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Combustion Chamber Clues

A_HRD

Well Known and Active Forum Website User
VOC Member
My trusty Rap was running fine today when I stripped the heads off to replace leaky upper seals on a couple of the pushrod tubes. (Those little blighters don't seem to last long do they?).

When I removed the front head, the piston showed some interesting markings adjacent to the carb showing a clear strip of piston like it had been washed clean by petrol. The front head shows a similar phenomenon in the same place. See pics - which I hope I have attached properly. The outside of the inlet valve is also oily - so it could be the valve-guide seal is on its way out too?

I'd be interested to receive opinions as to what is happening here.... In the meantime, I'll carefully strip the head down and photograph it as I go.

Peter B
Bristol, UK.

PS The rear head combustion chamber is dry with a mixture of dark grey and brown deposits - light brown on the head of the exhaust valve. Oh, and if anyone is interested, the pistons are Suzuki; they must have been in for over 15 years now, eh Trev? There is no wear and no scores in the bore (despite no air filters), I can still see clearly the 30 degree criss-cross honing marks in the cylinder!
 

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eddie

New Forum Website User
Non-VOC Member
hello, the piston looks like oil is getting past the rings, I would drop the piston down the bore and pour some oil down the bore to see if it leeks past the rings. I would do this on both bores then time how long they leak past the rings,This shows up broken rings or poor ring spacing. hope this helps.
 

clevtrev

Well Known and Active Forum Website User
VOC Member
Peter, the oil control rings , i.e. the thin wafer rings are now Kaput, simply get new ones. I could make them last 15,00 miles, some male them last far shorter. Measure the width of the ring, I will measure a new one.
 

Black Flash

Well Known and Active Forum Website User
VOC Member
Hi Pete, I am not much of an expert, but judging from your pictures, it seems to me that the headjoint between liner flange and head is not 100%. not an answer to your question I know....
 
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van drenth

Well Known and Active Forum Website User
VOC Member
Peter, for me it looks that the valve is hitting the piston. When i did run with Suzuki pistons, which i still regret that i did, the pockets of the valve area on the piston did have to be enlarged and depthtened. So check or fit some compression plates to avoid hitting.
Regards. van Drenth JC
 

A_HRD

Well Known and Active Forum Website User
VOC Member
Update:

Thanks for the replies. I removed the cylinders and pistons. The rings were intact but they are badly worn - just as ClevTrev (and a PM from Chris - thanks) said they would be. New ones are on their way. No wear anywhere else, so that is good. Bernie is right, the heads need a gentle lapping back on to the cylinders - I find this is usually the case after years of heating and cooling cycles...

I have looked at pistons and valves very carefully for signs of contact but can't find any. I put the valves in the lathe and rotated them by hand with a DTI affixed; the 1.5 thou run-out I would put down as normal manufacturing tolerance. Nevertheless, I will fill the valve pockets with plasticine on a trial re-assembly and see just how much clearance there is! It will be no problem to mill some more out if necessary. I don't want to use compression plates because my cylinder bases are o-ringed giving a 90+% metal-to-metal contact allowing the heat to move south more quickly... and no leaks. :)

Anyway, the aim is to get the Rap back on the road for the Annual Rally....

See you there, and thanks very much for the tips and help.

Peter Barker
Bristol, UK.
 

redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
Peter, the oil control rings , i.e. the thin wafer rings are now Kaput, simply get new ones. I could make them last 15,00 miles, some male them last far shorter. Measure the width of the ring, I will measure a new one.
Hi Trev, obviously you were correct in your diagnosis but can you tell me why you came to that conclusion from the photos i.e. was it because of the clean area, the dirty area or both? ...the more I learn, the more I realise I have to learn.
Regards
Kevin
 

clevtrev

Well Known and Active Forum Website User
VOC Member
Hi Trev, obviously you were correct in your diagnosis but can you tell me why you came to that conclusion from the photos i.e. was it because of the clean area, the dirty area or both? ...the more I learn, the more I realise I have to learn.
Regards
Kevin
It was simply a case of, `been there, done it`.
 
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