Tony.... did you ride the bike much before the partial seizure? As in...are you sure the smoke started afterwards or could it have been like that before and you just didn’t run it hard enough or get it hot enough because of the ignition problem?
E: Engine Nip up Comet
- Thread starter Tony Wilkinson
- Start date
two thoughts based on my own hard learned experience.
Valve guides: "the last owner who informed me he had indeed fitted the new valve and guide but had not investigated further " I would be checking to see of once fitted that the upper and lower valve guides were line reamed to ensure alignment, if they are NOT in line the valve stem at best will wear excessively on one side allowing oil to run into the combustion chamber - or worse, the stress will eventually cause the valve stem to break, the valve head falls and Bingo massive repairs!
Mixture: Nipping up is caused by overheating and as our air cooled motors rely to a reasonable extent on the cool fuel being sucked into the motor to assist with cooling (strange but true) and if the mixture is lean then nip up is possible. Check the spark plug - the outer edges of it should be sooty black and the centre electrode, near its tip should be a tan colour. If the centre electrode is white you have a lean problem that needs to be resolved- in this case you should RAISE the needle in the carb by one notch (lets more fuel in) - if the clip is already in the bottom notch , then you need to increase the size of the needle jet by 2 steps ie if you have a 106 installed, go to a 108.
You should also check the fuel level in the float bowl for that will also impact on lean running - instructions are in last months OVR.
Phil Irving is reputed to have said more damage is done by heavy oil than light. I use 20W-40 multi in my comet.
Valve guides: "the last owner who informed me he had indeed fitted the new valve and guide but had not investigated further " I would be checking to see of once fitted that the upper and lower valve guides were line reamed to ensure alignment, if they are NOT in line the valve stem at best will wear excessively on one side allowing oil to run into the combustion chamber - or worse, the stress will eventually cause the valve stem to break, the valve head falls and Bingo massive repairs!
Mixture: Nipping up is caused by overheating and as our air cooled motors rely to a reasonable extent on the cool fuel being sucked into the motor to assist with cooling (strange but true) and if the mixture is lean then nip up is possible. Check the spark plug - the outer edges of it should be sooty black and the centre electrode, near its tip should be a tan colour. If the centre electrode is white you have a lean problem that needs to be resolved- in this case you should RAISE the needle in the carb by one notch (lets more fuel in) - if the clip is already in the bottom notch , then you need to increase the size of the needle jet by 2 steps ie if you have a 106 installed, go to a 108.
You should also check the fuel level in the float bowl for that will also impact on lean running - instructions are in last months OVR.
Phil Irving is reputed to have said more damage is done by heavy oil than light. I use 20W-40 multi in my comet.
I did not ride the bike much before the partial seizure as I had problems with the ignition. I did not do more than four or five miles at a time. I never noticed any smoke before the nip up.
Thanks for the advice. Oil is a subject all of it's own and everyone has a different opinion.
I did actually do that as I had no idea if the engine had done many miles before I owned it. I think the timing may have been out before I changed the ignition as the exhaust was (is) very blue.
Sorry folks - got a bit carried away - a 108 needle jet in an Amal Mk1 will result in 8 stroking - waaaay too rich. Stay with the 106
My Amal Concentric was already jetted for a comet and it runs great with it so for the moment I will leave well alone.
I hear what you are saying but as an engineer I like things to be right, call me anal if you like. Also I get fed up with the ' experts ' keep pointing out what I already know. Things like ' engines a bit smoky mate, it must be knackered '.
Anyway, continuing the thread. I have now stripped the top end. The bore looks absolutely fine and the piston will be OK with a bit of a clean up. As I thought the nip up area on the worst part just seemed to be catching the oil ring. I measured the oil hole in the liner and the top of the hole is a good 3-4mm BELOW the bottom edge of the oil ring so I am assuming this is also fine. I have yet to strip and check the cylinder head but will do that next.
Can anyone identify the piston please as I might want to change the rings?
Can anyone identify the piston please as I might want to change the rings?
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