CD-11’s compression issue

[frankr]

quote labrador guy:

Are you pulling our leg here? Why does only one motor in the pictures have the line going from the lower main bearing to the upper main bearing? The two motors in the foreground have the spot for the soft plug that has the serial numbers on it but the ones in the background don’t have a spot for those plugs. You are telling us a scenario that seams impossible! You do have everyone’s attention though!

dale

Looking at some pictures in the Johnson service manual, it appears that early models had the oil line on the other side of the block. Some pictures show the SN plug cavity as you see it here, some pictures (early) show no cavity. Later on, the cavity was moved to the upper cylinder to make room for a fuel pump.

Sorry, I don’t know the specifics, but it all seems legitimate.

[ede]

Seems silly to ask, but are the pistons installed correctly

[jerry-ahrens]

Good point Ed, on the piston’s being installed backwards.Also, those 5 1/2 motors are notorious for having broken shock absorber springs in the lower unit, which brings up Mumbles post about how fast the motor can be cranked over. it seems like every time I find one of the old CD motors, the spring is broken… I have an equally tough time finding one with a good powerhead. Just my bad luck I guess.
Ed’s post about installing piston’s reminds me of a V4 that was brought in to the local Marine Mechanics students at our local Vo Tech class. The dealer had been fighting this motor because it would barely even try to run, if at all. It turns out that someone during a rebuild had installed all four cross flow pistons backwards. This was after the students had worked on it for some time, and finally looked inside the cylinder with an inspection camera. This may not be the issue here, but an interesting story anyway!

[fleetwin]

Just my simple mind, but what difference would piston dome orientation make when it comes to compression readings? Perhaps the engines won’t run due to incorrect piston installation, but it does not seem like compression readings would be affected. Nonetheless, it only takes a few minutes to inspect for correct orientation.

[fisherman6]

I also find many sprung/broken shock absorbers in these 5.5 and 7.5 motors. Mine go back in after a good tight squeeze in a big C-clamp looking like this.

Then I use a brass shear pin instead of the steel drive pin and all is well. Just carry a couple spares.
-Ben

OldJohnnyRude on YouTube

[skanders01]

I am grateful for all of the input I have received here. Believe me, I thought long and hard before posting such an improbable situation. Seems as strange to me as to everyone else who responded and probably read for that matter. Here are some answers to the posted questions…
1. Not pulling anyone’s leg. I am truly struggling with this issue.
2. I have put several gages on the compression tester. and verified the compression test gage accuracy against several gages. The bleed valve holds pressure. I acquired a 1972 Johnson 40 HP with a boat I bought to mount three of my 5.5’s on. The compression test read 60 PSI on moth this motors cylinders. All this done, I will still buy and test with another compression gage to verify. Also hooked up the compression tester to my air compressor and it tested out in sync with the various pressures (compressor gage) I cycled it through.
3. Measured all cylinders compression prior to breakdown for rebuild. All were in this 30-35 PSI range. Also miced/measured all cylinder bores and pistons prior to rebuilding. All were not too far out of spec.
4. All compression tests (and starting attempts) were done using a cordless drill on low speed spinning the crankshaft.
5. The motor powerhead with the oil return head was different from the other three and I don’t know why. New oil return lines are new and on the opposite side on the others.
6. Can highly recommend a resource for Johnson "Sea Horse Green" if anyone is interested. PM me…
7. In hindsight wished I would have resolved compression issues prior to rebuild and cosmetics as well.
8. Honed them with a newly purchased ball hone. Seemed to work well. If anyone needs one for a +/- 2" cylinder this lightly used one with significant life left is available reasonably. PM me. Need to find $ for new powerhead! 😆
9. All shock absorbers were fine.
10. Backward piston installation is an interesting thought….even though I do know which is the correct orientation….can’t hurt to check; not beyond making a mistake or three.
11. To all of the "they should not even start/run" comments, two of the three I actually did total teardown/rebuild on won’t/don’t. The fourth motor I have ran and still runs fine, just went through cosmetic restoration on that one. The rub here is that it tests out at the 30-35 PSI range.
12. Believe me, I won’t be tearing these down before I solve this mystery.

I can’t begin to thank everyone for replying/commenting in an attempt to assist. Need all of it I can get! As long as you all will keep replyin I’ll keep tryin. I’ll update soon.

Regards,

Kyle Anderson
Trempeleau, WI
skanders@triwest.net
920-764-0501

[fisherman6]

Kyle, Are you saying that your compression gauge is giving you a reading of 60 psi on both holes of your 1972 40 horse? Someone will correct me if I’m off base but I believe that motor should be about twice that. If that’s the case them maybe your 5.5s were and are actually are at 60-70psi which isn’t out of the ballpark at all for these. They’ll certainly run at that pressure. My next step would be a leak down test and verify piston orientation.
-Ben

OldJohnnyRude on YouTube

[aquasonic]

Kyle, the correct piston orientation is when the lower sloped side of the piston faces the exhaust cover. If you were unaware of that during assembly, then odds are you have one or more pistons installed backwards.

Regarding the compression test method, I am not sure if your cordless drill is generating enough RPM for an accurate reading. When I rope over a motor, there are approximately 5-6 piston strokes that occur in a fraction of a second. Try roping one over and see if that changes anything.

[Mumbles]

quote skanders01:

4. All compression tests (and starting attempts) were done using a cordless drill on low speed spinning the crankshaft.

You might have answered your own question about the low compression numbers. Try turning the motor over with the drill set at high speed and see if the numbers improve.

Hard starting/poor running is another can of worms. These motors like to start on the rich side so opening the needle valves a quarter turn or so more from their optimum setting will help them start especially if it is cold outside. I’m sure all major ignition parts have been replaced along with new plugs. NGK B6S are good plugs for these motors.

About the compression test on the forty horse, was the motor being roped over or spun with the electric starter? As stated, those numbers should be well over 100 PSI.

When checking air pressure gauges, I like to compare them to a known good one which has been calibrated by an instrument technician. I find the cheapo smaller gauges don’t read accurately in the upper or lower pressure ranges so I set them for around 80 PSI which is approximately what these and other trolling motors should read. My favorite tester to use is my digital one, which is accurate at any pressure range.

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[bob-d]

Mumbles, what brand digital is that?

[Author]

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