850 Head, cylinders and pistons are out. Can you comment on photos of piston tops and combustion chambers?
- Thread starter DennisMo
- Start date
marshg246
VIP MEMBER
- Joined
- Jul 12, 2015
- Messages
- 5,845
How many miles? IMG_8814 appears to have wet oil - is that true? The plugs, what little I can see show the mixture about right, so I would say you're burning oil and if that's wet oil in IMG_8814, a lot of oil. My guess would be a bad valve seal on both sides and maybe a slow intake area drain and/or running the bike on the side stand.
With the head level and the chambers up, pour in some gas and see if the valves leak. I'm suspicious of the valve in IMG_8814 that looks rusty under the carbon.
With the head level and the chambers up, pour in some gas and see if the valves leak. I'm suspicious of the valve in IMG_8814 that looks rusty under the carbon.
Thanks Greg.
As for mileage that is a tough one as the previous owner figured 10,000 miles or so.
I put on perhaps 2000 miles since I bought it. This looks like far more wear.
But I did run the bike in stop and go traffic for two hours plus in high summer heat and oil
pressure went down to 2 psi from its usual 10. I suspect the inlet valve seals got impaired and no longer did their
work. But maybe more damage than that. I don't have the experience to know what the effect of severe over heating is
on an air cooled engine. Ie what gets compromised. Knowing my luck, bore wear is an outcome....
For automotive, one never lets overheating happen...
There is a ridge of carbon oil crud on the outer top diameter of the combustion chamber in IMG_8813.
I have never seen that in automotive.
Rockers and valves come out tomorrow.
Thanks again
Dennis
As for mileage that is a tough one as the previous owner figured 10,000 miles or so.
I put on perhaps 2000 miles since I bought it. This looks like far more wear.
But I did run the bike in stop and go traffic for two hours plus in high summer heat and oil
pressure went down to 2 psi from its usual 10. I suspect the inlet valve seals got impaired and no longer did their
work. But maybe more damage than that. I don't have the experience to know what the effect of severe over heating is
on an air cooled engine. Ie what gets compromised. Knowing my luck, bore wear is an outcome....
For automotive, one never lets overheating happen...
There is a ridge of carbon oil crud on the outer top diameter of the combustion chamber in IMG_8813.
I have never seen that in automotive.
Rockers and valves come out tomorrow.
Thanks again
Dennis
Don't pull the rocker shafts out. You can get the valves out without disturbing the rockers. You just need the correct valve spring compressor tool.
Bikes being stood for ages, sometimes years, then getting sold to new owners who then ride them around without doing a full engine recommission/rebuild usually run into problems like this. Cylinder bores/ valves/ seals compromised by corrosion and age hardening. One cylinder is usually worse than the other because the engine when stopped has a valve/s open. A decent photo of the cylinder walls please.
Last edited:
marshg246
VIP MEMBER
- Joined
- Jul 12, 2015
- Messages
- 5,845
The ridge of carbon near the top of the cylinders is correct.
Do you have the workshop manual - you need to at least read through it once!
- Joined
- May 7, 2005
- Messages
- 7,639
A worn out +20 bore on a well maintained engine would be 80k miles not 12k miles.
Both valve guide seals leaking would be most likely, the bore being worn normally starts showing on the driveside and stays there until mileage builds up. But if the mileage of 12k is correct it means a rebore every 6k miles which is criminal.
Both valve guide seals leaking would be most likely, the bore being worn normally starts showing on the driveside and stays there until mileage builds up. But if the mileage of 12k is correct it means a rebore every 6k miles which is criminal.
Its surprising how many people tear into a strip down of an engine without reading the workshop manual. Even if they have got one to read.
I would estimate at least 60% to 70% of posts on forums such as this one are easily covered by reading the manual.
- Joined
- Apr 16, 2025
- Messages
- 61
"oil pressure went down to 2 psi from its usual 10"
10 PSI is nowhere near what it should be unless at a very slow idle
10 PSI is nowhere near what it should be unless at a very slow idle
Similar threads
