Compression test

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Papacub
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Compression test

Postby Papacub » Thu Jul 06, 2017 6:08 am

Hi everyone,
I finally had a chance to run a compression test on my 58 loboy here's the results...
#1
75 dry 95 wet
#2
0
#3
95 dry 110 wet
#4
0
Any suggestions where to go from here? Should have the chance to wrench this weekend!
Also don't know if it's relevant but we had the cub sandblasted and painted last fall, I've read since then that sandblasting can cause issues. Thanks everyone for all the help so far, would really like to get this loboy running again before the next snow!

Gary Dotson
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Re: Compression test

Postby Gary Dotson » Thu Jul 06, 2017 6:18 am

First, look for stuck valves on #s 3 & 4. It's almost certain you'll find that as the problem. Once free'd up, repeat your test.

staninlowerAL
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Re: Compression test

Postby staninlowerAL » Thu Jul 06, 2017 6:47 am

Gary Dotson wrote:First, look for stuck valves on #s 3 & 4. It's almost certain you'll find that as the problem. Once free'd up, repeat your test.

I think Gary meant #2 & #4..... Could be intake or exhaust (or maybe both). Good Luck.

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Re: Compression test

Postby Eugene » Thu Jul 06, 2017 6:51 am

Cylinders #2 and 4.

Valve cover, hood, and spark plugs removed. Search function, there are several tips on how to unstick valves.

Cub valves frequently stick when the tractor has been setting for a long time. Proper preparation, I doubt that sand blasting material got into the engine.
I have an excuse. CRS.

Gary Dotson
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Re: Compression test

Postby Gary Dotson » Thu Jul 06, 2017 7:23 am

Thanks for correcting me, Stan! It happens, you know.

staninlowerAL
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Re: Compression test

Postby staninlowerAL » Thu Jul 06, 2017 8:20 am

Gary Dotson wrote:Thanks for correcting me, Stan! It happens, you know.
Yeah, to all of us!! I just call it fat fingering the keys. Dale, in a recent post, called it "finger fart". :lol:

Papacub
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Re: Compression test

Postby Papacub » Fri Jul 07, 2017 6:26 am

Would lack of compression in 2 cylinders keep the motor from popping off when you crank it over? Thanks everybody will send an update ( and probably a lot more questions! ) in a couple days!

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Re: Compression test

Postby staninlowerAL » Fri Jul 07, 2017 6:31 am

Papacub wrote:Would lack of compression in 2 cylinders keep the motor from popping off when you crank it over? Thanks everybody will send an update ( and probably a lot more questions! ) in a couple days!
IMHO, yes. Especially if the engine has not been run in a while. There's just too much working against it to run on 2 cylinders.

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Re: Compression test

Postby Shane Nelson » Fri Jul 07, 2017 6:39 am

staninlowerAL wrote:
Papacub wrote:Would lack of compression in 2 cylinders keep the motor from popping off when you crank it over? Thanks everybody will send an update ( and probably a lot more questions! ) in a couple days!
IMHO, yes. Especially if the engine has not been run in a while. There's just too much working against it to run on 2 cylinders.

:Dito: Agree with this. Think of it like having diarrhea and only being able to squeeze one cheek, you can't hold the compression in and it will take the easiest path.
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Re: Compression test

Postby LRiddle » Fri Jul 07, 2017 7:49 am

Chipmaker wrote:
staninlowerAL wrote:
Papacub wrote:Would lack of compression in 2 cylinders keep the motor from popping off when you crank it over? Thanks everybody will send an update ( and probably a lot more questions! ) in a couple days!
IMHO, yes. Especially if the engine has not been run in a while. There's just too much working against it to run on 2 cylinders.

:Dito: Agree with this. Think of it like having diarrhea and only being able to squeeze one cheek, you can't hold the compression in and it will take the easiest path.

:lol: :lol: :lol: :lol:
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Re: Compression test

Postby Jim Becker » Fri Jul 07, 2017 11:22 am

Papacub wrote:Would lack of compression in 2 cylinders keep the motor from popping off when you crank it over? Thanks everybody will send an update ( and probably a lot more questions! ) in a couple days!

As already stated, the odds are strongly against you. Particularly if one of the no-compression cylinders has a stuck intake valve. The two working cylinders will have little chance of drawing enough vacuum to pull any fuel in.

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Don B.
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Re: Compression test

Postby Don B. » Fri Jul 07, 2017 11:49 am

Think of it like having diarrhea and only being able to squeeze one cheek, you can't hold the compression in and it will take the easiest path.

Ah. And, sadly, I actually understood that.
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Papacub
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Re: Compression test

Postby Papacub » Sat Jul 08, 2017 9:49 am

Thanks everyone for all the great advice...I had the valve closest to the driver stuck open on #4...tapped it loose and soaked everything with pub blaster...next question, front to back which are the intake and which are the exhaust valves? I'd like to positively find tdc on #1 to triple check timing before putting everything back together...
Thanks again in advance you gentlemen are lifesavers!

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Re: Compression test

Postby staninlowerAL » Sat Jul 08, 2017 10:14 am

With the valve cover off, rotate the engine until both the valves next to the radiator are closed which is the compression stroke for #1 cylinder. The mark on the C/S pulley and the pointer should align. If you want to go a step further, while the piston is positioned as described above, remove the #1 spark plug and fill the remaining void above the piston with something like transmission fluid or other light weight oil until the liquid is at the top of the spark plug hole. Rotate the crankshaft back and forth a few degrees and find the spot that the liquid is at the highest point. That's TOP DEAD CENTER/compression stroke. (If the liquid will not hold steady it's leaking past the rings or the valves on that cylinder.) Compare with the timing marks on the C/S pulley and pointer. The distributor rotor will be pointing to the #1 spark plug wire on the distributor cap. Or you can check with a timing light connected to #1 with the engine running.

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Re: Compression test

Postby Eugene » Sat Jul 08, 2017 10:28 am

If you haven't moved or removed the distributor, rotate the engine until the TDC mark on the pulley lines up with the pointer. Check to see if the rotor lines up with the #1 spark plug wire tower on the distributor cap. If the rotor points to #1 spark plug tower you are on the correct TDC. If the rotor points at the #4 plug tower, rotate the engine one full turn until the mark and pointer line up. You are then on #1 cylinder, TDC.

If you use Stan's method for determining TDC, be sure to crank the engine over several times with the spark plug out to clear excess fluid from the cylinder.
I have an excuse. CRS.


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