right cylinder running too hot

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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John Carney
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Location: JERSEY

right cylinder running too hot

Post by John Carney »

Hi, I've recently rebuilt the top end of a 1961 model 31. Still trying to get the idling spot on, but on starting, and very short run, the right hand cylinder seems to be very hot ( too hot ? ) and the left hand one I can touch with a bare hand. Oil seems to be returning to the tank OK. Has anyone any thoughts on this. Thanks in advance, John
Groily
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Re: right cylinder running too hot

Post by Groily »

Assuming not a tight piston or a ring problem, and that the exhaust valves and other mechanical bits are in decent shape after your ministrations.

Possible problem with gasket on inlet manifold on that side letting air in and causing weakness? That does happen now and then. What do the plugs look like, compared to each other?

Or, what is the firing interval between the sparks, especially if you're running a magneto? A significant error (which is not uncommon) can certainly cause overheating due to the timing being retarded on one side, sometimes by quite a few degrees. A degree off 180:180° at the points is 2°on the crank . . . This can only really be checked by strobing both sides, which is very boring, but sometimes frighteningly instructive!

On bikes of other marques, there was sometimes a problem with 'induction bias' - which was addressed by a special spacer between carb and manifold to 'aim' the mixture more towards the weaker cylinder (where weakness was diagnosed and there was no other reason).
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1608
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Re: right cylinder running too hot

Post by 1608 »

I tried a bias gasket on my Huntmaster, but hasn,t made any difference. At risk of teaching egg sucking etc. Did you fit and gently tighten the inlet manifold before torquing down the heads.
John Carney
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Location: JERSEY

Re: right cylinder running too hot

Post by John Carney »

Hi, No worries about the risk of teaching to suck eggs, i'm trying to learn things as I go along. I did fit and gently tighten the inlet manifold before torquing the heads. Plugs look similar, slightly sooty and left hand one perhaps a little oily but no more than that considering it's newly rebuilt I would think. Rechecked rockers and clearances and they seem to be getting oil OK. It's got new rings but I'm hoping that's not the problem. It is running with a magneto and I've noticed that the advance retard cable is very worn so I'm wondering if that may have been causing it to run retarded and if so could this cause it ? I was also messing with the pilot jet adjustment at the time so perhaps i've cause the problem myself ? Thanks again and I'll let you know if the issue persists.
Groily
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Re: right cylinder running too hot

Post by Groily »

John Carney wrote: Sun Nov 14, 2021 1:36 pm I've noticed that the advance retard cable is very worn so I'm wondering if that may have been causing it to run retarded and if so could this cause it ? I was also messing with the pilot jet adjustment at the time so perhaps i've cause the problem myself ?
A sloppy camring due to sloppy cable would normally affect both sides the same John, so probably not the reason, or not on its own. Worth resolving that though, so it can be eliminated as a possibility.
Pilot jet adjustment will have an affect across the throttle range, yes, but again, should affect both sides the same 'cos with a single carb opening the screw weakens for both sides, closing enriches. If the plugs look broadly similar, probably not a mixture thing.

May just be that one bore is slightly tighter than t'other with the new rings in though. If the rings are marked TOP, then it matters to have put them in the right way round, if they're parallel and unmarked, no issue.
Might be worth giving it a few more miles, with a hand hovering over the clutch lever just in case it decides to nip up on you? As always, there's 'hot' and there's 'HOT' and it's hard to say where the acceptable limits are. Could also try colder / harder plugs I suppose? Dunno what you're on, but NGK B7ES should be good, B8s at a pinch (I run my Mod 31 on them), or Champion N4 or even N3? Non-resistor, no protrusions on the nose . . .

(Oh, and I never had much luck with bias gaskets on the A10 I had for years either 1608 John.)
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