2.5l mercury overheating

**Smalltownbassin** · 07-04-2022, 04:01 PM

Short version- Melted #4 piston, swapped powerheads, trying to melt #4 in the new one too.

Long version -

After finally getting the "Frankenbeast" (modified 200EE with 2 piece plate) tuned i decided to install a BOB's LWP cone. Shortly after the cone install i melted #4. Originally i blamed myself for either leaning it out or making a mistake on the cone install. Since i had another new PH to drop on, i went ahead and swapped them out. But...after the meltdown on port side with the other PH i installed a head temp sensor in both sides of the new PH and sure enough its trying to melt #4 also. Once i learned this i went ahead and rebuild the carbs. No sign of any fuel delivery issues whatsoever. Still overheats. Then i go into the lower to look more closely. I went ahead and pulled the epoxy plug i poured in the gallery which was fairly easy to remove and i poured another which is bonded very well now. I also replaced the entire pump, not just the impeller. Still overheating. Then i pull the PH to check adapter plates and gaskets. Everything is clean and free of obstructions. Water pressure is good. Poppet operates normally and has been rebuilt regardless. Fuel and water flow are good. Then today, i went ahead and installed a spare lower with another new pump. Still overheating. Pulled the thermostats just to see what happens. Runs cooler on muffs but still overheats on the water at around 4-5k. WP is at around 15psi at that rpm when i get the alarm. Funny thing is that the heads read 150 with no stats on the water when it alarmed. In my mind a fishing motor shouldnt even get that warm with no stats in it but im not certain. Ive never run into a gremlin this hard to find. Im pretty sure ive eliminated the carbs and the lower as the source of my issue which means that the CDI ignition must be the cause. I used the CDI spec sheet to test all the components. At rest everything measures dead nuts in the middle of the "good" range. At idle, my trigger pulse is 10V. Seems kinda high to me but again, im not sure. Bias checks good also. Could this still be a SB issue? If so, this would the 3rd set in less than 1 year so whats killing my SB's so often. All components are new out of the box CDI brand that ohm good. Im really hoping one of you can point me on a more direct path here cause im pulling my hair out on this one. two powerheads, same cylinder. Electronics, fuel or water? Ive had this thing ripped apart several times now and i think ive eliminated fuel & water as the cause. Leaning on the electronics now as a last ditch effort to find it before i hold another funeral for a black motor.

TIA

**25two.stroke** · 07-04-2022, 04:13 PM

No thermostats in your engine? What do you have instead? Washers, nothing, 1/8 holes, 1/4 holes....?

**Smalltownbassin** · 07-04-2022, 04:16 PM

That was just a temp test we did today to see how it reacted to more flow. The results just point out that its not a water flow issue so im pretty sure that isnt the cause. Stats are going back in it.

**25two.stroke** · 07-04-2022, 04:18 PM

At 150 head temp and 120+ water temp I am slowing down and not pushing my motors till they cool down. Even nikisil motors, I don't like my heads 150. If head temp is 150 and fuel is the issue the egt is typically up at 1250 to 1300 if the motor is cooling correctly...and at that egt stuff melts real quick.
You can overheat from water moving too fast and too slow through your block; as you probably know. On fishin motors removing the tstats is NEVER a good idea (imo) if you have stock adapter plate dumps for the water. It does not regulate the water right. 15 lbs at 4000 is fine if the water is flowing through the entire block (diverters installed, poppet in and open at that psi, tstats probably closed due to low water pressure).

**Smalltownbassin** · 07-04-2022, 04:23 PM

Trust me, i agree 100% to what you just said. We just pulled the stats to see how quickly it heated up and no surprise, it still got hot. There is absolutely nothing wrong with the water flow in stock form. Still hot. Its flooded with fuel too, still hot.

**Smalltownbassin** · 07-04-2022, 04:26 PM

Never had a temp issue until now. Ran frankenbeast for months getting it tuned before i melted #4. This new PH is picking up where the other left off but this time i know water and fuel are ok. Could a firing issue show in #4 only? If the SB or trigger is doubling down would it only show in one hole opposed #1 & #4 that the trigger coil controls?

**Smalltownbassin** · 07-04-2022, 04:36 PM

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**Smalltownbassin** · 07-04-2022, 04:38 PM

Port and starboard piston tops in pics above. #4 looks like a lean out. But after the carb rebuilds I can be sure now it's not.

**LakeFever** · 07-04-2022, 04:56 PM

You sure your not pulling unmetered air into that hole? Carb base gaskets? Case halfs or? Sealed up properly? Maybe run cooler plugs see if that helps?

**transomstand** · 07-04-2022, 05:23 PM

Might want to index the flywheel and check timing on each cylinder. Also have some concerns that the alarm goes off at a head temp of 150. I think the alarm switch is calibrated to close at 240.

If you're gettin 15 psi@WOT with no stats, there's plenty of water. What's it make with stats in it?

**Smalltownbassin** · 07-04-2022, 06:46 PM

Originally Posted by LakeFever

You sure your not pulling unmetered air into that hole? Carb base gaskets? Case halfs or? Sealed up properly? Maybe run cooler plugs see if that helps?

Yep, im sure its not sucking air.

https://youtu.be/yrYWPx0NZe0

**Merc 2.5** · 07-04-2022, 06:58 PM

Originally Posted by transomstand

Might want to index the flywheel and check timing on each cylinder. Also have some concerns that the alarm goes off at a head temp of 150. I think the alarm switch is calibrated to close at 240.

If you're gettin 15 psi@WOT with no stats, there's plenty of water. What's it make with stats in it?

Only reason we took stats out ,cause I wanted see motor run cool at idle in driveway , but done same thing on river just took longer. My concern also is why is that temp sensor goin off at 150 or 160 , 20 seconds after alarm went off I was checking head temp and read 155. I read sumthin earlier that said alarm at 240

**Smalltownbassin** · 07-04-2022, 06:58 PM

Originally Posted by transomstand

Might want to index the flywheel and check timing on each cylinder. Also have some concerns that the alarm goes off at a head temp of 150. I think the alarm switch is calibrated to close at 240.

If you're gettin 15 psi@WOT with no stats, there's plenty of water. What's it make with stats in it?

I indexed the flywheel to #4 looking for a timing issue or double fire. I didnt witness any issues with the light. Its set to 23 BTDC. I thought the head temp sensor was going to go off at 180ish but yeah the #4 hole (measured at head) will climb up to the high 160s almost 170 if i let go that far on muffs. Its making some real heat before #2 gets warm enough to open the stat. Every motor ive owned barley gets to stat opening temp if ever on muffs. Anything over 145 concerns me. With stats in it still runs at 15psi. At idle with stats in all 6 will climb up to the 140-150 range.

**Merc 2.5** · 07-04-2022, 07:01 PM

What kinda Temps is everyone else runnin? I know lots folks on here run temp guages off heads

**transomstand** · 07-04-2022, 07:06 PM

Originally Posted by Smalltownbassin

I indexed the flywheel to #4 looking for a timing issue or double fire. I didnt witness any issues with the light. Its set to 23 BTDC. I thought the head temp sensor was going to go off at 180ish but yeah the #4 hole (measured at head) will climb up to the high 160s almost 170 if i let go that far on muffs. Its making some real heat before #2 gets warm enough to open the stat. Every motor ive owned barley gets to stat opening temp if ever on muffs. Anything over 145 concerns me. With stats in it still runs at 15psi. At idle with stats in all 6 will climb up to the 140-150 range.

15 is borderline at WOT with stats, I'd prefer at least 20. There should also be a huge pressure drop with stats out.