40hp Evinrude running on one cylinder @ low speeds but both at high speed

[sojo81007]

My 1966 Evinrude Big Twin has developed a unique problem. It usually starts up and runs fine
but does so only on the top cylinder. About 1/2-2/3 throttle the other cylinder will suddenly cut in
and the engine opens up and runs fine at high planing speeds.

I believe it is fuel related because I left the bottom spark plug in but pulled the boot and put a
spare plug in to see if there was spark while the engine was running. There is spark. In 2013
I replaced the entire ignition system (used Atom modules instead of points) and have had no
issues with it.

I thought possibly my fuel pump needed replacing so I did that and while the new pump works
great the problem persists.

I'm planning on rebuilding the carb because I am also having an issue fo the engine occasionally
not starting. Draining the fuel from the carb usually enables the engine to fire right up and run
well so I'm assuming the needle/float/seat are suspect.

Could this possibly be a bad leaf valve? Is it possible that one or more of the valves is
failing and not getting fuel into the lower cylinder when the vacuum pressure is low... but it
increases at higher RPMS and everything is fine?

Compression 88/90 on the cylinders which I believe is well in the proper range for this era of outboard.

 

[racerone]

When a cylinder suddenly cuts in it is ignition related !-----Concentrate on that first.---Run with a timing light to find out which cylinder it is.

 

[sojo81007]

It's the bottom cylinder. Again, I'm able to remove the bottom spark plug boot while the engine is running and it's performance is unchanged.
To see if it was ignition related, I took another spark plug, hooked it to the bottom boot, and started the engine. While holding that plug against
the block I'm getting consistent spark, so the ignition system is working fine even at idle speeds. That's why I'm assuming it's fuel related.

 

[racerone]

OK------Do this simple test.---Will spark on the bottom cylinder jump a gap of 1/4" or more yes or no.------Tuned up many motors like yours in the late 1960's when working at a dealer !

 

[sojo81007]

I bought an adjustable spark tester to make sure I was getting the spark test right. While I can get spark on both cylinders with the tester, the points have to be almost touching to see spark. The top cylinder is better but it's slightly less than 1/4". Thank you for your advice, now where do I go? The coils, wires, plugs, etc have been replaced, along with the Atom ignition modules. Someone suggested that maybe a coil has worked loose and has eased away from the magnet on the flywheel. Not sure about that or if it could possibly be the ignition modules.

 

[racerone]

If spark does not jump a gap of 1/4" you have a " no spark condition "----You could try to intall the points back in there as a test.

 

[sojo81007]

When installed the Atom modules I put them on the exterior of the powerhead rather than under the flywheel so I could easily swap them if they failed. I tried switching the modules between cylinders to see if the spark would change. It did not, at least not in a visible or obvious way. I guess I may have to pull the flywheel and see what is going on under there since I don't know how to further test the modules. If everything is fine there and the coils test fine, then I guess the modules are to blame. Any thoughts?

 

[sojo81007]

Update:

I have removed the flywheel and tested the coils. Everything checked out with the readings I found online.
Checked the spark plug leads. Fine.
Replaced the wires that went to the ignition modules.
The only thing I could find amiss under the flywheel was a connecting screw was SLIGHTLY loose. Fixed that.
Replaced the plugs with new properly gapped plugs.

It didn't fix it...was still running on one cylinder.

So I went ahead with the carb rebuild.
All was well inside the carb and the rebuild went smoothly

The only thing I did notice was that the oil return hose was
disconnected on the low side. Installed a new hose.

Installed a new fuel pump and all new hoses (clear to the tank.)

After the carb rebuild it started up and run roughly (needed adjusting)
and died at an idle. Tried to restart and it simply will not run. It has
good visible spark but still will not run.

The new plugs are coated with a black oily coating and are quite wet.

I've tried to get the spark to jump 1/4" but can't get that. I tried the tester
on my lawnmower which runs great and it won't jump 1/4" either.

Any additional thoughts? Last time I did a compression test it was 88/90
on the cylinders...Since I have fuel and spark, it would seem maybe there
isn't enough compression?

Or is it possible that the ignition modules, responsible for the timing, have
malfunctioned and while there's spark it is improperly timed?

Thanks!

 

[lindy46]

I'd put some points back in and see what happens.

 

[pro-crastinator]

Update:

It didn't fix it...was still running on one cylinder.

So I went ahead with the carb rebuild.

After the carb rebuild it started up and run roughly (needed adjusting)
and died at an idle. Tried to restart and it simply will not run. It has
good visible spark but still will not run.

The new plugs are coated with a black oily coating and are quite wet.

I've tried to get the spark to jump 1/4" but can't get that. I tried the tester
on my lawnmower which runs great and it won't jump 1/4" either.

Any additional thoughts? Last time I did a compression test it was 88/90
on the cylinders...Since I have fuel and spark, it would seem maybe there
isn't enough compression?

Or is it possible that the ignition modules, responsible for the timing, have
malfunctioned and while there's spark it is improperly timed?

Thanks!

Ignition is not working correctly
Carb rebuild fouled your plugs - thats why it wont start on 1 cyl

Forget everything (compression, reed valves, etc..) and focus on the ignition and try not to go to anything else until that is solved.
The "water is getting muddy" with this diagnosis.

 

[racerone]

?????------If spark does not jump a 1/4" gap you have ignition problems.------------Not sure why that is not understood !

 

[oldboat1]

might try a search on the AOMCI public forum for "Atom Computer Ignition". Upwards of a jillion posts -- might find some troubleshooting info for your installation.

 

[sojo81007]

I will try to find how to diagnose the computer ignition modules...I did use an ohm meter and got the same reading from both modules...not sure if that would be an indicator of some kind or not. I'm thinking it almost has to be the modules if the ignition truly is the issue. As far the 1/4" thing...I under stand what you've said but I took the same tester and tested my lawn mower and it wouldn't jump 1/4" spark and it runs fine and has since new.....You would think that even with somewhat weaker spark the engine would still run, albeit poorly.

I guess I'm at a bit of a loss....the coils check out fine, the plugs are new, the wires read fine, the magnet on the flywheel appears to be in good shape...the coils are spaced properly. The carb linkage has been synched according to the factory manual.

 

[oldboat1]

Check the AOMCI info. and analyses.

 

[geoffwga1]

I will try to find how to diagnose the computer ignition modules...I did use an ohm meter and got the same reading from both modules...not sure if that would be an indicator of some kind or not. I'm thinking it almost has to be the modules if the ignition truly is the issue. As far the 1/4" thing...I under stand what you've said but I took the same tester and tested my lawn mower and it wouldn't jump 1/4" spark and it runs fine and has since new.....You would think that even with somewhat weaker spark the engine would still run, albeit poorly.

I guess I'm at a bit of a loss....the coils check out fine, the plugs are new, the wires read fine, the magnet on the flywheel appears to be in good shape...the coils are spaced properly. The carb linkage has been synched according to the factory manual.

Lawn mowers and outboards are completely different animals.

 

[thdrduck]

As stated a few times... put the points back in, if it works, problem solved. Points aren't so bad to mess with.

 

[sojo81007]

I don't have a good set of points/condensers for the unit and am not really interested in going back to them. I've had really good success and better performance with the Atom modules, so far anyways. I did find from reading the antique outboard forum that most people run a ground cable from the armature to the block...I did grease the armature mount which may have contributed to the problem after I checked everything and ran new wires. I will attempt to run the wire, and if that fails some people reverse the wires on the modules with success. I would find that solution odd as it was running fine a few weeks ago.

After looking at the motor again I do think I'm going to grab a compression tester and check it again...there appears to be an oil residue seeping out of the main crank gasket on the bottom cylinder near the shifter safety lever. Also, when I was turning the flywheel slowly by hand I hear air leaking as the compression stroke is happening. I'm not much of a mechanic, but I don't think this is good. I'm beginning to think that I have a low compression situation. I hope I'm wrong, but this old machine is 49 years old and I use it as I would a new one...it's not babied.

 

[tomhath]

Why are you messing with a compression tester? The problem is a very weak spark on the lower cylinder.

 

[sojo81007]

Is it normal to hear air leaking out of an outboard when the flywheel is rotated slowly?

 

[racerone]

?????---Depends on whether it is air leaking from cylinder to the crankcase.---Or air leaking from cylinder to the exhaust.---Or air leaking to atmosphere ??-------Hard to tell without being there to see this 49 year old motor.