Perry carb on a Rossi engine.
 

Hi, I have 2 Rossi .40 engines, both without carbs. I am considering purchasing 2 perry carbs for these engines. the Original Throat diameter of the rossi carbs is .270 in which is about 6.8mm. the newer rossi engines state that the have "10mm carbs for power, as opposed to the 8mm carbs for fuel economy" does this mean that I could actually get a larger bore size such as the .315 in (8mm) dia. throat carb, and get more power ( or even the .340 (8.63mm) or .375 (9.53mm) dia throat)? has anyone used a larger carb on these rossi .40 engines, and what kind of success have you had with them? Or should I stick with the stock size?? any input would be appreciated!! I have asked the Rossi importer, Rossienginesusa and thier response was less than helpful , it seemed that they weren't sure, so they suggested that I stay with the stock size? any ideas/suggestions?? Thanks, Craig.

RE: Perry carb on a Rossi engine.
 

I've had a similar experience with the AXE/Rossi importer...

Check this guy. He can probably help you out. http://www.bobsengines.com/

RE: Perry carb on a Rossi engine.
 

The larger the CFM (cubic feet per minute) ability of the larger throat carbs, the weaker will be the fuel draw. Translated into reality, you can actually lose top end power with a larger carb because you will have to open up the needle valve farther in order to pull enough fuel to run. Then, whenever you climb, the mixture will lean out much more than it would have with the stock carb. To compensate for the leaning out during climb, you will be forced to open the high speed needle valve even further. Now, while the engine will run fine when the model is climbing, it will be nearly blubbery rich when flying level.

I would stick with a Perry carb that is pretty close to what the stock Rossi .40 had originally. They could have used larger carbs back when they made the engines that you have. After all, larger holes through the carb do not really cost anything extra. But, instead, they stuck with an 8mm wide opening and I'll bet that fuel draw was the reason. Making tremendous power on the test bench means nothing if you can't get that power when you fly. Smaller carbs have lots of advantages, such as:

1. Superior fuel draw.

2. Faster engine spool-up from idle.

3. Requires less fussing with HS needle valve from flight to flight.

4. Repeatable performance under most conditions.


Anyway, those are my thoughts.

Believe me, I've ran engines with some monstrously over sized carbs. I'm well familiar with the good and bad of doing so. Me, these days, I just wanna fly, not tinker or rebuild a cantankerous model/engine that is ticklish and finicky to run.


Ed Cregger

RE: Perry carb on a Rossi engine.
 

Hi!
The 8mm carb will work just fine on your Rossi if you use 10x6 to 11x4- 11x5 props!
9mm will work too if you intend to run your engine with 9x6 or 9x7 APC props (17000-18000 rpm)

RE: Perry carb on a Rossi engine.
 

Ok, so going way over sized isn't good. however, Jaka mentioned that an 8mm carb would be good for a 10x6 to 11x5 prop, which is what I typically use. does this mean that I would probably be better off going up from the stock .270in (6.8mm) carb to the .315in (8mm) carb for good performance, or as stick with the completely stock size carb as Broken Wings suggests?  I am toying with the idea of getting one of each, .270 (stock size) and .315 (one step up in size) knowing that one is absolutely safe, and seeing what kind of performance I get with one size up!

Thoughts? 

BTW, I am planning on using them for sport flying, no thoughts of racing, however, my flying style is that the engine is usually WFO 80-90% of the time.

Thanks again, Craig.

RE: Perry carb on a Rossi engine.
 

Oops, it was NM2K, not Broken Wings  that suggested sticking with "Nearly stock size", sorry for the mix up there.  NM2K, you mentioned the 8mm carb size, so you think that would be a good step up from the stock size of 6.8mm.  or just stick with the Completely stock size?  they have them both, 6.8mm and 8.0mm.

Thanks, Craig.

RE: Perry carb on a Rossi engine.
 

Well, I screwed up. I thought the 8mm was the stock size carb.

One thing to keep in mind is that even having the same size venturi opening, different designs/makes/models of carbs will have different air flow rates. This makes predicting how the performance will be affected a little iffy at best. I agree that several different sizes of carbs would be the best way to go for determining the best carb size for your flying style.

As Jaka pointed out (I hope it was Jaka - durned memory!), the smaller the prop size, the more you can get away with using carbs with higher flow rates (air).


Ed Cregger

RE: Perry carb on a Rossi engine.
 

If it was my engine, I'd go with the 8mm for general sport type flying...anything larger and you're asking for fuel draw problems as Ed described.