Re: Carburetor debate

Tommy makes a good point and the signal strength issue can be improoved by running a higher float level. There are some tuning tricks when running a very large carb and the result is positive with no significant drawbacks.

More specifically: I have run and set records with a 34mm on a 60cc engine altho for this I usually run a 28mm simply because the formula one hard cornering wont allow as high a float level.

My mod (secret other places, not here) is to make a new emultion tube that is taller then the stock one for two reasons,

First, it is where the fuel will flow when the carb floods from the level being to high, this makes it tolerate a higher float level better

second, The fuel will discharge from the top of the tube into the venturi but from a higher elevation thus giving the mixture more time and opportunity to vaporize before falling to the intake floor. This is important because at lower rpm with a big carb, velocity is low making vaporization difficult. Once the intake floor is wet with fuel it is hard to revaporize this back into the airstream without washboards, floor air jets and little duckbill jumps but you likely wont have to mess with these other more extreame alternatives.

Re: Carburetor debate

That rocks!

My best, most reliable ride was a 15 Bing, Korado top end, 15/36 and a Boss. 0-15 was snailville, but after that nothing but blastation all the way to 48mph. I'm redoing that on a lighter frame with Mikuni power as we speak...

Re: Carburetor debate

) Cupermcnewbster ( /

I'm surprised this turned into a legitimate debate.

Peter, you must also keep in mind Sarah is tiny, and her magnum is also small, and the overall weight is probably helping her out a tad bit. Its like elliot doing insane speeds on his maxi, 65+ with a boss at like 16x36; the entire setup including rider was very light.

So I guess it comes down to what you want out of the build, but I think me, justin and other cuperteens are basically trying to go all out all the time, and try to push each bike we build to their limits. We've found 19/21 phbg works great on a 4 petal polini, and with the right pipe and gearing, they can really fly. Same goes with the gila. I've run both a 21phbg and 24 oko, and there was more power throughout the range with the larger carb. Again, it may be tuning, but most of us on here aren't the greatest tuners, so the carb thats easier to make rip is usually the one you should pick.

But tommy proves the point with dyno stuff. A bigger carb equals more power, so go big!

Re: Carburetor debate

77 dyno results are the same

elliots 24 mikuni setup had a solid 8 hp over the whole band where the 21 dellorto just barely peaked at 8.

i run a 24 oko on my puch polini (really because i hav no other carb) and the thing runs fine!!! low end great.

Re: Carburetor debate

There was an equation in "The 2 Stroke Tuner's Handbook" or whatever its called for determining your optimal carb bore. Your carb size depended a lot on where your power band was.

Re: Carburetor debate

From _Two Stroke Performance Tuning_ by A. Graham Bell, p. 99 (or 93 in the adobe version linked in the wiki)

"The other advantage of using a carburetor of the right size is this: a high air velocity through the bore lowers the air pressure (i.e. creates a partial vacuum), which makes the fuel more volatile and easily vaporised. You probably already know that water will change from liquid to gas (steam) at a lower temperature on the top of a mountain than at sea level, because the air pressure is lower. Likewise, with petrol, vaporisation improves with a decrease in air pressure. Additionally, the high air speed in itself assists in breaking up the fuel and vaporising it. Properly atomised fuel blends more evenly with air filling the cylinder and power goes up due to the improved combustion which results."

Re: Carburetor debate

I always have to read that shit 3 times, think for fifteen minutes then read it again. And 45% of the time I have no idea.

Re: Carburetor debate

My experience= 50cc tccd + 16mm phbg= 50mph gps'd good power throughout the range (w/tuned clutch)

50cc tccd + 21mm phbg= 55mph gps'd about the same low end and midrange, but pulled harder on top..

Re: Carburetor debate

I recall a few hills that people were having trouble climbing but not many... I was on a blastorific vespa small frame though...

I live in Cincinnati and I know all about hills. My bike definitely climbs hills better with the 19 PHBG vs. the 15mm Bing that I used to run... No question...

Re: Carburetor debate

) Cupermcnewbster ( /

portland was flat.

cuperteens moto: go big or go home. This applies to everything.

Re: Carburetor debate

Maybe I ate too many weed brownies that weekend? I didn't think it was especially hilly and wouldn't have noticed much because the scooter that I was riding was fast as fuck and would shred any hill that got in it's way... I thought I remembered a big hill on one of the rides... Maybe not? You cuperteens are feisty little dudes.

Re: Carburetor debate

) Cupermcnewbster ( /

ok, so there were a couple little mini hills, but really not much. My bike was setup for medium speed, and it was still up front even on the biggest hills. I should note this is in no was dissing the rides as they were amazing.

we ride hills in cupertino some of the time.

Re: Carburetor debate

Portland IS a really cool place to ride for sure. There were some nice twisties on the Saturday ride. I'm glad that I was able to stay up front for it...

Re: Carburetor debate

dgma you gotta do a litttle porting to get that extra bit on the low endzorz.

Re: Carburetor debate

As new carborator technologys improove, better signal strength, bigger carborators are being used. See what david vizard "the wizard" says about this overcarborated four stroke engine.

http://www.gofastnews.com/board/technical-articles/795-product-review-1-aeds-1250-carb-too-big-350-a.html

Re: Carburetor debate

^ moped "technology" plateaued about 20 years ago.

Re: Carburetor debate

lol @ "carborator technologys"

Re: Carburetor debate

Zach: I would be interested in seeing what the results would be if you were to compare two carburetors of the same type, but different sizes. The 24 mikuni is a flat-slide? No? And the dellorto 21 is a round slide. Flat slides perform differently than round slides, but I would also add that mikunis are far better carburetors than dellortos. I believe that you will always get a much more consistent power spread with a properly tuned mikuni than you will with an equally tuned dellorto.

If you can show me dyno results with, for example a 16mm PHBG and a 21mm PHBG on the same setup, I'll concede my side of this debate. Or a mikuni 16mm roundslide and a mikuni 22 or 24 roundslide.

Re: Carburetor debate

^mikuni flat slide vs dell round slide, yes.

but a mik round vs a dell round, eh i bet its about the same. i think mikunis have a longer venturi. so possible slight increase.

i think a bigger butterfly valve on a "big" cylinder would be real cool.

Re: Carburetor debate

Kansas John says:

The problem is when you don’t have enough sucking power to get good consistent velocity through the carb and intake. And that is also when you’re going to run into tuning problems.

So let's say you do have a lack of sucking power (vacuum/velocity) through the carb. You clearly need smaller orifices (intake/carb throat and some parts I don't know the names of) throughout the carb.

So a troubleshooting step could always be: stick on a smaller carb. Got that. How can you tell if you went too far? That is: How can you tell if you now actually have great throughput (vacuum/velocity) but you're still not delivering as much fuel as you can burn and scavenge? How will it run and can it be overcome by re-jetting?

Re: Carburetor debate

mikuni vrs dellorto.

i vote dellorto.

Re: Carburetor debate

The sucking power (as you call it) had to be greater with older carb designs but there are now carbs that don't require as much sucking power so you can go bigger which will give you less pressure drop (resistance) threw the carb which causes the engine to move more air and thus consume more fuel and make more power.

Velocity as you know helps vaporization and a bigger carb creates less velocity. Many carbs used to spill fuel into the throat by sipping fuel right off the bottom of the venturi, these carbs need more velocity to lift the mixture upward as the flow travels horizontally so it dosen't just wet the floor of the port. Newer designs lift the fuel off the port/venturi floor much better at lower speeds for better low speed vaporization. Because of this these carbs can be bigger and make more power.

Re: Carburetor debate

Have you ever run a mikuni carb on a moped, Justin? I'm not trying to be snide, I'm just wondering.

Re: Carburetor debate

So Tommy, are all the Latebirds going 28+?

Also I know this is stepping off of the main subject and I don't mean to thread-jack but since we are on the subject of atomization and velocity and what not....... What does everyone think of long vs. short intake length.

I, for a brief time, had a rather long intake and noticed a difference in bottom end torque most likely due to better atomization and perhaps pulse tuning. I am on a reed setup however, so I don't really know how much pulse tuning can be achieved.

Re: Carburetor debate

) Cupermcnewbster ( /

i like dellortos, tomorrow I'll see how I like mikuni.

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