compression, how low, is to low?

[Howard]

Okay, so to start with, I just got a 1981 XS11 special (I posted previously about transmission troubles). Over the course of the last month or so, I stripped the bike down and flipped it over to perform transmission repairs. While I had the bike torn apart, I removed the carb stack with the intention of preventively rebuilding it. However, as soon as I got the carb stack off, it was apparent that it had very recently been rebuilt, presumably by the previous owner. So, without any adjustments to the carbs, I set them aside and completed the transmission repair. Well, last week, I got the bike fully reassembled. All the fluids filled, new air and oil filters, etc. when i fired it up i had a minor misfire and an irregular idle. I ran the bike down the road and back, and it mostly cleared up, but the misfire was still there now and again. The bike ran like a dream before I started, so I was hesitant to fiddle with the carb. And I mean perfectly, you bump the starter instantly running, with gobs of power, smooth, even power through the whole rpm range. You literally could not ask for it to have run better. So, I began running through everything else on my troubleshooting list to see if there was anything obviously out before I attacked the usual suspect. the carbs.... Everything looked good until I got to compression testing. I tested warm@WOT, and the results are as follows.

cylinder #1-55psi #2-60psi #3-60psi #4-65psi

The service manual calls for a MINIMUM compression of 128 PSI with no more than 14PSI variation between cylinders. I'm located just over 6000 feet above Sealevel, so it's to be expected that pressures will be slightly lower but not that low. Now, as aforementioned, the bike ran perfectly before I tore it apart, so I'm inclined to think the misfire is not compression-induced. But in any case. At what pressure are you all rebuilding /junking an engine at? and are there affordable piston kits for the 1100s, or do I need to be looking into a used motor swap?

thanks, and sorry for the wall of text, im not very good at condensing.

[cajun31]

Okay, so to start with, I just got a 1981 XS11 special (I posted previously about transmission troubles). Over the course of the last month or so, I stripped the bike down and flipped it over to perform transmission repairs. While I had the bike torn apart, I removed the carb stack with the intention of preventively rebuilding it. However, as soon as I got the carb stack off, it was apparent that it had very recently been rebuilt, presumably by the previous owner. So, without any adjustments to the carbs, I set them aside and completed the transmission repair. Well, last week, I got the bike fully reassembled. All the fluids filled, new air and oil filters, etc. when i fired it up i had a minor misfire and an irregular idle. I ran the bike down the road and back, and it mostly cleared up, but the misfire was still there now and again. The bike ran like a dream before I started, so I was hesitant to fiddle with the carb. And I mean perfectly, you bump the starter instantly running, with gobs of power, smooth, even power through the whole rpm range. You literally could not ask for it to have run better. So, I began running through everything else on my troubleshooting list to see if there was anything obviously out before I attacked the usual suspect. the carbs.... Everything looked good until I got to compression testing. I tested warm@WOT, and the results are as follows.

cylinder #1-55psi #2-60psi #3-60psi #4-65psi

The service manual calls for a MINIMUM compression of 128 PSI with no more than 14PSI variation between cylinders. I'm located just over 6000 feet above Sealevel, so it's to be expected that pressures will be slightly lower but not that low. Now, as aforementioned, the bike ran perfectly before I tore it apart, so I'm inclined to think the misfire is not compression-induced. But in any case. At what pressure are you all rebuilding /junking an engine at? and are there affordable piston kits for the 1100s, or do I need to be looking into a used motor swap?

thanks, and sorry for the wall of text, im not very good at condensing.

Found this on another motorcycle site.

Altitude Reduction Factor
1000' .9711...……………..Actual reading x 1.0289 = Approxmiate Sea Level Compression Reading
2000' .9428...……………..Actual reading x 1.0572 = Approxmiate Sea Level Compression Reading
3000' .9151...……………..Actual reading x 1.0849 = Approxmiate Sea Level Compression Reading
4000' .8881...……………..Actual reading x 1.1119 = Approxmiate Sea Level Compression Reading
5000' .8617...............Actual reading x 1.1383 = Approxmiate Sea Level Compression Reading
6000' .8359...............Actual reading x 1.1641 = Approxmiate Sea Level Compression Reading
7000' .8106...............Actual reading x 1.1894 = Approxmiate Sea Level Compression Reading
8000' .7860...............Actual reading x 1.214 = Approxmiate Sea Level Compression Reading

Using the multiplier against your figures indicate they are very low even after adjusting for altitude. Might want to try putting a spoonful of oil in each cylinder and then immediately check compression again to see if you get a marked increase. If you do that could indicate ring problems. Because the bike is new to you I would do a valve clearance check. With all the cylinders being low I would suspect ring wear or even possibly you just have some rings just not moving around freely. What we have all found over the years is that when these bike sit for long periods of time the rings get crud around them and until you run the bike for a while they don't free up and your compression is low. If you have marvel mystery oil available to you, get some and squirt a good amount in each cylinder and let it set for a couple of days then crank the bike without starting it to move it around a bit. Then run the engine until warm again a do another compression check. If still low you might be looking at a top end rebuild. I just posted this information on another thread. Several members have reported that these kits are OK. Search Results for “xs 1100” | Page 2 | CRUZINIMAGE.NET

[DEEBS11]

You look like you are the victim of a Chinese made compression tester. They always read low. However, the fact that each cylinder is close is a good sign. I would not worry about compression at the moment. Even though the carbs were clean looking they should still be cleaned. Crap lies where you can't see in the smallest orifices. Especially the pilot circuit.

[LAB3]

If a bike hasn't been on the road while you can expect those compression numbers to be low. The rings have set awhile in the cylinders and need to be worked to get them back up to snuff, give it about a hundred miles of riding time and recheck it.

As the the occasional misfire, if the bike ran better before you took the carbs off I'd go over the boots on both the intake and exhaust side of the carbs and make sure everything is lined back up the right way. Even the smallest air leak will give you some grief. If they check out OK try running some Seafoam or Berrymans fuel system cleaner in the gas tank. No need to exceed the recommended amount printed on the can!

[DEEBS11]

All good suggestions. What I do for a quick pilot orifice carb clean is remove only the pilot fuel screws (remember how many turns out) and pilot jets. Blast some carb clean through those orifices' and also the air pilot jet where the air box connects to the carbs. Blow some compressed air through and put it back together.

[DiverRay]

OK, everyone missed the elephant in the room. When you pull the carbs and put them back on, you MUST check them for sync!
Lab3 is correct about trying to get a good compression test, the bike needs some miles on it before trying. I had an '81 MNS that was up for sale. I had not been riding it much, as I was using other bikes for most my rides. When I went to do a cold compression check, everything was about 70. After a 20 minute ride, let cool just a bit, and all were back up to 140 range.
Again, you need to check sync with gauges to make SURE all 4 carbs are pulling the same vacuum, at idle as well as part throttle.