Compression

TomC750

TomC750

Senior Member
Searched the forum and other places for acceptable compression numbers for my '41 Merc. Mine range between 70# and 80#. Any help?
 
It'll probably start and run OK with those numbers. Before I changed the engine in my '51, it was between 85 and 100 on all eight and it was OK to drive daily. You may be OK (for a while) if they're all about the same. Hopefully, a couple dosed of Marvel Mystery Oil and some driving will cause the situation to improve. Maybe a couple of shots of Sea Foam down the carb, as well.

Don't expect any "seat of the pants" thrills, though.
 
tubman said:
It'll probably start and run OK with those numbers. Before I changed the engine in my '51, it was between 85 and 100 on all eight and it was OK to drive daily. You may be OK (for a while) if they're all about the same. Hopefully, a couple dosed of Marvel Mystery Oil and some driving will cause the situation to improve. Maybe a couple of shots of Sea Foam down the carb, as well.

Don't expect any "seat of the pants" thrills, though.
Click to expand...
Tubman, thanks for the response. This vehicle does not smoke or burn oil. It wants to run strong intermittently, but has a tuneup problem I have not figured out. I do suspect a valve sticking occasionally when listening to the exhaust at idle. But there is something else. I am going to put an ignition analyzer on it tomorrow.
 
Check for a weak condenser. Prepare to have to try one or more FLAPS condensers before you get a good one. I hear Vertex magneto condensers are very good, but they have to be special ordered.
 
tubman said:
Check for a weak condenser. Prepare to have to try one or more FLAPS condensers before you get a good one. I hear Vertex magneto condensers are very good, but they have to be special ordered.
Click to expand...

I have no idea what a "flaps" condenser is and the Mallory is good but I would try a "TUBMAN TRASHCAN" for peace of mind!
 
My ‘41 is 60lbs on all 8 and it burns a quart every 40 miles, but I still drive it to car shows several times in the summer. Post some pictures of your Merc, I almost bought a two-tone 1941 Mercury w/jump seats but the paperwork was wonky so I passed.
 
Sorry, "H.A.M.B. term". "FLAPS" - Friendly local auto part store.

For the record, I am now out of condensers. Any future supply depends on whether I can talk my partner into making some more when I get back to Minnesota.
 
Stores like N A P A. Standing joke here is : Never Any Parts Available. The guys who work there are actually GR8 & get what they don't have next day.
 
Regarding concern about valves, attach a vacuum meter at you carb. If the indicator needle stumbles, jumps etc it could indicate a sticky or badly seated valve. This is not unusual for engines that don't get enough use. If so, next step might be to add some MMO .... a bit down the carb and a quart in the oil. At purchase my '47 had same symptom and the MMO cleared it up.
 
Drbrown said:
Regarding concern about valves, attach a vacuum meter at you carb. If the indicator needle stumbles, jumps etc it could indicate a sticky or badly seated valve. This is not unusual for engines that don't get enough use. If so, next step might be to add some MMO .... a bit down the carb and a quart in the oil. At purchase my '47 had same symptom and the MMO cleared it up.
Click to expand...
I had a vacuum gauge on my engine, it fluttered a little bit, I did run some Seafoam through it, no change. It is not one valve that is a problem, it stumbles on all cylinders intermittently. I am going to do the MMO however.
 
tubman said:
Check for a weak condenser. Prepare to have to try one or more FLAPS condensers before you get a good one. I hear Vertex magneto condensers are very good, but they have to be special ordered.
Click to expand...
Charlie installed a new condenser, (NAPA) possible it is bad I suppose. I had my old ignition analyzer on it this morning, rather inconclusive. blurry trace, jumped around quite a bit.
 
Last edited:
tubman said:
It'll probably start and run OK with those numbers. Before I changed the engine in my '51, it was between 85 and 100 on all eight and it was OK to drive daily. You may be OK (for a while) if they're all about the same. Hopefully, a couple dosed of Marvel Mystery Oil and some driving will cause the situation to improve. Maybe a couple of shots of Sea Foam down the carb, as well.

Don't expect any "seat of the pants" thrills, though.
Click to expand...
When it pulls, it pulls strong, even when missing intermittently.
 
AKCJ said:
Tubman
Dang!
I was just looking to see if I could order my very own trashcan.
Please keep us posted.
Click to expand...

Well you can, but they're now $100. My arthritis has gotten so much worse that I can't make them any more. I had several requests from the guys with after-market ignitions who don't mind paying the price for "the look", so I paid my partner to build 25 more for me. I think I have about 10 left.

And that, boys and girls, is how there came to be a $100 automotive condenser.:eek:
 
tubman said:
Check for a weak condenser. Prepare to have to try one or more FLAPS condensers before you get a good one. I hear Vertex magneto condensers are very good, but they have to be special ordered.
Click to expand...

I have had extremely good fortune with Hunts Magneto's. I get their 32mfd. condensers and put them inside a factory "body" to maintain the authentic appearnace.
 
my rebuilt '41's shows about 90psi on all cylinders and runs like a top with plenty of power...relatively speaking. :) and the original 6v electrical starting circuitry turns it over like a champ too!
 
At the auto shop I worked at every compression tester had a different reading and it was a 30 pound range difference including high dollar name brand tool companies.
the lowest readings were from ones with long rubber hoses (the rubber hose expands with pressure and the length in
essence lowering the compression ratio.
My favorite one for gasoline engine has a steel reinforced hose similar to hydraulic hoses.

The diesel tester has wire rods inside the hose to reduce its volume

Another factor is carbon that gets dislodged when the spark plug is removed, ideally the plug’s are loosened a turn and the engine is started and rpm’s sharply increased a couple times to flush and dislodged carbon out the exhaust so it can’t give you a erroneous result if it gets caught under a valve during the compression test.
 
solid wire plug wires, not resistance or radio suppression and the plugs also copper core is OK. plugs at about 0.025"
 
You must log in or register to reply here.
Back
Top