advanced carb tuners: evaluate this statement

"Using a carburetor with an overly large venturi can lead to an airstream with insufficient velocity at WOT. The insufficient velocity of the airstream means that an insufficient quantity of fuel is drawn from the main jet, and thereby to a failure to maintain the fuel/air mixture at the proper richness. Those, oversize carburetors may lead to lean WOT conditions, and thereby by to seemingly 'superior' top end performance, which ultimately, inevitably leads to seizure."

I've heard this argument at least four or five times, but I've never heard it discussed properly. An example of where it came up recently was in the "busted polinis" thread, where someone was suggesting the seizures came from "overcarburetion" (using 21mm carbs on a kit designed for 15mm carbs). What do people think of it as an argument, generally?

Re: advanced carb tuners: evaluate this statement

) Cupermcnewbster ( /

No, I don't buy it. The carb would have to be quite a bit bigger to "overcarborate"

I run a 21 on my polini and its fine. If I were trying to run a 24 mikuni, that would be overkill and might lead to the above symptoms.

Re: advanced carb tuners: evaluate this statement

the alexander graham bell book reccomends a 26mm carb for 50cc applications

Re: advanced carb tuners: evaluate this statement

It has been proven without a doubt that a larger 19mm carb on a polini setup can easily pull a polini puch to over 1mph per cc. reliably and seize free for thousands of miles.

however this chart from dellorto would rate a 21 carb too large for a polini, given a polini produces 6hp. And a 19 right towards the upper edge.

I have seen the 21 work on some bikes but I think the 19 is a bit better, more velocity = more torque.

Then again I think some of our polinis are pulling over 6hp ;)

Re: advanced carb tuners: evaluate this statement

charts aside in reality i think you'd have to get to like a 30mm carb to get that effect.

Re: advanced carb tuners: evaluate this statement

Shouldn't you tune the main jet of the carb for WOT conditions anyways so this would not even be an issue.

Re: advanced carb tuners: evaluate this statement

I'd really like to see a 24 flatslide on a heavily ported polini. I am curious to see if it would work.

Re: advanced carb tuners: evaluate this statement

now THAT'S interesting. where does the chart come from - the dellorto tuning guide? do you have a link?

. . . so what that chart's saying is, unless my 65cc metrakit and bullet pipe can put out 6hp, my 19mm carb is too large. huh. I'd really like to know why they say that.

Re: advanced carb tuners: evaluate this statement

what this is saying is, if the carburetor bore is too big, than no matter how big a jet you put in there, since the air stream is insufficiently fast at WOT, an insufficient amount of fuel will be drawn out of it at WOT. you could not even have a jet - it'll still be lean.

Re: advanced carb tuners: evaluate this statement

im not too into messurement, but based on feel with 70cc setups theres no need to go over 19 or even 16... the smaller carb seems to have a harder draw (higher density or velocity of intake) for low end performance

also based on feel...you can get just as much top end performance out of a 19 as a 21 & also with a more fluid acceleration

Re: advanced carb tuners: evaluate this statement

I always thought my 19mm dellorto was better on my 65 metra than others running a 21.

I am still curious on the polini though. I think the mikuni is a much more advanced carb and if tuned correctly could produced enough power for you guys to blow your polinis up even quicker. Taking it to an rpm range it hasnt seen before, but this is a total guess.

The mikuni flatslide is very smooth and responsive.

Re: advanced carb tuners: evaluate this statement

Does anyone have approximate HP numbers for kits? I just bought a 65cc MK and was thinking about trading my 15mm bing in for a 19mm phgb. This thread gives me pause.... any advice?

Re: advanced carb tuners: evaluate this statement

b4 a bunch of ppl decide any 15mm carb is better than a 19mm. after seeing that chart I want to make somethings clear.

It should be noted the chart recognizes the carbs despite being different sizes to essentially all be of the same design. So if are deciding between a 15 SHA and a 19 phbg the bore size is a minimal upgrade compared to the tunability and proper fuel metering during all throttle ranges and rpms the 19phbg provides.

While one might think the 19 PHBG too large it will make up for this in how it delivers the mix. And ultimately be much more responsive, produce more power, and have better low end (torque) than the SHA 15mm.

And for the record I have never seen a 65 metra that did not like and perform supremely better with a 19phbg over an 15mm carb, bing included.

Tommy I really want to try a 24 flat slide one day on a pumped up polini as well. I can only envision good things.

Re: advanced carb tuners: evaluate this statement

Elliot, thanks for being able to put into words the thoughts I have in my head.

I hope you stay in school forever so you cant afford expensive parts, cause once you start spending money on your mopeds you are going to destroy us all.

Re: advanced carb tuners: evaluate this statement

just a thought... don't bigger carbs get better gas mileage?

Re: advanced carb tuners: evaluate this statement

Moped Game Sally Struthers /

what the hell is a gas mileage?

Re: advanced carb tuners: evaluate this statement

No. bigger carb = more air + more gas = less gas mileage

Re: advanced carb tuners: evaluate this statement

i got my info from Alexander graham bells tuning guide in theory and practice

Re: advanced carb tuners: evaluate this statement

The intake tract works as an entire system to deliver air to the cylinder. Y'see the interesting thing about intakes, is that no matter what you do, you can never ever get more than 14.7 psi of pressure differential across the entire intake. The carb is designed for a certain flow rate based on that pressure drop. If you streamline the intake tract, it has the effect of increasing that flowrate and therefore require a carb capable of more flow. If your engine is ported with 4 petal reeds, it is flowing way more than the stock crappy 2 petal that the 15 bing is meant for. At that point, you have enough flow that you will keep a steady diff. p across that bigger carb. The flow will be directly related to your horsepower. If you can rev a polini to 16 grand with a huge 4 petal reed, and make 12 or 14 horsepower out of it, you better account for that on the chart.

I dont care how skinny you are, you cant go 60 plus miles per hour on anything with less than 7 or 8 horsepower. You guys that are pushing 65 on your metrakits and polini's have to be making more than 8 horse. My calculations, and testing, have shown 60 mph on a scale model pinto requres 8.6 horsepower to overcome estimated losses.

Re: advanced carb tuners: evaluate this statement

is that authot = alexander graham bell, title = "tuning guide in theory and practice"? i can't find it on amazon . . .

Re: advanced carb tuners: evaluate this statement

) Cupermcnewbster ( /

Mikuni+Polini=awesome. I really want to try it. When I phase out my current polini, I'll try it. Worst case I blow it up.

Re: advanced carb tuners: evaluate this statement

No. bigger carb = more air + more gas = less gas mileage

That being said, how many mpg are you guys getting with bigger carbs and kits?

Re: advanced carb tuners: evaluate this statement

too many numbers ahhhhhhhhh. damn u graham and ur technical equations. and scale moped models lolz.

gas mileage, i have no idea.

Re: advanced carb tuners: evaluate this statement

https://www.amazon.com/dp/B000JKT3FY/

Re: advanced carb tuners: evaluate this statement

thanks - too bad it' out of stock though

Re: advanced carb tuners: evaluate this statement

🇮🇹💦 Of the Loin /

elliot why dont you try a 16 or 17mm phbg. same tuning + smaller carb.

Re: advanced carb tuners: evaluate this statement

they say the 50v engine from a 76 mobylette puts out 1.6BHP, and goes 30mph with a lightweight rider. so in order to double that, you must multiply the horsepower by at least 6? theres an exponential decline for you.

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