E: Engine Nip up Comet

Tony Wilkinson

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I bought my 1950 Comet a few months ago from a Dealer. After some running problems I fitted an electronic ignition as the mag was weak.
I then had the engine nip up on me on a long straight. I was doing about 60mph, whipped the clutch in sharpish for a few seconds, let out and it started and ran ok since.
Now although the bike has loads of compression and runs really well, it smokes when hot, mainly on down gear changes.
I got with it a load of receipts for new parts bought a couple of years ago, including a new exhaust valve and lower valve guide.
I only assumed these had been fitted by the previous owner.
Cutting a long story short, I managed to get hold of the last owner who informed me he had indeed fitted the new valve and guide but had not investigated further as the piston looked new.
I bought a little endoscope thingy off ebay to look down the plug hole. The bore looks recently rebored and piston does look new and has no carbon on it. However I can see a faint light grey line up the bore where I am assuming the nip up occurred.
I intend over the Winter to investigate this by stripping the top end down. I have a lot of experience but Vincent are new to me. I am thinking the oil ring it probably tight in that place on the piston, hence oil getting up the bore.
Has anyone had a similar problem I am asking or any advice from people more knowledgeable than myself please.
 

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vibrac

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My Trials Comet had a rebuild and then a week or so later it nipped just as you describe. A few days later was a long distance trial I had built it for so although compression might have been a little down I carried on. No problems on the trial so I kept using it and the motor just kept getting better. I used it through various iterations and as a solo and finally after many miles stripped it to fit a big bore the bore was clean as clean and the piston was fine.
 

Tony Wilkinson

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My Trials Comet had a rebuild and then a week or so later it nipped just as you describe. A few days later was a long distance trial I had built it for so although compression might have been a little down I carried on. No problems on the trial so I kept using it and the motor just kept getting better. I used it through various iterations and as a solo and finally after many miles stripped it to fit a big bore the bore was clean as clean and the piston was fine.
Thanks for that Vibrac. I would leave it alone as it runs good and strong. I have done a couple of hundred miles on it since the nip up. Just the smoking I am a bit concerned about, plus the fact I would like to check it out anyway. If only for peace of mind.
 

Robert Watson

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Smoking on the overrun can occur when the engine is sucking oil down the INLET guide. On the suck bit of the "suck, squish, bang, blow" cycle it is the valve that is open so if there is oil there, that is when it will get sucked in. The exhaust is open on the blow bit so not much chance of sucking oil down there.

I would do a leak down test. It will show you if you have any leaks. At TDC with everything closed it pressures the cylinder. You will hear any leaks.

Out the carb, Intake valve
Out the exhaust, Exhaust valve
Out the cases Piston.
 

Tony Wilkinson

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Non-VOC Member
Smoking on the overrun can occur when the engine is sucking oil down the INLET guide. On the suck bit of the "suck, squish, bang, blow" cycle it is the valve that is open so if there is oil there, that is when it will get sucked in. The exhaust is open on the blow bit so not much chance of sucking oil down there.

I would do a leak down test. It will show you if you have any leaks. At TDC with everything closed it pressures the cylinder. You will hear any leaks.

Out the carb, Intake valve
Out the exhaust, Exhaust valve
Out the cases Piston.
Thanks, I will check. When I looked in the cylinder with the endoscope, there was oil in the exhaust pocket on the piston but not on the inlet side.
 

Bill Thomas

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A lot of blokes run the pistons with not enough clearance, And a Comet has a hard life.
Yours looks a nice Bike.
As Vibrac says, You could get away with just running steady for a bit.
Do you run thin oil ?, My Comet has been Knackered for years and I now run it on 50 single grade,
To keep the smoke down.
Weak carb' setting can also make them NIP !.
Good Luck, Bill.
 

Tony Wilkinson

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Non-VOC Member
A lot of blokes run the pistons with not enough clearance, And a Comet has a hard life.
Yours looks a nice Bike.
As Vibrac says, You could get away with just running steady for a bit.
Do you run thin oil ?, My Comet has been Knackered for years and I now run it on 50 single grade,
To keep the smoke down.
Weak carb' setting can also make them NIP !.
Good Luck, Bill.
I will change the oil when I strip it. Oil was fresh when I bought bike. Seller said he uses Silkolene straight 50. I have changed the carb from the fairly worn original 229 to a (Burlen) Amal Concentric MK1 and it runs much better.
 

Tony Wilkinson

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Non-VOC Member
Piston skirt oil feed through the liner too high injecting oil into the rings ...........Very common.
Yes Greg I have read about this oil feed in the liner. When I strip the engine I will measure to see where the rings are in relation to the oil feed hole... This is one of the problems of not having a long history with the bike. I bought from a dealer but the owner before that had the bike for 18 years and never really used it. He told me that the previous owner to him had rebuilt the engine and had it rebored with new piston. This bears out as the piston does indeed look new and the bore still has clear honing marks. I am concerned that the nip up has smeared/scored the piston and may be stopping the oil ring in particular from doing it's job properly.
I will post my conclusions soon.
 

Cyborg

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Piston skirt oil feed through the liner too high injecting oil into the rings ...........Very common.

Back in the late 70’s I had a C Rapide as my daily rider. It behaved really well except for some oil smoke. Drove me nuts.... Turned out the oil holes were too high. When I was sorting out my B twin basket case a couple of years ago there were at least 8 cylinders in the mix. All 8 had the oil hole too high.
 
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