1958 7.5HP Johnson prop

[gmarshall43]

Is the prop from a 1958 7.5hp Johnson the same as a prop from a 1958 5hp Evinrude? I'm working on a 7.5hp Johnson and it starts up and runs well in neutral and the motor revs up good when I increase the throttle but when I put it in gear, it doesn't run right. The best way to describe it is the motor seems to bog down and doesn't run at full power when I turn the throttle up to full. I took the prop off and it will throttle up normally in gear. I took the prop off and used it on a 1958 5hp Evinrude and that motor runs normally at full throttle. I tried using the prop from the 5hp Evinrude on the 7.5hp Johnson but I get the same result as before. The two props look identical. Could I be using the wrong prop on the 7.5hp Johnson?

Gary

 

[racerone]

When prop is off ,the motor behaves like it is in NEUTRAL even if it is in gear.----Sounds like motor is running on one cylinder,----Or blown head gasket.----Or shock absorber below water pump is binding.---All easy to check out in my opinion.

 

[F_R]

5.5 and 7.5 props are different but look similar. But that is not your problem. Most likely, your motor is running on one cylinder.

 

[flashback]

+1
You probably have good plugs in it, but that's the first place I would look.

 

[gmarshall43]

I did find a bad plug and I took the plugs from a good running 1958 5hp Evinrude. I believe they use the same plug. It started up fine and ran good. I stopped and started it several times and it ran good in a barrel. I let it sit for several days and tried running it again but it started to run like before. It just didn't throttle up like it should. It will run good for a seconds in gear but feels like it bogs down in gear and will stall out. I checked compression and both are even. I've got spark on both cylinders. Could the carb be the problem? Could it be a timing problem? I don't have a problem starting it and idles good. Just when I put it in gear when the problem occurs.

 

[racerone]

Does spark jump a gap of 1/4" ( 8mm ) on a test device ?-----Have you considered that shock absorber below water pump might be the issue ?

 

[racerone]

edited

 

[Crosbyman]

see pict

 

[gmarshall43]

I didn't check to see if the spark jumps the 1/4". I used one of those plug checkers you get at Auto zone that lights up and it lights up on both cylinders. I did not check that shock absorber. That will be something new to me. If memory serves me right, I have to remove the power head to be able to remove the lower unit, correct? I have a lower unit from a 1958 5.5 Johnson. Will that work on the 7.5hp motor?

 

[Crosbyman]

why not just check the shock absorber ... spring may be broken but some will weld it a and install a brass drive pin on the prop do searches on AOMCI site ...removing PH is not complex and a chance to pop in a new impeller

Topic: OMC Pinion Shaft Shock Absorber – Antique Outboard Motor Club,Inc

www.aomci.org

 

[Crosbyman]

utube evinrude 5.5hp shock absorder - Search Videos

www.bing.com

 

[F_R]

I didn't check to see if the spark jumps the 1/4". I used one of those plug checkers you get at Auto zone that lights up and it lights up on both cylinders. I did not check that shock absorber. That will be something new to me. If memory serves me right, I have to remove the power head to be able to remove the lower unit, correct? I have a lower unit from a 1958 5.5 Johnson. Will that work on the 7.5hp motor?

5.5 and 7.5 use the same lower unit.
I still think your motor is cutting in and out on one cylinder.

 

[oldboat1]

Don't use an inline spark checker. Buy an open air adjustable tester at an auto parts store, and measure the spark to determine whether the ignition is doing its job.

 

[F_R]

I don't see anything indicating you have had the flywheel off and checked the ignition. Got old, cracked coils?? Spark arcing through the side of a coil?? Or arcing through a plug wire where it ic clamped to the armature plate?

 

[gmarshall43]

Well I finally got back to looking at this lower unit. I was able to get it all apart. The shock absorber is a little pitted but I can't tell if the spring inside is damaged. I did notice that the pin that holds the gears in place is no longer sticking out like it should. It is flush with housing. Look at the picture with the arrow pointing to it. Would this cause me my problems? Can you tell by the pictures if the shock absorber is still good?

 

[racerone]

Picture #2 does not show any damage from binding.-------That pin LOCATES the front bearing for oil flow in the bearing.----Look at it carefully.-----So just drill for a second pin.