1948 8N low compression

[Doug 13]

Great forum, thanks everybody. I have
read almost all posts that deal with low
compression problems on the 8n. Wet
compression test was not any better than
dry. #1=25 #2=25 #3=0 #4=25. I had
soaked all holes in MMO for a week.
Valves all move and are not tight. With
#1 up #4 seems to be in overlap. Cam
gear as viewed thru governor hole is
metal not fiber. I guess I'm left with
cam gear key sheared if it has one. I
probably need to remove front cover and
the head. Looks like it was rebuilt and
painted. Ran for a time then just quit.
Owner burned out start switch and
starter trying to get it going. Thanks,
Doug

 

[shaun wallace]

There is nothing wrong with cam or key. Thats normal. Im thinking maybe carbon or not enough lash on valves.

 

[Bob - MI]

Good news and bad news. Good that dry and wet compression readings are the same. Rules out rings to a large extent. This also points to valves which you can get to more easily than the rings.

If the valves are all moving then the cam is turning. A 1948 should have a front mount distributor and you can pull the cap off and see if the rotor is turning as well.

Have you noticed a lot of blow-by coming out of the crankcase breather?

I would be pulling the head and taking a look at the valves this would give you a chance to see all pistons/cylinders as well as replace the head gasket.

 

[HarryVanderpool]

Was the throttle plate in wide open position during the compression test?
If not, remove the air cleaner connection from the carb and bliock the throttle plate in wide open position and retest.

 

[wore out]

Borrow or buy a "cylinder leakdown tester" and you will be able to instantly find out where the compression is leaking, past the rings or valve(s).

If you are unfamiliar with the term "cylinder leakdown tester" GOOGLE will fill you in.

 

[Jim L WA]

The owner burned out the start switch? Do you mean the solenoid? The starter motor I can see maybe. So, how are you turning it over for the compression test?

 

[Doug 13]

(quoted from post at 11:20:20 11/15/21) The owner burned out the start switch? Do you mean the solenoid? The starter motor I can see maybe. So, how are you turning it over for the compression test?

I mean the start switch on top of the gear shifter cover. I just jump from switch wire to ground. Starter motor works ok and has no paint so I think it was recently replaced.

 

[Doug 13]

Ok, I will get a leak down tester. Heard low pressure compressed air thru spark holes might work also. Thanks!

 

[Doug 13]

I had forgotten to open throttle plate. I will retest. Thanks for the reminder.

 

[Doug 13]

I will check my front mount distributor. I figured it was spinning as I have great spark. I have never checked the crankcase breather while cranking. I will check valves for carbon and lash. Thanks to all for your suggestions.

 

[ASEguy]

When doing cylinder leakage, remember to have the cylinder being tested on TDC of the compression stroke or you will get false readings.

 

[Jim.ME]

In addition to having the throttle open. How fast was the starter cranking it over during the compression test? How many revolutions did you make for each reading? Did you have all the plugs out during the test?

 

[Doug 13]

Yes I had all plugs out. The 6 volt battery was installed with the charger on it. I got only 3-4 slow revolutions before the 10 year old battery died. I will get a new battery and try again with throttle open. I will check valve lash first. :

 

[Jim.ME]

(quoted from post at 10:56:55 11/18/21)
Yes I had all plugs out. The 6 volt battery was installed with the charger on it. I got only 3-4 slow revolutions before the 10 year old battery died. I will get a new battery and try again with throttle open. I will check valve lash first. :

Sounds like that could be part of your low readings, it needs to be spinning good for each cylinder, and the same for every cylinder. Crank until the gauge stops rising, i.e. the 5th stroke doesn't increase the reading from the 4th stroke. Does your gauge have a Schrader valve and is is holding? Little things can throw the readings off.