81 XS1100 burned cylinder

[BIGmackie]

Gentlemen,

I'm new to this forum and this bike model, but far from new to motorcycles or wrenching on them. I took on this project for a friend who is interested in learning mechanics and got this bike in non-running condition from the previous owner who tried to turn it into a bobber.

Here's what I've gotten myself into:
'81 XS1100 that didn't run due to "carb issues" my friend rebuilt the carbs on it, but didn't try to start it until he consulted with me. The first thing we did was run compression and found that the right two cylinders had about 130 psi compressions and the left two had 30psi. Suspecting valves, I pulled the valve cover and found the it was shimmed in spec, so I pulled the head and discovered the far right cylinder (#1?) had a burned piston (melted to the cylinder wall). I'm suspecting that it ran hot in a lean condition. We're considering going with a big bore kit to fix this problem, but my question is this: Is this a common problem with these bikes, or likely due to incorrect jetting and the removal of the airbox/ addition of cone type filters? What are the other common maladies that come from this type of malfunction that we should address while the bike is in pieces?

Thank you for any help anyone can give me, I'm good with bikes but not familiar with this model, so any intricacies I need to know about will help.

[PandaRescue89]

Thanks Mac,

This is the noobie learning on the bike, This forum is great, busted open my first carb(s) just by following the pictures

Thanks,
Alex

[MPittma100]

Melt down

There are several ways to burn a piston (excluding nitros oxide) in any engine.

Running the engine too lean or a much retarded ignition timing. Too lean can be incorrect jetting or a vaccum leak. In your case, make sure that you have correct jetting in all carbs and check the timing.

Inspect the vacuum/mechanical advance mechanism inside the left engine cover for proper operation. Inspect all of the carb boots on the cylinder head side for damage or holes. Common problem there is that the gasket between the head and boots fail. Silicone them back on and do not use a gasket.

Big bore is an option. If the one cylinder is ok. A replacement of one piston and new rings is the most cost effective fix.

MP

[skids]

Short pipes with no mufflers can cause very hot combustion at some rpm ranges and crappy scavenging at other rpm ranges. I would look for another engine with known good compression. It sound like rebuilding that one could be a gamble.

[DAVINCI]

There are several ways to burn a piston (excluding nitros oxide) in any engine.

Running the engine too lean or a much retarded ignition timing. Too lean can be incorrect jetting or a vaccum leak. In your case, make sure that you have correct jetting in all carbs and check the timing.


MP

much retarded ignition timing

Incorrect. The opposite is true.

[BA80]

much retarded ignition timing

Incorrect. The opposite is true.

+1 I meant to say something last night but got sidetracked.

[natemoen]

+1 I meant to say something last night but got sidetracked.

What kind of beer was it?

[BA80]

What kind of beer was it?

Cold.....

[MPittma100]

Mispelled Advanced

Sorry about the "much retarded timing" statement. Yes, "too much advanced timing" is correct. Spark knock or preignition occurs which can raise the cylinder temperature to piston melting levels.

MP