Compresson relief on outboards?

[pikefisherman]

I was talking with someone that told me that all small engines have a compression relief, and that a compression tester on out boards is useless, true, or not true? This person has a lot of experience and schooling on small engines and chain saw motors.

 

[Faztbullet]

The newer 2 stoke outboards will have a compression relief slot or hole cut in sleeve on exhaust. This usually results in a 10 psi decrease in cranking compression and is done to improve idle of engine. The engine will regain this compression when it is near full rpms. A compression test on a outboard will only tell you 2 things, (1) If cylinders are close to each other in compression (2) If there is enough compression to support combustion as engine can have good compression and cylinders be scuffed/worn.

 

[bonz_d]

Not true. Though as I recall the very early 25,30 and 35hp Johnson and Evinrudes did have it on their pull start engines. I have a 1969 9.5hp Johnson and it does not.

 

[pikefisherman]

So is their a low point as a rule for all Johnson motors say to 25 hp? What is good what is very good what is bad?I have a 1980, 4.5 I will be working on soon.

 

[hardwater fisherman]

My 78 johnson 4hp has around 110 psi and my 76 40hp evirude has around 135. I have a 63 evinrude 28hp with 125. And my 9.5s are lower at around 68.

 

[Faztbullet]

Not true. Though as I recall the very early 25,30 and 35hp Johnson and Evinrudes did have it on their pull start engines

Sorry but is true as I stated its newer engines, OMC motors use a holed drilled thru sleeve and merc uses a milled slot. Yes some of the the older "big twins had compression relief valves

 

[Chris1956]

Gee, are we discussing compression relief or idle exhaust relief? My old 35 'Rude had two valves that provided compression relief, when the pull start was used. Lost of outboards have exhaust ports located above the water line, to provide for idle exhaust backpressure relief.

 

[gm280]

Of all the OBs I owned, none of them had a compression relief. I did own an ATV that had a compression release so if the battery went south you could manually pull start it. But unless you set the release before pulling, you would nearly break you arm.

 

[racerone]

?????-----As stated many newer outboards have a hole drilled in the cylinder sleeve.---This small hole goes directly to the exhaust manifold.----Results in lower compression at cranking speeds.---Results in smoother idle.---Results in quiet idle.------This hole has little affect at full throttle !

 

[pikefisherman]

Thanks great information, this motor will run great and start good when warm, I noticed the fuel bulb was always too hard to squeeze in the morning and the motor was hard to start, I changed the ball to a Mercury bulb witch is softer,I just went out to try and the motor started right up cold.
Compression was at 75 on both cylinders the last time I checked last summer.

 

[Chinewalker]

75 psi for the 4.5hp Johnson is fair, but not great. It is a development of the old 3/4 hp models, none of which would ever be classified as beasts. That said, the 4.5 of that era does NOT have any sort of exhaust relief setup.

 

[Faztbullet]

Gee, are we discussing compression relief or idle exhaust relief?

Compression relief as idle exhaust relief is different. Exhaust relief is for reducing backpressure and helps the exhaust scavenging..

 

[bonz_d]

Compression relief as idle exhaust relief is different. Exhaust relief is for reducing backpressure and helps the exhaust scavenging..

Just for my own education which engines are these so I can check it out?

 

[Faztbullet]

Just for my own education which engines are these so I can check it out?

Most engines with compression relief slot/hole are looper motors and your 35hp has a idle relief, it is the rectangular casting with 2 round hole and the holes on lower midsection....

 

[scousekiwi]

Hi, I know this post is old but can anyone help with locating this compression relief hole on a johnson 25hp. It is impossible to pull start so I am wondering if it is blocked. Is this possible? It starts with the electric start but only just and the battery needs to be tip top. It is literally impossible to pull start and I also have a 30hp which is easy.

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