72cc bbk

[gabito503]

Does the 150 carb works good in the 72cc kit?

[diynuke]

May 30, 2018 21:46:21 GMT -5 gabito503 said:

Does the 150 carb works good in the 72cc kit?

no absolutely not.   its way too big for such a small engine   so the fuel air mix won't mix properly so it will run like crap.
stock carb is good.

[humanshield]

I know a guy who has a scooter repair business and this guy builds and races a lot of vehicles.
He knows his stuff. After many conversations with him that I can say.
He's ALWAYS insisted that a 50cc scooter with a 100cc BBK will run MUCH faster with a 150cc carb.

I have yet to try it.

Seems to me it would be too big, especially for a 72cc kit. But then, it depends on how much air the motor can flow and the RPM capability. If the head is worked and the motor can flow enough air, the cc's are not as important as the total volume of air flow. Some say the carbs on scooters are intentionally smaller than they can handle.

[benji]

Carb size is relative to speed and volume of air. Typically larger motors and higher revving/performance motors (like dirt bike motors) require larger carburetors than smaller or lower revving/low performance motors. There is a chart someone made that shows "optimum" carb size vs. hp output, which seems like a better way to choose than going by engine size. A stock 90cc scooter/atv motor comes with a 14mm carb, and a motor off a stock kx65 (65cc) comes with a 24mm round slide carb. The difference between those two motors is power and rpm levels.

[90GTVert]

Some of the big bore engines can benefit from big carbs and some can't. I would venture to say that you will get little more than an increase in tuning difficulty from a 24mm carb on a typical build with a 47mm bore.

More displacement, more RPM, larger valves and ports are the things that will usually make a large carb more necessary/successful.

I think 18-20mm works fine for most 72cc engines. Plus, if you have a stock head and put on a gigantic 150 intake, you've got quite a mismatch. You can match it up, but I don't think it's ideal by any means.

You can read about theories and formulas for inlet tracts and carburetion from books like Four-Stroke Performance Tuning by A.G. Bell or How to Port & Flow Test Cylinder Heads by David Vizard or Performance Automotive Engine Math by John Baechtel. Been a bit since I read those, but I'm quite sure none said to put the biggest carb and intake you can fit in place as a general rule. Most of the stuff that they go into is beyond what we'd really want to get into anyway and is so hard to work with in tiny little ports.

If the head is stock, think of the big carb and intake like this... Imagine you have a glass of water and a straw. Do you think it would do you any good to have a standard straw in the glass that balloons out at the top? It would probably take a fraction of a second longer to draw fluid up, because of it's increased volume, but the bottleneck of the smaller section wouldn't let it flow any more than if the whole straw were that uniform size. Remember that small changes in diameter may be large changes in area... 20mm circle = 314mm2 while a 24mm circle = 452mm2. So you've got just an 20% increase if you think about 20mm vs 24mm, but examining the area you end up with a 44% increase.

[GrumpyUnk]

90:Remember that small changes in diameter may be large changes in area... 20mm circle = 314mm2 while a 24mm circle = 452mm2. So you've got just an 20% increase if you think about 20mm vs 24mm, but examining the area you end up with a 44% increase.

Add in the stroke... and the volume change becomes a Wow. 39mm to 44mm to 47mm ... 49cc 63cc 72cc. Almost but not quite double the displacement with a 47. If the connecting rod lives through that, pity the 50mm/52mm rods for their sacrifice to the HP/Torque gods/goddesses... Hard to see how they can have a good life, being squished heavily and then stretched by more mass being decelerate... Use good oil.
tom

Oh, back on topic, too much carb can be worse than too little carb.  In the latter it will limit power, but should run fine.  The former may not run for ... well, may not run well at all, no matter the rpm range.  Stumble, hesitation, bogging, chugging, misfire, and more can be attributed to an oversized carb.  The air flow through the venturi might just not be enough for good atomization and mixture, and that leads to poor economy and low power.  IOW, it just ain't right...
tom

[fugaziiv]

When doing our in house testing, we've found that the 24mm stock GY6 carb (102 jet) works well on a 52mm kit with or without a stroker. With a 50mm kit, you can use the 24, but it's a bit large and you may need to jet down. On anything smaller, the 24mm GY6 CVK is a bit too much carb for the application.

Keep in mind that in all of our testing, any build 50mm or larger will be installed with aftermarket heads. We don't recommend stock heads for use on 50mm or larger applications.


Matt