Blow By - What is it??

[Dualie]

Hey Everyone,

I was running my 89 200 Merc carbed O/B in the driveway and a freind of mine said that I have BLOW BY in one of the top set of carbs.

I saw smoke coming from one of the set of three if that makes sense, the other two sets are not smoking at all.

Can someone explain what this is to me.

Thanks so much!!
Dualie

 

[Paul Moir]

Re: Blow By - What is it??

When the compressed fuel/air charge is ignited, the pressure of the resulting explosion is contained by the combustion chamber and piston. The piston is sealed to the combustion chamber's walls by springy metal rings which push against it. Any gasses that leak by the piston or it's rings from the combustion chamber and into the crankcase are called "blowby".
The reeds stand between the crankcase and the carburetor's throat.

To verify that you are indeed looking at blowby, perform a compression test. It will tell you if the combustion chamber is adequately sealed. Another possibility is lean running, which can cause the fuel/air charge to ignite inside the crankcase before it reaches the combustion chamber. The resultant misfire is called a "lean sneeze" since the engine appears to sneeze.

 

[Texasmark]

Re: Blow By - What is it??

Ok Paul, you opened yourself up for this one. d

I have always wondered why a 2 cylinder (especially) 2 cycle misfires in N and doesn't in F or R.

I realize that the engine is loaded by the prop when in gear, but what's going on in the combustion process to cause this. Sounds like you might know about that.

I have doubted it to be a rich-lean issue as once it's loaded it runs fine and you changed nothing to effect the fuel air mix.

Answer appreciated.

Thanks,

Mark

And while you are here, my wife's family came to Louisiana from NS many years ago. She's one of those folks we Texans call Coonasses. Don't know what you call them up there. d

Mark

 

[Paul Moir]

Re: Blow By - What is it??

I don't know Texasmark, but I'll advance the theory that it's mostly a simple carburation / timing error.

Say you rev to 2000 RPM in neutral. The throttle plate is opened just a crack compared to 2000RPM under load because the RPMs are not governed only by the carburetor and ignition timing, but by the load on the engine. So not only is the carburetor not delivering nearly the amount of fuel that the engine normally sees at 2000 RPM, but the ignition timing is way earlier too. Considering that, it's not hard to see why it would misfire.

I'll have to do a little further reading.

Speaking of which, your wife's family didn't come to Louisiana a long time ago did they? Are they expulsed Acadians?

 

[waterinthefuel]

Re: Blow By - What is it??

And I thought blow by was what a crotch rocket does to you when he passes you doing a wheelie and about 130mph...i.e. That motorcycle sure did blow by me today.....

Shows how much I know....

 

[Texasmark]

Re: Blow By - What is it??

Yeah, I think they are/were expulsed; know they were Acadians. Knowing my wife (rebel) gotta be the right answer. d

You may have it on the missing in the engine; makes sense.

Thanks,

Mark

 

[dingbat]

Re: Blow By - What is it??

Blow by is pressure from the combustion chamber that has gotten past the rings and has pressurized the block. It’s pretty typical of the motor with bad rings