Need help with low compression!

spikeinout2 · 03-08-2018, 06:47 PM

I have a 1980 amf harley davidson 4 wheel 2 stroke. Has new top end and all new seals replaced, and still only getting 70 psi of compression.
What could be the problem?
Can anyone help? Please!

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Lochlin · 03-09-2018, 10:29 AM

First things first.....make sure your compression tester is good. This winter I redid the top end on my son's quad and the compression still was garbage. I opened it back up, checked everything again, put it all back together and there was still no change in compression. I tried a different tester and, viola, compression was perfect. Try a different tester before you go nuts.

I am assuming the cart doesn't run at all at the moment? Any spurts of life here or there when turning over?

The new seals really shouldn't affect compression much as those are more for good vacuum and don't affect compression much. Compression is pretty much piston / cylinder. You may want to open it back up and make sure you didn't throw a ring or anything. Make sure that there are no scars or anything on the surfaces where the cylinder connects to the case. You can use a touch of sealer on the head gasket if need be. I have been told you can double up head gaskets as well but I have never done it.

I am assuming the cylinder is new so you should be good there. Bottom line is that you are leaking air there somewhere. Ya just have to find it.

spikeinout2 · 03-10-2018, 07:33 AM

No air leak. I did air leak test with 10 psi, introducing it thru the intake side and pluged off the exhaust port and spark plug hole.
I have not tryed a different compression gauge yet. The cart dose start and run fine.

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spikeinout2 · 03-10-2018, 07:39 AM

I did open up the engine, and looked at the piston rings, and cylinder wall. No damage found. Piston ring gap is .007.

Is there a break in procedure on these 2 stroke engines?

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Lochlin · 03-10-2018, 09:13 AM

I bet your tester is bad (they actually do wear out). 70 is kinda low to be running good.

Do you happen to have one of those testers that have multiple attachments for different spark plugs holes? Meaning the gauge side is consistent but you can snap on different leads for the spark spark side? Those can leak where the o-rings are connected the pieces. That's the one that was giving me bad readings on my son's quad. I like the solid, non-interchangeable ones.

spikeinout2 · 03-10-2018, 10:06 AM

Yes i do have the tester that has different attachments.
I will try a diiferent gauge.
It will be about a week before i get to it.
I will let u know the out come.
Thanks for info.

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spikeinout2 · 03-15-2018, 10:57 AM

It was a bad gauge. It did not have the shadder valve at the end of hose where it screws into the spark plug hole. With a good compression gauge I am getting 110 psi of compression. Compared to 70 psi with my bad, piece of junk gauge.
Thanks for your help!

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Lochlin · 03-15-2018, 02:35 PM

03-08-2018, 06:47 PM	#1
spikeinout2 Not Yet Wild Join Date: Mar 2018 Posts: 12	Need help with low compression! I have a 1980 amf harley davidson 4 wheel 2 stroke. Has new top end and all new seals replaced, and still only getting 70 psi of compression. What could be the problem? Can anyone help? Please! Sent from my SM-G935V using Tapatalk
spikeinout2 Not Yet Wild Join Date: Mar 2018 Posts: 12

03-09-2018, 10:29 AM	#2
Lochlin Gone Wild Join Date: May 2011 Location: Pittsburgh Posts: 2,953	Re: Need help with low compression! First things first.....make sure your compression tester is good. This winter I redid the top end on my son's quad and the compression still was garbage. I opened it back up, checked everything again, put it all back together and there was still no change in compression. I tried a different tester and, viola, compression was perfect. Try a different tester before you go nuts. I am assuming the cart doesn't run at all at the moment? Any spurts of life here or there when turning over? The new seals really shouldn't affect compression much as those are more for good vacuum and don't affect compression much. Compression is pretty much piston / cylinder. You may want to open it back up and make sure you didn't throw a ring or anything. Make sure that there are no scars or anything on the surfaces where the cylinder connects to the case. You can use a touch of sealer on the head gasket if need be. I have been told you can double up head gaskets as well but I have never done it. I am assuming the cylinder is new so you should be good there. Bottom line is that you are leaking air there somewhere. Ya just have to find it.

03-10-2018, 07:33 AM	#3
spikeinout2 Not Yet Wild Join Date: Mar 2018 Posts: 12	Re: Need help with low compression! No air leak. I did air leak test with 10 psi, introducing it thru the intake side and pluged off the exhaust port and spark plug hole. I have not tryed a different compression gauge yet. The cart dose start and run fine. Sent from my SM-G935V using Tapatalk
spikeinout2 Not Yet Wild Join Date: Mar 2018 Posts: 12

03-10-2018, 07:39 AM	#4
spikeinout2 Not Yet Wild Join Date: Mar 2018 Posts: 12	Re: Need help with low compression! I did open up the engine, and looked at the piston rings, and cylinder wall. No damage found. Piston ring gap is .007. Is there a break in procedure on these 2 stroke engines? Sent from my SM-G935V using Tapatalk
spikeinout2 Not Yet Wild Join Date: Mar 2018 Posts: 12

03-10-2018, 09:13 AM	#5
Lochlin Gone Wild Join Date: May 2011 Location: Pittsburgh Posts: 2,953	Re: Need help with low compression! I bet your tester is bad (they actually do wear out). 70 is kinda low to be running good. Do you happen to have one of those testers that have multiple attachments for different spark plugs holes? Meaning the gauge side is consistent but you can snap on different leads for the spark spark side? Those can leak where the o-rings are connected the pieces. That's the one that was giving me bad readings on my son's quad. I like the solid, non-interchangeable ones.

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03-10-2018, 10:06 AM	#6
spikeinout2 Not Yet Wild Join Date: Mar 2018 Posts: 12	Re: Need help with low compression! Yes i do have the tester that has different attachments. I will try a diiferent gauge. It will be about a week before i get to it. I will let u know the out come. Thanks for info. Sent from my SM-G935V using Tapatalk
spikeinout2 Not Yet Wild Join Date: Mar 2018 Posts: 12

03-15-2018, 10:57 AM	#7
spikeinout2 Not Yet Wild Join Date: Mar 2018 Posts: 12	Re: Need help with low compression! It was a bad gauge. It did not have the shadder valve at the end of hose where it screws into the spark plug hole. With a good compression gauge I am getting 110 psi of compression. Compared to 70 psi with my bad, piece of junk gauge. Thanks for your help! Sent from my SM-G935V using Tapatalk
spikeinout2 Not Yet Wild Join Date: Mar 2018 Posts: 12

03-15-2018, 02:35 PM	#8
Lochlin Gone Wild Join Date: May 2011 Location: Pittsburgh Posts: 2,953	Re: Need help with low compression!

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