Carbed 4-stroke OHV engine science....

[PhilosophStein]

So after a few months of riding my MB200 and delving deeper into working on 4 stroke engines, I've noticed a few things... Below are the questions and anyone who knows scientifically what is going on, please chime in:

Note: I have a stock 212cc Pred in the MB200 and it has maybe 60-70 hrs on use on it with no issues whatsoever.


1) Why does the engine backfire after a long stretch of WOT (that is say 300 feet +) running and I suddenly twist the throttle shut (like close it faster than the normal spring return would)?

2) It may be just my imagination, but it seems like 5 minutes or so before I run out of gas (which I define as the bike slows in rpm then dies and I cannot restart it next pull), it seems like the engine has more "pep" or torque? It almost seems like it has beefier throttle response?

3) Why is it bad to run a really small short pipe exhaust header with an OHV engine with stock valves?


That's it for now, I might have other questions later on.

 

[bpoirier]

1) The engine pops after a long strech do to the fact you are pouring the fuel to it WOT. Then slamming the throttle shut. Lots of fuel there but the rpms have dropped off. So there is excessive fuel in the intake. You could also have an exhaust leak around the gasket.
2) The engine will run a bit leaner when the fuel gets low. I think it helps make it run at a more optimal air fuel ratio. Therefore increasing throttle response.
3) You can run a short pipe but will have to adjust the main jet accordingly. The short pipe has a scavenging effect. Moving more air and thus needing more fuel. The exhaust valve is already hot and the short pipe with leaner jetting exaggerates this.

 

[jbrocks]

Hey thanks PhilophStein for asking these questions, I may have wondered myself but am too ignant to formulate the questions. And thanks to you bpoirier for the answers ahead of the pop quiz! I woulda fail'd fowshow.

When my engine backfires, part of me thinks I'm screwin' up, but part of me just likes the sound of it:biggrin: The answer to question 3 explains why I'm now up to .41 jets in my 212s and they roar (also at 4900ft elevation).

Question 2 I would rephrase to "....5 minutes before I'm a dumbass riding like the Great Ape on his crady lil monkey bike and I runs outta gas like a dam foo...?"

I even got the answer for than one