1989 48 HP Johnson Outboard No WOT With Air Silencer On?

Highdog1

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Apr 18, 2015
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boobie - it was bogging bad but turned out to be a twisted cylinder. The engine over heated from bad fuel pump not long after I got it. After I pretty much rebuilt the motor it's ran great. I've had some wiring issues where I'd shut off the key but motor would still stay running. Got that wiring fixed and was doing good til this recent issue. Some minor fuel leaks but those were just bad worn out connectors. I also replaced fuel tank, bulb and hoses few months back. Carbs rebuilt in late April.
Thanks
Thanks
 

boobie

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Nov 5, 2009
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This is just a thought with all that's going on but maybe you might pull the head and have a look ??
 

Highdog1

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Apr 18, 2015
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boobie - I've not thought about that. Got good compression so wasn't really focused on the power head yet. Runs great without the air silencer. Going to install those gaskets, re adjust the carbs when I have it on the water, and possibly change out that fuel tank. I've been doing some research on those caps (in my post above about the attwood 6 gallon) and many complaints about them not self venting. Hoping it's just a fuel / air issue , maybe carb, but I will do a drop test as well. If not, guess I will have to tear it down more. Will report back. Thanks for your response.
 

Fed

Commander
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Apr 1, 2010
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Just to be 100% clear, plastic cover on is no good, plastic cover off it's OK. Correct?
The metal air box base is on all the time?
 

Stumpalump

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Dec 5, 2013
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The 48 is the same as the 50. The 50 is basically the early 80's 60hp rated at the prop vs the crank. Exhaust tuner is tuned to run 3 or 400 rpm slower. The 60 ran 56D main jets and the 48 used 49D jets so common wisdom was to slap 56D jets in the 48hp and scream it to 6500. That's all fine but to run those bigger jets you need to mod or remove the air silencer and open up the cowl vent by the lift handle. All these series engines will run 200 rpm higher with a modest cowl mod. Run yours with the cowl off and see then check your jet size. Four 3/8 holes thru the metal back of the air box will keep it quiet and not let the fuel that spits out of the carb escape. If you leave the front cover off or drill it then oil and gas makes a mess in the lower pan.
What WOT rpm are you pulling? It should run 6000 on a cold day with no weight in the boat and 53-5500 with the cooler and fat chicks in the summer. Cruise rpm is 4200-4500. What a great engine and why they still race the same powerheads today in the SST45 stock motor class. I had a 60 and 48 spl at the same time and kept the 48. It is only a smidge slower on top but it pulls better and holds cruise rpm better because of the exhaust tuner.
 

valvebounce

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Mar 18, 2014
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Unfortunately,just blowing the carbs out with air most times is not enough,2 stroke oil can set like varnish,especially in the air and fuel passages that aren't visible.
I suggest you get a can of carb cleaner when you have another look at the carbs.
Use safety glasses,that stuff will melt nearly anything.(Tried it on the mother in law,but I needed too many cans,Haha)
 

Highdog1

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Apr 18, 2015
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Stumpalump - My tach is not working at the moment. So I'm not really sure the exact rpms. Think it just something in the wiring , or may just need to replace. I've not tackled that yet lol but the carbs are a low speed adjust and high speed fixed. They came with the motor and have worked fine unless they've gotten dirty or in need of a rebuild, no issues with otherwise. Like you said, it's a great motor and runs great when no issues going on. From what I've read on the 48 sp it was Johnsons "work horse" (for commercial) boats. It surely has good pull, and the top end on it is great too , plenty for my pleasure fishin. I dont use it for pulling or sking, just fishing. It wants a lot of gas on a cold start, but after get it warmed up, is idles great and will idle all day if I let. Been really happy with it. The air box has 3 holes that are quarter size, maybe little bit larger. And I was a little worried about justing drilling some extra air holes, not sure if I would be masking another problem. It's always been enough before. Hoping to get it on the water today and give it a test drive. Figured if new gaskets and fuel tank doesn't help, possibly exhaust issue?!
Thanks for your reply.
 

Highdog1

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Apr 18, 2015
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valvebounce = I hear ya there lol.. I will give them a good soaking. On that fuel tank issue , I pretty much talked myself into a new one. Saw they had a good sale going on at West Marine for the 4th. With a fuel gauge too. I got the attwood 6 gal that I have now at wally world and had already gotten new bulb and replaced fuel hoses. Well while I was there the guy came over to assist and they basically had that attwood 6 gal (little higher in price) and I told him that cap does not self vent. He said no they don't. You got to get the fuel line assembly with a Fuel Demand Valve that connects into the hoses. Well I wasn't about to buy a whole new fuel line assembly that included the hoses, bulb, connectors and that FDV. Asked if I could just buy it separate , that I just replaced all that stuff. They didn't sell them separate. So that may be the problem going on here. I never knew that I needed that valve with that gas tank I have now. Apparently, the venter it needs and control pressure that can basically flood the engine without being installed. So I decided to buy the self venting in the cap Scepter 6 gal they had on sale. Really hoping this may be the whole issue going on. It sure isn't helping the situation. Been rainy here last few days so not had a chance to get it on the water. yet. Gonna try today if it stays nice.
Thanks
 

Highdog1

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Hi everyone - Did my gaskets and new fuel tank yesterday and the problem seems worse now. Did a drop test on both cylinders and they just stahl out. It still idles pretty good, but harder to start. I've adjusted the carbs again and no difference even with the adjustments with silencer on or off. Rechecked my compression, it's good and where it was before. Spark good. Although I noticed a small amount of gas / oil mix around two bolts on the block above the linkage and behind the primer solenoid (not leaking or spraying) so I've re torqued those bolts. Only other thing I can come up with is exhaust issue? Any suggestions would be appreciated.
Thanks
 

oldboat1

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Apr 3, 2002
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If adjusting the needle makes no difference, you likely have a carb problem. To clean the carb, you need to remove the core plug at the top, and soak the carb top (soak top and bottom). Then blow out all passages with carb cleaner, and use some soft wire to make sure they are open. The idle orifice at the top has to be squeaky clean, and you should use the carb cleaner on it after soaking it in place. They are brass, but if it's plugged run some soft wire through it along with the carb cleaner. I use lacquer thinner for soaking -- need something serious to get through varnished fuel.
 

Stumpalump

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Dec 5, 2013
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You will need a vented fuel cap. Those new tanks generate pressure and fuel will blow past your needle and seat. I think you can install an older cap or drill out the new ones. Not sure why they sold you a new EPA tank in the first place those are for newer engines. Like said pull the carbs and clean them. It's a little complex and scary but just plan a little time. It's quite a few steps but just a lot of easy steps. Why do you keep thinking exhaust?
 

Highdog1

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Apr 18, 2015
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oldboat1 - I will soak them as you suggest. On these carbs the idle orifices on these are not brass but a yellow plastic/nylon that screw out. The same with the yellow nozzle/ high speed fixed jets that are in front of the butterfly center of carbs. It's also yellow plastic. They're fixed in there. I wasn't sure if soaking them or how long being that they're plastic. How long should I soak for? Pretty sure I got some of that thinner lying around.
Thanks
 

Highdog1

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Apr 18, 2015
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Stumpalump - I just got a new fuel tank with a self venting cap. With the screw knob in the center of the cap. Yeah, the standards have changed on those and with the other tank I had , I didn't know about needing that "fuel demand valve" with it cuz the cap on that one doesn't self vent. Before it ran great with silencer off, it would idle good and low speed good. No top end with the silencer on. I thought is was a fuel issue going on too much gas, starving for air since it would run great with the silencer off, just not with it on. The only other thing I could think that could effect the "vacuum" was somewhere in the exhaust. I replaced the fuel tank, put a new gaskets on around the air box and manifold around the reed valves. It idles worse and neither cylinder will pass a drop test on it.
Thanks
 

Highdog1

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Apr 18, 2015
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oldboat1 - Yes you are correct sorry about that, they are brass on the inside. They little screw out pieces look yellow/plastic?
Thanks
 

oldboat1

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I've soaked them for an hour or overnight -- think an hour or so is plenty, followed by some spraying and poking. Main thing is to let the solution get into all the nooks and crannies.
 
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