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Old 09-18-2009, 03:47 PM   #1
jpoteet
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Default GX620 Compression PSI?

Hey guys.....

What should a Honda GX620 V-Twin compression test at? I am almost ready to purchase one of these but obviously want to check compression out before I pull the trigger. Any help is much appreciated!!!!
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Old 09-19-2009, 01:21 AM   #2
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Default Re: GX620 Compression PSI?

Sorry....I should of specified that I am purchasing a used GX620 V-Twin. Just need to know what the compression should be at.....anyone out there have any idea? Hopefully the engine will spec out ok....looking forward to throwing this in my G16A. Thanks to all for the help.
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Old 09-19-2009, 08:23 AM   #3
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Default Re: GX620 Compression PSI?

115 psi somewhere in that neiborhood. I think dont quote me.
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Old 09-19-2009, 08:36 AM   #4
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Default Re: GX620 Compression PSI?

I couldn't find it on the net and don't have a service manual for it, so I looked in my 20 hp B&S service manual, this is what Briggs has to say:

Briggs & Stratton does not publish compression pressures, as it is difficult to obtain an accurate reading without special equipment. An assessment of the engine's compression can be made as follows:
Remove both spark plugs and insert a compression gauge into either cylinder (one cylinder at a time). Turn engine over with engine starter until there is no
further increase in gauge pressure reading. Record this reading. Repeat procedure on other cylinder and record that reading. Divide the difference between the
cylinders by the compression pressure of the higher of the two cylinders. A difference over 25% between the cylinders indicates a loss of compression in the
cylinder with lower pressure. See example below.
Engine #1 Engine #2
Cylinder #1 65 PSi 175 PSi I
Cylinder #2 60 PSI 55 PSI
Difference 5 PSI 20 PSI
% Difference 7.7% 26.7%
If compression is poor, look for -
1. Loose cylinder head bolts
2. Blown head gasket
3. Burned valves, burned and/or loose valve seats
4. Insufficient valve clearance
5. Warped cylinder head
6. Warped valve stems
7. Worn bore and/or rings
8. Broken connecting rods

I know they both have compression release mechanisms, Briggs is on the camshaft, so that may be the reason they don't publish a number............
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Old 09-19-2009, 09:48 AM   #5
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Default Re: GX620 Compression PSI?

Flint hit it right on the head the compression releases will not give you an accurate compression reading from the starter rpm's
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Old 09-20-2009, 01:08 PM   #6
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Default Re: GX620 Compression PSI?

Hey guys thanks for all the help!!!! So will the cylinders test somewhat the same or should I expect there to be a slight difference between the two? Sorry I am not an engine expert and trying to make heads or tails of what has been posted here so far.
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Old 09-20-2009, 01:47 PM   #7
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Default Re: GX620 Compression PSI?

Follow the above procedure, and as long as there is not more than 25% difference between the two it should be OK. I know I would like to see a firm published number but the compression release on these engines makes it difficult to judge, at started motor RPM's.
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Old 09-20-2009, 07:11 PM   #8
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Default Re: GX620 Compression PSI?

Good call Flint
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Old 09-21-2009, 10:02 AM   #9
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Default Re: GX620 Compression PSI?

When checking compression in any engine, it's true that a large difference between cylinders is a sure sign of a problem but the absolute compression is also very important as well. There are a number of things that could cause ALL cylinders to have even but low compression (like the engine being operated without a proper air filter for a while), and in this situation, the engine will still need service to correct the issue. In other words, consistent compression among cylinders is only PART of the story.
I'd guess that 120psi would probably be a gold standard for the compression reading but you're probably fine with anything over 100psi. Would be a good idea to call a bike shop to confirm before going ahead and buying though.
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Old 09-21-2009, 10:43 AM   #10
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Default Re: GX620 Compression PSI?

Quote:
Originally Posted by Katshot View Post
When checking compression in any engine, it's true that a large difference between cylinders is a sure sign of a problem but the absolute compression is also very important as well. There are a number of things that could cause ALL cylinders to have even but low compression (like the engine being operated without a proper air filter for a while), and in this situation, the engine will still need service to correct the issue. In other words, consistent compression among cylinders is only PART of the story.
I'd guess that 120psi would probably be a gold standard for the compression reading but you're probably fine with anything over 100psi. Would be a good idea to call a bike shop to confirm before going ahead and buying though.

Nice post. Thats why I said 115. If you test them they should be no more than 15 psi of one another. If you really want to know the health of the bores you should do a leak down test. Motors can show good compression and have leak down beyond 10 percent per cyclinder. Once the bore has exceed 10 percent in leak down it should be re-ringed. I would think as long as there over 95 your good.

The correct way to check cranking compression is to start the motor and get it warm. Then with a fully charged battery and carb butterflies wide open. Connect compression gauge and turn over until gauge stops moving. Usually about 7 to 10 revolutions.
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