Model AA cyclinder compression

U

Unclepalmer

Junior Member
This old truck has ALWAYS started. But not now. I have replaced all plugs, points, condenser, starter switch, set timing, etc.... all the usual. Getting spark, gas, ckd wiring connections. Now i am checking the compression and all 4 cylinders are 49-50.
I watched a couple of YouTubes and one guys was 69 and another guy has 39-60 -ave50. 76 apparently is the factory spec. but the other guy said 64... So help me out on what the compression should be. Does this seemingly low compression indicated a leaky head gasket? that should be replaced?
thanks!
 
Last edited:
Hello, maybe recheck all the things you changed , sometimes the repo parts are worse than the worn used parts.Might check the points and cap 1 st some people have had trouble with repo parts .
 
Gas! Do you have gas at carburetor? Try removing gas cap.

Good luck. It should be something simple.
 
yep, got gas, even drips out if i choke it too much. Yeah, i keep thinking/hoping its something simple but its not giving in that easy!
 
It will start with 50 si, especially if the compression is even across all 4 holes. Your readings may improve after it has run and you get some oil on the walls.

Did you change all of those parts because it wasn't starting, or was it OK prior to changing out the parts and just won't start now?
 
Also is the cylinder head gasket was leaking from what I have seen one will read a lot lower than the rest ,50 pounds probably do to engine having some miles on it, if it ran well before without oil burning and rods and mains good,I wouldn’t start taking head off .Valves can give low compression too if need ground.You can check this by putting a small amount of oil in each cylinder and recheck compression, if it goes up will indicate rings .Please let us know what you find out. Sure you will get more reply's on this.
 
It woudlnt start so i started changing one thing at a time. The ignition switch had gotten real lucy-goosy when you turned key off it kept running or you could pull the key out so i changed that first. then the coil, then plugs points condenser. then black jumper wire to coil. so now to compression. i so do not want to pull the head.
 
My engine that had been sitting for 30 years had 45-50-50-50 psi in the cylinders. Once it got gas and spark it started right up. I doubt it's a compression/head gasket issue.
 
When you change things in the dash panel sometimes things can "touch" when it all goes back together. Has happened to me more than once...
 
Don't pull the head. You have compression.
Do you see the amp meter deflect (slight discharge) as the engine turns over? That would at least indicate that the points are opening and closing (but timing could still be off)
If your problem started after changing the ignition switch I would start there.
Perhaps when changing the ignition switch, the ampmeter connection was disturbed.
If you don't see the amp meter deflecting, check points, then connect a jumper between the two terminal box terminals (bypasses the Amp Meter). You won't see amp meter deflection, but if the amp meter is bad it will allow everything to run.
 
Compression is good enough for the engine to run and is in the correct range for a stock engine. Start with the simple things like checking sparks at the plugs. You said you were getting sparks, but where? The end of the spark chain is at the plugs. It sounds like you are getting enough gasoline for the engine to at least try to start.

Did you change the timing or is the point gap off which would change timing? You can check the timing without changing it if it is correct.

The only thing I would do to the head is check the torque on the nuts. They should be 55 foot-pounds. Start at the center nut and work outward in a circle from there. I have always used 20 foot-pound intervals but I recently read to use 10 foot-pound intervals from an expert. He said that if one nut is torqued too much in relation to the rest it can crack the block.
 
Maybe pull all that apart, turn on the switch, let everything hand in the air, and step on the starter???
 
These old engines don't need much compression to fire off. They just need enough to draw in a fuel mixture. If it has fuel and sparks at the plugs then I would be wondering if it's sparking at the right time. Recheck your spark timing and make sure it is at full retard. Unless it is getting too much fuel, the thing ought to pop at least once.

If your valve timing is off due to a bad timing gear, it would still have to be pretty close to have compression. Set the timing mark with the pin and look into the cylinder & see if No.1 is at TDC. It should also be blowing some air out as it comes up to the pin dimple in the gear.

Ain't these old Fords FUN!
 
we looked at that: the ammeter does not wiggle., so prob bad. however, it is passing power between the 2 terminals as we are getting power "downstream". i have an old one i can put in but point well taken. we did catch a couple of shorts inside the distributor lower plate. prob need to recheck that again.
 
have reset the timing with a timing wrench that i have used previously on the Coupe (that hadnt run in 10 yrs) then it started right up with basic tuneup.
the simplicity of them is what makes it so frustrating!
 
Pull the wire out of the top of the distributor and hold it about a half inch away from a head stud nut and crank it over. You should have a good fat blue spark that jumps between the two.
If not run a piece of fine grade sandpaper between the points to remove any oxidation.
 
"we looked at that: the ammeter does not wiggle., so prob bad."
Are you saying that the ammeter is bad or that the ignition system is bad? The bottom line is getting sparks at the plugs at the right time. Depending on how your ignition system is wired you may not see the ammeter wiggle when the points are opening and closing.
 
You have more than enough compression and apparently you have fuel my guess is the spark is weak test the condenser or replace it with a known good 1
 
You must log in or register to reply here.
Back
Top