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Honda GX160 ... only 50 Lbs compression

This is a discussion on Honda GX160 ... only 50 Lbs compression within the Small Engines forums, part of the Tech Support Forum category. Initially, when trying to start... one hears a slight "pop" sound now 'n again. When it DOES finally start........... it


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Old 09-23-2008, 11:41 AM   #1
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Initially, when trying to start... one hears a slight "pop" sound now 'n again. When it DOES finally start........... it barely idles........for awhile, then dies.
A Leakdown test shows there is NO leak.......... or next to None. A standard, regular compression test shows only 50psi, when there should be a minimum of 85. Valve clearance is correct.

It's got me baffled, ................. I have one idea, but I'd like to hear from you experts, please??

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Old 09-23-2008, 02:49 PM   #2
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Hi & welcome to TSF -

Does your engine have a decompression arrangement to allow easy starting?? These things are sometimes marketed as "easy start" or some similar terminology.

I ask as assuming that the engine isn't just worn out hence low compression - then the low compression might be caused by a stuck decompression start unit

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Old 09-23-2008, 03:14 PM   #3
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It MAY have, but as I mentioned with the Leak-Down test (aka: Differential Compression test) it shows to have 95+% compression, turning CW or CCW.

I'm thinking "Sticky" valve, and presuming it closes fine while performing the Leak-Down test, as it's sitting still with Air pressure closing it, but when performing a regular Compression test using the recoil starter, it Sticks, not closing properly. What do you think?
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Old 09-23-2008, 03:27 PM   #4
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Yes - it certainly could be a sticky valve and as this engine is OHV - you might be able to see what is happening by taking the plug out and the rocker cover off and pulling the engine thru and watch the valve movement and have a look at the valve / tappet clearances.

Given as you said, that it passes a static leak down test, I guess that eliminates a burnt valve or seat.

This is a fairly easy / cheap test - so worth a look as a start. If the valves are all OK - then we will have to look deeper into the engine
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Old 09-23-2008, 04:43 PM   #5
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Thks MrChooks, ........... well I've poured some Rislone Karbout into the Spk Plug hole............. I'll let it sit for a day or more hopefully, & see if that does the trick. Whadya think?
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Old 09-23-2008, 04:53 PM   #6
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Hi Fix - I am not familiar with Rislone Karbout - but I guess its worth a try as a 1st - at least you wont have to pull anything apart to try it.

My experience with "sticky" valves has nearly always had me physically freeing / cleaning the valve stem - as that is where the "crap" that fouls the free movement of the valve usually builds up.

Just a point - if you are lucky and the Risolene works - be sure that you change the oil before you put the engine to serious work again - as some will almost certainly leak into the crankcase and dilute / foul the oil.
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Old 09-23-2008, 05:02 PM   #7
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Thks, yeah...... Karbout is a product we've used here in the Frozen North (Canada) for 30+ yrs........... removes carbon & gunk that builds up in the Upper Engine.............. sometimes works :) Thanks for your help
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Old 09-23-2008, 05:17 PM   #8
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OK - well given you are in for an oil change anyway - is the Rislone product OK / safe to pour into the crank case as well -

Just a thought - but if it can clear gunk off valves - then it might also clear any muck that might be fouling the engine's decompression / easy start mechanism.

I've never done this - but if it is just a solvent - maybe you could drain the engine oil and sloosh a load of the stuff around in the crankcase / cam shaft area. You would have to make sure you got it all out afterwards and maybe do a double (1st oil change as a washout) oil change at the end.

Just a thought and as I said - I am not familiar with the product and have never tried this - but might be worth a try as my experience with these decompression / easy start mechanisms is that they are located down in the cam shaft area of the engine - so its a pretty big strip down to get to them.
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Old 09-30-2008, 06:51 AM   #9
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Thumbs Up

It could most certainly be a stuck valve because of sitting for a while but i work eith these hondas all day long and if any thing goes wrong with these things it is usually rings have been wore from tremendous amounts of use so i think it could be rings or a compression release
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Old 09-30-2008, 08:25 AM   #10
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50lbs compression is not at all unusual for a 4 cycle engine...

Stuck valves can sometimes be freed with Marvel Mystery Oil. It's amazing stuff, we've even seen it free up seized engines.
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Old 10-01-2008, 11:10 AM   #11
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93-5.0, you think the problem could be RINGS, even though the Static Compression Test (Leak-down) test shows 95% compression (Virtually No Leak)?
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Old 10-01-2008, 11:12 AM   #12
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K-B, It will barely run with 50Lbs & Honda Spec's. are Minimum of 85 to 120psi. I'll check out the Marvel Mystery Oil, thanks.
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Old 10-01-2008, 02:49 PM   #13
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I also have one of the l'ill Honda engines and have run it for years without any problems at all. I agree with 93-5.0 - that unless you engine has had an enormous amount of work (or run without oil) - then rings should be OK

See how these whiz bang additives work - but if your problem has come on quite quickly - I would doubt that it's worn rings - these things (worn rings & excluding catastrophic failure -which you would hear) generally just ware out over time - in other words a long slow degradation in performance and a noticeable increase in oil burn / blue smoke

If you engine was working fine - then all of a sudden you find you have low compression - then I would suspect stuck valve or decompression mechanism.

See how the Marvel Mystery oil etc works but if that fails - I think you are going to have to take the rocker cover off - check the valve movement / function then keep digging until you find "where the compression is going"
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Old 10-02-2008, 08:27 AM   #14
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The reason low compression is "normal" in 4 cycle engines is because of the automatic compression release, thus it is usually difficult to get an accurate compression reading.
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Old 10-02-2008, 11:49 AM   #15
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You would have to spin the engine over approx 1800 RPM's to get out of the comp release mode. A leakdown test by far the most accurate. I would look at the flywheel key for shearing and maybe try some starter fluid to see if it picks up RPM's, because fuel nowadays is the biggest problem with small engines.
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Old 10-02-2008, 01:53 PM   #16
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Quote:
Originally Posted by k2skier View Post
You would have to spin the engine over approx 1800 RPM's to get out of the comp release mode. A leakdown test by far the most accurate. I would look at the flywheel key for shearing and maybe try some starter fluid to see if it picks up RPM's, because fuel nowadays is the biggest problem with small engines.
Absolutely.
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Old 10-20-2008, 06:42 PM   #17
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Well, the Karb-out made no difference, so I ripped 'er apart. There's no gumming on the valves ('operate nice & smooth); decompression release works just fine......... no binding, nice 'n smooth. While I had it apart, I lapped the valves. Reassembled, & now, maybe another 5 Lbs to a total of 55 (as opposed to the Honda Specs of a Minimum of 85)........... & won't even fire. This engine hasn't many hours on it......... 'can still see the Cross-hatching in the Cylinder. Any OTHER suggestions folks????

Oh, & Pls. tell me K-B, why you think a sheared flywheel key would affect compression?
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Old 10-20-2008, 07:06 PM   #18
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Fix - this is bizarre - given all you have done, I am running out of ideas - if you have the valves lapped, the decompression isn't stuck and the cylinder walls are good - yet you still can't get compression and the damned thing wont start.

Look - here is something out of left field - and based on the fact that getting a good dynamic compression test with the decompression unit is difficult - based on the fact that if you have fuel, spark and compression - all engines just have to start -

Have you checked that you have a good spark?? Don't throw rocks at me for this one - but I am wondering if due to the difficulty in getting an accurate dynamic compression test and your leak down tests seem OK - whether the compression issue is clouding the picture here.

Have you tried to pull the engine thru with the plug removed and look to see, if with the plug grounded, there is a reasonable spark on the plug. I only ask because my experience with these l'ill Honda engines is usually very good and am starting to wonder if we have all been looking in the wrong place
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Old 10-20-2008, 07:20 PM   #19
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Quote:
Originally Posted by Fix View Post
Oh, & Pls. tell me K-B, why you think a sheared flywheel key would affect compression?
I think K-B was referring to a possible timing / firing disruption rather than an effect on compression. If the flywheel key has failed and the flywheel moves out of its position - the spark still appears, BUT at the wrong time and the engine just wont start or run at all - even though it all turns over nicely and you still get a spark on the plug when you test it.

As I said in earlier post - I am running out of ideas - so having a look at some of the more odd-ball fixes are worth a try.
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Old 10-21-2008, 10:20 AM   #20
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Thanks MrChooks. Yes I did pull the plug & chk for spark, & as I know that on rare occasion a "sparking" plug will fail under compression.... I tried a new plug as well--- Great Spark! As you say, it IS Bizarre!
I think the only other possibility is Weak valve springs? At the moment I'm waiting for a COMPLETE Service manual (as opposed to the free manual I was able to download off the net).......... MAYbe there will be some tips there, as well as specs on the springs.

(Oh, & I am well aquainted with the effect a sheared key will have on the timing..... I was just curious what K-B's reply would be regarding the compression,... thank you)

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