250 Yamaha 4 stroke not looking good

steplift20 · #1 10-25-2022, 03:36 PM

250 Yamaha 4 stroke only runs when you give it gas( push the throttle forward) stalls in neutral , won't run
Changed all the filters
Pulled the spark plugs ( this is where it gets ugly) the bottom plug ,the electrode is gone,broken off. Someone told my son that the motor is toast,
Is there anything that I can check? Anything I missed? I know he needs to replace a throttle cable but what happened to the plug? I'm all ears,thanks

phatdaddy · #2 10-25-2022, 05:05 PM

I d start with compression

Maybe put a borescope down in that cylinder

spareparts · #3 10-25-2022, 08:01 PM

compression test before you go any further, if that cylinder is low, youve probably got a dropped valve. Amazon bore scope that runs on your phone will show you the extent of teh damage. If its a salt water motor, its not worth fixing. Reman powerhead installed will run around $7000+

steplift20 · #4 10-25-2022, 08:48 PM

Ok will do that but what if the compression is good? Should all the cylinders be around 110 psi? And if that cylinder is bad than we are screwed I guess. But before that I have to check the compression. One step at a time. Personally I don't like 4 strokes and definitely do not like electric vehicles. Sometimes progress is not good.

spareparts · #5 10-25-2022, 10:20 PM

compression should be around 175 depending on the gauge. If its good, take a look with one of those bore scopes, if you dont see any damage, throw plug in it and see how it runs

steplift20 · #6 10-26-2022, 03:04 PM

Will do spare. When you relocate to New Jersey let me be the first one you tell.

phatdaddy · #7 10-26-2022, 05:12 PM

have a friend who has worked on outboards for 30 plus years, he said his dream job is opening a shop in Lake Eufala, Alabama and work on nothing but freshwater Mercury V 6 2 strokes

steplift20 · #8 10-26-2022, 06:57 PM

Phatdaddy , that's what I have on my wellcraft,a 150 2 stroke Mercury of course and I would love to know like spare and your friend how to work on it. I do know some things but would love yo know all of it. It's really a very simple engine ,just the basics carbs,stator,trigger,switch boxes. Plus the other electrical components I would work with a mechanic just to watch and learn.

phatdaddy · #9 10-27-2022, 08:35 PM

Find a Mercury factory service manual for that model. No a Seloc or Clymer or whatever those off brand books are, get the one published by Merc. It will tell you how to do anything you want to do step by step with pictures

When I got my four stroke, I had the dealer order me one for my motor, I think it was about 80 bucks, but well worth it

scook · #10 10-27-2022, 11:57 PM

I agree - you want the factory shop manual.

You might be able to get one online and download it to your computer (cheaper). I do prefer being able to leaf back and forth in a paper version, but you may be able to print the pages for what you want to do. With printouts, you won’t care if you get oil or grease on the pages and if you have an old laptop computer laying around, you can dedicate it to “shop computer” and have your manual handy. I did that for an old ford truck and it worked well.