help please

Fired up my 71 commando that i got last fall....seem to be running real hot on left cylinder....looked for air intake leak ..all seems good ....pulled carb and checked all seems good ...can anyone tell me how hot these motors should run ....left cyl 300....and right half that 130...did compression test left cyl 160....right cyl 110
Thinking maybe exhaust valve not sealing good ....any help greatly appreciated....thx

It sounds like the LH cylinder is pulling the load (hotter) and the RH cylinder is coasting (cooler) a bit. This will drive up the temperature of the LH cylinder and 300 degrees is a bit hot.

See why the RH cylinder is so cold, that will likely be the answer as to why your LH cylinder is so hot. I would check with valve lash (intake and exhaust) on the RH cylinder to see that you have proper lash.

Was this a good runner last fall or is this the first outing of a new bike to you? Your slides could also be out of balance causing the RH cylinder to work harder but the compression difference still needs to be resolved.

160 psi compression on the LH cylinder may be a tad high for a Commando but certainly ok. The 110 psi compression on the RH cylinder is problematic.

If the valve lash on the LH cylinder is proper you can re-run the compression test on the RH cylinder by first squirting a little oil in the cylinder. If the compression ratio on the RH cylinder jumps up significantly higher than the 110 psi then that is an indication that you likely have a ring problem and not a valve problem. If you still get around 110 psi on the RH cylinder after the oil then that suggests it is a valve or blown head gasket on the RH cylinder.

Let us know how you go with this.

thanks for your reply.....just picked up the bike last fall....apparently had been in storage 20 years....did the oil thing on the right cylinder ....with no change
I was thinking about checking valve clearance also ..will do that next before pulling head ...thx

Dances with Shrapnel said:

160 psi compression on the LH cylinder may be a tad high for a Commando but certainly ok. .

Click to expand...

Why do you say that 160 psi is "a bit high". ???

A stock Commando 850, with new pistons and a bit of running and everything stock (cam, compression) consistently gave 175 psi on both cylinders for me.
I'd expect my current 850 to give about that when finished too.

Might depend on the test gauge used, but this is a good brand. (remember to test HOT with throttle wide open)
Other engines I've had with more oem compression have given higher numbers.

If one valve is not sealing well, you can sometimes hear it when you kick it over (with the ignition off).
It can make a bit of a hissing noise.
Perhaps more audible with the exhaust off - which it will be if you need to take the head off.
Blown head gasket is not impossible either - especially around the front if those little bolts haven't been torqued.
goodluck.

Rohan said:

Why do you say that 160 psi is "a bit high". ???

Click to expand...

I did not say that, I stated

may be a tad high for a Commando but certainly ok

Click to expand...

I am going from recollection, nothing else. Perhaps I am a little off but it really is not critical in this diagnostics.

You could check to see if it has a RH4 head. They tend to crack at the intake runner. I had a RH4 head which did as I described. I found out it was cracked by spraying brake cleaner around the intakes.
Just my experience.

Badbird states that he fired up his 1971 model.
Therefore his bike would have a 750cc engine and I doubt it would have the RH4 head.

More tinkering today....adjusted valves...ran hotter on right side ....but still heat issue on left side .....comp test to day after running ...both cyl 150....170 with oil
pulled the head off and found one broken stud up front left side...have to order a few parts....just wondering whats the best head gasket to go with ??

thanks

Take a can of ether and spray it around the left intake. Perhaps a leak is causing the issue. Is your plug light compared to the other side?

badbird said:

More tinkering today....adjusted valves...ran hotter on right side ....but still heat issue on left side .....comp test to day after running ...both cyl 150....170 with oil
pulled the head off and found one broken stud up front left side...have to order a few parts....just wondering whats the best head gasket to go with ??

thanks

Click to expand...

The smart a** answer is "a new head gasket".

In all honesty I have heard lots of opinions on this and hopefully others chime in.

Me, personally, I would go with a solid copper and read up on here:

http://www.jsmotorsport.com/technical_compression.asp

for some ideas to assure a good seal. Spray gasket compound and threads and/or very fine copper wires looped on both sides of all oil passages is recommended.

Was the existing gasket blown or indicating loss of compression?

What are your plans going forward? Are you going to inspect valves, freshen up head while off or......? Has the problem been identified?

Dances with Shrapnel said:

badbird said:

More tinkering today....adjusted valves...ran hotter on right side ....but still heat issue on left side .....comp test to day after running ...both cyl 150....170 with oil
pulled the head off and found one broken stud up front left side...have to order a few parts....just wondering whats the best head gasket to go with ??

thanks

Click to expand...

The smart a** answer is "a new head gasket".

In all honesty I have heard lots of opinions on this and hopefully others chime in.

Me, personally, I would go with a solid copper and read up on here:

http://www.jsmotorsport.com/technical_compression.asp

for some ideas to assure a good seal. Spray gasket compound and threads and/or very fine copper wires looped on both sides of all oil passages is recommended.

Was the existing gasket blown or indicating loss of compression?

What are your plans going forward? Are you going to inspect valves, freshen up head while off or......? Has the problem been identified?

Click to expand...

The head gasket looked good...only thing we found was the broken stud...going to pull the valves out while head is off and check
Have to put it back together with new stud and head gasket ...see what happens ...fingers crossed

pete.v said:

Take a can of ether and spray it around the left intake. Perhaps a leak is causing the issue. Is your plug light compared to the other side?

Click to expand...

Did lots of spaying around intake ...seemed good....did spray head gasket and rpm when up

badbird said:

pete.v said:

Take a can of ether and spray it around the left intake. Perhaps a leak is causing the issue. Is your plug light compared to the other side?

Click to expand...

Did lots of spaying around intake ...seemed good....did spray head gasket and rpm when up

Click to expand...

If you sprayed ether and the rpm went up that indicates you may have an air leak. Could you isolate where you sprayed when the rpm picked up. Be careful with the ether while spraying around a hot running engine. You may want to try spray carb cleaner or an old propane torch where you can unscrew the air mixing nozzle and only have the small orifice blowing the propane out.

Are you sure the ether was not reporting to the mouth of the carbs?

Dances with Shrapnel said:

badbird said:

pete.v said:

Take a can of ether and spray it around the left intake. Perhaps a leak is causing the issue. Is your plug light compared to the other side?

Click to expand...

Did lots of spaying around intake ...seemed good....did spray head gasket and rpm when up

Click to expand...

If you sprayed ether and the rpm went up that indicates you may have an air leak. Could you isolate where you sprayed when the rpm picked up. Be careful with the ether while spraying around a hot running engine. You may want to try spray carb cleaner or an old propane torch where you can unscrew the air mixing nozzle and only have the small orifice blowing the propane out.

Are you sure the ether was not reporting to the mouth of the carbs?

Click to expand...

Sprayed around intake manifold numerous times....carb cleaner and ether ...no change...checked valves while head is off...found sticky exh valve on cool side
new head gasket ...valve and guide on the way.... and studs !


thanks