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Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 14306
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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I have a 50 HP Mercury outboard. It overheated while slowly

Customer Question

I have a 50 HP Mercury outboard. It overheated while slowly cruising on the lake. I shut it down and cleared the water tube, we have a lot of milfoil in this lake. Once I restarted it, it would not go past 3400 RPMs. I changed the water pump impeller and
put new plugs in it. Now it will get up to about 4000 RPMs but will not reach the limit. Do I have a bad coil pack or something more serious?
Submitted: 1 year ago.
Category: Boat
Expert:  Jason replied 1 year ago.

Hello my name is ***** ***** am I speaking with?

Whenever an engine overheat happens both piston and ignition component damage is suspect. The engine would need both it's compression and spark output tested to get you pointed in the right direction. Has that been done yet?

Customer: replied 1 year ago.
No, I am away from home with limited tools. Can I put a plug in the wire and check the spark that way?
Expert:  Jason replied 1 year ago.

No, that could give a false result. What you would need is either a timing light or an adjustable gap spark tester.

Click this link below, this s a video that shows how to test both. Both sets of tools can be had a most autoparts stores.

https://www.youtube.com/watch?v=eSmNMWTDtZE

This is just to get you pointed in the right direction. If it has low compression then there would be piston damage, and the engine would need to come apart to see how bad it is. If it has no spark, more tools would be required to figure out why.

Expert:  Jason replied 1 year ago.

Let me know what you think after watching the video

Expert:  Jason replied 1 year ago.

Now that you have watched it what do you think? Is that something you can do?

Customer: replied 1 year ago.
I can. I am going to the parts store to get spark tester. If that's not it then I'll pull it out of the water and take it home to my shop.
Expert:  Jason replied 1 year ago.

Okay let me know how you get for results.

Customer: replied 1 year ago.
OK, I checked the spark and I am getting it on all cylinders. It is not as bright as in the video but I am outside in the sun and the contacts are not enclosed.
Expert:  Jason replied 1 year ago.

Got it. Next step is compression. Let me know what you get for numbers there.

Also, do you have a serial number on this engine?

Customer: replied 1 year ago.
I have attached a photo.
Expert:  Jason replied 1 year ago.

Got it. Let me know what you get for compression numbers and we'll take it from there.

Customer: replied 1 year ago.
It will be tomorrow before I can do a compression test. I believe I can barrow one from the parts store.
Expert:  Jason replied 1 year ago.

No problem, there are no time limits here. Just post back when you have a result.

Enjoy the rest of your night.

Jason

Customer: replied 1 year ago.
I've checked the compression. I'm getting 60 - 70 - 60. Not sure I completely trust the tester.
Expert:  Jason replied 1 year ago.

That would be way to low. You are looking for 90 psi or better. If those numbers are accurate the cylinder walls are going to be scored up, and the engine is going to be shot. At this point getting another compression tester would be like getting another opinion. So definitely get your hands on a decent quality one and not a harbor freight special, and double check it. Does that make sense?

Customer: replied 1 year ago.
I have checked the compression with a good gauge. From top to bottom I get 75, 115, 115.
Expert:  Jason replied 1 year ago.

That's still going to be an issue, that 75 is to low. Do you happen to have a serial number on this engine, I want to run it and see if this one has a cylinder head or not .

Customer: replied 1 year ago.
See attached photo.
Expert:  Jason replied 1 year ago.

Got it. There is no head on this one

It's more than likely going to be a bad cylinder. But just for the heck of it If you place your palm over the #1 carb and crank the engine over, do you feel any blowback against your palm, or is it all suction?

Customer: replied 1 year ago.
I'll have to get someone to turn it over while I check. No one else is here right now. I'll get back to you later this afternoon.
Expert:  Jason replied 1 year ago.

okay

Customer: replied 1 year ago.
Do I take the air box off to get to the carbs or does more have to be removed?
Expert:  Jason replied 1 year ago.

Just the airbox

Customer: replied 1 year ago.
I see a screw at the top. Is there one at the bottom somewhere?
Expert:  Jason replied 1 year ago.

There should be 4 screws holding that air box cover on.

Customer: replied 1 year ago.
OK, some must be behind the oil tank.
Expert:  Jason replied 1 year ago.

Hmmm.... no, this is just the black plastic cover in front of the carbs. send me a picture of what your looking at.

Customer: replied 1 year ago.
Actually I looks like just one.
Expert:  Jason replied 1 year ago.

It doesn't match my manual, but looks that way to.

Customer: replied 1 year ago.
Did the test. I couldn't get my hand over the top of the carb well enough to seal it. I put a shop rag over it and couldn't see any movement.
Expert:  Jason replied 1 year ago.

Gotcha. At this point unfortunately the #1 cylinder is damaged as a result of overheating. Whether it's the actual cylinder walls, or the pistons, or both, can't be known until disassembly is done. But at this point the block would need to be torn down to see how extensive the damage is.

I think that should do it. I work for tips so I do want to make sure you are happy with my service before you go. If you had a further question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank.

Questions do not close out, so if you have to come back later on with follow up questions to the issue you still can even after doing a rating.

Thank you
Jason

Customer: replied 1 year ago.
Can you give me an idea how much it will cost to repair the motor?
Expert:  Jason replied 1 year ago.

It really depends on how much damage there is and if machine shop work is needed. But on something like this worst case scenario is going to be in the 1400/1500 range.

Expert:  Jason replied 1 year ago.

Hello, just checking in. The website has charged you but no rating has been put through yet in order to credit me for helping you out. Did you have any further questions on the issue?

Thanks,

Jason

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