35 hp evinrude misses

1976 35 hp evinrude. Magneto ignition with driver coil and exterior hv coils. Running test on muffs. Starts easy and idles well. On the rev up it tends to intermittently stumble. At steady higher revs it will miss intermittently.


1. Is testing on muffs inconclusive?

2. Spark appears strong. Plugs are dry.

3. Swapped out carb with a spare, no change.
Fuel is fresh, clean and present.

How can I zero in on the problem? (if there is one, i.e. muffs?)
Ignition? Carb/fuel? Water in combustion?

Any thoughts?

Thanks.

Put a timing light on it --------One cylinder at a time and observe how spark behaves as you speed up the engine.

Tried timing light. Spark is steady on both cylinders.

At this point I have new head gasket, new exhaust cover gaskets, new plugs, new points and condensers and a new carb kit w/ new float.

Starts fairly easy. Idles great. Misses hard on the rev up and no matter where you hold the rpm...misses-hard.

Pulled power head off. Base gasket was fine. Removed bypass cover. Top section of cover is very black/wet with oil. Lower cover section is perfectly clean. Upper piston is visibly wet with black wet oil, lower is clean, as seen through bypass ports.

Removed reed plate assembly. Reed plate and gaskets good, no black staining/carbon deposit on reed plate. Black oil is present @ back side of piston and upper crankcase, clean and shiny lower.

Piston rings appear present and clean, not stuck, as seen through exhaust and intake ports.

This is blow by on upper? Is this the cause of the miss?

Anyone? All thoughts are welcome.

Have you checked fuel pump diaphragm ?-------Have you inspected lower crankshaft seal ?--Have you tested the driver coil ?--------------Does spark jump an actual gap of 3/8" or more?-----The early seals were black rubber and turned hard like carbon.-----The replacement seal is a brown material and much better.

Tested driver coil for continuity and shorts to ground--all ok. Spark will go 3/8" easy.

Fuel pump, not a big suspect as symptom will persist while pumping primer bulb.

Would the lower crank seal affect conditions at the upper cylinder?

Ok then , no more coaching.

Ok. Thanks for trying.
Greg.

I am using a fuel tank that is reliable and has always performed well. Gas is fresh.

Yes, I did pump primer when it cuts out. No change.

Checked spark with tester. 3/8" jump, no problem.

Have not examined impeller or T stat. Will do that, however motor pumps water well and you can see when Tstat opens, as flow begins to take place at top of exhaust leg.

Other notes: Compression is 125 on both.
Fuel pump is fine, diaphram's fine.
There is ALOT of wet oil residue, inside the upper bypass cover/ intake chamber and upper crankcase.
Top plug is also oily. Switched plugs=same. Switched out hv coils=same, switched out driver coil=same.

I have had two pro marine mechanics examine this motor. Have chased every suggestion.

I think the best clue of all is the oil in the upper crankcase and intake chamber. Lower is clean,clean,clean. That wet, black oil should not be there. How does it get there??? Isn't that backflowing from combustion?
Rings look good, as seen through exhaust and intake chambers.

Is that the upper seal failing? It's dry at top of block. Engine idles great. I thought seal failure was most pronounced at idle.

One other weird thing: when warm, at idle, when choked, motor picks up speed instantly, for a few seconds (5) and then will begin to choke out. Usually, in my experience, choking a warm motor will instantly reduce rpm. It really goes for it. I have never seen that.

I have looked and listened for external crankcase leaks. No luck.

When an ignition goes dead on one cylinder, will it result in wet/black oil residue deposited on the intake side? Any opinions?

Is it possible for a driver coil to fail on only one side? Driver energizes both hv coils, alternating from one to the other. Points, condensers are new. Wiring has been checked and checked again for shorts to ground intermittent etc.

Getting ready to deep six this one.

Interesting idea. I will look into that tube item #27. What is going on with #92? It connects directly to the upper bypass cover and runs to a single reed leaf at the reed plate. It is set up to deliver (what?) from the reed plate to the bypass cover.

Yes I am running 50:1

Thanks for your help.
Greg

Running on muffs is not the correct way of testing. Get it under load, preferably on the boat and launched and check if still missing on higher revs. Muffs only for flushing and basic idle testing!!

Pictures show wet/black oil residue on inside of bypass cover and intake port. Note difference between upper and lower. What the heck?