We have a project in the shop a customer brought in - '37 flattie in a newer chopped frame. I cleaned the carb, we oiled the cylinders, set the points and got it running after sitting for 20-30 years. Smoked heavily but seemed to run well considering it just woke up from a long nap. We don't have any info on anything that old and I'm trying to find what the pressure range should for a compression test. We were at 50 PSI both front and rear.

Can anyone steer us to an online reference for for Flatheads ? Added a couple pics.

 

Kick only will be hard to get numbers like if it had a starter and how many compression strokes are you doing?. Throttle wide open during the test?

 

Throttle open, plugs out and 10 good kicks on the compression stroke.

 

50 is low but I only use that number to isolate a dead hole or consistency and you should get the majority of your final reading with the first stroke and slight increases after each kick. My next step after a compression test is always a leakdown test to tell me where its going. If it passes that test then your comp test is inaccurate.

To answer your original question, I dont have a spec but it could be in the shop manual. Rule of thumb for an engine is 90 psi for combustion and since it runs Im not sure what to say about your 50lb reading. Also the higher the compression ratio the higher the psi reading. My sport bikes are typically 175 - 200 psi because of the ratio.

 

While the numbers might seem low being consistent is a positive sign. I'd say the rings are still sticky or valve seats are dirty. Those were a low compression engine, only 7:1 if I remember right.

 

Looked in my shop manual and nothing that I can find about compression test specs. Compression ratio is in the 5:1 up to a whopping 5.7:1 for the 9.4HP "performance version". I dont believe that HP rating

 

Think those are the 45" specs.

 

I thought so too but it says 74 and 80" in the column. I figured its more like 30HP

 

Thanks for the advice and recommendations guys.That's the road we're heading down. Now that we had it running sounds like the owner will probably do the top end and put it in an aftermarket frame this spring.

 

The 9 . 4 was the old way of determine the HP a conversion chart i have some place dont think i used it since hi school

i agree its not the number its the fact they were almost the same number your good thats not an issue - it could have all 4 rings being compression and no oil ring and if it has one a oil ring its below the wrist pin and 3 compression above the pin and all one piece - the smoking is what you added to the cylinder and maybe some slumping of oil over the decades

the rings were also prone to sticking as they were very thick and the ring to piston groove is actually small and its before they were back drilling the piston ring lands with holes so carbon is the enemy - pore marvel mystery oil in the gas of the old machines couple oz each gas tank and even add pint to the engine oil

the marvel will loosen all the crusty carbon and free the rings if sticking - ride it easy 35 MPH for 10 min only 3 or 4 times you will feel it fixing its self

marvel is a great carbon remover