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1988 Mercury Classic 50 won't get my boat on plane

Dguysk01

Member
Hello all, this is my first adventure into the boating world and I need some guidance. My boat is a 1988 Bass tracker tournament tx17. The outboard is a mercury classic 50 4cyl 2 stroke. My issue is very frustrating: my boat won't get up on plane and won't go past 8mph. It starts and runs great, and feels like it has power right when I first gun it to get going, but stops accelerating very quickly. When it's topped out at WOT, the tach says around 3100 rpm and my gps says I'm going about 8 mph.

Things I've done to the motor since I bought it 2 months ago:
Replaced the fuel tank, supply line and primer bulb.
Rebuilt both carbs using OEM parts (they probably didn't need it as they appeared pretty clean inside while I had them apart but I wanted to be sure anyway)
Replaced the impeller and housing
Replace spark plugs
Tested compression (98psi on all cylinders)
Replaced the fuel filter

It starts and idles great, but it acts like it's not opening up all the way. What should I check next that could cause this?
 
I should add that it doesn't feel like it has any misfires or like it's cutting out, it just wilt rv 0sst about 3100rpm. Reminds me of plugged catalytic converters on a car/ truck.
 
Remove lower unit.----Use strong flashlight to inspect up into the exhaust housing.-----Not saying it is plugged , but no coins spent to take a look.
 
Have you recently changed propellers? I fished out of your exact boat for many years. 13" or 14" pitch would be the correct propeller.
 
I have not changed props, the one on it was on there when I purchased the boat. Is there a way to determine what pitch the prop is if there are no numbers stamped onto it? I've looked between the blades and on the hub near the nut and haven't found a single number stamped into it. That was the first thing I went and looked for when I got it off the river this weekend
 
And in regards to the exhaust housing, when I drop the lower unit to check it out, where exactly should I be looking for a restriction? The only thing I looked at when I replaced the impeller and housing was the shift shaft and drive shaft to line it up when re-installing the lower unit :/
 
Thank you for the replies, I have it figured out! Honestly it's pretty stupid that I let this stump me for a bit, but it's solved. The throttle plate was backed all the way off and the throttle wasn't opening past halfway! I adjusted it and now it feels like a new boat! It should finally be as reliable as I would like *fingers crossed*. My father has had an alumacraft lunker 16ss with a Yamaha-built 40hp mariner since before i was born, and he's literally never put a eyebrows to it since new apart from spark plugs every 5-6 years, and I'll be a monkey's uncle if it isn't the most dead nuts reliable boat I've ever seen. He stores it outside, uncovered year round but it fits up immediately every time. If this boat can treat me even half as good as that thing, I'll be tickled pink. Thanks again for the advice!
 
It appears I spoke too soon... I had the boat out this weekend for the first time since adjusting the throttle plate, and saw very little difference in performance. The throttle doesn't have any slack in it anymore and the engine is much more responsive, but on the water under load it still will not go any faster than 8mph at 3100rpm.

I also noticed the overheat alarm coming on after 2 or 3 minutes at wot, but going away once I slow back down. The water pressure gauge reads 4-5 psi st idle, and is around 10-11 psi at wot and when the alarm goes off. The water stream from the tell tale stops and it looks like exhaust/steam comes out. Note that the impeller and housing and as all related gaskets/plates/seals are new.

I removed the lower unit today to look for problems, and the only issue I noticed was the gasket on the bottom of the exhaust stack looked like it didn't fit properly and the plastic support under it is melted and deformed. I liked up into the stack and it looks very clean with only a bit of carbon around the edges at the very top by what I am assuming is the baffle/plate under the powerhead.

I pulled all of the plugs to check if one isn't running properly, they all look identical to one another.

What should I check next? I'm close to putting it back together and taking it to a shop, but around me every shop is either booked through fall or I've heard some bad things about. I'm not afraid to do the work myself, I'm a high level professional automotive technician by trade, but my experience and background knowledge of outboards is limited to what I've done with this boat here lol so the specific diagnostic steps aren't familiar to me. I appreciate any and all advice!
 
Much like auto malfunction it is best checked out with the proper diagnostic tools. I had the same problem and due to ignorance I did the "throw parts at it and then?" I managed to replace my wiring and finally my stator. The stator was the culprit and my wiring was crumbling and barren in places so that needed replacement also. I ruined one whole season of fishing and the next years vacation on that motor. I would have been much better off taking it to a pro.
 
The wiring appears to be in great shape, no cracked insulation or bare spots, but I could check the stator. I would think that if that were the problem it would be running rough under heavy load though. Could that cause a smooth running engine that just doesn't have power? If so, how could I verify that?
 
Well 98 PSI is quite low.----Remove intake bypass covers and inspect pistons and rings.----Sounds like the motor got hot and there may be an issue with a fitting at the TOP of the copper water tube.
 
Ok I'll take the copper tube out and check that out. I was told that harbor freight Compression testers are wildly inaccurate most of the time by a tech at the boat shop i called today, so i will bring my matco compression tester home from work tomorrow and see if the reading is any different. I didn't see any scoring or hot spots on the cylinder walls when I scoped it, so hopefully my HF tester is just reading low.
 
The wiring appears to be in great shape, no cracked insulation or bare spots, but I could check the stator. I would think that if that were the problem it would be running rough under heavy load though. Could that cause a smooth running engine that just doesn't have power? If so, how could I verify that?
Mine ran and started very nicely just had no hole shot or power to get it to go. After messing with it it would run strong and then go back to it's lethargic ways. I couldn't get a handle on it but if it was a car it wouldn't have been such a pain to diagnose. So check the stator with a proper diagnostic tool. The folks on here know what you will need.
 
I tested the stator, and according to the specs posted in the forum it is indeed faulty, with 146 ohms between the red & red/white stator leads, specs read 56-76 ohms. So that will be the next step. I brought home my good compression tester and double checked compression, got readings of 129psi, 140psi, 134psi, 129psi. Checked it 3 times and it stayed consistent, so I'm not worried at all about the internal condition of the engine at this point. I'll update once I get a new stator and get it installed. While I'm this far, I'll pull the water jacket cover and look for chunks of old impeller that could be causing a restriction in the cooling system
 
I'm a little concerned about the cheap compression tester now, because I just stuck it on there to see and it read 98psi across the board again lol
 
I'm a little concerned about the cheap compression tester now, because I just stuck it on there to see and it read 98psi across the board again lol
Says a lot about where those are made and who makes them. It also says a lot about Harbor Freight. You are not alone in your displeasure with those testers. Thousands of us have /or had them.
 
Well, I replaced the stator, as well as the exhaust tube-to-lower unit gasket, and it did not make a difference at all. While I had the lower unit off, I noticed what looked like soot up on the plate underneath the powerhead, similar to what you'd see from an exhaust leak on a car/truck. I also noticed that the water coming from the tell-tale is very very hot, like almost boiling hot. The mid-section of the outboard is hot to the touch while running as well. What do you think? I'd hate to spend the money on another outboard at this point, considering not only how much I've already sunk into this one but also the fact that I could buy a motor just like this one that runs excellent on the house but costs me hundreds more to make it work right. What would be the most likely culprit at this point?
 
Well ---I guess the big $$$ throw parts at it did not work.-----Was the exhaust tuner plugged ?----Did you inspect the part at the VERY TOP of the water tube ?----Do you have spark on all leads ?---Are all wires from stator / switch box hooked up to the correct coils ?----Are carburetors confirmed clean ?
 
The answers to most of the questions you just asked are in my previous posts. The exhaust tuner is not at all plugged, looks very clean actually. The very top of the water tube is properly attached and the seal looks fine. All 4 plugs are firing and all 4 plugs look the same, as in no fouling or 1 cylinder not firing properly. All wires from the stator to switch box are hooked up to the corresponding terminals, as they were before, thanks to my switch box having clear markings that match the wire colors. I rebuilt both carbs right from the beginning, i even used an ultrasonic cleaner. They were very clean to begin with, but i went ahead with it so i knew I had a good baseline. I did not in fact "just throw parts at it", apart from the impeller and carb kits i bought the moment I bought the boat. Literally everything else I've done to it has been replacement of parts that have failed or tested bad. Exhaust tube gasket was stolen and deformed, fuel tank had debris floating in it and the primer bulb wouldn't build up any pressure, the stator tested bad according to specs i found on this site. Should I have just completely disassembled it right from the beginning? As I stated in the original post, this is the first outboard I've ever worked on so I don't have a vast bank of previous experience related to this thing, I only know what i see right in front of me
 
Maybe an issue with reed valves.----Or bottom bearing / crankshaft seal.--I did the trouble shooting on one of these " low power issues " some years ago.----Owner gave me an extra bit of coins and said.----" thank you very much, you fixed it for 1/2 what I spent at the shop already " !
 
Do you think the 2 issues could be related? Or from the info I've provided thus far do you think the lack of power and overheating issues are separate?
 
I see you stated you redid the carbs. I didn’t see that you also looked at the fuel pump (s). Fwiw my ‘78 50 horse Merc has a fuel pump for each carb. In getting this old girl back up and running ( it sat dormant for several years), I had both carbs cleaned and all new gaskets and needles. At that point it ran very similar to what you describe how yours is running. It was at that point I looked at both fuel pumps (I should have looked at them closer when re-doing the carbs). They were not super dirty but not exactly clean either. The real culprit I think was some of the gaskets in each pump were compromised, deteriorating and breaking down from being 42 years old. I cleaned them up and installed new gasket sets in each pump and now it runs like a new motor.

Also, make sure your fuel line bulb is good. With age the check valve inside can stop working like it should and can give you grief.
 
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I have an update! I removed the water jacket cover and the exhaust manifold cover and baffle plate to look for debris and/or blown gaskets. Unfortunately i did not find any issues there, so i decided to pull the powerhead, and boy am I glad I did! Underneath the powerhead I found several pieces of old impeller fins, as well as 2 jammed up into a pocket in the bottom of the powerhead. I also found the gasket at the base of the powerhead was blown, allowing exhaust and water to meet. So that should solve my overheating concern!

Now to address the lack of power concern... I disassembled the fuel pump and found the diaphragms inside were rather stiff, could that maybe cause low fuel flow to the carbs and starve it for fuel without causing a noticeable misfire or stumble? Also, through the exhaust ports i looked at the pistons, and they definitely have some scoring that couldn't be seen from the spark plug holes. But with acceptable compression numbers, would it be worth going the extra mile and completely rebuilding the powerhead while I've got it off? It would add about another $700 to the cost, which is definitely less than a new or reman motor but still a lot, so i don't want to just jump into that unless the experts think it's worth it for this motor.
 
The carbs on this motor have plates on the side where I imagine there were fuel pumps on older models of this engine line, that's where the fuel inlets are. This engine has one fuel pump that is bolted to the block. After disassembling the fuel pump, i noticed the same thing about the diaphragms as you did, even though I didn't see any tears or holes in them they feel hardened
 
Maybe an issue with reed valves.----Or bottom bearing / crankshaft seal.--I did the trouble shooting on one of these " low power issues " some years ago.----Owner gave me an extra bit of coins and said.----" thank you very much, you fixed it for 1/2 what I spent at the shop already " !

The reed valves appear to be in good shape from what i can see through the intake ports, but there is fuel/oil mix all over the bottom of the engine, the only place I can tell it was coming from would be the lower crankshaft seal. If that's the case, that could explain the lack of power, couldn't it? I suppose it would lean out when the engine is building vacuum, correct? I was going to replace the crank seals whether I overhauled the whole powerhead or not because I'm right here, but if that could've been the cause of the power problem then I'd feel really good about it
 
The engine has 4 crankcases.---all sealed from each other.----Lower crank seal would do things to the # 4 cylinder.
 
The engine has 4 crankcases.---all sealed from each other.----Lower crank seal would do things to the # 4 cylinder.----The " boost venturi " ---is in place on your carburetors ?
 
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