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A Lack of Power - Revisited

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Bigschuss
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Location: Savoy, MA

A Lack of Power - Revisited

Postby Bigschuss » Sat Apr 23, 2005 7:56 pm

Last week I posted a request for help about my Cub's lack of power under load and white smoke belching from the oil filler. I thought perhaps coolant had gotten into the crankcase over the winter.

Several gentlemen responded with great advice and help. I drained the oil and found no evidence of coolant. I then pulled the pan and found everything to be VERY clean. There was no muck and no sign of any damage. I did a compression test and got 116, 110, 115, and 115 on cylinders 1 through 4 respectively. I then checked out the crankcase vent tube as Bigdog suggested, as it was as plugged up as a black bear heading into its winter den!

I replaced the vent tube but have not had a chance to test the Cub under a load.

I guess I have 2 follow up questions. I have all of the manuals for the Cub, but nowhere could I find the compression specs? Could somebody please tell me what the compression should be and if my compression results are acceptable? Secondly, could the plugged vent tube be the sole cause of my Cub's lagging under load?

I'm starting to think maybe the watery brown oil I drained before I plowed didn't have coolant in it, but just 3 or 4 months worth of condensation.

Thanks in advance, and a special thanks to Bigdog for that tip about the vent tube. As a beginner, I didn't even know that tube existed.

Blair in Savoy, MA
I wish coke was still cola, and a joint was a bad place to be...
- Merle Haggard

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johnbron
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Postby johnbron » Sat Apr 23, 2005 8:05 pm

I hope you cleaned out the hole in the block where the vent tube attaches. It is/was probably plugged too if the tube was that bad. I think your compression results look pretty good.
Then came Bronson

Jim Becker
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Postby Jim Becker » Sat Apr 23, 2005 8:05 pm

I would say any compression reading over 100, with no more than 10# variation is a very good reading. They can be considerably worse than that and still run OK. Simply cleaning the tube shouldn't do anything powerwise, but should get rid of the venting from the filler. There may be a powerful placebo effect from getting rid of the smoke.

WKPoor
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Postby WKPoor » Sat Apr 23, 2005 8:24 pm

Cranking pressure readings can be very missleading as to an engines overall health. Some I've talked to have gone as far as to say worthless in diagnosing. I would rather like to know what the leak rate is or just plain put air on the cylinder to see if and where the leaks are. Cranking prexs are good to know out of curiousity to know how even the cylinders are but may mask a problem with a cylinder. Some automotive manuals that give pressure reading info will state how many cranking revolutions the pressure should build on. I would still give each cylinder a leak test to see just how tight it is.

Eugene
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Compression Test

Postby Eugene » Sat Apr 23, 2005 9:11 pm

Blair:

Just my opinion. With the numbers provided in your post I'd say you engine (at least the valves and rings) are in excellent condition. You didn't say if the test was wet or dry - doesn't make a difference or change my opinion.

Air leakage (leak down test), you will get leakage past the ring gaps even with new or nearly new engines. Leak test will tell you where the air is leaving the cylinder, either past a valve or past the rings. Since everything appears to be in excellent condition, I wouldn't bother.

Eugene

WKPoor
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Postby WKPoor » Sat Apr 23, 2005 10:09 pm

Air leakage (leak down test), you will get leakage past the ring gaps even with new or nearly new engines. Leak test will tell you where the air is leaving the cylinder, either past a valve or past the rings. Since everything appears to be in excellent condition, I wouldn't bother.


I can't speak for Farmall Cubs but I have performed leak tests on engines that were perfectly tight. A new or freshly overhauled engine very well might have some leaks but after break-in they should be very tight. I will agree however that unless the problem is still there and all other efforts have been exhausted most likely the problem is somewhere else.

FYI- Differential compression testing is becoming more and more widely used in the automotive sector because it is more acurate and can reveal the source of the problem during the check. Cranking RPM can hide a leak that might otherwise cause an operational issue.

Eugene
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Lack of Power

Postby Eugene » Sat Apr 23, 2005 11:03 pm

Since the compression is good I suggest you look at two other likely areas; Ignition and carburation.

Ignition. Timing, weak spark.

Carburation. Fuel flow.

Air flow to the carburator: Remove the entire air cleaner and give it a good cleaning. Soak the air cleaner body in kerosene for several days - shake up and down every so often.

Vacuum leak on the manifold.

Mechanical drag, brakes, bad bearings in transmission/rear end. Coast the tractor down a hill.

OH. 120 PSI would be normal on a near new engine. New ring end gaps on (Cub) slower turning engines .007 - .017. Side clearence .003. As the rings wear the end gap increases.

Eugene


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