compression

Wed Feb 05, 2014 11:10 pm

I saw this in another thread, and I wanted to get a second opinion before taking the advice:

"you can take one plug out and install the tester and start it up , do the same the the other cylinders
the results will be the same as checking it with the starter, and save your arm :D "

Also, how much will changes in cranking speed change the readings on my compression tester? I'm asking because I got really low readings tonight (like 35 in each cylinder), but my battery was also a bit low, so the speed wasn't what it could be. I've got the charger on it and i'll check again in the morning, but I figured i'd ask you guys in the mean time. I've been doing some work on it - carb, valve clearance, clutch, wiring, plugs, fluids. It was running nicely before I took it apart, maybe a little low on power, but not THAT low.

Re: compression

Wed Feb 05, 2014 11:25 pm

Hand cranking compression tests results will be quite a bit lower than tests using the starter. You need 5 compression strokes per test to get the proper reading.

If the shop is unheated, wait and conduct the tests when things warm up. If you can't wait, bring the compression tester up to room temperature before conducting the tests.

Engine runs. 35 psi. Some thing wrong. Faulty compression tester or compression tester gasket, O-ring, not sealing would be my first guess.

Re: compression

Wed Feb 05, 2014 11:40 pm

i've got a wood stove in the shop, but it's undersized for the space.. only got up to like 45 degrees in there today. I brought the tester inside the house tonight, and i'll check in the morning with full battery and warm tester.

Re: compression

Thu Feb 06, 2014 11:26 pm

there was definitely a better result this morning, though still not great..

1: 80
2: 60
3: 60
4: 60

not sure if the low numbers are the tester's fault or the engine (the tester was warm, but it was also very cold in the barn). It ran fine before I messed with it, so i'm gonna leave it alone for now and just put everything back together.

on the plus side, my new home made wiring harness and battery cable both work great. hopefully my battery will charge now too!

I went oversized on the wiring with 10 gauge all around, and 1/0 to the battery. Everything is soldered, but I used a crimp connector at the battery post, is that gonna be ok in the long run do you guys think? Also, the light switch has a dim setting, but my volt meter doesn't see a difference in current from dim to bright.. the resistor coil is a little worse for wear..

Re: compression

Fri Feb 07, 2014 6:59 am

Compression is still low. Put a tablespoon of engine oil on top of each piston. Crank engine over a couple of times. Repeat for wet compression test.

Adjust valve lifter clearance. Also try SeaFoam to remove deposits from combustion chamber.

Borrow another compression tester, some auto parts stores have loaners. See if you get different, better results.

Re: compression

Fri Feb 07, 2014 10:08 am

your latest compression readings are still very low. being that low it should be hard to start. If you run the engine with a tester it may be a little higher but not much. If the reading is higher with oil squirted to the cylinders then the indicates worn rings. If no higher that indicates a valve problem. Or a combination of bad rings and valve problems. Optimum compression on a top notch engine should be 130 to 140 pounds with starter cranking with all plugs out and throttle wide open.

Re: compression

Fri Feb 07, 2014 11:41 pm

I haven't tried to start it in this cold before, but I never had a problem last summer. there was some smoke under load, but It plowed the hard sod up like a champ, so no major lack of power. I'm keeping my fingers crossed and leaning towards the tester being faulty. Next chance I get I'll call napa and see if they have a loaner. Maybe I messed up when adjusting the valves? I don't think so though.. didn't move them far. If i take the manifold off can i see clearly if they are seating?

unfortunately, I've already put the new plugs in and the hood is back on.. I'm anxious to start it up and see if my generator/wiring is working properly.

edit: oh, and I'll do a wet test when I get the loaner too

Re: compression

Sat Feb 08, 2014 12:07 pm

You can check the valve tappet clearance by removing the cover behind the carburetor.

You could remove the manifold, but that really won't help much. Wet test and another compression tester will tell you if the valves are seating.

I operated my Cub for two years with compression in the same range as shown. Worked ok for most tasks.

Compression testers. The ones that screw into the spark plug opening work better. They are not that expensive, might consider purchasing an other one.