# Compression on hmsk 80



## lsengines (Jan 4, 2016)

I put the Compression tester on biggie this morning, since I've changed the engine few days ago. Well it didnt turn out all that good.

The specs I beleive should be between 80 and 100 psi..I got 40 psi engine cold..kinda slapped me in the face since I tought that motor was running ok.

It started ok, idle ok and works ok too in the snow when I tried it..

Anybody know if I can get thru winter with that?


----------



## classiccat (Mar 1, 2014)

are those #'s from a recoil or electric start?


----------



## lsengines (Jan 4, 2016)

Recoil start. Cold engine.


----------



## lsengines (Jan 4, 2016)

Just thinking of this: could it be valves not seating right in valve seats? Im not a small engine pro..im more car trucks inclined..lol


----------



## Kiss4aFrog (Nov 3, 2013)

Unlike an airplane, don't trust your gauges. :blink:

If it feels like it's running ok, it's running ok. :hope: Are you doing it in an unheated area with the engine cold, cold ??

Might also be a compression release for easier starting.
What are your engine numbers, mod, ser, family ...

Might want to try it with the electric.

.


----------



## classiccat (Mar 1, 2014)

lsengines said:


> Just thinking of this: could it be valves not seating right in valve seats? Im not a small engine pro..im more car trucks inclined..lol


That is pretty low even for a recoil however it really depends on how hard you're pulling it over. 

your engine has a compression release that opens the exhaust valve a tiny-bit to help with pull starting.






electric start can overcome the release mechanism, or look into doing a leakdown test instead.


----------



## classiccat (Mar 1, 2014)

...and here's donyboy video explaining it


----------



## lsengines (Jan 4, 2016)

Thanks to you all it answers my questions..pretty hard to get true compression with decompression mechanism. .loll! Then again I tought the old motor was running allright.


----------



## lsengines (Jan 4, 2016)

I got two broken bolts holes for starter..so no starter for now :/


----------



## Grunt (Nov 11, 2013)

I agree with classiccat about doing a leak down test. The length of hose on compression testers has a direct impact on what readings you get also.

Does putting a little oil in the cylinder increase the compression reading? If the rings are good, the oil will only increase compression by about 5 pounds. I would say that if the engine starts easily, runs without popping and RPM's stay up when putting a load on it, your good to go. I'll do a compression test on my 8 hp tomorrow and post the results in this thread.( I'm guessing it will be around 50 to 60 psi with the pullstarter)


----------



## lsengines (Jan 4, 2016)

i did put a bit of synthetic atf in cylinder, but did not check compression after. I wanted to let it sit and work thru rings just in case of stuck rings into grooves.

Will check it tomorrow and will update.


----------



## Grunt (Nov 11, 2013)

As promised, I did a compression test on my 1989 HM80.
Using a Harbor Freight compression tester with a 12" hose.

Recoil starter and dry cylinder ------ 40psi
Electric starter and dry cylinder -----100psi

Recoil starter and oiled cylinder------45psi
Electric starter and oiled cylinder-----110psi

These are the readings on a cold engine I consider to run very well.

The compression release went off when reaching 60psi and an unknown RPM.

Curious to see the readings from other Tecumseh HM80 owners.


----------



## classiccat (Mar 1, 2014)

right after a valve job & a new headgasket, my '89 HM80 (Link) was getting 120 on the starter & 60 on the recoil.


----------



## lsengines (Jan 4, 2016)

Didn't try yet after oiling cylinder but I took it home today, and put it to work.

I blew hard snow, for a good 90 minutes non stop, at full throttle and it seems like the motor318 cc, 9hp) is working fine. I used a full gas reservoir. About a gallon I think, so I didnt baby it.

Heres a pic of it just before takin her out of the building..


----------

