#4 cylinder misfiring. Spark good. Compression 53 psi
 

Can't determine cause of misfire and loss of power in #4 cylinder. Idles ok, but misses when revved. Plug coated with carbon. Swapped plugs, same result after 70 miles. Pulled head and #4 black crusted. Valves "look" ok. What could be my problem? Would appreciate some guidance while I have the head off.
Thanks!

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

Things that should have been done first prior to head pulling:::

Ck to make sure its not an ignition or spark issue.//in line spark tester to #4 plug

A vacuum leak from the intake gasket /// smoke test motor or for the novice use propane or carb cleaner to test

Perform a leak down test


All this is assuming your #4 cylinder is not dropping out the correct percentage when grounding out the spark

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

Could it be a bad distributor body that has an open or cross firing on number 4?
JB

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

A spark tester will show that as a potential problem

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

Make sure the spring force of the moveable arm on your points has not failed or partially failed. You can determine this with your finger. If it floats off of the cam it will idle, but that is all. We had this on one of our tours.

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

Assuming Ignition is good it sounds like a valve sticking to me.
When running at an Idle it has time to close but soon as you rev the engine its slow closing affects the compression and creates a massive loss of compression that will not fire.

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

Thanks. Had to pull the head anyway due to head gasket leak under water pump. Might a broken valve spring cause the delayed closing. While I'm at it I will replace manifold gaskets. All ignition components incl Dist body, were replaced 700 miles ago. I'll test #4 with in line spark tester. Appreciate all the suggestions. I'm new to Model A's but learning fast!

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

Excellent point

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

Hopefully the distributor cam was lubed so the points rubbing block hasn't worn down in the 700 miles since they were installed. Points gap is .020".

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

Once I receive all the parts I have on order and reassembled the engine, and completed the excellent suggestions I'll report back. Thanks for the help!

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

I have had spark plugs get wet and rust causing them to not ground properly. Check it with an ohm meter. Plug was tight but not making good contact through washer or threads. Sounds impossible but it happened.

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

From experience ... very possible scenario:

No. 4 cylinder rings sticking in the closed position ...... very common vintage engine problem after lots of carbon build up.

Bottom of cylinder after engine use always has a smaller diameter .... & top of cylinder has larger diameter .... one can "see" this at the ridge.

Piston goes to smaller diameter at bottom of cylinder where rings compress and get stuck with carbon/oil crud in the ring grooves ..... when piston goes to the larger diameter top, sticking rings rings do not have enough mili-seconds to expand ..... hence, allow blow by & more carbon on plugs.

Ever consider how fast the piston is traveling up & down and how often ring expand & contract on every revolution.

Years ago, many poor who had more time than money used to clean rings and ring grooves and re-install same to prevent harmful plug fouling & loss of power.

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

When you suspect that there is a valve sticking, after everything is back together,
you can test compression with the schrader removed from the threaded end of the hose. The needle will jump up and then drop to zero. Works very much like a vacuum gage. This is more to find an intermittent issue - sticky valve or weak valve spring. Can do this by a long starter crank and/or by running engine for a short period. Similar to using a scope - watching in real time.

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

As long as you have the head off, remove the valves in #4 and check the guides and the seat area.

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

I don't follow what u mean. If the schrader valve is removed from the hose on an engine with good non sticky sealing valves the gauge will drop to 0 after each compression stroke normally

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

It is simply a check for an intermittent valve that is not sealing. Frequently, a conventional compression test might not show an open valve at cranking speed if the issue is marginally sticking open. At running speeds it will show up by dropping to zero with the gage. A compression test gets a reading by cranking until the pressure levels off to the best reading and if it had a zero reading on one power stroke, it might not show as with a broken spring. The plus is, if you find a floating valve, you are already to proceed with a cyl. leakage test, which answers the issue completely. This test is only for the head scratcher problem that is inconsistent.

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

Ok i got what your saying ...without the schrader installed maybe on one of the consecutive compression strokes the gauge may not register anything.or register inconstantly if the valve sticks. ..

Re: #4 cylinder misfiring. Spark good. Compression 53 psi
 

Mitch, That's the idea. Just another way to collect data that's hard to find.