1993 25 hp mercury runs but then stalls and won't start.

I have a 1993 25 hp 2 stroke mercury outboard that has been a wonderful motor until recently. It starts "ok" but has a rough idle and at idle it sounds like it's running on one cylinder but I'm told by the mechanic both cylinders have fire. At high rpm's it runs fine and never gives me a problem but sometimes (now often) after I've run it for a while and bring it back down to idle it stalls and it's a real BUGGER to get started. When it starts acting like this I usually have to wait 15 to 20 minutes befor it will start again. It has never left me stranded, yet, but I'm loosing confidence fast. And then sometimes, although usually not, it runs fine.

- Sounds like the carburator: I've had it rebuilt.
- Sounds like the coils or plugs: I've replaced them.
- Sounds like bad fuel: It's not, I'm religous about keeping the fuel fresh with the proper oil mixture and using Stabil and yes the vent is open.
- Sounds like the themostat: It's not that either, I had it and the water pump replaced at the same time.

I am at my wits end and at this point can't find a mechanic that can diagnose the problem but they say it will probably be at least another $500 in repairs.

Gentlemen - Your help and any sage advice would be greatly aprreciated.

Thanks, Jake.

Re: 1993 25 hp mercury runs but then stalls and won't start.

My guess is it's flooding. But instead of the shotgun approach, why not troubleshoot logically, one step at a time:

First, compression test. Report the results.

Next, determine for yourself whether a cylinder is dropping:

With the engine idling pull one plug wire at a time. Use an insulated spark plug boot tool to avoid shock. If the rpm's don't change, that cylinder isn't firing.

Then, spark test:
Get a spark tester (couple bucks at auto parts store).
Disconnect the spark plug leads,
One at a time, use a jumper wire to connect one to the engine block, connect the other to the tester, set it for 1/2" and clamp it on to the engine block.
While a helper engages the starter, look for a sharp blue spark.

Let us know what you get.

Re: 1993 25 hp mercury runs but then stalls and won't start.

I had the same symptoms once on a motor I had, turnt out to be the control box was knackered. Would work fine in the beginning, and as soon as the motor got warm the motor would be die.

Someone explained it as, the heat makes the control box expand thus causing a bad connection.

Re: 1993 25 hp mercury runs but then stalls and won't start.

Jtexas and Wiggy,

Thank you for the replies. I checked the compression and got a 135 psi for each cylinder. I don't what it should be, but those are the results. There's definitely spark in both cylinders. I did the compression test at dusk so I just pulled the cord one time with the kill switch on and there was good spark across both spark plug gaps and the plugs are not fouled at all. Regarding Wiggy's comment about this possibly being the switch box how do I test for that?

I also noticed that there is a thick black oily subtance in the bottom of the engine compartment that didn't use to be there. Not much, but enough to wipe it with your finger. Is that something to be concerned about?

Keep the advise coming and thanks again.

Jake.

Re: 1993 25 hp mercury runs but then stalls and won't start.

Compression is good - 135 is pretty good, but the difference between cylinders is more important than actual number, you want 'em to be within 5 - 10%.

You've confirmed that both cylinders have spark but a weak spark might jump the plug gap in the open but fail under pressure inside the cylinder, which is why you test a 1/2" gap. If it passes that test then you know the switch box is good.

Did you try the cylinder drop test?

Re: 1993 25 hp mercury runs but then stalls and won't start.

Admiral Jtexas,

I'm a little slow on the uptake but you have achieved student comprehension with your second response. I will do the 1/2" gap spark test (the tester actually looks like a spark plug if it's what I'm thinking of, right?) and the cylinder drop test. I completely forgot about the drop test. It'll be a few days before I get to it.

Looking forward, let's suppose I pass the spark gap test and the the rpm's do change meaning I pass the drop test. Then what? Any comment's on the oil?

Thanks, Jake.

Re: 1993 25 hp mercury runs but then stalls and won't start.

jtexas, has you on the right track.

Re: 1993 25 hp mercury runs but then stalls and won't start.

If compression and spark are good then your problem is fuel. Be good to know whether it's hitting on both at idle, also what the spark plugs look like after it's been idling.

When it's idling rough try pumping the primer bulb, see if that has any effect on RPM's. Also try activating the primer momentarily by pushing the key in.

Re: 1993 25 hp mercury runs but then stalls and won't start.

Admiral Jtexas,

I did the 1/2" spark plug gap test and it passed with flying colors.

I also did the drop test (and even though I used insulated pliers to pull the plug wire I still got a little shock if that's any indication of spark strength) and when I disconnected the top cylinder the engine continued to run, rougher, but it ran. When I disconnected the bottom cylinder the first time the engine stalled immediately. I tried it again and the engine ran for maybe 3 seconds and then stalled again. (Again wrt spark strength, while the engine was running spark was grounding against the block right out of the end of the spark plug wire.)

As for pumping the primer bulb, I think Wiggy would characterize my primer bulb as "knackered." I can't get the bulb to harden and I can hear full running back into the tank BUT this is new problem that just started today. It's time for a new fuel tank so I'm going to get a new tank and fuel line and try the primer test.

The current fuel tank is also a 1993 - I clean it religously every spring so my problem is not crap in the tank. As for turning the key, this a pull start so that's not an option for me.

As for how the plugs look, to my novice eye they look fine, I just don't think that's the problem but I'm open to advice and suggestions.

I also had the opportunity to see a demo on a new 25 and the choke allows you to adjust the idle speed. That feature definitely does not work on my motor. You can turn that choke knob every which way and the idle speed does not change. Any suggestions on the choke?

So what next? Contrary bugger started right up when I worked on it today, even with a knackered primer bulb.

Thank you, Jake.

Re: 1993 25 hp mercury runs but then stalls and won't start.

Admiral JTexas,

I forgot to add one other comment to yesterday's response. When the engine is giving me trouble starting sometimes the pull cord will come out about half way and then it gets so hard to pull it will slip right out of my hand. I didn't know if this was a symptom worthy of mentioning or not.

Jake

Re: 1993 25 hp mercury runs but then stalls and won't start.

JTexas,

I hooked it up to a new tank squuezed the primer bulb and it had no effect on the motor whatsoever. Today it's hard starting as well.

I am the end of my pull cord. Please help.

Thanks, Jake.

Re: 1993 25 hp mercury runs but then stalls and won't start.

I’m not familiar enough with your specific model, hopefully somebody will jump in here...

But your carburetor does have an idle adjustment, it’s the screw at the top of the carb throat.

Instructions for setting the idle adjustment: http://forums.iboats.com/bbBoard.cgi?a=viewthread;fid=36;gtid=1105882

Note that your “choke” is actually a “primer” – it opens a valve to let fuel drain into the cylinders to richen the mixture for cold starting. If it leaks it’ll flood your motor making warm starts difficult and maybe shutting off a running motor.