xs1100 low compression

[roger]

hello everyone, well my motor is missing on # 4 cyl. i put my hand over the back of the carb and its got no suction, i checked the compression and its only got 50#. the intake valve clearance is 12.7 and the exhaust is 15.2 could that cause the problem. all the other cyl run good. is there anyway to tell if a ring is broke?

[oldyam80sg]

Did you squirt some oil in the spark plug hole and see if the number goes up?

[roger]

Did you squirt some oil in the spark plug hole and see if the number goes up?

no i did not. what will that tell me

[oldyam80sg]

I think if it doesnt change then maybe a bad valve.

[LAB3]

Those valve specs are puzzling, they should be in the .05 - .10 mm range

[DEEBS11]

Roger, the "squirt oil in the cylinder" trick from OY8 is to help determine if you have a ring problem. An introduction of oil will temporarily improve a faulty ring seal and you would see compression raise a moderate amount. If you have a valve problem, the oil would not change anything and you would have the same compression.

The most common cause of sudden compression drop is a bent valve that is not sitting on it's seat. However, if that is not the case, you may have a blown head gasket or a crack in the head.

LAB3 made a good point about the valve gap. I'm not sure about the numbers you have stated 12.7 & 15.2. Inches or millimeters?

[roger]

Roger, the "squirt oil in the cylinder" trick from OY8 is to help determine if you have a ring problem. An introduction of oil will temporarily improve a faulty ring seal and you would see compression raise a moderate amount. If you have a valve problem, the oil would not change anything and you would have the same compression.

The most common cause of sudden compression drop is a bent valve that is not sitting on it's seat. However, if that is not the case, you may have a blown head gasket or a crack in the head.

LAB3 made a good point about the valve gap. I'm not sure about the numbers you have stated 12.7 & 15.2. Inches or millimeters?

thanks guys ill try the oil trick today. my gauge shows in inches and mm.

[cajun31]

You need to download a copy of the service manual for the bike. The section on valve adjustments has a table included for both the intake and exhaust valve settings. It involves an acceptable clearance as you measure and you have to have a valve shim tool to remove the existing shims to check what size you currently have and the tables suggest what shim size you need to either open the gap or close the gap as needed. When doing that you really need to check all of the valves. Tell us all a bit more about the bike concerning the history and what has been done to it. Is the low compression a recent event and has the bike been running or did you just acquire the bike?

[DEEBS11]

Roger, using a feeler gauge to measure your intake and exhaust valve on your #4 cylinder with the cam lobe facing away from the valve shim, what are you reading? It should 0.0 some thing. No gap is an issue. If the valve is not seated because of no gap, you have little compression. Keep in mind your compression reading should be taken with the throttle fully open.

[roger]

Roger, using a feeler gauge to measure your intake and exhaust valve on your #4 cylinder with the cam lobe facing away from the valve shim, what are you reading? It should 0.0 some thing. No gap is an issue. If the valve is not seated because of no gap, you have little compression. Keep in mind your compression reading should be taken with the throttle fully open.

on my gauge the intake is .005 or .127 the exhust is .006 or .152 im going to get a new mm gauge today this one is old and rusty.

[LAB3]

Here's how to use a zip tie instead of a valve shim tool. It's demonstrated on a Suzuki inline four but the concept works the same way. Many don't like the idea for various reasons but it works for me!

https://youtu.be/DhmGBkje-dY?si=EXIoTJeZ5n_lu1uc

[DEEBS11]

.005 and .006 are plenty of gap. Not really that unusual.

[Ranger_xs1100]

I wouldn't worry too much about precise readings at this point during troubleshooting. I'd just check to see how the low compression cylinder valve gaps compare with the other cylinders. if it's close to the others, then probably not a valve problem.

[roger]

your not going to believe this but i thought i would check the compression before i put a little oil in the cyl and the compression was 145. it must of had a valve or ring stuck and it turned loose.it had ben setting all winter and when i tryed to start it it wouldn't start, i pulled the carbs off and all four slides were stuck, i cleaned carbs good put them back on and it was missing on # 4 thats when i found the low compression.

[DiverRay]

To have a valid compression check, the engine MUST be warm! When I sold my MNS about ten years ago, the buyer asked me to do a compression check before he drove the three hours to my house. Cold, the numbers were bad and all over the place. Took the bike out for a five minute run, let it cool a bit, and re-ran the test. Everything was good, 145 to 150 across all four.