CD-11’s compression issue

[skanders01]

Well, I have finished restoring my four CD-11’s and much to my chagrin, the new rings, cylinder honing, and new gaskets did nothing to add any significant compression to any of the motors. Still all cylinders in the 30-35 PSI range. Two motors did start and run in a barrel, but the other two wouldn’t start. Good spark, good fuel, but the compression is not not there.

I assume that if I want these motors to run I will need to find replacement powerheads or have mine bored out and install new oversize pistons/rings. Not sure which would be a more cost effective option. I do want these all to run to take out a couple of times a year. Don’t need them to be pristinely restored to OEM specs.

Any other suggestions? Any leads on either replacement powerheads or oversize piston kits? These old guys have come a long way and would like to finish the race.

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Attachments:

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

[frankr]

30-35 psi is absurd on four different powerheads. They should not run at all at that pressure. Something is seriously wrong or your readings are not correct.

[opposedtwin]

You’ll burn through too much money trying to find os parts and paying someone to enlarge the bores. Find different powerheads.

Just out of curiosity,why 4 of the same motor? And also would recommend getting the mechanicals taken care of prior to or simultaneously with the cosmetic restoration.

[fisherman6]

Did you check the bores of any of these while you had them apart? Bore gauge and measure, or check ring gap in the bore with the new rings, etc.? A slight taper in the cylinder can still benefit from new rings, but a badly worn cylinder that has become slightly out of round can actually get worse with new rings than it was with the old ones that were already worn into the irregularities of its bore. In that case the cylinder must be bored and oversized pistons and rings used.

With compression in the 30-35psi range I’d bet on the rings not sealing hardly at all. Hard for me to believe they will run at all with those compression numbers. My instinct tells me this is due to some out of roundness to the bores that is preventing proper sealing. Try a leak down test and see what kind of seal you have if anything at all. I’m with Frank that having those numbers on 4 different powerheads is absurd

I would try running one of the ones you got running in a tank for several hours and see if it starts to come up. If it comes up some I’d run it on a boat under load and varying speeds for a tank of fuel or so for a proper beak-in and see where that takes it. New rings in a well-worn bore can be kind of a gamble in my opinion. Often it will work but it ends up being a slow break in process.

-Ben

OldJohnnyRude on YouTube

[aquasonic]

It does defy logic that four different powerheads have compression numbers in the 30-35 PSI range. Those are blown head gasket readings. Need to verify the testing guage or testing procedure. By the way, nice paintwork.

[frankr]

Forgive me while I play detective here. By any chance, did you replace the schraeder valve in your compression tester with a tire valve? That can reduce the indicated readings on the gauge by 30psi or more.

I agree with whoever said it above, 30psi is blown head gasket or blown piston readings. Even slap worn out rings will do that good. Well actually, it shouldn’t run long enough to get them that worn.

[fleetwin]

This is a shame after all that cosmetic work, for sure….
Are you saying that all these engines showed low compression readings (30-35PSI) before you did the rebuilds? This just seems like too much of a coincidence to me, all four engines worn to the same low compression readings, then getting the same readings after honing and new rings. My first suspect would be your compression gage. Did you check compression on the one engine that ran after you ran it?
What did you use to hone the cylinders, and how much did you hone them? Did you check the ring end gap on the new rings after you honed the cylinders? How well does the one engine run? Start and idle OK?
Again, I suspect a compression gage malfunction.
Please don’t go ripping these engines apart again, until you have confirmed low compression readings with another gage and made more attempts to get them all running….

[crosbyman]

all low compression…. common point is the pressure meter … have you tried another pressure meter

4 identical readings is near impossible what was compression …before all that fixing up ???

are you spinning the engines fast enough with opened full

Joining AOMCI has priviledges 🙂

[Mumbles]

The CD 10-12 models have the small starter on them and are harder to rope over than the later models equipped with the larger elliptical starter so they may not be getting up to the needed speed to get a good reading. New rings in a used bore will take a lot longer time if ever to fully seat in than new rings in a new bore so the motor should be run for at least ten hours break in time before accepting the compression numbers. I don’t think they would run well or even fire much below 60 lbs.

Why not try grounding the spark plug wires and spinning the motors over with a drill to see if the numbers come up? No, it wont hurt the motor for the short time needed as only one cylinder will be doing any compression.

[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

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