'93 25hp 2 stroke Spark Plug Issue??

[Jason2tpa]

Engine has been running perfect. But a couple days ago about 15 minutes into motoring the engine boggs down, very low sound, but continues to stay on. I turn back towards the boat ramp and at full throttle get no more than slow speed. Then it cuts back to near idle speed, then kicks back into slow speed. This repeats till i get to boat ramp, all the while having throttle at WO.

Thought water got into the tank bc its happened before. So i emptied tank, lines, carbs, etc. I removed plugs and they didnt look bad bc they are brand new. But heres the kicker....plugs wouldn't screw back in completely. I normally tighten by hand then 1/4-1/2 turn with socket. But now as soon as i make a turn with the socket it stops. This is not normal. And im perplexed bc i cant make sense of what would cause this. Btw, both cylinders can independently start the engine, even with the plugs not torqued down.

Your thoughts?

[SeaDawg3]

how far from the ramp were you?
regardless, running the engine at WOT with bogging issues was no good. sounds like dangerously lean and you may have hurt the engine in doing so.

assuming you have the correct plugs bc you mentioned they are new, what did the threads on the plugs look like when you removed them? was the engine hot when you took them out? pre mix ratio correct?
you`ll have to look into the cylinders and see if you can view any damage to the piston crown and cylinder walls.

did you try new plugs? maybe this could also me an ignition issue...

did you try a plug thread chase to clean the threads?

tell us what you find.

[Jason2tpa]

I have not looked at threads, next to do. Yes i pulled plugs after having idled a few minutes (is that bad). Mix is good (100:1). I know running it when bogging was not smart. But it was either that or stranded in middle of large channel at night on outgoing tide.

Think 50:1 would be better? Im heading to get new plugs, again, just in case. Ill look at the threads on plugs and case. Thks

[TownsendsFJR1300]

It is better to let the engine cool down a bit before pulling the plugs. The aluminum block cools faster than the steel plugs.

Hopefully you didn't remove any threads but a thread chaser can fix that..

Once the plug issue is fixed, have you done any other work / changes to the engine, set up, etc?

Fuel vent on the tank open/not clogged?

I would at least drain the carbs into a small CLEAN CONTAINER checking for any debris. Check your fuel filter as well...


I gather you run the boat on a regular basis, (IE; hasn't sat for a month W/O stabilizer)?

[Jason2tpa]

Doing a mental checklist of the fuel delivery-ignition system, the three are Fuel, spark, and compression. Yesterday i put new plugs in and tested for a spark. Good. Checked compression. Good (90 on both). Which leaves fuel. This is where the issue must be. So tracing the fuel delivery (new fuel in tank). The bulb hardens passing fuel thru filter into bowls (i can hear it fill). What i havent tested is the fuel pump function. My guess is its the original pump so it could have gone dead. To check i was told to remove the tubing going from pump to engine. Pull starter rope. I shud see fuel squirt with each pull. If it does squirt then i will need to test the pressure with a fuel pressure gauge.

Sound about right? As far as venting the tank...i use the old school non-EPA compliant cap that allows the caps vent to manually screw open/close. Ive been keeping this closed, especially while running the engine so water wont get in. Ill open the vent every so often to release pressure. Could this be a cause of my issue?

It is better to let the engine cool down a bit before pulling the plugs. The aluminum block cools faster than the steel plugs.

Hopefully you didn't remove any threads but a thread chaser can fix that..

Once the plug issue is fixed, have you done any other work / changes to the engine, set up, etc?

Fuel vent on the tank open/not clogged?

I would at least drain the carbs into a small CLEAN CONTAINER checking for any debris. Check your fuel filter as well...


I gather you run the boat on a regular basis, (IE; hasn't sat for a month W/O stabilizer)?

[SeaDawg3]

Ive been keeping this closed, especially while running the engine so water wont get in. Ill open the vent every so often to release pressure. Could this be a cause of my issue?

yes. the tank needs to be at atmospheric condition to allow fuel to flow, or let me say that as a strong pump pulls fuel from the tank and with the tank not vented, this will collapse the tank. or even a weal or small fuel pump especially pulse driven pumps with dead head and stop flow.

the tank needs to be vented during operation.

[TownsendsFJR1300]

+1 ^^^^^^. Open it up.

Your likely causing an AIR LOCK in the fuel system..



Also, generally, if the fuel pump fails, if you MANUALLY pump the primer bulb, your basically doing the work of the fuel pump. I don't think that's your issue, but something to be aware of...

BTW, I wouldn't think leaving that tank valve open while underway will allow water in the tank, it'd just pi$$ off the EPA. If you feel more comfy, fold some aluminum foil over the vent cap (not tight), just so it sheds any underway spray-BUT it'll still vent..

Please post back how it goes.

[Jason2tpa]

Would it be a safe assumption that if the issue is as simple as the tank not being vented that the tank would be somewhat collapsed? Like the fuel pump is trying to pull fuel from the tank and since its pulling from a air-vacuated space the tank would be pressurized?

+1 ^^^^^^. Open it up.

Your likely causing an AIR LOCK in the fuel system..



Also, generally, if the fuel pump fails, if you MANUALLY pump the primer bulb, your basically doing the work of the fuel pump. I don't think that's your issue, but something to be aware of...

BTW, I wouldn't think leaving that tank valve open while underway will allow water in the tank, it'd just pi$$ off the EPA. If you feel more comfy, fold some aluminum foil over the vent cap (not tight), just so it sheds any underway spray-BUT it'll still vent..

Please post back how it goes.

[Jason2tpa]

Sorry I just re-read Seadogs post. Sounds like he answered my question about a "collapsed tank"

Would it be a safe assumption that if the issue is as simple as the tank not being vented that the tank would be somewhat collapsed? Like the fuel pump is trying to pull fuel from the tank and since its pulling from a air-vacuated space the tank would be pressurized?

[TownsendsFJR1300]

Yep, that's I asked about in post #4, earlier...

And yes, if its sealed nice and tight, no fuel will flow...

[99yam40]

My 6 gal plastic tank will suck in if I do not open the vent, but will not starve the motor. it is a C40TLRX.

So maybe a weak pump would starve for fuel.
I would test vacuum and pressure output to see what it is doing

[Jason2tpa]

Good news guys. She's running great. It must have been a combination of an air lock and the tank not vented. I started by opening fuel pump. Diaphragm and gaskets looked good. Reinstalled fuel pump leaving the hose between pump and carb open. Cranked up (with tank cap vented) and fuel was squirting out good. Idled on the muffs for 10-15 mins. Then took it out for a test run. No stalling or bogging even at WOT. Funny how something as simple as not venting can screw everything up. Thanks everyone for your help. A good lesson learned.

[SeaDawg3]

awesome! what was the issue with the plug threads?

[rodbolt17]

the issue with the plug threads is simple too.
its called a fire ring crush gasket that technically can only seal correctly once.

once crushed its simply crushed and is now simply a washer.

means there is no 3/4-1 turn past finger tight.

back in the day you could actually buy them at the parts store.
may go back two days.

[Jason2tpa]

Ya know i guess there isnt a problem. Perhaps after a plug has been used, removed, then reinserted, it suppose to seat with less torque? I did a compression test and both cylinders are at 90. I recon if there was a threading issue id be losing compression. Your thoughts? Btw, ive looked in both owners and service manual for the proper compression and they dont say. Do you know? 25hp 2 stroke

awesome! what was the issue with the plug threads?