Compression rate

[otis71]

What is an acceptable compression reading on a JD?

[chuckthebeatertruck]

Generally on prewar bikes you can achieve 10 psi per point of compression at kicking speed.

a 4:1 makes 40 psi, 5:1 50psi

dont confuse this with more modern machinery. It’s ok to have lower kicking psi than you’d normally think.

[kitabel]

IMHO that's a safe minimum, but somewhat conservative, the intake closing point is very early, which traps more mixture.

[otis71]

Mine is reading about 45 in both cylinders but I don’t know the ratio. It recently failed to start but it sounds like that’s enough to run. Am I correct in that assessment?

[T. Cotten]

Golly Otis,...

45 divided by 14.7psi (one atmosphere at sea level) is only about three to one.

It might fire and idle, but I wouldn't expect much power.

I'd dump some oil on the rings and see if the compression comes up. (Then you know the rings are shot!)

....Cotten

[kitabel]

The static CR X atmospheric pressure for CCP sometimes comes out fairly close... by accident. Engines with low CR tend to have conservative intake closing points; high CR later closing. The 2 factors sometimes partially cancel each other out.
To predict CCP or cranking cylinder pressure in psi (leaving out kicking speed, ring condition, temperature, oil viscosity) the math is pretty complex.
A JD solved for 4.0:1 CR to give 45 psi shows valve closure at 69 degrees ABDC, I assure you, the JD cam is nothing that radical. A shorter intake duration will give higher CCP at the same CR.
Give me other variables and I'll run them through my Excel program and post the results.

My comments here: http://victorylibrary.com/tech/cam.htm

[chuckthebeatertruck]

Mine is reading about 45 in both cylinders but I don’t know the ratio. It recently failed to start but it sounds like that’s enough to run. Am I correct in that assessment?

Three things:
1) the fact both cylinders are about the same is good
2) 45 psi is on the low side even if you have a 4:1 CR, but in my experience on SINGLES it is enough to start. I've never quite gotten twins to start with less than around 60 psi.
3) kicking speed varies widely -- if you kick slow you will always have low values.


Now, here's where I'm going -- I don't really understand why you checked cranking pressure for a no start situation. Why do you think compression is the reason for no start? Are there other symptoms that indicate the bike suffered a major failure that would tank the compression ON BOTH CYLINDERS? That's the whole key here -- your readings front to rear are the same -- so a "failure" would need to affect both cylinders. What do you suspect happened to do that? If you don't have a clear picture in your head -- what is being investigated with these numbers?


Personally, I "check" for compression with my foot. Unless I feel a substantial difference from the last start, I simply don't think about it.

Instead on bikes between 4 and 6:1; I am more apt to suspect a weak spark -- triply so if it is a magneto bike.

A low compression motor allows you to get away with an incredibly weak spark. If it goes any further off . . you may get a spark in the air, but nothing in the chamber under compression.

A simple test at 4:1 or 5:1 is to gap a spark plug at .100 - .125. If your bike can jump that gap in the air with a nice blue spark -- you have plenty of spark to light a mix off at 4:1.

That done; you'll have compression and spark out of the way . . . time to concentrate on mix and timing.

BTW -- you didn't happen to have a stuck or sticking decompression lever?

[kitabel]

Low CCP also makes low vacuum

[otis71]

The bike always started very easy and then out of the blue after running the day before it failed to start. When I first got it, the bike would not start due to lost compression through the caps to access the valves. I lapped them and all 4 valves and it’s always started easily since. Because the spark looks strong and the carburetor wasn’t touched I decided to give the compression a check. Also the only tester I have to fit the 21mm plug hole doesn’t thread in and has to be held by hand and is subject to not the best seal. Also the kicker isn’t working 100% properly so between the two I would guess the readings are a little low.

[Steve Swan]

The bike always started very easy and then out of the blue after running the day before it failed to start. When I first got it, the bike would not start due to lost compression through the caps to access the valves. I lapped them and all 4 valves and it’s always started easily since. Because the spark looks strong and the carburetor wasn’t touched I decided to give the compression a check. Also the only tester I have to fit the 21mm plug hole doesn’t thread in and has to be held by hand and is subject to not the best seal. Also the kicker isn’t working 100% properly so between the two I would guess the readings are a little low.

you state the engine "...would not start due to lost compression through the caps to access the valves."

if you do not have a seal at the joint where the "inlet valve housing" sits in its recess in the cylinder, compressed air will leak past that joint and up to and past that ring nut holding the "inlet lever housing" in place and you will lose compression to the point the engine will not start or run properly or as you mention, quit running.

a compression test is not diagnostic, only records psi; nothing else. it's one thing to know you have low compression. it's completely another thing to know exactly where you are loosing compression.

having a seal at that "inlet valve housing" joint is imperative to the engine starting and running properly. the tried and true method to confirm there is a seal at that joint is to bubble test.

have you bubble tested to verify no air leaks around the ring nuts holding the "inlet lever housings" in place? i suggest perform bubble testing if you have not bubble tested.

bubble testing will also reveal if and where the inlet manifold system is leaking at any of its joints.

[otis71]

I have not bubble tested it though I have a tester I made for manifolds and will give it a go. I’m assuming all valves need to be closed for a good read? Also in your opinion the 45psi is low?

[T. Cotten]

...)I'm assuming all valves need to be closed for a good read?

As you turn the motor through, Otis,..

The regulator will rise and fall, and tell you when the valves are the most closed.

They need not close perfectly, as the regulator will keep the differential pressure up for the test.

....Cotten

[kitabel]

If the idle is stable and regular an intake leak becomes less likely.
A leaks not only allow compression to escape, it partially satisfies intake vacuum from external air, rather than requiring all air to pass through the carburetor.

[T. Cotten]

If the idle is stable and regular an intake leak becomes less likely.

Therefore, Kitabel,
To ascertain the likelihood, to avoid a tailchase, we employ the simple diagnostic of bubbles.

A leaks not only allow compression to escape, it partially satisfies intake vacuum from external air, rather than requiring all air to pass through the carburetor.

How much compression is there within the intake manifold? A reversion shock at most?

All I know is no two leaks behave the same.

....Cotten

[kitabel]

Where do you think compressed charge escaping through an open valve goes?