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E: Engine More Adventures with the Comet

manxman

Website User
VOC Member
The silver lining of the pandemic is that I finally have more time with the Comet, which was otherwise neglected in favor of a well-sorted (and faster...) Norton 750. I apologize.

I noticed it was smoking a fair amount from the exhaust at the end of the summer and made a note to check the plug and piston. Bottom of my list. It shouldn’t have been.

See the pictures below—fouled spark plug and sludge/carbon on the piston. (Using a cheap endoscope from Amazon.) Will need to keep going and have a look at the valves. The piston seems to have been enjoying an oil deluge. I expect I’m also running too rich but I’m focused on the oil for the moment.

I can’t tell if the cylinder wall scoring is exaggerated by the LEDs and the focal length of the scope. Will have to have a look at that too. My understanding is that there should be some intersected scoring, diagonally, on the cylinder wall as a result of the honing process, but I’m by no means an expert. Will try to get a few more pictures.

As a note, I never once had an issue starting (first or second kick) and the bike ran beautifully this summer, but for the smoke and some leaks from the Burman and primary (which I’m also addressing)... My error was not to check more regularly the spark plug, among others. I’m amazed a spark was coming through all that mess.

Anyway thought I would share. The downside is I have a lot of work to do; upside is I get to keep taking apart this lovely machine. Would welcome any thoughts.


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MartynG

Well Known and Active Website User
VOC Member
Been there - done that! It may well be worn valve guides allowing oil into the combustion space. This means stripping down the head to check. It sounds like a daunting task but work slowly and methodically and its not that hard.

Remember to check the valve stems themselves for unusual wear on their sides - I had that and it was caused by misalignment between upper and lower valve guides.
 

Cyborg

Well Known and Active Website User
VOC Member

manxman

Website User
VOC Member
Thank you all. Yes the oil was certainly too light. 40 weight when I got the bike but I didn’t bother to replace it. Haven’t had it long, but this summer I gave it its first real going-through and I imagine the lighter oil exacerbated the issues.

Martyn yes I need to do some surgery on the head! I’d love to see the valve faces and seats so at least I can know the culprit. Need to finish up the work on the shaft seals in the primary first.

And Cyborg yes I had the same thought (just from what I can read online). The cross hatches look to be too shallow of an angle and quite deep. Will know better once I pop it open (and likely discover other issues). Thank you for the link. If I need honing done I’ll have to find someone good in New England. But I’m hoping I can avoid that step.
 

brian gains

Well Known and Active Website User
VOC Member
before you pull the top end check your ET49 is not loose. My Comet used to 'make smoke' and after I found the threads partially stripped I replaced retaining nut, punched and no more issues.
 

erik

Well Known and Active Website User
VOC Member
If you think the Valve guides are the reason for the oil consumption then dismount the inlet stub and have a look. And with your camera you can see where the oil is entering the inlet.Erik
 

Bill Thomas

Well Known and Active Website User
VOC Member
Maybe you could fit metering wires, If not already fitted ?,
In the rocker feed bolts ?
With the C/Head off, Check that oil could drain away from the valve guide area ,
Hold the head as it would be on the Bike and make sure it can't build a puddle of oil above the guide,
More so when Mk2 cams are fitted and the top of the guide has been shortened.
Some times we have to grind a bit of head where the guide is to allow drainage.
 

Ken Tidswell

Well Known and Active Website User
VOC Member
The silver lining of the pandemic is that I finally have more time with the Comet, which was otherwise neglected in favor of a well-sorted (and faster...) Norton 750. I apologize.

I noticed it was smoking a fair amount from the exhaust at the end of the summer and made a note to check the plug and piston. Bottom of my list. It shouldn’t have been.

See the pictures below—fouled spark plug and sludge/carbon on the piston. (Using a cheap endoscope from Amazon.) Will need to keep going and have a look at the valves. The piston seems to have been enjoying an oil deluge. I expect I’m also running too rich but I’m focused on the oil for the moment.

I can’t tell if the cylinder wall scoring is exaggerated by the LEDs and the focal length of the scope. Will have to have a look at that too. My understanding is that there should be some intersected scoring, diagonally, on the cylinder wall as a result of the honing process, but I’m by no means an expert. Will try to get a few more pictures.

As a note, I never once had an issue starting (first or second kick) and the bike ran beautifully this summer, but for the smoke and some leaks from the Burman and primary (which I’m also addressing)... My error was not to check more regularly the spark plug, among others. I’m amazed a spark was coming through all that mess.

Anyway thought I would share. The downside is I have a lot of work to do; upside is I get to keep taking apart this lovely machine. Would welcome any thoughts.


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View attachment 38925

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View attachment 38927
badly worn guides, seized piston, stuck rings pull it down only way to find out
 

Cyborg

Well Known and Active Website User
VOC Member
If I need honing done I’ll have to find someone good in New England. But I’m hoping I can avoid that step.

Based on what’s shown in the photo, unfortunately you can not avoid that step if you want the rings to do their job properly. Those honing marks are an excellent indicator of the engine assembler’s skill. That means that everything else inside the engine that he/she touched is suspect. Like Mr. Tidswell says... pull it down. You will save money and heartache in the long run. If that is an original cylinder, you may want to consider replacing it with a new one. The cylinder liners on original ones have usually lost most of their interference fit in the muff. Also lots of crap built up between the liner and muff.... these two things combined lead to poor heat transfer.
 

Bill Thomas

Well Known and Active Website User
VOC Member
The Burman box just needs our special grease = Magic, = Semi fluid grease "Penrite",
700 grams from empty, But I just put a few Big spoons in mine because you can't drain a lot of boxes, Mine had normal grease and a little very thick oil,
Then I put some Penrite in it and all is well !!
And the primary leak is mostly too much oil in there, Just needs the chain to be dipped in the oil,
Or it comes out the back.
Then good for another 100,000 miles :D .
 
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oexing

Well Known and Active Website User
VOC Member
Once you got inside the engine, check cracked or weak piston rings - obviously - and check for a correct and working engine breather. Could that have slipped from seizure ? So then the timing would be wrong and internal engine pressure would lead to very oily pistons. I´d go for reed type breathers anyway.

Vic
 

manxman

Website User
VOC Member
Further down the rabbit hole... I started this work thinking I would just be fashioning some seals on the primary side!

I think at this point I need to do a full accounting, from the barrel up through the valves and feeds. Unless the camera is badly exaggerating the cylinder lining scars, I agree there's no option but to pull it all out. I'll know for sure once I get the head off.

I had a short this summer in the headlight which was easily solved, and I remember thinking the bike had been relatively trouble-free! But I'm looking forward to this. Will be fun. Will update if/when I find any more bodies buried inside.
 

manxman

Website User
VOC Member
The Burman box just needs our special grease = Magic, = Semi fluid grease "Penrite",
700 grams from empty, But I just put a few Big spoons in mine because you can't drain a lot of boxes, Mine had normal grease and a little very thick oil,
Then I put some Penrite in it and all is well !!
And the primary leak is mostly too much oil in there, Just needs the chain to be dipped in the oil,
Or it comes out the back.
Then good for another 100,000 miles :D .
And yes this Penrite stuff is held in high regard around here. My Burman is from 52, which I think is the first year they added a drain plug (just above the rear adjuster bolt). My plan is to put a heat gun to the box to gently bring up the temp, drain what I can, then pour some warmed Penrite in--not too much. Hopefully one the easier tasks in the next few months.
 

Vincent Brake

Well Known and Active Website User
VOC Member
can also be due short rides out and always put away on the left side stand, causing oil in the "feedback to tank" line dripping in the cylinder....
causes oily fumes when start again!!
nice
 

Bill Thomas

Well Known and Active Website User
VOC Member
A shallow groove starting at the lower guide retaining ring to lead any collection of oil down the rocker tunnel will frequently stop oiling plugs and smoking. It is essential with cut aback guides that are necessary when using MkII cams.

From Ted Davis, Long ago , He was a Vin' Factory worker and Racer.
Cheers Bill.
 

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