Flathead compression check
- Thread starter guy1unico
- Start date
deuce_roadster
Well-known member
So it spins easier.
JSeery
Member Emeritus
It is harder to get the engine to spin fast enough with the plugs in. The engine needs to turn over at least 3 or 4 times at a decent RPM to get a good reading. You will normally see the compression gauge jump up with each revelation and then finally level out. You also need to block the throttle wide open.
Question, why would you not want to take the other plugs out?
Question, why would you not want to take the other plugs out?
JSeery
Member Emeritus
Not sure I understand what that has to do with not removing the plugs to test? You ask the question if you need to remove the other plugs to get a good reading. The answer is yes. If you still what to check it without removing the other plugs, give it a try and see how it works out. It might, but I doubt it.
Mike51Merc
Well-known member
You should always check all cylinders because the differential in compression readings matters a lot.
RalphG
Well-known member
I didn't know you needed to open the throttle plates for a compression test but I did remove all the plugs. Makes it easy with a little jumper wire on the solenoid (and long arms) you can do every cylinder by yourself in a matter of minutes.
But why disconnect the coil?
Ross F-1
Well-known member
You can get a real good shock, all those wires are looking for a ground!
OP, if you tested earlier with just the one plug out, test again the same way. I doubt it will make a huge difference whether all the plugs are out. I've forgotten to block the throttle open a couple times, the results were identical when repeating with it open. IMO the biggest reason to pull all the plugs is so you don't wear the battery down, so that by the time you get to the last cylinders, the battery is on its last legs.
I assume you adjusted the valves on that cylinder?
RalphG
Well-known member
Not unless the key is turned on. And that is not necessary just to crank the engine. Not with the starter button/key switch setup anyway.
I've got a video of compression testing the 52 Merc. Thought I had it on youtube but can't seem to find it now. Shows how it is all done by one man leaning over the engine with a jumper wire in one hand and compression tester in the other.
All of the above to do it properly.
1.Warm up the engine.
2. all plugs out.
3. full open throttle (Open it once and block it there)
4. do comp test
5. If you have concerns about compressions add a little oil down each cyl.
6. spin it over and repeat test of each cyl.
If you have a low comp. and the oil raised it, you have bad rings. if nothing changed you have a valve problem. (it is not always black and white)
Under these circumstances you can look up Fords data and decide on the mechanical state of your engine by comparing the figures.
I borrowed a comp gauge once. It gave really low figures. It was because someone had replaced the schraeder valve with one with a heavier spring.
1.Warm up the engine.
2. all plugs out.
3. full open throttle (Open it once and block it there)
4. do comp test
5. If you have concerns about compressions add a little oil down each cyl.
6. spin it over and repeat test of each cyl.
If you have a low comp. and the oil raised it, you have bad rings. if nothing changed you have a valve problem. (it is not always black and white)
Under these circumstances you can look up Fords data and decide on the mechanical state of your engine by comparing the figures.
I borrowed a comp gauge once. It gave really low figures. It was because someone had replaced the schraeder valve with one with a heavier spring.
Last edited:
tubman
Well-known member
I had the same experience. When I went to the FLAPS, I was shocked at the price of the proper Schrader valve; it was at least 4 times a regular old one.
The reason I suggested disconnecting the coil was there was a chance the OP might try it with the other 7 plugs still in. Believe me, they can start on 7 cylinders! Not necessary on an earlier car, but since he didn't say....