1975 50hp johnson model#50ESL75B

hazelnutz20 · Jun 7, 2010

i have a 18' tri-haul fiberglass boat with a 1975 50 hp johnson model #50ESL75B. I have been out and the boat will only go about 12mph and the max rpm i got was about 3700. i have a 11-3/4" diameter with a 17" pitch prop. the throttle is opening all the way. I have been thru the carbs and everthing looks good. i went thru about 4 gallons of gas and i didnt go very far around the lake. i pulled the plugs out and there dark but it didnt smoke while it was running. all the hoses are alright and no gas leaks. it starts up just fine. i guess im just trying to find out if this is all im going to get out of this motor because i think i should get a little bit more out of it. thanks to anybody if they have sum sort of input for me to try

eavega · Jun 7, 2010

Re: 1975 50hp johnson model#50ESL75B

Have you checked the compression on the cylinders?

I would also check to see if you are only running on one cylinder.

Also, what is your boat rated for? a 50 HP seems a little light for an 18' tri-hull. Even my 15' is rated for 70 HP. I've seen 18' tri-hulls rated up to 115 HP, in which case the 50 would be woefully inadequate.

-E

papasage · Jun 7, 2010

Re: 1975 50hp johnson model#50ESL75B

sounds like you lost a cylinder . due to no gass or no fire or bad cylinder . crank on muffs put a finger on the cylinder hesd sround the plug aera . the coldest one pull the plug wire it wilnot go dead then pull the other if it dies that is the bad cylinder . pull plug wires check for spark if bith have spark thencheck carn you may have a high speed jet cclogged .i have pulled the bottom drain plug and run a wire streight in and find highspeed jet orfice .a wire the size of a spinnerbait will do . this will work with high speed jet stopped up. i have don that onthe lake a couple of times .after letting the motor set for a long time . you can remove the carb and take the bowl off where you can see the HS jet .also you have to much pitch on the prop .

hazelnutz20 · Jun 7, 2010

Re: 1975 50hp johnson model#50ESL75B

i dont know much about the boat and i think its rated for a 125hp i can tell the motor has been replaced once so im trying to figure out some things like the proper prop i need and stuuf like that so Ill try it out...

thanks

eavega · Jun 8, 2010

Re: 1975 50hp johnson model#50ESL75B

Before you go stressing over the prop, make sure you are getting the proper power from the motor. Check the cylinders and make sure they both have adequate compression and are both firing.

I will repeat again that you may be severely underpowered on that boat if its rated for 125 HP. I was previously told by others here on iBoats that a rule-of-thumb for engine power is at least 50% of rated power to plane reliably, and at least 75% of rated power to perform adequately. It does depend on boat design, weight, etc. but from my own experience trying to find a hull that would match up to my 50 HP Evinrude, 18' fiberglass tri-hulls are really pushing the limit of what those engines put out.

hazelnutz20 · Jun 8, 2010

Re: 1975 50hp johnson model#50ESL75B

thanks vega i dont really plan on doing much more than using it for a fishing boat i dont plan on crusing around the lake much so if i can get it running on both cylinders that would be great thanks for the info

ps do you know off the top of your head what kind of rpms i should be getting out of it?

eavega · Jun 8, 2010

Re: 1975 50hp johnson model#50ESL75B

My '72 50 HP Evinrude is rated at 5500 max RPM.

hazelnutz20 · Jun 8, 2010

Re: 1975 50hp johnson model#50ESL75B

well i checked the compression and i have 140 on both. Then i started the moter and pulled plug wires off and the top one wasnt sparking alot and never killed the moter the bottom one had lots of spark when pulled and it did kill the moter its getting gas to both cylinders im thinking coil may be bad but im not sure how to check and what im suppost to have for reading

eavega · Jun 9, 2010

Re: 1975 50hp johnson model#50ESL75B

At least now you know its an ignition problem on the top cylinder. At this point, you are gonna have to refer to a manual or someone more experienced with these motors. My present motor has not had any ignition problems, so I haven't torn into that part of it at all. My only experience with ignition systems has been with older, smaller motors (points/condensers). My next step, though, would be to switch out the coils. You know your bottom cylinder is working properly, so if you swap out components one by one, you will be able to isolate which component is failing.

Good luck

-Eric

hazelnutz20 · Jun 9, 2010

Re: 1975 50hp johnson model#50ESL75B

Thanks alot vega for all the info that you have provided me with.

hazelnutz20 · Jun 12, 2010

Re: 1975 50hp johnson model#50ESL75B

well i had the coils tested and there both good then they pointed me in the direction of the powerpack and i was wondering if theres anyway of testing it.

Brokeback Johnson · Jun 12, 2010

Re: 1975 50hp johnson model#50ESL75B

In my opinion there is no possibility of that motor pulling a 17 pitch prop on such a large boat. I would try a 13 pitch and possibly even an 11.

1975 50hp johnson model#50ESL75B

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