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| Why do two strokes four stroke ? |
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peter ogborne
| | Posted on Thursday, March 18, 2004 - 07:32 am: | 
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We have all experienced this , two reasons, one too rich a mixture.....two to much oil in the fuel.But why ? |
Ernie
| | Posted on Thursday, March 18, 2004 - 12:03 pm: |
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Peter,
It's all related to the fuel/air mixture. It can be either rich or lean. The reason that pops into my mind for the rich side is that the engine simply misfires. Since fuel is heavier than air the second time around some of the fuel has plated or puddled and there is more air seperated from the fuel. So what is now in the combustion chamber is closer to 14.7 to 1. Once the fire has lit all the fuel burns and it's now ready to do it over again every other time.
On the lean side it takes 2 intake strokes to get enough fuel to fire.
With our "show engines" with no load it is really easy to have happen on the rich side. Carefull adjustment of the fuel/air mixture usually can correct it. However if the base is loaded with fuel what happens is the fuel mixture is leaned out with the needle valve or what ever means the carb/mixer has to adjust it. Since all the extra fuel is in the base the actual adjustment ends up too lean. The engine runs real good untill the extra fuel in the base is used up. One can tell when this happens as the engine will sneeze and quit. If the mains are loose it will blow the grease out of them as well. Funny to watch if it isn't yours.
However some engines won't sneeze to tell you it's too lean. My Gray "U" is one of them. A lot of the old ads state something to like this.
"no backfire" "backfire trap in the intake" etc.
My Gray "U" has a screen in the transfer port. It acts like a miners lamp. Some of the Fairbanks Morse and Roberts engines had a piece of elongated honey comb in the transfer port. This had the same effect as a screen.
A 2 stroke sneezes when lean because the mixture in the combustion chamber wasn't rich enough to really light. When the piston started down it uncovers the transfer port and the incoming charge is ignited by the slow burning fire that was in the combustion chamber. This also ignites what is in the base. KERCHO! An engine with a backfire trap in the transfer port is just like a miners lamp the screen kept the fire from igniting the fuel/air in the base.
Also it's a good idea to chech for these screens/traps in the transfer port.
I restored a Fairbanks Morse engine a few years ago and almost missed the fact the the honeycomb in the transfer passage was really nothing but a big gob of rust. The result would have been just like the discussion last week with gaskets backwards and not cut out for the transfer passage. I woulnd't go to any great lengths to replace one of these screens. The Fairbanks Morse runs just fine. It even sneezes to let you know it's too lean.
Hope this helps
Have a good day
Stuck in the snow
Ernie |
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