Top end rebuild (inquiry)

cobrasneverdie

cobrasneverdie

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Hello guys,

I'm here to inquiry about when a bike would need a top end rebuild. As far as I know the reasons would be with ring and valve leaks. Am I missing anything else?
compression timing is the way to determine this as far as I know. Is there a way to improve compression? I do believe one side is less than the other, this was 2 years ago. The bike has accumulated 17,000 clicks on it. I'll have to do a compression test the proper way
 
cobrasneverdie said:
Hello guys,

I'm here to inquiry about when a bike would need a top end rebuild. As far as I know the reasons would be with ring and valve leaks. Am I missing anything else?
compression timing is the way to determine this as far as I know. Is there a way to improve compression? I do believe one side is less than the other, this was 2 years ago. The bike has accumulated 17,000 clicks on it. I'll have to do a compression test the proper way
Click to expand...

Reasons for a top-end rebuild may include:

Very low compression or large difference between cylinders. Worn rings/cylinders and valve sealing.

Lots of smoke on start up and running............could mean valve seals are worn out.

Large increase in oil consumption, and puffs of black smoke out the exhaust..........could mean the head gasket is passing badly. Spark plugs blacken quickly.Compression may still be normal.

Last but not least, if metal/plastic debris starts showing up in the sump, then top end work would be wise.

17,000 kms is nothing on these bikes. These engine are tough as nails. If the last rebuild was done properly, and you have kept the camchain adjusted, the engine should be fine for 40,000 to 50,000 kms.
 
PO hadn't ran the bike ina long time cause I guess he didn't know how to work on it. He had harley's in his garage. I did notice bits of metal debris when I changed the oil and thick soot of oil. Changed the oil and babied it no black smoke at all I'm probably horrible with tuning the bike, especially carbs. But I'm on here to learn these things.
 
Identify it is metal and not alloy. Sump plugs have magnets in them so any metal should be caught by the magnets.
 
cobrasneverdie said:
Hello guys,

I'm here to inquiry about when a bike would need a top end rebuild. As far as I know the reasons would be with ring and valve leaks. Am I missing anything else?
compression timing is the way to determine this as far as I know. Is there a way to improve compression? I do believe one side is less than the other, this was 2 years ago. The bike has accumulated 17,000 clicks on it. I'll have to do a compression test the proper way
Click to expand...
I bought a new xs650 back in 1976 and ran it on and off for 29yrs and 115000 miles. The top end needed attention after 40000 miles, as burning a lot of oil (a pint every 500 miles) and cam chain worn out. All I did was change the rings and valve guide oil seals, and camchain. The pistons and barrels were great so were left alone. Bike ran for another 50000 miles before it started to use oil again. Of course, it depends how your bike has been looked after but at 17000 miles I would suggest that its barely run in. If you can see smoke ,on the over-run and the camchain adjuster has run out of adjustment then a look inside the top end is advisable
 
The reason most of these motors need the top end gone into today is a failing front cam chain guide. It's not a mileage thing, it's an age thing. After 30+ years, many of those front guides are literally falling apart. The rubber face strip is separating from the aluminum base.

A lot of work is involved getting into one of these, what with having to pull the motor out of the frame, so it's best to do anything that needs doing while in there. Change the valve guide seals because, once again, you have 30+ year old rubber parts here. You will usually find at least a few of the valves leaking slightly so a hand valve lapping will be needed. All the valve faces and seats will probably show some pitting, that's just normal wear and tear on an operating engine. Only the ones that have pitted badly enough will show leaks, but the others will leak eventually as their pitting increases. Best to lap them all.
 
Kathos said:
Reason Im digging into mine would be valve pitting and cam chain noise.
Click to expand...
Hi ....Slight Cam chain noise (Clicking) when the engine is STONE COLD is Ok, but it should disappear when warm (after a minuite or so) if it doesnt then it needs adjusting. This is how I ran my xs 650b and I got 40000 out of the first cam chain and 50000 out of the second.... 5twins is right in pointing out the front cam guide as a ''weakness'' I did change mine after 90000 but should have done it earlier . Valve pitting was never a problem , buy good practice to lap the valves in if you remove the head..
 
what are the the signs and symptoms for cam chain guide

if I was to relap my valves what would I need to look at in order to inspect leakage, black smoke?
 
I always flip the head upside down after it removed, put gas in the chambers and let it sit. My new engine before i rebuilt it, leaked badly. After i cleaned it, lapped them in, it leaked very slightly after about 50 min, just damp. Good enough. Engine assembled, it passed a leak down with almost nil leakage. So far its a great runner with lots of power.

Leakage while running is loss of compression. Buy a leak down tester and youll hear quick whats leaking. Wont see much but loss of power, unless its real bad, prolly wo.nt run.
 
I take the valve leak test a step further. After filling the combustion chamber with solvent (I use kerosene), I blow compressed air in the intake and exhaust ports. Leaks will show as streams of bubbles emanating from around the edge of the valve. I feel this gives a much more accurate assessment of how the valve is sealing and the results are immediate. No waiting around for hours watching for leaks. It also shows small leaks better.
 
Heres my reason......
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Another thing to look out for, is if you run out of adjustment when setting the ignition timing. This can be an indication that your cam chain is out of spec (stretched or worn). That also means that your valve timing is off as well. Just won't run 100% like that. Replace cam Chain. :doh:

If your going to re-ring that piston you better make sure its still within spec. Mic it .750" up from the bottom. :laugh: If you can find the bottom. :laughing:
 
Last edited:
the guy told me the kicker was locked up, that's why I couldn't turn the motor over. It sure was!! 50$ basket case.
 

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