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What is Normal Compression

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bobinyelm
Join Date: Jun 2017
Posts: 16 Olympia, WA
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2017-07-03          196805


Going through my new-to-me Gator CX 4X2 and noticed not a lot of resistance as the starter spins the Kawasaki engine over.

I askem JD dealer, and they had no idea what was normal compression-the tech just said "not a lot."

I did a compression check and the first compression stroke made 30psi, and after about 5 strokes was up to 75psi. Further cranking accomplished nothing additional. That's with a very strong battery spinning teh motor over pretty quickly.

I did a search and one post mentioned 57 psi minimum is acceptable, and another post said 170psi is min-normal for each cylinder on a 2 cyl Kawasaki (that sounded high to me).

BIG difference!

I set the valves (tight 0.007", loose 0.005"), but the intake was a tight 0.005" and the exhaust was a loose 0.007" so the additional adjustment I made probably didn't alter the compression.

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What is Normal Compression

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dododo
Join Date: Aug 2005
Posts: 19
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2017-07-09          196825


Other than your testing the compression. What is wrong with it that caused you to check the compression? ....

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What is Normal Compression

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bobinyelm
Join Date: Jun 2017
Posts: 16 Olympia, WA
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2017-07-09          196826


Quote:
Originally Posted by dododo | view 196825
Other than your testing the compression.What is wrong with it that caused you to check the compression?


Nothing I am aware of, which is why I performed the check.

I recently purchased the Gator with 640 hours or so on it, and suspect it was not highly maintained, so wanted to do basic testing and maintenance (oil/trans fluid changes, valve adjust, disassembled and cleaned fuel pump and carburetor, compounded and waxed sun-damaged exterior plastic body, removed park brake and lubed cable, installed new seats (old ones were sun damaged), and a few more things.

I noted that the starter spun the engine over with minimal hesitation on each compression stroke, so I wanted to know what typical cranking compression is for these engines, and based on that I would decide what, if anything, to do. It's summer now, so suspected low compression would make the engine more difficult to start in cooler weather, so wanted to correct any problems before then.

The engine does start OK after maybe 3 seconds of cranking "cold" (will not tolerate even a bit of choke cold, though) and runs fine, and restarts hot instantly, with my only complaint being that occasionally forward and reverse gears "clunk" indicating that the drive is not completely disengaging at idle (a very LOW idle), though as I use the machine more, this seems to be correcting itself (my Gator sat unused for a few years before my purchase), so at this point, I am not taking action on this "complaint."

I am a retired airline captain and 40 year licensed aircraft mechanic as well as a designated FAA maintenance inspector, and I will own or operate, or certify nothing that is not absolutely 100% perfect to the best of my abilities, and if anything is wrong with my Gator, I would take whatever steps are necessary to repair or replace that item (or sell it to a less exacting owner is such repair were economically impractical).

Therefore, the testing was just part of my normal regimen. I know in our society (use it and throw it away) most people just use things until they break, then get new ones (I actually observed a niece over the week end trying to break her perfect iPhone 6 so her mother would buy her the latest iPhone 7), but I have a hard time with that philosophy.

While buying, and ordering a few parts for my Gator, I asked my John Deere dealer what the compression on my Kawasaki engine should be, and his answer was "not much," which was pretty unbelievable (not to mention unprofessional), so I thought I'd ask here.

....

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What is Normal Compression

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dododo
Join Date: Aug 2005
Posts: 19
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2017-07-09          196828


I get your interest in being on top of things and I would want you to be representative of all who maintain aircraft!

On the gator, the good news is if the rings are worn it is already on the the ground :-)

Good luck with your evaluation of the gator you have. I have only done compression tests when I have had mechanical problems with an engine and my compression testing tool was an inexpensive hand held and needed multiple runs to get an average reading. ....

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What is Normal Compression

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bobinyelm
Join Date: Jun 2017
Posts: 16 Olympia, WA
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2017-07-09          196830


Maybe I spoke too soon!

I went out today and the Gator would even fire.

Fuel pump was pumping; spark plug had good spark.

Screwed in the compression gauge and only 60 today.

Checked the valve clearance and made them a precise snug 0.006".

STILL 60psi only (was 70 last week)!

Pulled carb (again) and cleaned jets (again).

Starts and runs now.

The carb appeared flooded though when it didn't fire cold I did try a little choke. Maybe that's where the extra fuel came from in the inlet pipe?
....

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What is Normal Compression

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Trainpilot
Join Date: Mar 2015
Posts: 67 Ukiah, CA
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2017-07-10          196831


Fuel in the throat of the carb could also be from float level being too high. That would certainly contribute to the engine being prone to flooding. Gummy carbs can take quite a while to get cleaned up. I am still fighting carburation problems on a used motorcycle I purchased and have have had the carb off too many times already. What does your spark plug look like? (I'm asking general questions and stating what may seem like obvious things to check as not everyone on the forums has the same levels of experience) The guy who used to work on our utv's and quads likes to set valves a little loose, as he feels that helps with easy starting. I like them set to the tight side of specs, as I find it helps the responsiveness of the engine. I used to set the valves on my mini-trail 70 engine to 0.0000" hot. Back in college, that was for a time my only set of wheels. I would run that little motor at about 10,000 rpms for hours-thousands of miles with no valve issues. But if the exhaust valve does not close all the way, or does not spend enough time on the seat, it can overheat, or can prevent the engine from developing full compression. Have you tried adding a bit of oil through the spark plug and cranking a bit to coat rings before the compression test? If the readings come up dramatically, that is a sign of bad rings. ....

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What is Normal Compression

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kthompson
Join Date: Oct 2005
Posts: 5219 South Carolina
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2017-07-12          196850


If a shop's mechanic could not answer the compression question think I would find another mechanic. Not sure the model number of our Gator but it is I will say about 14 years old. Think it has decal like 4 by 2 or it and only decal to indicate any model.

But the transmission does sort of go clonk at lease sometimes when going into gear. The person who went over ours with me said it was a snow mobile transmission design and that is whey the engine must idle so low as if memory is correct you are engaging with no way to clutch. Hope have the reason correct.

Yes the starter spins mine very well. I do have to use the choke to crank when cold.

Do not use any gas with ethanol in it or it gums up the carb and has to be taken apart to be cleaned. Ran a good bit of sea form and other cleaners through ours with no success.

We had a JD dealer that was also a lawnmower dealer (no longer a JD dealership, they wanted them to go only green and their red products was the money, JD made a bad call there) and they have been great on the Kawasaki engine. Then only issue we have had is with gummed up carb and normal maintance. ....

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What is Normal Compression

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bobinyelm
Join Date: Jun 2017
Posts: 16 Olympia, WA
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2017-07-12          196854


Quote:
Originally Posted by Trainpilot | view 196831
Other than your testing the compression.What is wrong with it that caused you to check the compression?
[QUOTE=Trainpilot;196831] Fuel in the throat of the carb could also be from float level being too high. That would certainly contribute to the engine being prone to flooding.

XXXX
Absolutely, but until the last no start it was running flawlessly, so either something tiny got on the float valve seat, or the float stuck if the gas was not due to a couple of seconds choking. Nice super-low idle. I wish the idle needle was adjustable so I could confirm it's just right, but as smooth as the idle is, it cannot be far off.

Gummy carbs can take quite a while to get cleaned up. I am still fighting carburation problems on a used motorcycle I purchased and have have had the carb off too many times already. What does your spark plug look like?

XXXX
Spark plug looks normal grayt/tanish after a bit of running. Not black and sooty at all.

The guy who used to work on our utv's and quads likes to set valves a little loose, as he feels that helps with easy starting. I like them set to the tight side of specs, as I find it helps the responsiveness of the engine. I used to set the valves on my mini-trail 70 engine to 0.0000" hot. Back in college, that was for a time my only set of wheels. I would run that little motor at about 10,000 rpms for hours-thousands of miles with no valve issues. But if the exhaust valve does not close all the way, or does not spend enough time on the seat, it can overheat, or can prevent the engine from developing full compression.

XXXX
Spec is 0.005" and I set the a draggy 0.007" to be safe. In performance engines, too tight means a burned valve, but I guess in this engine it's not likely.

Have you tried adding a bit of oil through the spark plug and cranking a bit to coat rings before the compression test? If the readings come up dramatically, that is a sign of bad rings.

XXXX
I am aware of that diagnostic check, but didn't do it. If I secured the crank and did a leak-down compression check it would tell me instantly where the compression is going, but I need to make a 14mm plug adapter by braking out the ceramic on an old plug and brazing on an air male QD) for my aircraft compression gauge (it has an 18mm plug on it now).
[/QUOTE] ....

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What is Normal Compression

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bobinyelm
Join Date: Jun 2017
Posts: 16 Olympia, WA
TractorPoint Premium Member -- 5 Tractors = Very Frequent Poster

2017-07-13          196855


Quote:
Originally Posted by kthompson | view 196850
Other than your testing the compression.What is wrong with it that caused you to check the compression?
[QUOTE=kthompson;196850] If a shop's mechanic could not answer the compression question think I would find another mechanic.

XXXX
Funny you should say that. I was talking to a fellow today who was saying that my town's JD dealer has a questionable reputation, though they are a large outfit w/ many branches (in different communities) all over Washington State.

Not sure the model number of our Gator but it is I will say about 14 years old. Think it has decal like 4 by 2 or it and only decal to indicate any model.But the transmission does sort of go clonk at lease sometimes when going into gear. The person who went over ours with me said it was a snow mobile transmission design and that is whey the engine must idle so low as if memory is correct you are engaging with no way to clutch. Hope have the reason correct.

XXXX
Yes, same as snow machines I ran in Alaska. Centrifugal where fly-weights squeeze sides of the pulley together. The belt and transmission shaft are always turning from the drag of the belt. My idle barely ticks over.

Yes the starter spins mine very well. I do have to use the choke to crank when cold.Do not use any gas with ethanol in it or it gums up the carb and has to be taken apart to be cleaned. Ran a good bit of sea form and other cleaners through ours with no success. We had a JD dealer that was also a lawnmower dealer (no longer a JD dealership, they wanted them to go only green and their red products was the money, JD made a bad call there) and they have been great on the Kawasaki engine. Then only issue we have had is with gummed up carb and normal maintenance.

XXXX
When I got the Gator, the carb was beyong gummed up. The gum had solidified into concrete that needed to be picked and scraped out, and even STRONG carb cleaner wouldn't budge it even a little. It LOOKS perfectly clean now, but disassembled still smells like varnish.
[/QUOTE] ....

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