57' Johnson seahorse 7.5hp delay to full power

[Medicman613]

Hey all, i rebuilt a 1957 johnson seahorse 7.5hp motor and installed a new impellor, new points and condensors, and also new gasket around the carb. (Carb was cleaned but was extremely clean to begin with. Motor starts one pull and idles perfectly. The problem i have is that when you give it full throttle it is very delayed to build up speed and once it gets to top speed it will run there for a minute and then drop off a few km/h. The carb is adjusted as per the original manual and the fuel is mixed as per the original manual. Any thoughts as to what i can try next?

Thanks,
Curtis

 

[racerone]

Did you inspect the " shock absorber " in the lower unit.-----Found right below the water pump.

 

[oldboat1]

Which model do you have? 7522 would have the shock absorber (would have removed the powerhead to service the water pump with that model).

Also might try your spare prop, and check transom height against shaft length (poss. hub slipping, or ventilation issues due to mounting height). Your high speed air/fuel mix might need adjustment. (High speed needle adjustment would be set at WOT, and can vary between motors and operating conditions.) Additionally, the needle could be vibrating out of adjustment when running. Might tighten the jam nut to snug up the needle.

(Pretty sure the manual calls for 16:1 for the 7 1/2, where 24:1 is used on other '57s. That what you found? Would be important for high speed running.)

 

[Medicman613]

If that jam nut is snugged up, the needle wont turn at all making adjustment on the water impossible. 16/1 is what i have found.

What is the shock absorber? Never heard of that... also i did have to remove the powerhead to service the water pump (stupidest design ever by johnson might i add)

 

[racerone]

Why is it a " stupid design " ?-----Great stuff as there is no shift adjustment for newbies to get wrong.---And it means regular inspection of the mechanical seal is also done. ------Motor is now 60 years old and will be running long after the new offshore motors are in the landfill !

 

[Crosbyman]

5-6 screws wasn' t that bad was it ? I actually found the process quite interesting for a new hobbyist myself. that Utube Video took away all fears.
I eventually got my 7.5 to purr like a kitten all day ​and give me confidence of many return trips back to the launch point of my fishing trips...but my 75 ETEC didn't strand me 10 miles out . The engine were 50 years apart... what a pair !

 

[Medicman613]

I guess when you look at it that way racerone it does make sense! When i looked at the simplicity of newer motors to replace the impellor (drop lower unit, disconnect linkages and your done) i thought great! Nicr and simple until i realized you had to pull the powerhead haha. But i figured it out and we are all god now. Just cant figure out my full throttle delay.

 

[F_R]

No Title

Is the delay something that happened when you replaced the pump? Did it run hot before you replaced it? Could be a few possibilities. The shock absorber is below the water pump and disturbing it sometimes makes it bind up if it has expanded. Expansion often is the result of a broken spring, but not always. Could be because of 50 years of dredging bottom. If not broken, just mash it back together and run it.

 

[lindy46]

Cutting out on one cylinder?

 

[Medicman613]

Difficult to say... i just ordered a new carb rebuild kit so i can play with the seals for the needle adjustments. I also ordered a new lower unit gasket\ seal kit. Ill take apart the lower unit tonight and take a look at that ahock absorber. Anyone know of a place to order parts other than iboats? Selection here for that motor is limited.

Thanks a lit everyone i know this forum would help me again.

 

[Medicman613]

Is the delay something that happened when you replaced the pump? Did it run hot before you replaced it? Could be a few possibilities. The shock absorber is below the water pump and disturbing it sometimes makes it bind up if it has expanded. Expansion often is the result of a broken spring, but not always. Could be because of 50 years of dredging bottom. If not broken, just mash it back together and run it.

My grand father said that was an issue he had with it 20+ years ago.... the delay that is.

 

[oldboat1]

The Evinrude model I mistakenly had you in would have interchangeable l.u. parts -- if parts for the Evinrude are more readily located. The gland nut holding the needle is also available if yours is stripped -- needs to be tight enough to hold it from vibrating loose, but not so tight you can't adjust when underway. Use a couple of packing washers on each needle.

(think I would try another prop as well, just for grins -- should be able to pick up one pretty cheaply. Interesting that the running issues are long standing ones. Think checking compression wouldn't hurt.)

 

[Crosbyman]

any chance you could be loosing fuel pressure from a tank loosing built up pressure ?? try soap and water on all potential leak points

 

[racerone]

I think those motor used a solid one piece prop.---No rubber drive hub.

 

[oldboat1]

^^Yup -- Solid. (Took a look at one of mine.) Thanks, R.