Another reason not to use an RC carb locked in place

[Bob Reeves]

Received an email from a gentleman running a Saito 40 using the RC carb with the throttle locked in position. This started me thinking about how the RC carb works and why using the RC carb to slow down the engine is a really bad idea.

At the small intake settings we use the idle needle closes off a portion of the inlet nipple which restricts the amount of fuel the engine can take in on the intake stroke. I have posted a photo of a 40 intake showing the small area we ended up with someplace on here, if I can find it I will add the link. One item we discovered some time ago is you do not want to do anything that in any way restricts the ability of the engine to draw fuel. This is the reason for a non uniflow clunk tank and all synthetic fuel, you want that engine to be able to take as big a gulp of fuel as possible.

Add the variable air leak between the barrel and carb body and you have a system that is doomed from the start.

Found it..

http://stunthanger.com/smf/index.php?topic=13536.0

[Bob Reeves]

Posted this in the general section but wouldn't hurt to have a link to that post as it is related to all the reasons not to use a locked down RC carb on a 4 stroke.

http://stunthanger.com/smf/index.php?topic=31504.msg310079#msg310079

Crude drawings of how an RC carb works and why modifying your RC carb will provide much better fuel delivery.

[Randy Ryan]

Received an email from a gentleman running a Saito 40 using the RC carb with the throttle locked in position. This started me thinking about how the RC carb works and why using the RC carb to slow down the engine is a really bad idea.

At the small intake settings we use the idle needle closes off a portion of the inlet nipple which restricts the amount of fuel the engine can take in on the intake stroke. I have posted a photo of a 40 intake showing the small area we ended up with someplace on here, if I can find it I will add the link. One item we discovered some time ago is you do not want to do anything that in any way restricts the ability of the engine to draw fuel. This is the reason for a non uniflow clunk tank and all synthetic fuel, you want that engine to be able to take as big a gulp of fuel as possible.

Add the variable air leak between the barrel and carb body and you have a system that is doomed from the start.

Found it..

http://stunthanger.com/smf/index.php?topic=13536.0

Bob, I understand your reasoning however I am not in agreement with your "big gulp" idea. That idea carries penalties for the ability of the engine to draw fuel. The larger the venturi the lower the throat velocity therefore the higher the pressure at the fuel jet, simple Bernulli effect, (high velocity; low pressure, low velocity; high pressure). This will draw less fuel and could even cause difficulty drawing fuel in maneuvers. Fuel charge velocity is important too because the charge has intertia which helps pack the cylinder at the end of the intake stroke, the higher the velocity the better able to fully charge the cylinder. Charge velocity is very important, so really some experimentation needs to take place to find that magic throat area.

[Bob Reeves]

Not sure I get exactly what you are trying to say. Are you saying 'B' will draw fuel better than 'A' in the below drawing? This is a representive of what you end up with, RC carb being represented by B and choke screw represented by A.

[Randy Ryan]

Not sure I get exactly what you are trying to say. Are you saying 'B' will draw fuel better than 'A' in the below drawing? This is a representive of what you end up with, RC carb being represented by B and choke screw represented by A.

Other way around Bob, A will draw better than B. The smaller throat increases air velocity and lowers the pressure in the throat. Lower pressure, better draw.

[Bob Reeves]

OK now I see what you are saying, think misunderstanding is with my explanation. Let me try this.. The smaller venturi creates a higher vacuum on the fuel inlet which helps draw fuel from the tank. The larger venturi can have greater flow but doesn't draw as much of a vacuum on the fuel inlet. Just by looking at any factory CL venturi I'm not the only one that thinks higher vacume is what is needed for our intakes.

[Randy Ryan]

OK now I see what you are saying, think misunderstanding is with my explanation. Let me try this.. The smaller venturi creates a higher vacuum on the fuel inlet which helps draw fuel from the tank. The larger venturi can have greater flow but doesn't draw as much of a vacuum on the fuel inlet. Just by looking at any factory CL venturi I'm not the only one that thinks higher vacume is what is needed for our intakes.

Nope, not the only one, the lower the throat pressure the better the draw with a suction or uniflow system. the larger throat doesn't necessarily mean greater flow, it does mean lower pressure drop across it because it add less restriction then a smaller one. With any given engine we have a fixed volume of air to do the carb work.  My Saito .56 gives me .56 Cu In of air through the carb to do that. If the velocity is low two things happen, fuel is not efficiently drawn and the charge will have little energy and will not "pack" the cylinder. Conversely a  high velocity charge will draw fuel well and have the energy to continue flow even at BDC of the intake stroke. I'm not sure where the magic number is, but somewhere closer to the high velocity end is my feeling. I have nothing to base that on other then opinion at this point.

By the way, what is reasoning behind your opposition to uniflow for a 4 stroke?

[Bob Reeves]

<snip>
By the way, what is reasoning behind your opposition to uniflow for a 4 stroke?

Because when I first started messing with 4 strokes I wasted a week running back and forth between my shop and the circle in my front yard trying to make a metal uniflow tank work as well as a simple clunk tank. I built 3 tanks including one chicken hopper, tried several plumbing configurations and never had one give me the consistency of a conventional vent clunk. At that point I said why bother, the clunk works.

With a little help, I developed a system for Saitos that I can share and explain to anyone, as long as they follow the guidelines it's plug-n-play to a power train that is able to compete with the best at the highest levels. My Saitos have won Intermediate PA, Advanced PA and Expert profile at Brodaks, I would say the only reason a Saito hasn't won Expert PA is because no one has ever ran one.

Have no reason to change anything or do further experimenting. Sometimes I get wrapped up in trying to explain the why's and wherefore's of my setup, not because I'm trying to sell it but because I'm trying to fully understand why it works.

[Randy Ryan]

Because when I first started messing with 4 strokes I wasted a week running back and forth between my shop and the circle in my front yard trying to make a metal uniflow tank work as well as a simple clunk tank. I built 3 tanks including one chicken hopper, tried several plumbing configurations and never had one give me the consistency of a conventional vent clunk. At that point I said why bother, the clunk works.

With a little help, I developed a system for Saitos that I can share and explain to anyone, as long as they follow the guidelines it's plug-n-play to a power train that is able to compete with the best at the highest levels. My Saitos have won Intermediate PA, Advanced PA and Expert profile at Brodaks, I would say the only reason a Saito hasn't won Expert PA is because no one has ever ran one.

Have no reason to change anything or do further experimenting. Sometimes I get wrapped up in trying to explain the why's and wherefore's of my setup, not because I'm trying to sell it but because I'm trying to fully understand why it works.


Well my interest is purely curiosity.  I played with a clunk but it was way too big. Mike Alimov, who actually owns the engine I'm working with, Has a very nice uniflo setup that runs very consistently. I'm ever the experimenter tho and while I like his setup, I went a different way. I have however returned to a uniflo tank and am looking forward to testing it very soon. I am also planning a carb that is similar to a Cox TD setup with a clean unobstructed throat and radial spray holes. I don't really know if it will be of any advantage but theorize the cleaner flow will aid in keeping the runs more consistent because the air will flow smoother to the head. That said it could also be a drawback in not creating enough turbulence for good charge mixing.

[Bob Reeves]

I've stayed out of the other thread because it's pretty obvious Mike and I have different view points and you seem to like Mike's ideas. I will say my engines burn almost a full ounce of fuel more than the numbers Mike posted. I can't get a full pattern on a 56 with a 4 ounce slant front Sullivan because it will only hold about 3 1/2 ounces of fuel. I had to go with the 4 ounce tank with all tubes forward (like a bottle), it will hold almost a full 4 ounces. In my pea brain burning more fuel = more power but what do I know.

You and Mike may be happy with the runs you are getting, I wasn't when I was playing with that type of setup but you only have to please one person, yourself.

Good luck and hope it works out for you.

[Randy Ryan]

Bob, I sent you a PM

[proparc]

By the way, what is reasoning behind your opposition to uniflow for a 4 stroke?

I don't have a philosophy concerning 4 stroke plumbing. I use uniflow on 2 strokes because it works good. I don't use uniflow on 4 strokes because it sucked. Regular suction works good on my Saitos and uniflow sucks.