'93 25hp 2 stroke...violent sputter

[Jason2tpa]

Here we go again guys. Here's what has been done to the engine so you dont have to look through my previuos threads. Replaced one of the 2 carbs. The older one (lower) has been cleaned with new kit. New fuel pump innards. Tank has been cleaned and dried. Fuel filter is clean. No issues starting now I have the 2nd carb choke valve. New fuel.
Was out running boat last night. Ran great for about 5 miles. As I was near the boat ramp i could hear it start to sputter at 3/4 throttle. This sputter im describing as violent. Noticeable shake to the engine and i can feel it through the tiller. I came off plane briefly then jumped back on same issue. This came out of nowhere ya know. It was late so I stored boat till today to work on it. I pulled the carbs out,dumped the bowls, cleared the lines, filter, and pump. Pulled plugs. Nothing major..they werent covered in fuel or oil. Put it all back together. No issues starting. Same result once I ran it.
I recently switched to synthetic oil and made it richer 50:1. Idle speed and mix were set to spec. Could this be a timing issue? One thing i have not checked is igition timing at TDC. Or could it be CDI issue? It cant be fuel, compression or spark. Tjose have all been checked. Im shaking my head.

[Jason2tpa]

Coulpe more notes. Before launching today i put engine on muffs to see if sputtering had stopped from the previuos night. No problems at all. No skipping or sputtering.
Also Before i pulled lower cylinder plug i noticed the outside of plug thst is uncapped was whitish. Like a oxidation color. The top plug looked normal.

[Jason2tpa]

One more note. This just occurred to me. I have a tiny tach installed. The wire wraps around the lower cylinders coil wire. It was working last night when engine was running good. I noticed when flushing engine this morning the gauge was stuck on 1200. This is a first. When i ran boat today it also was stuck at 1200. The tachs wire is wrapped around the plug cover coil that i mentioned is whitish. Coincidence? Cause i don't believe in coincidences.

[99yam40]

take it off to see if that does anything.
If plug wire is bad/leaking spark it may have jumped to the tach wire and fried it.
I would not think it would, but who knows for sure

[Jason2tpa]

Good suggestion thanks. Does this sputter im describing...is this a sign of bad timing/misfire? Its not bogging with low pitch, which is why my guess is its not a fuel issue. It feels and sounds like its skipping, even in neutral idle.

take it off to see if that does anything.
If plug wire is bad/leaking spark it may have jumped to the tach wire and fried it.
I would not think it would, but who knows for sure

[rodbolt17]

ign timing on that motor is mechanically fixed. if no one monkeys with it it wont change nor come and go.
typically that white powder indicates water on a plug.

[99yam40]

Coulpe more notes. Before launching today i put engine on muffs to see if sputtering had stopped from the previuos night. No problems at all. No skipping or sputtering.
Also Before i pulled lower cylinder plug i noticed the outside of plug that is uncapped was whitish. Like a oxidation color. The top plug looked normal.

You might want to explain what you mean by outside of plug.
The ceramic is white already, so is it the metal body of the plug that the plug wrench fits on, the threaded portion, or maybe the tip that the plug wire make contact on?

My guess Rod was thinking it was on part of plug that is exposed inside the combustion chamber

[Jason2tpa]

Thats the hiccup...if youve read some of my previous posts this engine was missing parts, some weren't installed correctly, etc. i have to assume that the timing has been fuddled with.

However, I spoke w 2 different small engine techs independent of each other and they both came to the same conclusion: ignition system. Either the magneto or coils. They said Rust or corrosion has probably built up causing a randomly bad connection. Which does explain why the engine would start sputtering 5miles into a ride. Its a '93 and im certain its the original coils and magneto. So as soon as i can locate the special tool that unscrews the nut holding the flywheel ill take a close look at the system. Your thoughts?

Btw, I was referring to the spark plug insulator and shell.

ign timing on that motor is mechanically fixed. if no one monkeys with it it wont change nor come and go.
typically that white powder indicates water on a plug.

[rodbolt17]

you can stand 10 ft back and chunk large amounts of cash at it or trouble shoot some stuff.
no special tools required to remove the flywheel.
it does not have a magneto so you can rule that out.
it does have a pulser coil for triggering both cylinders. it has a charge coil to charge the CDI and two ign coils.
all easily tested while running.

lets review.
it will run well for 5 miles.

means compression/sealing is most likely ok. I would carefully inspect the plug electrodes for signs of water burn.
if I did not see signs of water burn I would hook up my test gear and turn and burn till it broke, checkingcharge coil output,pulser coil outputs and CDI outputs.
but that's just me.

[99yam40]

Proper test equipment is needed for ingition system troubles shooting, but it will tell you if there is a problem with the ignition system without pulling the flywheel or replacing parts

DVA adapter and regular meter will work and is the least costly is you have to buy.

5 minutes of running upper RPMs could get motor into an overheat situation or one of the coils to break down from heat maybe.
Test to eliminate or confirm the ignition system voltages is the problem.
If it is then it will tell you which component has the problem.
If it is not spark then you head in a different direction without having to replace parts

You did have someone do things to the carbs that normal people would not have done, and you only replace the top one.

I found the main jets on my C40 that I picked up 3 hand, had been drilled out took me a year to finally find out what was causing the high speed miss

Test what you can 1st always

[Jason2tpa]

Describe what water burn looks like. Ive never heard of that.

you can stand 10 ft back and chunk large amounts of cash at it or trouble shoot some stuff.
no special tools required to remove the flywheel.
it does not have a magneto so you can rule that out.
it does have a pulser coil for triggering both cylinders. it has a charge coil to charge the CDI and two ign coils.
all easily tested while running.

lets review.
it will run well for 5 miles.

means compression/sealing is most likely ok. I would carefully inspect the plug electrodes for signs of water burn.
if I did not see signs of water burn I would hook up my test gear and turn and burn till it broke, checkingcharge coil output,pulser coil outputs and CDI outputs.
but that's just me.

[Jason2tpa]

All great tips ill use. Seems much more efficient to test ignition system with a meter thsn pulling flywheel. Although im certain this has not been inspected for god knows how many years. As far as condition of lower carb, the older one, I had a yamaha mechanic break it down, clean, and install the kit after I purchased engine. He said it looked good.

Proper test equipment is needed for ingition system troubles shooting, but it will tell you if there is a problem with the ignition system without pulling the flywheel or replacing parts

DVA adapter and regular meter will work and is the least costly is you have to buy.

5 minutes of running upper RPMs could get motor into an overheat situation or one of the coils to break down from heat maybe.
Test to eliminate or confirm the ignition system voltages is the problem.
If it is then it will tell you which component has the problem.
If it is not spark then you head in a different direction without having to replace parts

You did have someone do things to the carbs that normal people would not have done, and you only replace the top one.

I found the main jets on my C40 that I picked up 3 hand, had been drilled out took me a year to finally find out what was causing the high speed miss

Test what you can 1st always

[99yam40]

All I am saying is test things, it is dumb not to.
Taking things apart just for the heck of it is a good way to break things

You cannot see if someone did something stupid all the time , that is why it took so long for me to find the drilled out jets.
I had to buy new ones to compare the hole sizes after everything else was tested and I went through carbs way too many times looking for something wrong

Rod already told you what water would look like on the plugs

[Jason2tpa]

Picking up a multimeter today. From what I understand, I can test the resistance from the coil pack to the boot and test from CDI to coil pack without having ro remove the flywheel. All other ignition system components are under the flywheel. Correct me if im wrong please...if I potentially have a ignition issue, specifically intermittent loss of spark and/or reduction of spark strength, mustn't the CDI be powered? Meaning the flywheel must be turning so the engine would be on.

I am a DIY kinda guy but diagnosing the ignition system is new to me. So any step by step advice is greatly appreciated.

[Jason2tpa]

By the way, if it turns out there is a leak in the plug wire assembly (between coil pack and spark plug cap), this wire cannot be purchased separately from the coil pack. A new coil pack is $165. So my question is there a way to replace this wire without buying a new coil pack?