Carburation woes

Could it be that your inlet valves are adjusted too tight?

Mike

Did you check all the jets and carb channels were clear, if it wants choke it sounds like it might be running lean.

Hi John,

It sounds like you have a tight wire advance on your mag. The mag runs the same way as the engine on a 1936 Model 18 so the advance/ retard cable should come out nearest the oil tank for slack wire advance. If it comes out nearest the engine it is tight wire advance. Regarding the carb, strip it all down, spray generously with carburettor cleaner (aerosol) and leave overnight if possible then use an airline or bicycle pump to make sure all airways are clear. When you re-assemble it, screw the slow running adjuster fully in then back 1 and half turns. You will need to flood the carb when starting from cold.

Regards, Richard.

Previously richard_cornish wrote:

Hi John,

It sounds like you have a tight wire advance on your mag. The mag runs the same way as the engine on a 1936 Model 18 so the advance/ retard cable should come out nearest the oil tank for slack wire advance. If it comes out nearest the engine it is tight wire advance. Regarding the carb, strip it all down, spray generously with carburettor cleaner (aerosol) and leave overnight if possible then use an airline or bicycle pump to make sure all airways are clear. When you re-assemble it, screw the slow running adjuster fully in then back 1 and half turns. You will need to flood the carb when starting from cold.

Regards, Richard.

Yep. I think you are correct. Tight wire for retard on pre war bikes.

It depends on the exact year as some singles had the inlet and exhaust cams configured as a train so the inlet went in the same direction as the crankshaft, the others had them separately run from the crankshaft so the inlet went in the opposite direction.

Best plan is to pop the points cover off and turn the engine over with the kickstart and note which way the points turn. If the cable pull is in the same direction as rotation then it's slack wire advance; if the opposite then it's tight wire.

Previously john_carter wrote:

Thanks to all for your thoughts... I've already dismantled/reassembled the carb and given it a good clean with carb cleaner and cleaned all the jets but I will do again and leave it overnight.

Dan, from the plug condition you could well be right.. I'll give it another go when I've re-cleaned the carb..

Richard.. do I understand correctly that as the advance retard cable comes out nearest the engine I have a tight wire advance? So if the lever is closed the spark is retarded and when the lever is pulled open it is advanced?

Mike.. didn't understand your comment re the inlet valves being too tight.. can you elaborate please?

Thanks again..

Hi John,

Yes that's correct, this is not the best set-up as when you are riding the vibration can make the ignition go to the retard position very gradually so you don't notice straight away. Make sure the lever is tight enough or put a strap or similar to stop it moving. The sv and push-rod ohv engines had a major re-design in 1934 and ran the gears as a train ( inlet same rotation as crankshaft )and this system remained until the re-design for the 1948 models where both cams run off the crankshaft.

Richard.

P.S. Does it have the original engine ?

John:

Re: inlet valve tightness

It was just a thought that if the inlet valve clearence is out of adjustment it is possible that when the valve is supposed to be closed and sitting on its seat, it is being held off thus letting some of the combustion fire leak back into the inlet track/carb.

Mike

Previously John Carter wrote:

Hi Richard

Yes the engine is the original. (Confirmed by Chris Streather) and thanks for giving me such a crucial piece of information... I might have figured it out eventually but my only prior experience with advance and retard controls was an old Triumph I had many years ago which had a slack wire advance. It didn't even cross my mind that they could be the other way round. None of the books and manuals I have mention it.. Thanks again..

Hi again John,

You're welcome. Even though I've had these old Nortons for 45 years, there's hardly a week goes by with-out learning something new! Norton were unusual in running the gears as a train and using a clockwise (viewed from the drive end) mag. This may have been done to make the gears less noisy but not really sure. Mechanically it puts all the load on a couple of teeth on the half time pinion and exhaust cam wheel rather than spreading it between the two camwheels. If you can find one the E.M.Franks Norton Motorcycle Maintenace book is probably best or Pitmans Norton Motorcycle Maintenance (W.C. Haycraft) are well worth looking out for.

Regards, Richard.