vacume

[Rich Novak]

How much vacume does a marine engine produce.
I want to set the carburator using the vacume guages.
1972 225 HP omc
My classic car has about 15 # at idle.
What should the boat have.
Thanks, Rich

 

[gm280]

That is a good question. But since a 2 cycle OB is both a vacuum AND a pressure type crankcase operation, I don't think using a vacuum gauge will work. You have a vacuum when the pistons are going down the cylinders, but that also creates a pressure for the crankcase fuel/oil mixture to be pressurized to flow into the cylinder for the next power spark. So I don't see how you could use a vacuum gauge to adjust your carb. Otherwise the reed valves would not operate properly. Even the fuel pumps use that pulsing operation to pump fuel to the carb. So I don't see it working. I could be total off course, but I honestly don't thing so... Lets see what other have to say first!

 

[alldodge]

How much vacume does a marine engine produce.
I want to set the carburator using the vacume guages.
1972 225 HP omc
My classic car has about 15 # at idle.
What should the boat have.
Thanks, Rich

Hey Rich, your 225HP I think is a 307 cu in V8. With that your vacuum should be the same as you car around the same 15 in Hg vacuum. If your pulling a much less then that and timing and everything else is OK I would suggest a compression test. More then likely you just have a tired engine

 

[Bondo]

Ayuh,.... Is there even a vacuum port on it,..??

Most boat motors ain't got any vacuum ports,....

 

[gm280]

Hey Rich, your 225HP I think is a 307 cu in V8. With that your vacuum should be the same as you car around the same 15 in Hg vacuum. If your pulling a much less then that and timing and everything else is OK I would suggest a compression test. More then likely you just have a tired engine

Okay. I was visualizing a 225 HP OB and not an Inboard 4 cycle engine. Guess I really shouldn't have gotten out of bed today.

 

[alldodge]

Okay. I was visualizing a 225 HP OB and not an Inboard 4 cycle engine. Guess I really shouldn't have gotten out of bed today.

It's all good I do this kind of stuff quite often, most folks just figure I'm going to mess up also sooner or later

 

[Bondo]

Okay. I was visualizing a 225 HP OB and not an Inboard 4 cycle engine. Guess I really shouldn't have gotten out of bed today.

Hey,.... if ya ain't makin' mistakes,.....


Ya Ain't Tryin',......

 

[Scott Danforth]

The amount of vacuum developed will be in direct proportion to engine RPM, throttle position, sealing of your rings, overlap of the can, and of course, load to a small amount. However like Bondo pointed out, there is no port to take a reading.

Set the idle mixtures so it runs just a tad rich with the flame arrestor in place, adjust idle speed per your manual (about 650 rpm in neutral in the water)

 

[Redrig]

I used a vacuum gauge to help dial in my carb .

On my 302 , there is a plug on one of the runners of the intake manifold , I just used a brass barb to adapt to that and hooked the gauge there. (the plug is dead center in this pic)

 

[Rich Novak]

I know how to do it I don't know how much vacume a boat motor should have.
I red in a post that boat motors don't produce much .
Thanks, Rich

 

[Rich Novak]

Hey Rich, your 225HP I think is a 307 cu in V8. With that your vacuum should be the same as you car around the same 15 in Hg vacuum. If your pulling a much less then that and timing and everything else is OK I would suggest a compression test. More then likely you just have a tired engine

Thanks, that is what I needed.
Although someone in a different post said boat engines don't make much vacume that is was confused me.
Thanks again, Rich

 

[Bondo]

Thanks, that is what I needed.
Although someone in a different post said boat engines don't make much vacume that is was confused me.
Thanks again, Rich

Ayuh,.... For distributor vacuum,....

Boat motors run as fast as ya push the throttle Open,....

Open throttles don't make as much vacuum,....

At idle, throttle plate closed, ya they make the same vacuum as you'd see in a comparable pickup,...

 

[jerryjerry05]

This was from a post a short time ago. Rick explains the negative of vacuum on a marine engine.Ricks in blue.



Nor will any Marine ignition distributor. The diaphragm vacuum advance is not only unnecessary, but it will serve no purpose for a Marine engine that produces very little manifold pressure (vacuum).
In addition, the automotive advance curve will be incorrect for his Marine engine.

Should he hook it direct to a plug on the intake or a vacuum line on the carb?​

No... in order to be USCG approved and to protect his engine, he needs a Marine ignition system.

I had an old I/B with twin 350 Chev's and it came with car dist. with the vacuum hooked to the intake.
How should it be hooked?? I say to the intake. His neighbor says to the carb. He says the intake will have too much vacuum.​

Just the opposite.
At all times when above hull speed, the Marine engine is under load.
With the throttle plates open for heavy engine demands (beyond what the automotive engine would require for similar speed), this prevents any substantial manifold pressure that would bring the advance further.

Even if the diaphragm vacuum advance did bring on more advance, this would be during loads when we least want more spark lead.
More spark lead would most likely cause detonation damage.

He needs to bite the bullet, and buy the correct ignition system.
Like said, the advance curve is most likely incorrect for his marine engine.
Short of overheating and low to no oil pressure, ignition induced detonation is the Marine engine's worst enemy.
It can take out the tops of pistons, and can take out valves along with it. And it will do it with little warning to boot.

I removed the pipe plug and the motor seemed to have a lot of vacuum??​

Sure, if no load on the engine, it will produce similar manifold pressure to that of a car/truck engine. But these marine engines do not operate above hull speed with light loads.
A heavy load demands that the throttle plates to be more fully open.
When the plates are more open, manifold pressure diminishes rapidly.

Marine loads are significantly greater than auto loads given the same speed.
In order to compare to automotive, the car/truck would need to be pulling a heavy trailer up a never ending hill, and in a higher than usual gear.

.
What would it do if it's left unhooked? (I think I can answer this) timing won't advance and cause big problems.​

No... this will still have a mechanically advancing system using flyweights and flyweight return springs.
The diaphragm vacuum advance is used on auto engines during manifold pressure that says it's OK to advance further.
It also prevents the auto engine from back-firing during deceleration, of which the Marine engine never experiences.
Trust me... the Marine engine will see no real benefit from diaphragm vacuum advance.

The owners one of these guys who thinks "I can fix it, I don't need to ask any questions" Until it's past the point of no return.​

And he may end up at the point of no return sooner than later, if he allows this system to cause detonation damage.
I would NEVER install an automotive ignition system on a Marine engine.
It just isn't worth it!

 

[Rich Novak]

Thanks guys, it helped a lot.
Rich