XR75 New Rebuild smoking

4551 Views 3 Replies 2 Participants Last post by mderbish, Dec 13, 2016

mderbish

Discussion Starter · Dec 12, 2016

Hi,
Just finished my 1978 XR75 Engine rebuild. all new seals, new piston, new rings, cylinder smooth and in spec, rings positioned properly, re-lapped the valves.

The engine is running good, but it is smoking (white/blue). Lots when I rev it. I did but a little oil in the cylinder during the rebuild so that may be the cause, but I'm worried it is something else.
I did replace the exhaust valve seal during the rebuild (intake has no cam seal).

Ran it for about 20min, still smoking. How long does it take for the oil to burn out of the cylinder?
I check the plug and it was a little oily. Spit out oil spray when running w/o muffler then I first started it. Now I see a little oil seeping out of the exhaust muffler pipe seal.

Compression is OK (~120).
Do new rings and cylinder need to break in to do a good oil seal?

Is there any way to diagnose the cause before I drop the engine again and pull the head?

Thanks -Mike.

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mderbish

Discussion Starter · #3 · Dec 12, 2016

storminnorman07 said:
Was the cylinder honed before install of new rings and piston? What was piston to cylinder wall clearance?

I didn't hone the cylinder.

Thought I could get by without. The cyl looked new and measured well in spec.
The old piston and rings were at the end of their wear tolerance and compression was low ~90, but no smoke.

The new piston and rings improved the wall clearance and is now in spec. Compression went up with the new piston and rings to about ~120.

Will this improve with wear? Or is my only option to drop the engine and pull the top end again?

-Mike.

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mderbish

Discussion Starter · #4 · Dec 13, 2016

mderbish said:
I didn't hone the cylinder. Thought I could get by without. The cyl looked new and measured well in spec.
The old piston and rings were at the end of their wear tolerance and compression was low ~90, but no smoke.

The new piston and rings improved the wall clearance and is now in spec. Compression went up with the new piston and rings to about ~120.

Will this improve with wear? Or is my only option to drop the engine and pull the top end again?

-Mike.

Just read why you hone a cylinder.... Need to drop the engine and hone the cylinder. Too bad you can't pull the head/Cyl on these bikes w/o removing the engine from the frame.
Live and learn.... rebuild and learn more.....
-Mike.

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