Gallery, Projects and General > Project Logs
Webster IC Engine build log.
dbvandy:
--- Quote from: NickG on May 16, 2011, 01:26:00 PM ---Nice 1 Chris, I will be extremely chuffed if my first i.c. turns out as good as that. :thumbup:
I think it might be worth a try with the butterfly valve still, am sure I've seen one working somewhere. I see what doug is saying about the vacuum, but then again, it's still pulling the same air through per revolution so it should still work I reckon.
It doesn't have to be as complex to make as a butterfly either. It could be something like the attached diagram.
Nick
--- End quote ---
Here's the problem... in a "normal" carburetor you have at least two jets, a low speed and a high speed to control the mixture. IF you restrict the airflow with the vapor carb, you would also have to adjust the mixture to enrichen it due to the lack of air volume through the tank which is needed to produce the vapors. Once the engine is tuned and running correctly and he has his carburetor sorted, the mixture valve will control the revs very precisely.
I had planned to put a throttle screw in the Otto carb, but found out I did not need it after I got it running on it.
Doug
NickG:
Ah right, I understand. I wasn't aware that model carbs had two jets. The only real experience I have of carbs is SU type where opening the throttle richens the mixture at the same time by moving the needle down on a cam (quite complex at small scale?) Guess the other thing a proper engine does is advance the ignition too so there are quite a few compromises in model engine designs.
NIck
dbvandy:
What the little model carbs (and most of the ones you make from plans) have is a needle valve that goes down into the orifice (jet) as the throttle is closed to adjust the mixture. We are at the mercy of fluid dynamics with the vapor carb and that, in part, is what makes it so forgiving.
With a normal carb you change the size of the jets to change the mixture, smaller hole ==> less fuel ==> leaner mixture. In the OS2a carb (and all of the others) there is a needle that in essence "plugs" the hole that the fuel is coming through as the barrel throttle valve is closed.
This adjustment is what gives us so much hassle because those carbs are designed to run on nitro methanol (alcohol) which requires more liquid and thus the jets are larger. So, when we put gas through them, there is too much and they either run rich at idle or rich at WOT, there is no in between.
I tried for two weeks or more to get one of the two RC carbs to work with either motor reliably, but never could on either motor. When I discovered Jans vapor carb online, studied how it worked and made my own, it was a glorious day. This is what mine looks like:
The mixture screw on the top is the high end and meters the maximum amount of fuel that will come through the carb and the screw on the side meters the fuel at idle by plugging the hole that the fuel is pulled through.
There is a really good writeup on the workings of the RC carb here: http://www.oocities.org/ericperez_2000/Carb_101.html
Worth a look.
Doug
lordedmond:
the ones I used in the past were very simple ( although theses were for 95 nitro with a dash on methanol and a smidgin of castor oil )
they had a flat throttle slide with two needle jets one above and one below the flat slide , the one below was flush with the side wall of the venturi , the one above had a beak one it at 45 degrees facing downwards that was the main jet the other was the low speed jet.
the throttle slide had just a plane hole in it
you could at that time buy from the model shops replacement needle valves complete its those that I used
so the order of things from the air in was jet with a 45 beak looking down ,throttle slide, and a jet flush with the side wall
to set up I used to close off the bottom jet open the slide and adjust the main mixture, then slowly close down the slide and open up the lower jet to get it to idle
Theory is that the top jet with a beak would tend to richen up with increased air flow wears the bottom one would weaken
Stuart
dbvandy Doug check your email
raynerd:
Guys, I`m really sorry, I didn`t read these last two replies until today. I just missed them totally!! Sorry. Lots of good info. The info really helped in my understanding of how a carb works
I`ve still not had time to test anything on mine as it is currently in bits ready to be put back together on a new base. I`ve made a couple of changes. I`ve profiled the fly wheel a bit which I think looks a bit better. I`m considering painting the flywheel...I can`t make my mind up. I`ve also replaced the brass inner section of the fly wheel with a steel one, threaded it M4 and drilled right through into the crank shaft so that it won`t slip at all and is nice and solid. Not the best photo, looks quite good in person but..
paint or not?
I also had an issue with the timing of the ignition. Although it was easy enough to undo the grub screw on the magnet holder, I realised I needed a way of adjusting it whilest running. To do that I had to get rid of the base mounted sensor holder and make a bracket for the sensor that could be swung around the axis of the crank shaft. I`m sure there are many neater ways but I`ve not used my rotary table in a while so I went for this option. Basically I bushed the hole in the side frame that the crank goes through and purposely left 2mm sticking through the front side. I then turned down a large diameter of ally, drilled it 12mm the OD of the bush and then reamed it so that there was a recess in which the magnet holding disk could sit. I then mounted it on an arbour:
I then took down the thickness of the back side and milled a V in the top, this was so I could get an allan key in to tighten up the magnet holder in an approximately correct position. An arc slot was then milled through the side and a M4 hole cut into the side frame. The sensor will be fed up into the support from the bottom, the support and sensor can then be rotated (on the bush OD) during running to get the best setting. This is just a dry pose you can hopefully see the idea.
I`ve cut the new base to size, I just need to mark out. I`ve got a little more polishing and then I`m ready to put it back together again!
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