3.5 Merc Cuts out after a few minutes

[Daibheid]

Year is approx '94, Serial Number: 06202173 Model Number: 1003203

I'm hoping someone can help with this one. This is the simplest form of 2-stroke engine so there's very little to go wrong/check but it's beating me!

Symptoms - starts and runs OK at low revs but when throttled up to 50% or more after 2-3 minutes starts to miss and then cuts out. Will not run again for about 5 minutes. Runs better on high throttle with a little bit of choke but will still die even with choke.

Additional info/diagnostic efforts: Hadn't run for a couple of years but has low hours. I stripped and cleaned the carb - twice, replaced the inline fuel filter, and even ran with with the fuel tank connnected direct to the carb to rule out the filter and the fuel tap. I also ran it without the fuel cap to rule out air-locking as everything pointed me to fuel starvation.

I put it in a bucket and ran it until it quit and then whipped off the carb bowl (having removed the screws and wedged the bowl in place earlier) and confirmed it still had gas in the carb when it quit. I ran it again until it quit and immediately confirmed a healthy spark to rule out an overheating coil.

The only other piece of info I can add is that when running it appears to give a misfire that borders on a backfire every 10-15 secs unless a little choke is added.
Beats me with such a basic engine as I've confirmed it appears to have everything it needs to run so I'd really appreciate any help you can offer!

Thanks
Dave

 

[Laddies]

Re: 3.5 Merc Cuts out after a few minutes

Dave, you definately have a lean condition, you need to ck the carb high speed jet either it or the fuel passages for it are plugged, we had one apart 3 times to get it clear and call ourselve professnals

 

[Redneck Yacht Club]

Re: 3.5 Merc Cuts out after a few minutes

Hook a timing lite too it and run it, see if its lossing spark at top end.
The wait 5mins thing seems to point that out.

 

[Daibheid]

Re: 3.5 Merc Cuts out after a few minutes

Dave, you definately have a lean condition, you need to ck the carb high speed jet either it or the fuel passages for it are plugged, we had one apart 3 times to get it clear and call ourselve professnals

Thank Laddies - I really appreciate the 3 times apart encouragement 'cos after two careful strip and clean sessions I'd have ruled that out. I'll give it at lease another thorough clean out.

That's a good idea red on the strobe - I'd thought baout it but found in the past that a strobe will trigger at voltages thet won't create a spark. But it's worth a shot to at least see if the spark goes away completely.

Great suggestions guys - Thanks.

I was beginning to suspect a crankcase seal as I saw a bike engine behave like this one time and eventually had to strip it completely and found a crank-seal with a small crack that would seal to varying degrees at different times.

It'll either be next week or a month or two before I get to spoend the time on this again but I'll come back and post when I sort it.

 

[Daibheid]

Re: 3.5 Merc Cuts out after a few minutes

Dave, you definately have a lean condition, you need to ck the carb high speed jet either it or the fuel passages for it are plugged, we had one apart 3 times to get it clear and call ourselve professnals

I just wanted to complete the follow up on this. Laddies you were absolutely right. I did tear down and clean the carb one more time but to no avail. I then gave it to a small bore outboard specialist who concluded the same thing, cleaned it and claimed it was fine in a barrel. Sadly on the boat it was same old same old.

So with the assurance of your professional experience I took it apart like the manufacturer never intended and I then blew everything through and rodded out all passages. Finally I put everything through an Ultrasonic cleaner. (a small $40 unit sold for cleaning jewellery)

Result? I ran it for 20 - 30 minutes and then got to shut it down. Very satisfied and hopefully this will help someone else sometime. Thanks again Laddies - I know my way around 2strokes and would not have persisted without your insight. I most likley would have stripped it and replaced crankcase seals!