QD-13 Compression

[OldAmerican]

Ran a compression test on my '52 10hp Johnson and only got 35lbs/cylinder. So ran a lead-down test and it only showed a 10% loss in both cylinders. Doesn't add up but then this is my first adventure with an old 2-cycle. Anyone have some wisdom to impart?

 

[HighTrim]

Re: QD-13 Compression

Pull the head and inspect the head gasket for damage between cylinders.

You are confident in this gauge?

 

[OldAmerican]

Re: QD-13 Compression

Gauge has never failed me before but then I have loaned it out a time or two since I last used. :-( Considered the blown head gasket possibility but should have heard some leakage into the adjacent cylinder during the leak down.

 

[HighTrim]

Re: QD-13 Compression

Perhaps try the gauge on something else?

Also check the head for square while off with a straight edge and dress before re installing.

 

[OldAmerican]

Re: QD-13 Compression

Started to pull the head just to see if there was cylinder wall or piston damage but there are several bolts that are way too difficult to loosen. Fearing I will twist off so want to be sure there is a problem before I risk creating an even bigger problem for myself.

 

[oldboat1]

Re: QD-13 Compression

wait -- You're doing this because you have operating problems? These oldies can run well on what seems to be low compression, if both cylinders are about the same (no more than 10% difference, as a rule of thumb). A cold engine will give lower compression of course. Can you get the motor running, or is that why you are into the testing?

 

[boobie]

Re: QD-13 Compression

Get a second opinion. Try another compression gauge.

 

[OldAmerican]

Re: QD-13 Compression

long story. I gave the motor (along with a 12' aluminum boat) to my daughter and son-in-law. They were out on the lake at full throttle when it made a loud noise and lost power. I haven't attempted to run it since. Feared the worse (no/insufficient oil in the gas) so drug it back home to troubleshoot. Have not attempted to run. Too bloody cold out there for that now!!!

 

[boobie]

Re: QD-13 Compression

When you get a chance, try and start it once and see what happens.

 

[OldAmerican]

Re: QD-13 Compression

Roger. Perhaps I am worrying about nothing. That would be a pleasant surprise. Now if we can just get a break from winter.

 

[oldboat1]

Re: QD-13 Compression

dosn't hurt to get lucky now and then. oil mix should be 24:1, I think, but it would be good to check on that before trying to fire it up again. If you know there is corrosion at the head bolts, it wouldn't hurt to soak them and others for future reference. One of the best solvents is ATF fluid. It also wouldn't hurt to spray some fogging oil into the cylinders through the plug holes, then slowly turn the motor a little with a couple of pulls. Put the plugs back in after you do this. Cold compression will probably be improved by this, by the way, after the rings are lubed up a little. If you're getting a 35 compression rating now, it will probably double if the engine is warm and gets some lubrication. Guessing, though, that compression of 40 - 50 lbs or so in both cylinders would give good performance.

 

[HighTrim]

Re: QD-13 Compression

dosn't hurt to get lucky now and then. oil mix should be 24:1, I think, but it would be good to check on that before trying to fire it up again. If you know there is corrosion at the head bolts, it wouldn't hurt to soak them and others for future reference. One of the best solvents is ATF fluid. It also wouldn't hurt to spray some fogging oil into the cylinders through the plug holes, then slowly turn the motor a little with a couple of pulls. Put the plugs back in after you do this. Cold compression will probably be improved by this, by the way, after the rings are lubed up a little. If you're getting a 35 compression rating now, it will probably double if the engine is warm and gets some lubrication. Guessing, though, that compression of 40 - 50 lbs or so in both cylinders would give good performance.

Sorry but going to have to disagree with you on that one. She will not run on 40 to 50 psi. All of my good QDS are in the 90 to 100 psi range. That is with an accurate gauge. They will run with less, but not as well.

 

[oldboat1]

Re: QD-13 Compression

You may be right on that, HighTrim -- think I probably place more emphasis on balance between cylinders, and I'm most likely to check compression on a cold motor (as in one I may have just picked up). If OldAmerican's motor fires up and gets to operating temp, a compression check at that point would no doubt be the one that matters.

 

[racerone]

Re: QD-13 Compression

Pull the 2 bypass covers off and have a look at the pistons and rings.----Might have to make 2 simple gaskets if you damage them.

 

[dazk14]

Re: QD-13 Compression

If you don't have another gauge...You can do the finger blow off test - the PO Folk way.
pull plugs, move wires out of the way, ground them.
Put finger over plug opening to try and seal the compression.
Pull starter rope - yes, it's awkward. Give it a solid/quick tug.
If it blows your finger off, you have decent compression.
If you can prevent the compressed air from escaping, low compression engine/issue.

Somehow the "blows your finger off" phrase doesn't sound good.LOL I did say ground the wires - although none of us do that, including my friend, Lefty.

On your gauge, is your shraeder valve at the base of your rubber hose loose? That will cause a low reading even if the upper shraeder valve at the gauge is working.
If it is leaking, you'll wind up with 1st pull pressure just repeating itself as the volume in the hose creates an artificially low compression ratio, especially dramatic in small engines.

 

[dazk14]

Re: QD-13 Compression

Added shraeder valve info above