Motorcycle Repair: Yamaha XS 400, exhaust pipes, valve clearances

Question
Mark,
I have a 1982 XS 400SJ.(twin) The bike starts fine, idles good, runs up to 6000 RPM smoothly, but with no power to carry a load. I have checked valve clearances,timing is good, and seems to advance normally. Carbs are clean, getting good fuel. I did notice left hand cyl. did not put out as much heat (at the tailpipe). I am out of ideas. Any thoughts?

Thanks in advance!

Answer
Hi Tab,

Sorry for the delay in replying. I usually reply within a couple of hours.

Perform a cylinder compression test. If compression is not within service limits, perform a leak-down test. These tests will determine the condition of the engine and help locate the source of defective parts, if any. Repair accordingly.

Compression and valve clearances must be within service limits beofre the carburetors will work properly.

Perform a spark plug test by grounding the plug against the engine while starting the engine. You should see a healthy blue arc at the plug's gap. The arc may not be present under compression, so try the following:

With a cool engine started and running, carefully touch the exhaust pipes with your fingers near where the exhaust pipe down tubes connects with the cylinder head. You may notice that as the pipe temperatures rises, one of the pipes remains cool. If so, either an ignition or fuel problem exists.

I return tech files by email. There was no email address with your question as requested and explained in my instructions.

Respectfully,
Mark Shively