1961 G12 jetting mystery

tpeever · Post by **tpeever** » Tue Sep 13, 2011 11:25 pm

I have been slowly fettling my 61 G12 after a complete rebuild and feel like I am still a ways away from optimum fuel delivery. Some advice and thoughts from those more experienced than I would be greatly appreciated.

I am running a sleeved 1 1/8 Monobloc carb currently set up with a number 4 slide, 20 pilot, 0.106 needle jet, needle position 5, main jet 420. These settings still seem a bit lean as the bike hesitates a bit off idle and the plugs look lean. This has me puzzled as standard settings appear to be 390 main and needle position 4. I have found contradictory information about throttle slide size. Some sources say 3.5, other say 4. The number 4 slide was in the bike when I bought it. When I first tried these original settings, they were far too lean. I have new Armours silencers on the bike which are quite loud so are presumably pretty much straight pipes. At the intake end, I have a K&N pancake filter which is also pretty free-flowing. Would the combination of these silencers and the K&N be much less restrictive than stock and explain the need for such rich settings? Could these symptoms be caused by too a low float level? I have not checked the float level. Has anyone had a similar experience? I can keep testing larger main jets but feel like I may be off track somehow.

I am testing the bike at about 1000 meters elevation and am also running an electronic ignition that was recently strobe-timed so the ignition timing should be correct. Thanks.

Tobin

1608 · Post by **1608** » Wed Sep 14, 2011 12:47 am

The main jet size has nout to do with the hesitation or flat spot on acceleration from standstill. I have always thought that was symptomatic of blocked idle jet or perhaps low fuel level causing weak mixture.You say the carb has been sleeved, has the jet block been replaced with the washer.
Rob, whats an air intake tube.Edited by - 1608 on 13 Sep 2011 11:52:38 PMEdited by - 1608 on 13 Sep 2011 11:58:22 PM

tpeever · Post by **tpeever** » Wed Sep 14, 2011 6:51 am

Thanks Rob. Yes I have a 389/50 Monobloc carb I originally tried putting the needle in position 4 and it had a distinct hesitation off idle. Position 5 was better and the 420 main jet was better than the 390 whihc makes me think I am still too lean. By "air intake tube" do you mean a velocity stack? If so, what size was generally fitted on these bikes?

Rob Harknett · Post by **Rob Harknett** » Wed Sep 14, 2011 12:14 pm

I will have to check this with the Amal parts list. I guess you know 1960/61 did differ to 59 and 62. For 60/61 under special details it lists. air intake tube pt. no. 389/089
ferrule 6/132A. Parts list 102/2 which I do not have. This is the bit you screw on the air intake and polish up like chrome. No sizes are given in parts lists but if you aint got the correct tube you wont get correct air velocity will you ?? I'm only the pre war officer, and those old bikes will run without a carb ( almost) one of my old bikes got me back to camp, just, then I discovered the carb was almost about to drop off, nuts undone to the last thread.

Rob Harknett · Post by **Rob Harknett** » Wed Sep 14, 2011 12:27 pm

I do mean air intake tube, this is what Amal call the part, if its given another name that differs from what Amal call the part, things can get confusing. you will find air intake tube named on the amal parts lists you will not see the other names it is given

Malleon · Post by **Malleon** » Wed Sep 14, 2011 1:06 pm

On the GP carbs. it is a velocity stack, and on the 376 series it is named 'air intake', so perhaps Amal could not make up their minds on the terminology.

dave16mct · Post by **dave16mct** » Wed Sep 14, 2011 1:52 pm

Always called it a bellmouth myself.

petert120r · Post by **petert120r** » Wed Sep 14, 2011 1:59 pm

My guess is that at 3000 feet above sea level the thinner atmosphere is causing the symptoms. Have you tried partially closing the air slide at medium to high throttle settings? If there's an improvement that would confirm a weak mixture and you could then play around with main jet sizes and needle position.

Remember that the pilot jet controls the idling mixture, the slide quarter to half throttle, the needle half to 3/4 and the main jet full throttle.

The K&N would cause the mixture to be slightly richer.

There must be some information somewhere about carb settings at altitude.

Another thought - I assume you've checked the usual suspect of an air leak between the carb and manifold or manifold/head?

Edited by - petert120r on 14 Sep 2011 1:19:28 PM

Rob Harknett · Post by **Rob Harknett** » Wed Sep 14, 2011 5:27 pm

I've always used belmouth Dave, but I was asked what the tube was so used what amal state, for obvious reasons, like thats what you will find the part named as,
for a 389 carb. Malleon, Maybe amal thought velocity stack sounds much more impressive than air tube. Like trying to order spares, if you use the correct name you do better than " the thing a me jig " that screws on the end etc. BUt things do get renamed by us that seem more exceptable, like bellmouth.

tpeever · Post by **tpeever** » Wed Sep 14, 2011 6:56 pm

quote:
My guess is that at 3000 feet above sea level the thinner atmosphere is causing the symptoms. Have you tried partially closing the air slide at medium to high throttle settings? If there's an improvement that would confirm a weak mixture and you could then play around with main jet sizes and needle position.

Yes, I have tried this and have done a ton of playing around with main jet sizes and needle positions!! When I slightly close the air slide, performance improves. This observation, coupled with the appearance of the spark plug, and the hesitation formed the basis of my lean diagnosis. It was still running way too lean with factory settings of needle position 4 and 390 main.

quote: The K&N would cause the mixture to be slightly richer.

I have run the bike without an air filter as well and there isn't any detectable difference. However, I am not running any sort of air intake tube which I suppose might make a difference as someone has suggested. I run a similar carb on a 67 Triumph TR6C with a K&N filter and without a velocity stack and throttle response is nice and crisp with factory settings at the same altitude. That Monobloc carb has a 340 main jet in it. Why does the G12 require such a large main jet compared to the TR6C?

quote: There must be some information somewhere about carb settings at altitude.

I added the information about my elevation as I thought it might possibly be be relevant. However, the more I think about it, the more I think it can't be the issue. I haven't noticed any large differences in carb settings from published settings with any of my other bikes running similar carbs. Increases in altitude should make a bike run richer anyways correct? I think 3000 feet is not high enough to warrant large changes in jetting.

quote: Another thought - I assume you've checked the usual suspect of an air leak between the carb and manifold or manifold/head?

This is a good idea and a possibility. I have not checked this but will do it. I suppose a very slight air leak may give the symptoms iI am seeing. I have some very thin paper gaskets between the manifold and the head. I think these are the stock gaskets that I bought from the spares scheme but they are very thin compared to others that I have seen. I am thinking these thin gaskets might not be able to absorb any slight variations in the mating surfaces of head and manifold. I have a thick gasket between carb and manifold so I doubt whether I would have a leak there.