Johnson 50hp 1972 only getting 8mph

[bag007]

Hi quick question.... Just got hold of a 1968 sportcraft with a 50hp 1972 Johnson. First time out on it it fouled the plugs.. so i adjusted the carbs.. It was running fine prior to this but wont push the boat faster then 8mph. I know the prop is the same as the origional. according to previous owner). But what was the origional pitch for the prop.
Also what else besides the prop may cause the speed to be low... I exected 20-30 mph... or am I expecting too much.... Could it be incorrectly trimmed would that casue such a slow speed.. I did notice that it peuse dthe Bow up. and the transom down.. Would triming it correctly increase the boats speed signoficantly??
Thanks ..

 

[DJ]

Re: Johnson 50hp 1972 only getting 8mph

Believe it, or not, you are only getting power out of one cylinder.

Things to check.

1. Spark. You can get a spark checker-cheap-at the auto parts store. You should get a clean, crisp blue spark about 7/16" long.

2. Compression. Should be over 100 psi and even, or very close.

If those two are good, you have fuel delivery issues. Could be fuel pump or fouled carbs.

 

[pduquette]

Re: Johnson 50hp 1972 only getting 8mph

Definitely agree with DJ. Motor will purr pretty smooth, take throttle as quick as you give it in neutral, then dog out in gear! You most likely burned out a coil at the time the plugs burned.Its a common problem when an old (magnito style) outboard is awaken from along sleep. An easy fix if you are handy and pick up a manual to guide you.

 

[samo_ott]

Re: Johnson 50hp 1972 only getting 8mph

What RPM was is getting up to? Can't be very high! The original prop was 13 3/8 x 17

 

[tashasdaddy]

Re: Johnson 50hp 1972 only getting 8mph

what size an model 68 Sportcraft? did the motor set up for a long period?

 

[jay_merrill]

Re: Johnson 50hp 1972 only getting 8mph

The business with the stern squatting and the bow coming up at 8mph porbably isn't a trim issue. All that is goiing on is that the boat is trying to make the transition from being a displacement hull to a planing hull. At that point, the bow is riding up on the bow wave and the stern is sitting in the trough that follows it.

When you say that the motor ran fine before you adjusted the carbs, do you mean it planed the boat? If so, you may have made the adjustment incorrectly. Also, are you running UL type plugs? If so, switch to J style plugs after you get the problem solved. I had a fouling problem with my '72 65hp when I first used it and my OMC dealer recommended the J plugs - he was right because the problem went away as soon as I did. What he told me is that the ULs are ok for people who run their motors at higher power settings most of the time but those who idle more and run at moderate power settings will see more fouling.

I agree with the others that, if you were only idling at the dock or on the muffs when you felt the motor was running ok, you may have running on one cylinder and didn't know it. The spark checker is a good idea and an inductive timing light, put on one spark plug wire at a time, can sometimes show an ignition problem. If you see breaks in what should be a steady, constant frequency flicker of the light, you have ignition problems on that cylinder. Often times, the ignition coil is the culprit and your local OMC dealer will test it for you if you bring it into the shop. They are easy to remove and the benefit is that you can buy a new one on the spot if it is bad.