Compression for Honda GX390

[TimB]

I picked up a 13 HP Honda several years ago for, I think $5. I was missing the gas tank and air filter

I am trying to decide if it is worth putting back together and getting it going.

First thing to check is compression.

After squirting some motor oil in the cylinder, this builds 59 psi. Is that a reasonable amount of compression for an engine like this?

Thanks,
Tim

[KE4AVB]

Other opinions will probably differ than mine but a wet compression of 59 is too low and leak down test is order to locate source of the low compression. Normal dry compression is around 70 or better on most engine even with active ACRs.

[TimB]

I have a leak down tester. I will see what I can find out.

What is an "ACR"? automatic compression release?

[KE4AVB]

Yes, It an automatic compression release. Part of the camshaft assembly to starting easier especially with a recoil starter. And boy do you know when it fails on a 390. First failed one I ran into nearly broke a couple of my fingers on the kickback. Learn quickly to watch for this.

[creia]

You did remember to open the throttle completely during your compression test, correct? It will make a big difference if you did not. My son and I work on the old Briggs and Tecumseh L-heads from the 1960s and 1970s. We typically get a compression reading of around 90-100PSI on them if they are not worn out or highly worn.
Michael

[TimB]

I did not open the throttle creia, I will try again and do that.

I am hoping that makes it easier for air to get in, so the compression reading might go higher ??

Thanks,
Tim

[bgsengine]

I did not open the throttle creia, I will try again and do that.

I am hoping that makes it easier for air to get in, so the compression reading might go higher ??

Thanks,
Tim

Yes and crank the engine over multiple times until needle stops moving to get a true compression reading

[creia]

I did not open the throttle creia, I will try again and do that.

I am hoping that makes it easier for air to get in, so the compression reading might go higher ??

Thanks,
Tim

Yes it should be higher. Post your results after you have rechecked compression.
Michael

[Arkie]

I take off the exhaust valve push rod or jump the ex valve rocker off so as the ex valve stays shut and compare to the first compression reading. If ex valve lash is less than .008 can cause the ACR to operate and lower compression.

Point is I try to disable the decompression release when doing compression tests on a small engine.

If compression is still low afterwards I then usually do a leak down test to see if I'm whipping on a dead horse.

[TimB]

Hi Gents, was not able to spend time on this again till last weekend ......

I just put the compression gauge on again, and it built up to 103 lbs. I am not sure why it went from ca. 50 to 100+. Perhaps adding some motor oil to the cylinder and some activity and time loosened up the rings.

The 103 psi was with the throttle closed. Opening the throttle did not seem to make a difference.

However, this engine is loose on the garage floor, and I had a hard time pulling the cord once some compression built up. It might build more pressure - I will bolt is to a board the weekend and give it another try.

This engine has an electric starter on it (wires are missing so it will take me an hour or so to put a battery on it),

is it possible it was not built with an auto compression release?

Serial No. is QAE2 GCAA-4061812

Thanks,
Tim