Question for engine experts

[Clancy Arnold]

I have received two 25 size engines for the Ringmaster Trainer I am building and one thing is obviously different on the two engines.

The OS 25 FP RC engine has a carb. with a .176 dia. throat,
The Blue Bird 25 RC engine has a carb. with a .288 dia. throat.

What differences in performance or setup should I be aware of?

Clancy

[Jim Kraft]

If the spray bar/needle valves are about the same restriction, and you do not have good runs because of low fuel draw on the Blue Bird, You might want to put several layers of pantyhose over the intake on the carb. That should choke it down enough to help.

[Alan Hahn]

Typically the OS FP-LA series have had quite small carb openings---compared to OS's equivalent ball bearing engines. I wondered if some of the reason was that OS tried to keep the bushing engines power down as a marketing tool for people to step up the the more expensive engines.

Now it is true that the FP/LA line of carbs were air-bleed---a cheaper unit to manufacture compared to the twin needle carb of the ball bearing engines. The gist of this is that the air bleed carb spraybar only expends half-way into the carb, so effectively the opening isn't as obstructed as the two-needle carbs. So maybe that could explain part of the reason the diameter is smaller.

I am not sure what type of carb the Blue Bird engine has, but maybe it is a two-needle version.

[Randy Powell]

Clancy,

I don't' know about the characteristics of the individual engines when compared to each other, but the larger venturi area (in the carb) will, up to a point, create more power. Up to the point that the engine cannot draw fuel. There is also an issue of consistency. Larger choke areas are somewhat more inconsistent.

[Clancy Arnold]

Thanks for the help.

The OS 25 FP has an air bleed carb with a single needle and the Blue
bird 25 has a two needle carb but the difference in restrictions are minor compaired to the throat diameter.

It is in the teens outside so no test runs today.

Clancy

[Alan Hahn]

Another comment is most, if not all, RC carbs are designed to run with muffler pressure. This helps a lot with the oversized openings.

[Clancy Arnold]

Thanks to all for comments.

I think Alan Hahn has the right hint.  The Blue Bird is complete with muffler and is set up for muffler pressure.  That is the way I will try it first.

The OS has a 2B carb that is a single needle with air bleed but I have the carb off of a OS 25 ball bearing engine that I crashed and it was a 2B carb with two needles and it has a .225 dia. throat.  I would put the two needle carb on put it is broken at the mounting screw holes.
Clancy

[Brian Hampton]

I've got a new Bluebird 28 that's almost finished running in but it's got a much smaller carb than what you said yours is. But did you measure the bore down inside the barrel? Mine's .218" in the barrel, minus the area of the spray bar (an average of the two diameters) gives it an area of .024 sq inch. And I'm almost certain the 25 is an AAC like mine. Bluebirds were sold as Brat in America, very nice engines.

[Clancy Arnold]

Brian
Thanks for the comments.  I just went back and measured it again.  This time making sure I am measuring the hole in the rotary drum and it measures .261 inches dia.

I plan on running it on muffler pressure and because of space I will have the tank mounted inboard on the profile body. 

Time will tell,
Clancy

[Phil Spillman]

Hi Clancy, If your don't need the RC aspect of the engines I know that you can replace the carb on the FP .25 with a NVA and venturi insert from Tower Hobbies! The existing muffler will work just fine either way! I have been enjoying both the LA .25 as well as the FP .25 on some older designs and these have come alive with both engines mentioned! I use 58' lines eye to eye, an APC 9 X 4 prop and either 10 11 11 or 5 11 11 fuel in each. This is 5 or 10% nitro methane with even split oil of the same percentage in each.

Phil Spillman

[Clancy Arnold]

Phil
For safety and ease of training new pilots, I will be using one of my U/Tronics Single channel control units and a servo on the throttle.  If they get dizzy or confused then we can cut the throttle and land.  Also great to practice takeoffs and landings.  I have done as many as twelve in a single flight. 

Clancy