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Muffler/Backpressure

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Patrick (Sussex

10-20-2002 19:29:13

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My 50 side mount needed choke to run smoothly under a heavy load. I replaced the muffler to manifold clamp because it was leaking badly - and voila...the tractor runs much better. I read in the archives that the Ngine needs backpressure to run correctly. Why?

Many thanks..

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Patrick (Sussex, NJ)

10-21-2002 10:38:24

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Re: Muffler/Backpressure in reply to Patrick (Sussex, NJ), 10-20-2002 19:29:13

Thanks for the explanations.

As for changing anything else...hmmm...replaced the manifold nuts with brass'uns...maybe there was the fix for my intake leak. I thought I had eliminated that possibility by replacing the manifold gaskets and properly torqueing down the old nuts...

I've also been running a little Marvel Mystery oil in the gas to clean the valves etc....maybe that's having an effect as well.

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Gary B

10-21-2002 09:40:03

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Re: Muffler/Backpressure in reply to Patrick (Sussex, NJ), 10-20-2002 19:29:13

When a carburetor is adjusted for a certain amount of backpressure, removal of that backpressure causes what amounts to a leaner condition. Actually, the backpressure causes more of the exhaust gases to remain in the cylinder. Since exhaust is inert, less gasoline was needed in the cylinder for a complete burn. When the leak developed, causing less backpressure, more fuel is needed in the cylinder, so the mixture must be made richer to compensate. How much this effect is noticed would depend if you were closer to lean or closer to rich in the first place.

Gary

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chuck

10-21-2002 11:59:35

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Re: Re: Muffler/Backpressure in reply to Gary B, 10-21-2002 09:40:03

Gary, All the air moving through the carburetor is coming out at a set air/fuel ratio. The inert gasses in the cylinder are exactly that-inert. They do not affect air/fuel ratio. They do however absorb energy and affect the peak temperature during combustion. This is the effect that car engines use in their EGR system to knock down Oxides of Nitrogen levels. That being said, unless you have a real serious restriction in your exhaust system (like cutting more than 50% of the flow area) the effect of an exhaust leak on the way the engine runs (especially a low performance engine like an N's) would be hard to notice. A difference of a couple of psi on the exhaust side will hardly have any affect on the density of gasses left in the cylinder.

chuck
436244

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Larry

10-20-2002 20:02:17

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Re: Muffler/Backpressure in reply to Patrick (Sussex, NJ), 10-20-2002 19:29:13

Well, it probably doesn't. What you have is a vacuum leak - more than likely between the manifold and the block - possibly between the manifold and the carburetor.

We run these tractors (for many years) with no exhaust system ( save for the manifold) and everything works and runs fine.

When you need "extra choke" that means the engine needs an enriched mixture. So you must be pulling more air into the system.

The question of having a muffler has to do with very small engines (lawnmower's) that needed a muffler to keep (in therory) cool air from entering the exhaust valve chamber, which in turn would keep the exhaust valve from burning.

My advice? Forget about this idea - find the leak and everything will be just fine.

Regards.

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Wayne Mo

10-20-2002 19:52:54

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Re: Muffler/Backpressure in reply to Patrick (Sussex, NJ), 10-20-2002 19:29:13

I do not see how a new clamp and resulting higher back pressure would make a noticeable difference. Use of a choke to run better usuallyl means a vacuum leak somewhere. Are you sure you did not do something else to eliminate a vacuum leak, like tighten the manifold bolts or something. Sometimes a worn engine will need choke to run because the intake valve guide clearance is so large. It is usually a sign of a worn engine.
Getting back to the muffler, you could run a straight pipe and get good performance. You could run with no exhaust system at all likewise but you would burn the exhaust valves. I always thought any increase in back pressure has a negative effect on performance.

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