Low Compression?

[Billy Ricks]

Don,
I haven't run the stock CV's in a long time. It's been about 13 years since I did a compression check with those carbs. That was after missing a shift and burying the tach needle. That spun a couple of the threaded valve adjusters right out of the rocker arms. I did a compression check at that time to make sure I hadn't bent any valves or anything. I want to say there was about 20 lbs of difference between the readings with the slides down vs up. It's been so long though that I can't say for sure. I was disappointed in the readings I got with the slides down. After raising the slides the numbers were better.

[Don Lobacz]

Thanks, Billy. That is a significant difference.

[saaz]

The final compression numbers should ultimately the same, slides up or down. The key is how many revolutions does it take to get to the max reading...one or two, and things are dandy...if it takes a few turns, then things are not as healthy... Slides up gives a better chance for consistent readings and observing how the pressure is reached. If the bike still performs well, hopefully the readings are still within acceptable limits.

[Anonymous]

I got my screw in gauge. With a warm motor here are the readings. Cyl. #1 - 140 #2 -114 #3 - 116 #4 - 148. These were w/slides propped up and wide open butterflys. I checked one cyl. w/slide down and the numbers for that cyl. dropped 3 psi. I put a little oil in cyl. # 3 and the numbers dropped by almost 40 psi. I don't know what was going on there!
Maybe too much oil eh?

[Billy Ricks]

Most likely too much oil. You didn't happen to notice any up in the hose to the guage did you? If you had bad rings you would expect to see the numbers go up. If they don't go up, or very little, the rings and cylinders probably aren't in too bad of shape and valves are the problem. The only other thing I can think of, and it's not likely, is the extra oil didn't allow a valve to seat right and that led to the 40 psi drop.

A leak down test would tell you exactly where the problem is. Unless you know someone with a kit and a little know-how you may have to either bite the bullet and pull the head or have a leak down test done. I've seen guys make there own valves to fill the cylinder with air out of an old spark plug. You fill the cylinder with a certain air pressure and see how long it takes to drop. You can hear where the air is escaping. If it is valves you can hear it coming out the intake and/or exhaust ports. If it's getting past rings you can pull the oil filler cap and hear the air getting into the crankcase. Naturally some air is going to get past the rings and valves. That's why you need the kit to guage what is normal and what's not.

[saaz]

It only takes a little squirt of oil...teaspoon or so. If the centre cylinder readings improve, then ring problem. If not much improvement, valves (they run hotter on the inside cylinders as well)

2&3 can often run lower as the inside cylinders get hotter on air cooled 4s. Your readings are quite a bit lower..I usually see only a 10psi drop on mine (both when it was standard at 80,000kms and when a big bore kit was fitted just after that..I currently have 145,000km on the bike)

Unless performance is down a fair bit, keep on riding. Before the rebuild, mine was at 100psi and using oil..smoky but still went ok!!

[Don Lobacz]

That 40# drop sounds like a fluke (sometimes the o-ring on the gauge adapter doesn't seal right). If you recheck it after running and consistently get these numbers, then I would agree with Saaz that it should be OK to run like that. A topend teardown is really not that hard, either, and that will let you decarbon the valves/combustion chamber, lap the valves, replace the valve seals, and inspect the bores.