Compression

[Chicagoxs]

I am picking up a 79 midnight special tomorrow. Can anyone tell me what healthy compression is for these motors

[BA80]

There's no such thing as a 79 MNS.

I do have some Arizona beach front for sale though.

[Old Red]

It’s probably a 1980 that was built in 1979. What is the serial number?

[BA80]

I did a compression check a little bit ago and it's 115/105/105/115.

Typical readings.

[Chicagoxs]

I don't have the vin. It may not be a 79 mns, the ad expired. I'm pickin up another xs to bring back to life tomorrow and forgot which model it was exactly.

[TopCatGr58]

Hey Chicago,

You probably know that the FSM says that 140 is stock new level. If at or below 90 and the engine/rings are shot. Those levels are probably okay, and if the engine has been sitting a long time, the rings may be stuck, etc., and they may improve with both the application of MMO as well as several hundred miles to reseat them.

Speaking of comp levels, Greg, have you rechecked yours since you cleared that clogged exhaust system? I bet they are actually higher than what you recently measured.

T.C.

[BA80]

Hey Chicago,


Speaking of comp levels, Greg, have you rechecked yours since you cleared that clogged exhaust system? I bet they are actually higher than what you recently measured.

T.C.

Nope. It's running like crazy so I never rechecked. I figure between 100 and 120 is where the greatest number of these 35 - 40 year old engines is gonna be. As I said in my original thread, the compression guage I was using probably wasn't the best anyway

[TADracer]

One thing you should consider is that compression is sometimes like torqueing a pattern of bolts. Its not so much that you hit a certain number as much as it is that all cylinders are giving readings that are within a few percent of each other. A lot of things can affect the 'number' .

[crazy steve]

Keep in mind that low/uneven readings don't necessarily mean the motor is bad. A lot of these suffer from lack of maintenance and if the valves haven't been adjusted in a long time doing that can bring readings up a bunch. I had one motor that doubled/tripled the numbers on a few cylinders after adjusting the valves. Once adjusted, the numbers were excellent. Generally, if the bike has 40K miles or more on it and the seller doesn't know when the valves were adjusted, they'll be tight with a loss of compression (usually the exhaust valves).

[MPittma100]

Compression

Look for 130+. All should be within 10 psi of each other. Check only with carbs at full throttle. If any holes are at zero, there can be several causes.

[kboehringer]

MMO Works with proper valve adjustment

Can anyone tell me what healthy compression is for these motors

Chicago,
I would not necessarily use a compression test as a buy/don't buy test of a bike I was going to buy. If the compression is great that would probably show that the valves have been maintained properly and the bike is in good condition. On the other hand, bad compression could merely mean that you have a good motor with badly adjusted valves.

I've purchased three XS11's in the last year (all clunkers) and none of them had good compression.

The first XS (The Duck) is now at 140-135-135-135 after a proper valve adjustment, a little Marvel's Mystery Oil in the slugs, and several hundred miles of driving. That bike had probably sat outside for several years.

My second purchase, (Dead Duck) had basically 35 pounds across the board. It's had a treatment of MMO and is now in the 90# range across the board. All intake valves and one exhaust are poorly adjusted. Once the valves are properly adjusted I am certain it will jump up into a much better place. Add some heat from driving and I bet I'm close to stock like The Duck.

Personally, I though this MMO treatment to the rings and rings being "STUCK" was a bunch of baloney. I'm a pretty experienced motor head and this just didn't make sense. However, the MMO worked well to my huge surprise. I guess, many of these bikes are left out in the elements and they are much more likely to get some water/condensation in the bores freezing the rings than a small block Chevy or some other factor that makes them more likely to get "STUCK" rings. Whatever the cause, it's an easy fix.

KURT

[dbeardslee]

I though this MMO treatment to the rings and rings being "STUCK" was a bunch of baloney. I'm a pretty experienced motor head and this just didn't make sense. However, the MMO worked well to my huge surprise.

I used MMO on Betsy when I first got her. Not sure she needed it, but I figured why not? When I fired her up afterwards she did a pretty good impersonation of a WWII destroyer laying a smoke screen for the D-Day invasion. If you use it, make doggone sure you're outside when you start it up.