Johnson 7,5 hp 2 stroke overheated

[lmcjmgo]

Hi Guys,
I have a Johnson 7,5hp J8RCSS 1980 that I have overheated due to a water pump problem. I already repaired the water pump and changed the head cylinder gasket; however, I could not improve compression (55 PSI both cylinders).

1) is it difficult to change rings in that small motor or is better to buy a new motor?
2) When I change rings, do I need boring or honing the cylinders?
3) Do you think that honing cylinders is enough? how I can know what is needed?

Thanks, Folks

JMG

 

[Crosbyman]

tried another meter... ?

tried the engine... results= ???

 

[puffitu]

What type/name brand of compression gage are you using?

 

[lmcjmgo]

One that I bought in Amazon

Orion Motor Tech Professional Engine Compression Tester, 8PCS Cylinder Gauge Test Kit Petrol Gas Auto Tool : Amazon.ca: Automotive

Orion Motor Tech Professional Engine Compression Tester, 8PCS Cylinder Gauge Test Kit Petrol Gas Auto Tool : Amazon.ca: Automotive

www.amazon.ca

 

[lmcjmgo]

tried another meter... ?

tried the engine... results= ???

It started after the 3rd pull fast. I test it at home.....

 

[oldboat1]

Use the screw in attachment in the kit. For testing at home, use a plastic trash can, with water about half way up the motor leg. Can't tell much running on muffs.

 

[F_R]

I see the problem. The check valve needs to be right at the spark plug port, not way up at the other end of the hose. Let's just say that the hose holds one cubic inch of air. That cubic inch is added to the volume of the combustion chamber. So, if the combustion chamber holds one cubic inch, and add the hose, the piston is compressing the air into a two cubic inch space instead of a one cubic inch space. Bottom line is it is going to give false (low) readings. Not a big deal with a car engine, but is a significant deal with that little 7.5.

 

[Sea Rider]

If the spark plug's orifices have small carbon built ups on them add some oil drops to the threaded nipple as shown in post 7 and screw it fully in till the rubber o'ring sits fully in.

If the compression tests shows poor low PSI readings must tear the whole powerhead down to access both piston's rings along having a close visual inspection of the cylinder sleeves condition.

If not deformed, oval, with deep scratches could install new standard rings which isn't complicated. A motor with very low compression readings is hard to start, is that your case ?

Happy Boating​

 

[lmcjmgo]

If the spark plug's orifices have small carbon built ups on them add some oil drops to the threaded nipple as shown in post 7 and screw it fully in till the rubber o'ring sits fully in.

If the compression tests shows poor low PSI readings must tear the whole powerhead down to access both piston's rings along having a close visual inspection of the cylinder sleeves condition.

If not deformed, oval, with deep scratches could install new standard rings which isn't complicated. A motor with very low compression readings is hard to start, is that your case ?

Happy Boating​

Thanks a lot for your prompt answer.

1) when I changed the head power-cylinder gasket both cylinders look well. There are not any scratches on the cylinders, the pistons and rings look well too; moreover, when I move the flywheel I can hear the vacuum sound like the rigs are tied. Yet it started fast after three pulls.

I see the problem. The check valve needs to be right at the spark plug port, not way up at the other end of the hose. Let's just say that the hose holds one cubic inch of air. That cubic inch is added to the volume of the combustion chamber. So, if the combustion chamber holds one cubic inch, and add the hose, the piston is compressing the air into a two cubic inch space instead of a one cubic inch space. Bottom line is it is going to give false (low) readings. Not a big deal with a car engine, but is a significant deal with that little 7.5.

Very good point. I will check with another tester. Sorry for my Spanish in the video; however, you can see the compression test and let me know if I am doing something wrong.

 

[lmcjmgo]

Guys, this is the test that I did yesterday... let me know what you think? it started after 4 pulls. sorry for the Spanish.

1) Manly I want to know if it is good enough to use it?
2) running on the lake pushing a boat will have the same power as in the tank test?

 

[F_R]

I can't understand Spanish, but I say that motor is OK. Get it out of that can and on a boat and let us know how it goes.

 

[Sea Rider]

Some issues :

With removed cylinder head gasket won't be able to visually check the piston's rings condition, will see the piston and piston'r ings with removed exhaust cover if that motor counts with one.

Compression Test should be performed with a warmed motor and carb fully opened, with minimum 3 to 5 vigorous rope pulls, check as stated to see if the compression varies or not and by how much. A motor with very low compression will be hard to start specially during first morming start.

A motor running as a champ on muffs, barrel without load, that's not pushing anything that floats it's a different escenario that when doing same at load pushing something. Only then will know if the motor is really running healthy and strong or not...

Happy Boating​