DD Caddy low compression in one Cylinder

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AirPat
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DD Caddy low compression in one Cylinder

Post by AirPat » Mon Dec 07, 2015 10:36 am

Gotta a question for you guys. I had a blow by issue with my Cadd motor and blowing quite a bit of smoke out of the breathers but none out of the exhaust. Did a compression check and discovered one cylinder with about 30 psi and one with a 110 psi that are not next to each other but on the same side. All other cylinders are at or around 200. I took the valve cover off and everything looked ok. Ran out of time to take the head off. Oil looks good and didn't hear any noises or knocks when motor was running. What do you think I will find when I take the head off? A little about the motor: Built by myself having never built a motor before. Bought a book (forgot the name) and used it as a guide to build it and haven't had any problems until now. Has a mts cam (forgot which one) but it was recommended by the guy on the phone. Edlebrock intake and carb. Cylinders are slightly bored over when machine work was done. All parts where new when built and block, heads and crankshaft were sent to a machine shop and checked/ repaired prior to assembly. Has 4 duck seasons on it and not really used much in the summer. One more thing....I put a little oil in the cylinder and it raised the compression about 30 psi to 60 psi.

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Deano
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Re: DD Caddy low compression in one Cylinder

Post by Deano » Mon Dec 07, 2015 12:03 pm

You didn't say any different so we'll presume you are running a 472/500 with the factory rockers/valve train.

If the pushrods, rockers and springs all appear to be straight, intact and operating with the same full distance of travel, that would likely indicate that your problem is lower than that. Additionally, your excessive blow-by would imply the same thing.

Generically, adding oil to the cylinder and improving the numbers the way you did would imply that at a minimum you may have a ring problem. It is also possible that it could be a piston ring land problem. These things are tough and while it may seem like such a problem would stop it in its tracks, many times it does not (depending on what pistons are being used).

A couple potentially relevant things that you didn't mention are:
1 - Which heads are you running ? the early 76cc, small chambered or the later 120cc, open chambered
2 - Which pistons does it have in it ? factory, KB, Silvolite, Egge, or other ?
3 - size of breathers ? one on each side ? any EGR or PVC being used ? any other crankcase venting ?

If by chance you have the early heads and are running the dished KB pistons, they are very temperamental and it only takes a little bit of detonation to wreak havoc with, if not outright break, the top ring land. They are known to have problems in that regard due to their design.

There are others here who may see something I overlooked, but without other info, and basing a guess on your description it would seem that rings and/or ring lands would be the safe bet.
"The suppression of uncomfortable ideas may be common in religion and politics,
but it is not the path to knowledge; it has no place in the endeavor of science."
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crowhater
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Re: DD Caddy low compression in one Cylinder

Post by crowhater » Wed Dec 09, 2015 10:13 am

perform a leak down test to narrow down your compression loss. A compression test is not a leak down test.
18x8 Alum, 496 BBC, 2.6 CH4, 4B 82" S-blades

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