G12 running very hot on left cylinder

[LynP]

Hi Griffin, yes I did nick the manifold up before tightening the heads, but one of the things that bothers me is because I had the barrels powder coated and the right side barrel was tight going down, I am wondering if there was not enough play around the studs in the barrel because of paint in the holes stopping any movement, so perhaps the manifold did not pull up as tight/square across heads as it should. I don't really want to remove the heads and barrels, but if I can't solve the in any other way that's what I will do and clean stud holes in the barrels. Thank you for your help with this, LynP

[mdt-son]

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?

There are two causes for erratic running - carburation and timing. I suggest you pull the carburetor, give it a good ultrasonic cleaning, blow through all bores with compressed air (jets out), replace jets and needle, make sure the throttle slide doesn't rattle, check carb to manifold gasket, and don't overtighten the carb or it will deform. The nuts should be tightened just a tad more than finger tight. Check the carb flange for flatness. Carbs do warp!
If your petrol tank has rust inside, make sure you have an external fuel filter fitted. That's a good precaution at any rate.

Check the points housing and shaft. Could there be movement between contact breaker shaft and gear wheel? Is the ATD in good nick? Do a stroboscopic test of your ignition timing (you will need a helper for this).

Not directly related, but also important is oil flow to the cylinder head. Make sure the left side cylinder head is lubricated. Oil removes a lot of heat and if oil supply fails, the head will overheat.

Do not run the engine static for more than 2-3 minutes. For adjustments, do them in succession, allowing the engine to cool down in between.

Paint residues in the cylinder stud bores are unlikely causing a problem, as long as cylinder mating faces are clean. Bolt stress will easily overcome friction caused by paint as the cylinder is pushed down.

- Knut

[LynP]

Hi mdt-son, thank you very much for taking the time to give such a detailed and helpful post. I have not been able to work on the bike since my last post. I covered many of the things you suggest like overhauling the carb, flattening the flange etc. when I did the complete engine rebuild. The first time the bike was started, once I sorted the 180 degree out timing; (plug leads wrong way round) and the carb tuned it ran fine, even with the choke cleared but it was only run for a short time. The next time it was started was when the over heating started and the spitting back occured, which is one of the reasons I suspected an air leak maybe because of cooling down and heating up again. I am also thinking that the other cylinder which was much cooler may have an electrical issue despite a change of plug, so I will change the plug again and also the lead.
I probably won't get back to sorting it out for a few weeks yet, but when I do I will recheck it all and keep your ideas in mind.