G12 running very hot on left cylinder

[LynP]

Hi, My 1961 G12 was started up a few weeks ago after a rebuild that included a complete engine rebuild, after the first warming up I re-torqued the heads and checked the valve clearances, the next time it was started it ran fine. Having completed a few little jobs such as replacing the petrol pipes to replace the temporary ones fitted for the earlier start ups and fitting an ignition isolating anti sumping valve I started it today following a few weeks layoff. Having run it for 4 or 5 minutes to check for oil return, leaks etc. it started to run very hot on the left cylinder to the extent the left hand pipe coloured up badly; (the right pipe is still showing no colouring up) and there was a marked difference in the heat of the exhausts leaving the silencers. I am concerned about the big difference in temperatures, why such a bias. All jetting is for an air filter that is fitted. Any thoughts would be appreciated. Thank you, LynP

[John Carney]

Hi LynP,

I had a very similar issue with my 1961 AJS after a top end rebuild. ( n.b. I don't have an air filter or different jetting )There were some suggestions made by members on this forum at the time so maybe have a look at that feed in the topic search. I remember checking oil return on the rockers with the engine running on the hot side, finding no issues and in the end I took it for a short run and on checking the cylinder / exhaust temp on my return they were even. Personally I never leave the bike running for more than a couple of minutes to warm up and the issue never reoccurred. Hope this helps, cheers, John

[LynP]

Thank you John for your reply, I will have a look at the posts as you suggest. As the the bike ran fine when I started it before, maybe I ran it longer this time and it got hotter. I will check the valve clearances again, change the plugs and then when I get chance take it for a run and see how it goes.

[MalcW]

How do the plugs look? Could it be that a combination of, say, slightly retarded timing, running with no airflow through the fins and an issue on the RH side means the LH is actually OK the the RH side is running cool for whatever reason? But as has been suggested, as long as the oil is circulating OK, take it out for a test, it'll probably be fine when you get back.

Malc

[LynP]

Hi Malc, thank you. I will do as you and John suggest. I have not taken the plugs out yet as I was a bit cheesed off as I was ready to take it for its first run out so I put it back in the garage until I had chance to check it over. I intend to do the valve clearances again and chance the new old stock N5 plugs for the new N4c ones I have. The oil is circulating nicely so will take it for a run as soon as I get the chance.

[Andy G]

Are you running separate pipes, or siamesed?
Quite sure there's no blanking rag left inside...
And presumably you have a single carb with splitter manifold to heads? ie, no air leak that is isolated to just the one cylinder?

[LynP]

Hi Andy, separate pipes and single carb. The bike ran fine on the two previous startups and the exhaust gases were checked by hand as they left the silencers and felt as if the were about the same, this time there was an obvious difference in temperature, so something has changed. When I fitted the inlet manifold I nicked it up before tightening the heads up however, it is a possibility that there an air leak and so if it gets hot again I will investigate that area and also do a compression test for head gasket seal. It may be a little while until I have chance to do much with the bike but I am grateful for you taking the time to give me some other things to think about, thank you.

[griffin]

Is your ignition timing spot on for both sides ? I once had a magneto that differed by about 4 or 5 degrees from one cylinder to the other. Perhaps check the timing on the side you are having problems with .

[LynP]

Hi Griffin, thank you for your thoughts on my over heating issue. My bike is a coil ignition, when I overhauled the distributor I checked the shaft for straight so as to make sure there way no variation on the points gap. My son came yesterday, he wanted to see what was up and so the plugs were changed for new N4c's. The N5's that were in were clean on the hot side and dark but not really bad on the other side, the leads and caps are new.
We started it up as it warmed up and as the choke was cleared is started spitting back and there was a noise that I think sounded like an air leak. I was getting hot as before, closing the choke half way it was better, so running weak? So may next week when I get chance a compression test first then valve clearances which I had not had time to do yet.

[griffin]

We started it up as it warmed up and as the choke was cleared is started spitting back and there was a noise that I think sounded like an air leak. I was getting hot as before, closing the choke half way it was better, so running weak? So may next week when I get chance a compression test first then valve clearances which I had not had time to do yet.

LynP
Ah yes that is a possibility, did you follow the correct method of tightening up the manifold to the heads ?
Griffin.