2.5l mercury overheating

[Smalltownbassin]

The couple of triggers I've tested at cranking speed check out at 4 to 5 volts. My service manual states 4 to 6 volts. Not sure about the 10v that your getting?

Might try a bias and kill test, or mess with the timing light some more.

Im going to try again to witness the double fire an evening this week when its getting dark out. Seems like i can hear it start doubling down on the water and then clear up for a split second before stumbling again. Had Merc2.5 in the boat with me. It was more obvious to me because im used to what it should sound like behind me but Merc2.5 also mentioned it as we were taking the boat out. It never dies though. Sounds and feels similar to a rich stumble but i can clearly hear an extra sharp POP every revolution. Its noticeable to me but almost imperceptible to anyone else who doesn't sit behind my wheel often. Gonna replace the key switch regardless. Its served its purpose for 20 years. I have a good OEM trigger to use also once i get the new SB's. Ill put all this in the video im finishing up and fingers crossed i wont have to film "Troubleshooting part 3" lol. I cant thank everyone enough for helping me narrow this down. This site is worth its weight in gold! ill update once i get another timing light reading. Thanks again!

[Smalltownbassin]

I've read thru this thread again. SO Small town I do not see where you have said it sounded alarm. You say its overheating. What are you basing it on? Have you verified temp alarm sender. Many have commented on pressure and temps. My take is similar to most . 15 lbs no less on water pressure at 5000 or more. 3/4 to 7/8 on temp gauge is typical in warm water. No 4 can melt and be unrelated to water temp. One posted about an air leak. Good point. A cracked reed change or intake manifold. Warped intake or something maybe gasket doesn't line up properly.
Are the carbs on it original to engine? And set up per manual?
I hope you find something to narrow down your search.

After melting #4 and not running a port head temp, i installed a port head temp sender and getting the alarm at the same rpm range i smoked the last one. I checked the sender with a multimeter on continuity and a small hand torch. Slowly ( at a distance) heated the sender until it made and i got the audible continuity beep on the meter. Sender works. After melting #4 in one motor, the alarm going off now is legit. SOB is getting hot. Hotter than this motor has ever been and you should know that this isnt the first time i had it bolted up. I pulled it when i finished the last one to save this as a spare. Its a James Perry cut block and i got quite a bit of money invested in it so i wasnt going to run it. I built it and bolted it up to break it in only. Used to run around 125 on muffs. Now, exact same motor runs 155 on muffs and 4 will continue to climb. Mechanically there isnt anything at all wrong with it. The build is clean and tight. Common parts are carbs, electronics, mid and plates. The lower was common too but after running the spare lower ive ruled it out as the cause. Ive also been through the carbs multiple times and the last was on video. Reeds are all clean and in tact. Cages arent damaged. Gaskets are new and tight. No air leaks. Mid and adapter plates are clean and clear of any debris or obstructions with new gaskets. Water tube is clear and upper grommet isnt folded or loose. Good water flow and pressure. I even adjusted the carb bowls to the point of flooding to rule out fuel. Had .086 mains in all 6 and overheating. Those 86's were needed in the other smoked motor because it was so dang thirsty. After speaking to James about this he recommended 80-82 in his block. I put them in and temps are the same. The fuel is already doing all the cooling it can even with 86's in it. When i smoked #4 the first time i was just cruising at 5k for less than 5 minutes when i noticed a sudden 200 rpm drop so i killed it. Trolling motored back to ramp, got it home and pulled the heads. Found what i found. Swapped powerheads with new gaskets, same temp issues but this time it alarms out before i melt one since i put a sender on the port side. Ive pulled the heads of this motor twice already and im starting to see significant scuffing and some scoring in #4 only. I assume the piston is trying to swell beyond the rings liking from the heat and doing some damage. Ive literally pulled the powerhead, lower unit, adapter plates to go through this thing 4 times now. 5 times actually for the lower unit because i was dead set on it being the nose cone. Once i tried the spare stock lower with the exact same issue i ruled it out. You know the rest.

[rgsauger]

Damn. We’re going to name this thing Christine! Good luck. Keep posting your results.

[LakeFever]

For what its worth ive melted plenty if two strokes down and that piston looks like lean burn down to me which will overheat the motor beyond the cooling capacity of a healthy cooling system. In Mikuni carb world id say your suffering mid rang lean burn because its so slight and suggest a fatter pilot tube and needle jet but ive never tuned a merc carb so i dont know what parts you need to fatten up the mid range is it just the main?

anyways id fatten it up to the point of fouling spark plugs and see how the engine cooling functions. If it stays cool, slowly start leaning it out one jet size at a time and read the plugs. Tedious process, but im guessing this will bring it home. I also took a look at the earlier pics that piston is fubar you need to replace it unfortunately.

[Merc 2.5]

For what its worth ive melted plenty if two strokes down and that piston looks like lean burn down to me which will overheat the motor beyond the cooling capacity of a healthy cooling system. In Mikuni carb world id say your suffering mid rang lean burn because its so slight and suggest a fatter pilot tube and needle jet but ive never tuned a merc carb so i dont know what parts you need to fatten up the mid range is it just the main?

anyways id fatten it up to the point of fouling spark plugs and see how the engine cooling functions. If it stays cool, slowly start leaning it out one jet size at a time and read the plugs. Tedious process, but im guessing this will bring it home. I also took a look at the earlier pics that piston is fubar you need to replace it unfortunately.

To fatten mid range on wh carb , u pull the vent jets In front of the carbs , usually 96s in 200 carbs , but I'm pretty positive those have been removed

[LakeFever]

From what it sounds like mechanically speaking your cooling system is functioning properly but iirc stb said the issue began after installing the low water pick up yes?

anwyays id still fatten the fueling up til its fouling plugs and dial it back. This is such a pita i gave up doing it many years ago because the time involved jetting a ported and piped motor is both a challenge, and a huge risk. I burnt down three engines in three days trying to tune it. Fortunately the oem motors were getting bigger and mor powerful at this period so i left motors stock after the mid 90’s and worked the other stuff; clutching/suspension etc.

back on track im now modding my first 2.5 and having nightmarish memories as this thread unfolds. Hope mine doesn’t dont this lol. Efi too

It would be helpful to swap electrics over from a perfectly fine running motor before tuning this one again just to clear the doubt in back of mind. It can be hard to overcome that. With the fueling another key aspect is make only one change at a time and test. Impatience and guessing can be costly here. Man i feel for you guys this is a tough deal to work through. Keep morale high

[Smalltownbassin]

WH-46 bodies that served well and Merc 2.5 is right. The 0.096 backdraft jets are removed for more mid range flow. Ive got several sets of different era carbs (WMV & WMH bodies) i could try but i think id be wasting my time. These 46's carried the "frankenbeast" for months without any issues at all until i added the LWP cone. The timeline is whats screwing with me. Everything was tip top until i did that mod and all of a sudden BOOM...there goes #4 at mid range rpm. But, after all the changes, new pumps, gaskets & double checks, then swapped lowers the problem isnt the lower. Its just a coincidence that these things happened close to each other i think. The "frankenbeast" was truly an experiment of mine so it was highly possible i leaned it way out trying to tune it but that should have happened when i had 80's in the WH-46's to begin with. I jetted it up to 86's based on the power range over 5K. Once i got more fuel to it she pulled HARD up to about 6200 before tapering off and leveling out. Pretty heavy boat with a 25 trophy plus so that WOT rpm was about tight. She was healthy. All 6 holes looked like the 3 on starboard side when i pulled heads to inspect my creation after break in and jetting. I did all the port and chest work myself so it was a cool learning experiment but.....James Perry has his work down to a science. This new block thats trying to take a dump isnt much more than a 200PM and way less thirsty than the other one. He said 80-82's would be safe for longevity with WH-46's and he knows through experience what this port map wants. Aside from the fact that ive already run this block after building it to break it in with zero issues before swapping over to the beast when i finished it. If my timing light shows clean fire tonight ill try a set of WMV-18's from a stock 2.5 200 and jet them up like you said to guarantee its not a fuel issue with different bodies. but i think ill be replacing electronics before addressing the cause after kicking this can around with you guys. Seems like all roads lead to switchboxes at this point but i also need to address the cause of the failures too or im always going to be dumping my wallet out on it. Frustrating to say the least. Especially when you were taught by very competent builders. In no way would i consider myself on their level but im pretty confident in my mentors (Merc 2.5, James P., Dez Nuts, and everyone here) and abilities at this point. Im always learning and willing to adopt new ideas to expand my knowledge but when we are all scratching our heads on what should be a fairly simple problem to address, i start second guessing everything ive learned. SMH

[Smalltownbassin]

https://youtu.be/nfSBIB4WC6s

I posted this private link on the "Frankenbeast" build thread. If the WH-46's were an issue, it would have popped this day. I was jetted with 80-82's in this video and i let her have it this day. If anything i tried to blow it up i was so excited lol! Hear that smooth as silk idle and acceleration too. Something in the electronics definitely changed between then and now. No extra POP in the ignition in this video.

[Smalltownbassin]

https://youtu.be/0au41qIuHh0

At 13:45ish you can see it. Because of where #1 is in relation to #4 on the flywheel, #1 is visible in the light cone on the right, while im witnessing #4 double fire you can clearly see #1 disappear from its spot on the right exactly as it should if #4 is doubling down somewhere else in the rotation.

If you dont know how to slow down playback on youtube, hit the gear at the bottom right icon bar under the screen and select 0.25 playback speed. Thanks for all your help! We found it!

[Mrzip]

I have never known #4 to get hot due to water.. It was always # 6.. I sure have seen inside a bunch motas in my life.

[rgsauger]

Are your switch boxes old? Double firing will damn sure kill one.

[LakeFever]

Thanks for the update glad your hot on the trail now! Looking forward to learning the cause

[Merc 2.5]

One thing we did was swap out the flywheel and I could hear a big difference in sound of the motor at idle and when reved up , the motor did smooth out , of course didn't fix the issue ,but there was definitely sumthin up with it

[Smalltownbassin]

One thing we did was swap out the flywheel and I could hear a big difference in sound of the motor at idle and when reved up , the motor did smooth out , of course didn't fix the issue ,but there was definitely sumthin up with it

Pretty sure thats just the difference in a heavier rotating mass. Helps keep it turning through the double fire and hides the sound a bit more. Ive several flywheels that can prove it.

[Smalltownbassin]

Are your switch boxes old? Double firing will damn sure kill one.

Less than a year.