1967 20hp lossing high rpm

[dan-n-kim]

I have a 1967 20 hp Johnson model num. FD-22E. I was given the boat and motor from my dad. When the boat wasn't outfitted for fishing the boat was scary fast! Now not so much. I know the weight of the equipment is the issue for lack of speed, but my problem is the rpm when I'm on plain. It gets up to between 4400 and 4800, depending on the conditions that day. After a few minutes of running the rpm's start dropping to about 3900 and off plain i come. I was thinking of changing the pitch of the prop to be able to get the rpm's higher but i don't know what the max is for the motor. I've changed the plugs, compression @ 120, filters clean, high test gas 50:1 mix. Any thoughts?

 

[crxess]

Re: 1967 20hp lossing high rpm

First find out what is robbing your power after running a while. then if needed change props.

Check fuel vent/pump/Timing/Mechanical advance/Carb to advance mechanical parts.

Overheating? Thermostat ever been changed?

If it gets on plane @ 4800, it should be able to stay there in its current configuration.

 

[dan-n-kim]

Re: 1967 20hp lossing high rpm

Never thought of the thermostat. Would explain the hot water coming out.(duh me). I'll check online for one seeing the age of the motor. Any good sights you would recommend? As for the prop there is a 9x10 on it. takes forever to get up on plane ( i have to step to the center of boat to get bow down) what is the max rpm?? I was told no higher than 5100 but 5500 was max.. Thanks.

 

[crxess]

Re: 1967 20hp lossing high rpm

You can probably order the thermostat right here form iboats.
5500 Is probably WOT max. with High cruise @ 4800-5100 rpm.

For better planing results, Balance the load. Move the Tank forward if possible along with anchor/ cooler/ etc.

See how your rpm's are with it balanced better and not overheating. If still needed you might try a 9x9p prop. Much smaller pitch and you might lose all your speed.

 

[dan-n-kim]

Re: 1967 20hp lossing high rpm

Thanks. I'll get one ordered and try it. I'll post the results.

 

[dan-n-kim]

Re: 1967 20hp lossing high rpm

Well the thermostat is bad, got one on the way. My concern is when i put the cover back on and test in tub the water gets hot quick! powerhead hot to touch still as well. should i pull the head and clean or try to back pressurize the jackets??? Help?

 

[crxess]

Re: 1967 20hp lossing high rpm

Not a bad idea. Head gasket is cheap and the task is not bad.

My '736hp had a lot of scale blocking the head passages and some in the block openings. Pumped water ok.

 

[dan-n-kim]

Re: 1967 20hp lossing high rpm

I back washed through the exaust and water pickup and made a few more holes in the plate where it sucks water. Put the top back on (without thermostat) fired it up and now @1700 rpm in tub i can hold the head and the flow is better, water still cool as in the tub!!(except in the exaust port). So now i wait for new thermostat and try again this weekend!!!! Thanks.

 

[dan-n-kim]

Re: 1967 20hp lossing high rpm

Just got back from the water. The boat ran like a champ!!!!! Ran @ 4400 rpm at first them after a few minutes of running it got to 4650 rpm. Went around the lake 3 times and never missed a beat. I'm probally gonna get that 9x9 prop to get the rpm a little higher so i can get back into it a lil quicker after slowing down to turn and such. I can't thank you enough for such a simple fix!!!! Thanks again!!!!!