

the needle position is standard, as recomended in the manual, I dont remember what that was without looking it up again, I did try it fully raised but it needed a larger main jet it runs just fine now.

petert120r · Post by **petert120r** » Tue Sep 20, 2011 4:36 pm

Further thought on this subject.

There should be a 1/16" heat insulation spacer between the carb and manifold. Its absence could cause partial fuel evaporation when hot which might explain the symptoms of lean running.

Paper gasket between spacer and manifold, just the carb "O" ring between carb and spacer.