Low compression and low power, cheapest fix?

[BossMaverick]

I think I'll take Crazcnuk's advice and wait to see what Hastings has to say. I really don't have any problems with my lower end (minus an occasional 1-2 hard shift) and I have under 40k. If Hastings makes the right rings, I might just take the top end off, do some measuring, and see if I can just get by with rings.

The history of my bike was that it got a lot of miles on when it was new (judging from the different early 80's state park stickers on it) then not so much as years when by. The last owner just used it for occasional cruising with his wife around their lake but decided to downsize to a new 750ish V Twin. My guess is that my bike has stuck rings and/or has run lean sometime during those years (but hopefully just stuck rings).

[crazy steve]

There's no reason you can't check the compression on a XS engine that's out of the bike as long as the starter is still on it. All you need is a car battery and a set of jumper cables; pull all the plugs, screw your gauge into a cylinder and put the juice to her. Repeat as needed.

I will note that you'll get much higher readings with the carbs off. Removing that restriction will bump the readings up to around 200 psi on a good cylinder. I checked the one I just recently got and had 3 between 190 and 210, with one at 120. That one obviously needs attention....

'78E original owner

[BossMaverick]

I'm waiting till payday at the end of the month to take apart the top end to see what I have to work with. After reading everyone's comments again just now, the general feeling I'm getting is that I will probably need another set of jugs and pistons due to my 2 cylinders with low compression, right? If thats the case, I would rather hold off on taking it apart until I know I have some cheap good jugs and pistons lined up just in case I need them.

I was right about the engine off the local bike being priced way too high since he has it and I need one. He said something like $600 for the engine alone when I stopped in and asked.

[red bandit]

Maybe someone already said this. Do a compression test. Then squirt about 3-4 pumps of oil from an oil can in the cylinder in question. Redo compression test. If the numbers go up it's piston/ring/ cyl. related. If numbers don't go up, it's head related. A set of pistons, cylinders, head just sold for $16 plus about $25 shipping on e-bay. Add said off a running bike. I wouldn't buy anything until you know for sure what the problem is. But that's just me.

[BossMaverick]

The weather was nice enough and the curiousity got the better of me so I ran a compression test this morning. The last time I checked the compression the engine was still warm from running while this morning it was about 20 degrees F and it had not been running. If I recall correctly, the bike had right at 36,000 miles the last time I pulled the plugs and tested the compression. I ran the compression without adding fluid and then with a straw full of sea foam thru the spark plug hole (my oil is outside and was too thick to play with easily).

Here are the results:
#1: 150 160 (w/s.f.)
#2: 145 160 (w/s.f.)
#3: 160 180 (w/1.5 shots of s.f. since I miss half of the first shot; spark plug showed signs of running slightly lean)
#4: 145 170 (w/s.f.)

I'm sure the second numbers would have been a lot higher had I used oil instead of sea foam. My compression tester is cheap and reads high (the needle sits at 20 without pressue). The last time I checked the compression the 140-145 number sicks out for two cylinders but it seemed like the other two were 160+. I did notice more black on the plugs then the last time I checked (the ceramic was clean but the metal was a dry black). The #3 plug showed signs of running slightly lean on the ceramic but was still black on the metal portion. I was planning a carb rebuild this winter or spring so that'll take care of the #3 lean problem.

After the compression test, I ran it and let it warm up. Like normal, I did not see any blue smoke on start up or while reving it. To tell the amount of blow by, I plugged the breather hose with my thumb for about 3 seconds (I previously re-routed it from the airbox to the back of the rear fender due to the amount of oil going into the airbox). There was a hiss when I relieved the pressure, not a great amount of pressure like I imagine a hole in the cylinder would put out but still a good amount. I know the vast majority of my oil goes out the breather tube but it wouldn't suprise me if I was burning a little too.

So what do all of you think? The mileage now is just shy of 39,000.

[BossMaverick]

It just dawned on me that the last compression I did was right after I adjusted my valves. The valves were way off before I adjusted them to the point it was started to run bad (the previous owners must not of adjusting them in a long time). I'm thinking that the valve seats cleaned up a little with the mileage since the last check and somewhat even out the compression that way.

[DiverRay]

I would pour a little seafoam in each plug hole, and let it set for about a day with the plugs back in two or three turns. Then go back and do it again, after turning the engine over a little with the starter. This will free up any stuck rings. Then the next day, put about three ounces of ATF (automatic trans fluid) into each hole, put the plugs back on, and wait for two days. Try a restart and see if the breather tube shows less signs of pressure build up. I would also pull the cover the tube mounts on, and clean out inside. you may have some oil or sludge build up, and the oil may not be able to drain back into the crank like it's supposed to.
Less than 40K is WAY to low too need rings, IMHO

[DenyP]

Maybe someone already said this. Do a compression test. Then squirt about 3-4 pumps of oil from an oil can in the cylinder in question. Redo compression test. If the numbers go up it's piston/ring/ cyl. related. If numbers don't go up, it's head related. A set of pistons, cylinders, head just sold for $16 plus about $25 shipping on e-bay. Add said off a running bike. I wouldn't buy anything until you know for sure what the problem is. But that's just me.

That was a heck of a deal, just couldn't pass it up. I have low compression on #4. I figure it makes sense to clean them up, do a valve job and hone the jugs so I can have every thing I need on the bench when I pull her appart. (Ya I know best laid plans etc - **** still happens)

Deny

[eroellig]

My 2-cents worth.....

When I got my bike, I had a couple of low readings (on 27k miles) as well. This bike had sat for 3 solid years, with little use in years previous. The prevailing wisdom here was that I had stuck rings. While I had the bike torn down (bearings, brakes, etc...), I would shoot penetrating oil into each of spark plug hole now & then and turn the engine over. When I was ready for first start, I changed the oil to get rid of the junk I'd been squirting in there. The stuck ring un-stuck not long after. I've had decent compression readings since.

YMMV

Eric

[BossMaverick]

So you guys are saying that I could still have stuck rings even with me putting about 4,000 miles on it last summer? I was always under the impression that putting miles on loosens up rings.

[Crazcnuk]

No, they can stay stuck for quite a while sometimes.

Back in the late 80's I had a 650 Midnight Maxim, it ran good, no problems. Just for the heck of it I did a compression test, one day, and found all 4 were ~95lbs.

I put a few tablespoons of diesel in the sparkplug holes, overnight, drained and changed the oil the next day. Took it for a ride to warm it up then tested again and all 4 were ~135lbs!

I must have had 2-3000 miles on it at the time. That was the first time I encountered stuck rings. Now, I do the 'soak' to any engine I get that has sat for any length of time.

[BossMaverick]

I will give Sea Foam in the cylinders a shot tomorrow then. I will give a couple days to soak and report back with my findings.

Thanks guys.

[Crazcnuk]

With Seafoam, since it is fairly thin, it may be an idea to put a tablespoon at a time, 3 or 4 times over 2 days, as it may run through the rings fairly quickly.

Before you change the oil, afterwards, start the bike and let it warm up and the Seafoam will give your crankcase a clean.

[BossMaverick]

Well I discovered one problem tonight. I was replacing my spark plug wires and took ohm readings without the wires on. Turns out that one of my coils is shot. That could explain some of the low power.