• It's time to vote for the bikes you want to see in the 2025 XS650 Calendar! Vote here!

The meatball mechanic: Low Compression fix.

gggGary

gggGary

One down, one to go.
Top Contributor
XS650.com Supporter
Messages
31,971
Reaction score
66,820
Points
813
Location
Baraboo, WI, USA
Per recent thread about my daughter's car wreck I just sold a 77 and pulled out a 78 to get it running to sell. As always I pulled plugs and ran a compression test, Left 110 Not great but OK on a bike that hasn't run for a while. Right side 20 pounds, CRAP! I put it on TDC compression stroke for the bad side, zeroed down the air compressor, clipped the chuck on the compression tester hose and slowly added air pressure. I could hear the air was going out the exhaust valve and not really anywhere else. I was really NOT wanting to do an engine overhaul so I checked valve clearances and that valve had 20 thousandths of an inch clearance. Way too much clearance, still that by it's self wouldn't cause a 20 pound reading. But it did kinda confirm a sneaking suspicion. So with nothing to lose I turned the engine to open that exhaust valve fully. I pulled that exhaust pipe off, got out my rifle cleaning kit,
Basically like this.

gun cleaning kit.jpg



fitted a small brass bore cleaning brush, bent it so I could get it in between the valve and seat from the exhaust port and scrubbed all the way around the valve and seat. The valve is clearly visible and reachable through the port. I then blew compressed air in the plug hole to blow the carbon out. I figured (hoped) there were just some chunks of carbon on the seat that were keeping the valve from closing. I put the compression tester back on and got 140 pounds. :D I then checked that valve clearance again and sure enough .06" right on spec! I hit all four valve stems with some PB blaster and sprayed some in the plug holes too. both sides are up near 150 now. Next step, get r running and see what I have!

By the way the rifle cleaning brushes work great for scrubbing between engine fins and other hard to reach nooks and crannies too.
 
JWMX
My compression tester has an air hose type coupling so you can disconnect the gauge from the hose. I couple my air compressor line to the spark plug hole and slowly raise air pressure with the piston at TDC on the compression stroke. It's just air going in and it has to go out some where:
Hear air hissing at the crankcase breather; bad piston rings, scored bore etc.
Hear air hissing at the muffler; bad exhaust valve seating
Hear air hissing at the carb; bad intake valve seating
WARNING the engine may turn when you add air. If it's in gear on the side stand it could ruin your day. in gear with rear brake applied will stop it from turning. Exact TDC isn't critical but the further away you get, the more leverage the air pressure, piston will have to turn the crank.
 
Last edited:
Check YouTube for "leakdown test" and you'll get the idea. I just learned this myself but you basically put the cylinder at TDC and then put compressed air in the cylinder and see where it comes back out. Could be exhaust, intakes, radiator, oil dip stick etc... Each leak tells a different story. I just used thison my Yukon to give it a checkup. You start the process with a dry and then wet compression test.
 
If you look at a professional leak down tester it just tells you how much air you are losing. I'm not convinced it's that important if you already know you have a problem. A lot of compression test hoses will also plug into your air line so just us that with a little pressure.
 
gggGary said:
JWMX
My compression tester has an air hose type coupling so you can disconnect the gauge from the hose. I couple my air compressor line to the spark plug hole and slowly raise air pressure with the piston at TDC on the compression stroke. It's just air going in and it has to go out some where:
Hear air hissing at the crankcase breather; bad piston rings, scored bore etc.
Hear air hissing at the muffler; bad exhaust valve seating
Hear air hissing at the carb; bad intake valve seating
Click to expand...


You deserve a feather in your cap on this one Gary! First time I've ever seen anything like it posted.....potentially a true labor savor. Come up with some minimalizing the handling of firewood from the standing tree to the woodstove please.
 
nj1639 said:
LOL...defeats the purpose.
Click to expand...

But that's the way of minimizing it that everybody else uses :)


It occurs to me that a compressor really isn't necessary to check for this. But it would help in confirming it. Just look for really excessive clearance on the valve, like Gary saw.
 
xjwmx said:
But that's the way of minimizing it that everybody else uses :)
Click to expand...

True, but what happens when the light doesn't go on when you flick the switch

I used light the woodrange, cook my bacon and eggs boil the billy for a cuppa and to wash up, and all that within 1/2 an hour, + it would heat up the room and the oven would be hot enough to cook a batch of scons. Unless you have been there it is hard to believe. mind you i did chop the kindling and have it by the stove from the night before.
 
Fire wood warms you twice.

Gary and everyone bring back a junk engine, please squirt some oil in the cylinders and kick her over with the plugs out to splash out the excess oil. This will get oil on the sealing rings and valves and eliminate a dry start. Sometimes, and pray it never happens, a perfectly good old engine will hang a valve open on the first dry start and the piston will bend it. I'd say about one in fifty, which is enough for preventative care.

Tom Graham
 
The short path from tree to stove is getting ahead. I have three or four years in the drying racks. I go to the woods on cool crisp fall days. I can hardly wait to start cutting this fall. All the wood but crotches is hand split. For the few pieces I can't split the 660 does the job. Have total of about 20 years of wood heating under my belt. The furnace never runs unless we leave for a few days.
 
Tomterrific said:
Gary and everyone bring back a junk engine, please squirt some oil in the cylinders and kick her over with the plugs out to splash out the excess oil. This will get oil on the sealing rings and valves and eliminate a dry start. Sometimes, and pray it never happens, a perfectly good old engine will hang a valve open on the first dry start and the piston will bend it. I'd say about one in fifty, which is enough for preventative care.

Tom Graham
Click to expand...

+1 on that, I have started pulling the valve covers and hitting the valve stems between the springs with PB blaster before the start up too.
I am getting pretty good about pulling plugs and doing a compression test then spark test before I ever try to go fire so the engine has a chance to get the oil spread around and everything a little juicy before it has to deal with the stress of running. And ALWAYS smell the oil, if it looks thin and smells like gas it has to be changed before a start up.
 

Similar threads

O
HELP!
Replies
4
Views
241
Jan_P
J
T
My 75 won't start.
2 3
Replies
52
Views
1K
jaybar6
jaybar6
T
Low compression.
2
Replies
28
Views
2K
toglhot
T
7
'77 XS360 flooding on left side at wide open
Replies
5
Views
392
74Torino
7
L
Any Advice To Get This Engine Started?
Replies
18
Views
1K
jetmechmarty
jetmechmarty
Back
Top