1992 40 HP Evinrude won't idle

[crybdr]

I'm still trying to figure out what is wrong with my non-idling 1992 40HP Evinrude. It was running fine earlier this summer, then sat for 6 weeks, and has been progressively running worse (stalling at idle and low speed).

Here's what has been done up to this point. This engine is now being run on very fresh recreational gas - with some Seafoam added.

1. Checked for spark on both cylinders with an in-line spark tester - OK
2. Checked compression of both cylinders - both measure out to about 125 - OK
3. Fuel pump is working
4. Primer pump is working
5. Rebuilt both carbs
6. With both rebuilt carbs installed, problem persists (top cylinder seems to be not firing). Verify fuel in both bowls - OK
7. Did a 'drop test' - pulling the top cylinder spark plug boot results in not much change in engine performance.
8. Suspect top cylinder/carb issue (manually choking bottom carb by hand raises RPM, manually choking top carb kills engine)
9. Substitute top carb with a surrogate (different make), seems to solve top cylinder misfire?
10. Receive and install used rebuilt carb. Engine starts. But, problem persists...... :-(

I have the manual for this motor - and at this point, it seems like I'm starting to get into potential manifold/ignition issues?

Before getting into that here are the things I can try while the boat is still in the water:

A. The used/rebuilt carb last installed came with jets - I could try changing those out with the jets from my original carb (the new carb is for a 40-50). Could that make a difference?
B. The spark plugs have looked fine. I could switch them to see if the symptoms change. Or, I could buy new ones and gap and install.
C. The fuel mixture screw position is unknown to me for the latest carb I received. That's a little out of my element, so I'd need my boat dude out here to assess and adjust.

Plug wires?

Weak fuel pump?

Weak spark?

What else could I be looking at before throwing in the towel and looking for a tow? Thanks for looking and for any help.

 

[jakedaawg]

plugs are cheap...$2 a piece...I have had bad ones right out of the box....but probably not your issue. You really should start with an open air gap spark test set to 7/16".

When you rebuilt the carb was it fully disassembled and cleaned? In cluding the little jets?

Have you done a fuel sample into a clear container? Just because you have new fuel does not mean its good.

Did you change any linkage? Have you checked idle timing?

I would go back to the original carb, thoroughly clean it and install. You probably just need to adjust them.

 

[crybdr]

Open air gap test is probably in order - the spark test gizmo that my mechanic had showed good spark. And the fact that his 'surrogate' carb fired has me completely flustered.

I've made no linkage changes. I have not checked idle timing.

The original carb has been rebuilt professionally (a carb shop that does old school muscle car stuff - did a 'smoke' test on it as well) once, and then, my mechanic looked at it a second time.......all checks out with new components. It was completely disassembled and rebuilt. The 'new' (used and rebuilt) carb exhibits the same behavior when installed.

Thanks for the advice.......

 

[crybdr]

Well, I'm still here pondering. Some progress was made today..........

Today I installed a new (rebuilt) carb off of eBay. I disassembled and cleaned it again making sure to pay particular attention to the idle uptake tube. I used a syringe and small gauge needle to gently apply 99% isopropyl into the 3 idle ports at the top of the carb mouth. I put lots of spray carb cleaner through all of the holes - dried it out, and reassembled. The service manual has a nice section on rebuilding this carb - everything looks good.

I also installed brand new plugs ($6) as well.

Now, the engine will run freely with the idle lever at about 30%. It runs smooooth without killing (that's much better than before) - it starts to chug a little around 10% idle lever, but, once I put the lever down, it slowly dies.

I tried an open air spark-gap test today. I was by myself, using a purchased device that has a clip that goes on the spark plug end, a screw adjustment to set the gap, and a post to insert into the spark plug wire boot. With that setup, I'm about 8 feet away and looking into reflections in the water. Should that 7/16" spark be 'bright blue' and obvious at that distance? Both plugs exhibited no visible spark today at that viewable distance - smaller gaps didn't show spark either - which makes me wonder if I'm too far away to see it. I will have a helper tomorrow to help retest. Is 7/16" the break even point of working or not working?

So, here are my thoughts now, looking for feedback on whether or not I am on the right track:

1. Fuel pump is probably not an issue - I can get that motor humming at about 30% idle lever without any hiccups (standing and watching it run for 2-3 minutes) - it runs smooooth.
2. Compression is not an issue - 125 on both cylinders, well matched.
3. Ignition may be an issue - I need to witness an open air gap test at 7/16" - will do that tomorrow.
4. Slow idle speed may be an issue - is it correct to say that the engine should idle fine with the butterfly valves completely seated and closed? This is where the digital and analog world confuse me. Also, while the 'slow idle' speed is 'set at the factory' - I have no idea where these needles are on these used carbs. It's also confusing that these 'slow-idle' speed adjustment screws also determine whether the engine is running 'rich' or 'lean'. If anyone can explain how these relate - it would be helpful.

Any other issues I could look at while I'm standing in waders diagnosing? Lync and Sync? I am all ears.....

I've already got a 'tow' scheduled

 

[crybdr]

Tested just now - I have good spark at 7/16" - now I'm on to slow idle adjustment I guess.

 

[crybdr]

Well.......crap.

Good compression.
Good spark jumping a 7/16" air gap.
Rebuilt carbs that have full bowls when the engine dies.

She just won't idle- what am I doing wrong?

I've seated the idle screws on both carbs to what the service manual says, 2 turns out from gently seated.

Also did the lync and sync prescription from the manual to a 'T'.

I feel defeated. :-(

 

[racerone]

Most carburetor " problems " turn out to be something else.---Have you removed flywheel to check the key ?----Have you inspected reed valves ?-----Have you considered the bottom crankshaft seal?---Have you made sure that the motor is warming up ( thermostat ) missing / stuck open ?--Note, the electric primer is NOT a pump.--It is a valve that opens when key is pushed in.

 

[crybdr]

Thank you for the advice.

I'm unable to do much more that what I have tried. The boat is still in the water, and it is literally 100 vertical steps from the house down to the dock. I'm only able to bring what I can fit into a backpack down and work. I've learned a ton and have gained more confidence methodically wrenching on this thing - per the service manual.

It's encouraging/discouraging that the primer is a valve, and not a pump. It tells me that the fuel pump is okay, if I can keep the engine running by occasionally bumping the key.

What you suggest probably needs to be done in a shop - It's simply beyond what I can do in the water.

I have a tow scheduled with a neighbor tomorrow to get it to the landing and pull it up to the service/storage place. Lots of boats got taken off the water last weekend - the water is starting to get a little cold for waders.

I'll update to let you know what they say is the issue. Thanks again.

 

[Michawilden]

I had the same problem with the same engine, same year. It was a johnson. Tried multiple settings on the mixture screws but I could not get it to idle properly with the butterfly valves closed. It seemed to not get enough air to keep running. Eventually, I adjusted the roller on the carb linkage to just slightly touch. With the butterfly valves slightly open it idled fine and I could adjust the idle rpm with the idle screw.
I know it is not the proper way to set the linkage but it worked fine for me. In your case, you can try to screw your low speed mixture screws more in, like 1/4 or 1/2 turn, this can give a higher idle rpm but may cause a lean situation. If it starts to cough, it is to lean. The factory setting is not always the proper setting.