Where to start to find this problem?

A buddy of mine just purchased a used boat and it runs well on muffs but when you put it in the water wont get over 4000 rpm,s and not doing over 15 mph. I have done a little research but not exactly sure where to start. Some say possible clogged exhaust, possible stopped up high speed jets, and I had one once that I believe the brain box was weak and caused this same problem. It's a 50 hp Mariner on a 16 foot Bass Tracker so it should be able to hit more than the numbers it's doing. Wont even get on plain. Any suggestions? Thanks

Most likely high speed jets plugged up. Here is a inexpensive tip to try first. Find or borrow if you don’t have one a portable gas tank that will hook up. About 3 gallons of gas and halt a can of seafoam. Mix that good with motor on muffs crank and let run about 15 minutes at idle. Shut off let sit overnight. Next day do same thing. Do this 3-4 hours with about 6-8 hour rest period for that stuff to work in carbs. Then take to lake and you can leave it hooked to trailer. Just back in enough to run motor in gear. Just put in gear and gas it up and let it run for a bit under load. Then launch and run the stew out of it. That should clean it out pretty good and loosen up any carbon on rings and some in exhaust. If that didn’t improve it then you start deeper trouble shooting. I bet this used boat has been sitting up a lot with old gas.


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My buddy bought it knowing it had a issue and from the conversation I heard with previous owner it was running fine and set over winter then started having this problem. Supposedly was taken to mechanic several times with no luck and the guy got tired of dumping money in it with no results.????? I'm just trying to help my buddy out so he doesn't get into a money pit.

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Originally Posted by Thomas Armstrong

My buddy bought it knowing it had a issue and from the conversation I heard with previous owner it was running fine and set over winter then started having this problem. Supposedly was taken to mechanic several times with no luck and the guy got tired of dumping money in it with no results.????? I'm just trying to help my buddy out so he doesn't get into a money pit.

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I would check the easy stuff first, make sure that the throttle cam is working and both carbs are opening equally, make sure that the timing is set properly, and make sure that the cable is not bound up or something and only opening part way.

It would also be a good idea to run a compression and power balance test to see that all cylinders are contributing.

Thanks, he,s new to boating and bought it without my advice. My advice would have been drive it before you buy it! I've made that mistake myself in my younger years.

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Originally Posted by Thomas Armstrong

Thanks, he,s new to boating and bought it without my advice. My advice would have been drive it before you buy it! I've made that mistake myself in my younger years.

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Another thing to check is that the timing advance arm is moving fully.

Had that problem on my boat and the dang mud bees stuck a mound right where the timing arm stop is, so it only opened half way.

I would also replace the Fuel separator Filter beside the tank.. then the Filters on the Motor, clean them also. Restricted flow from the tank can even be old Fuel lines collapsed on the inside

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Originally Posted by Fish on Line

I would also replace the Fuel separator Filter beside the tank.. then the Filters on the Motor, clean them also. Restricted flow from the tank can even be old Fuel lines collapsed on the inside

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:hesaid

I had the same problem with a 40 mariner. Took carbs off and had them professionally cleaned and then rebuilt them. The bottom carb jet was bad. Looked like someone tried to modify it.

Start with the basics. Buy or borrow a fuel line and bulb, modern gas even "ethonal free " has a serious issue with delamination of the fuel lines. It's a VERY common issue that has cost way to many boaters unnecessary expenses. JMO as a retired mechanic

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Originally Posted by Cray

Most likely high speed jets plugged up. Here is a inexpensive tip to try first. Find or borrow if you don’t have one a portable gas tank that will hook up. About 3 gallons of gas and halt a can of seafoam. Mix that good with motor on muffs crank and let run about 15 minutes at idle. Shut off let sit overnight. Next day do same thing. Do this 3-4 hours with about 6-8 hour rest period for that stuff to work in carbs. Then take to lake and you can leave it hooked to trailer. Just back in enough to run motor in gear. Just put in gear and gas it up and let it run for a bit under load. Then launch and run the stew out of it. That should clean it out pretty good and loosen up any carbon on rings and some in exhaust. If that didn’t improve it then you start deeper trouble shooting. I bet this used boat has been sitting up a lot with old gas.


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If it was a high speed jet issue then it would do the same thing on the muffs

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Hope this is not the case as mentioned above but several years ago I tried out a used boat that done the exact same thing. It would idle all day and wouldn't get on plane at all. We took it back to the gentleman's house and he had a compression check ran on it the next day and one cylinder had zero compression. Hopefully his will be a fuel issue and nothing major. I wouldn't buy a used boat today without running a compression check first. They don't make a cheap powerhead.

I have a 1989 Mercury classic 50 and had a similar problem. Tried rebuilding the fuel pump, new fuel lines, draining the fuel tank and new fuel filter. No change. Finally took it in and they replaced the switch box and it ran like new.

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Originally Posted by RobAnderson

If it was a high speed jet issue then it would do the same thing on the muffs

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If OP was just talking about spooling up up the rpms in gear on muffs then the jet could still be clogged. You spin up a motor on muffs to runaway speed at probably half throttle so a partially clogged jet could still be in play.


I would 100% check compression first. Then buy a real spark tester ($7) from the auto parts store and make sure each plug wire is sending out a spark that can jump a 3/8" gap. If it passes both then move onto fuel delivery or do a link and sync.

Keep in mind too that electric ignition components like the powerpack/brain box usually have a high speed and a low speed side. Meaning they can run great at 3000 rpms or below but whatever goes on in the inner workings of those can fail on the high side and the motor will never climb much past that. I've had it happen to me twice on two separate motors. It's unfortunate because those are always really expensive parts and sometimes they can bench test ok and still be bad...and sometimes they bench test bad but still seem to work. But I'd always save those for last unless you find a real cheap used one on Ebay from a good seller who took it off of a good motor. CDIelectronics.com makes aftermarket ignition components and should have a troubleshooting document on their site which will have the test specs for all of your ignition components, so I'd use that to bench test any component before you replaced it.

Bad news, #3 cylinder had 0 compression. Thanks for all the information though. He just learned a expensive lesson!

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Originally Posted by Thomas Armstrong

Bad news, #3 cylinder had 0 compression. Thanks for all the information though. He just learned a expensive lesson!

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Now pull the head. Sometimes if one cylinder has no compression, vs just a very low number, but the motor still runs it can just be a bad head gasket. And if that's the case that would be good news all things considered. Resurface the head, clean the mating surfaces, and put a new gasket on and he just may be in business. I wouldn't get his hopes up though...but don't give up all hope either.