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Post by geh3333 on Jan 30, 2015 5:33:14 GMT -5
What I'd like others to chime in on is about : will a bigger carb pull more fuel from a main jet then a smaller carb ? That is if the same size main jet is used in both and only at top end . This was touched on before but not in depth.
First it is said that the bigger carb will pull more fuel from the main jet at top end . another member and I had a conversation about this and I would like to hear others opinions.
Here is my theory . you have your 24mm carb with a 125 main jet and a 30mm carb with a 125 main jet. Your 30mm will be less restrictive meaning a few things . first as the engines piston is on its down stroke it creates all the vacuum needed to pull in air and fuel from the carb. Being that the 30mm carb is all round bigger at both ends and through out, the vacuum on the venturi will be weakened . however since the pilot on the 30mm is bigger and many of the 30mm carbs have an accelerator pump you should be able to get the same fuel volume at low end as you can with the smaller carb . next the 30mm carb being less restrictive should allow the piston to drop in the cylinder slightly quicker causing a slightly quicker crank rotation." This may be minimal however ,I believe enough to make a difference in performance ". So now mid range . as the butterfly opens the vacuum rises not only through the carb but at the venturi , pulling in more air through the carb and more fuel through the pilot and now the main jet as the needle rises. At this point I still believe the vacuum in the 30mm is lower then the vacuum would be in the 24mm carb. So I would expect there would be more fuel being pulled through the main of the 24mm then the 30mm , however since the 30mm has a bigger pilot all should still be quite equal when it comes to total fuel consumption. So let's go straight to top end . we now have an engine running at full capacity and pulling in most if not all fuel from the main jet . so how can the vacuum running through the 30mm carb be enough to pull more fuel from the main 125jet ? Well at this point as you have combustion happening in the chamber the piston is dropping quick enough in the cylinder to exceed the vacuum created by the smaller 24mm carb. Just like sucking the same volume of air and fuel through a straw very slowly or really fast , sucking in faster creates a stronger suction on the venturi then a slower weaker suction.
Another topic that relates to the above would be fuel capacity relating to the combustion chamber. How much air and fuel can the piston pull in before and during ignition ? This all depends on a few things like bore size, head size , cam and valve lash. The longer the valves stay open on the down stroke the more air and fuel will be let in and burned during the combustion period. You would think " if the air fuel is set correctly and u are burning fuel properly shouldn't the piston pull in the same volume of air and fuel no matter what ? " I believe it is possible while using a bigger carb and while at top end while suction may exceed the smaller carb there is an opportunity for slightly more burnable fuel and air to enter the chamber , possibly as the valves are opening and closing. The problem with proving or disproving this is that no body here has the equipment to do those kinds of tests so this will remain undetermined for awhile.
Any thoughts or opinions are welcome and wanted.
Thanks George.
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Post by lain on Jan 30, 2015 8:29:19 GMT -5
I'm no pro, but just putting my pennies in here.
I would think that a larger carb would pull the same amount of fuel through the jet, however I think while it would pull the same amount it would be more in volume since the body is bigger, but the stream of fuel would remain the same as the main jet isn't any bigger and cannot change it's shape to allow more fuel.
Again, I'm no pro, just going on what I remember of what I learned in science class a decade ago. Not that it is the same thing but I think that because I have watched the flow of water in a larger container and a small container both with the same size hole both flow at the same amount however the larger container forced out more with each pump due to the volume behind it adding pressure and pushing it, and it was plastic so it could bend so not a good example but yeah just thinking...
I wish we had a large scale replica of parts like carbs to explore the function of them visually like with car's parts in the science museum.
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Post by dmartin95 on Jan 30, 2015 9:05:09 GMT -5
What I'd like others to chime in on is about : will a bigger carb pull more fuel from a main jet then a smaller carb ? That is if the same size main jet is used in both and only at top end . This was touched on before but not in depth. I too have wondered about this... In fact, I think you and I have briefly touched on this subject in the past. I purchased a 30mm Hoca Carb to go on my 180cc power kit I bought from PFS. I figured worst case scenario I would decrease the Jet size but I've had a couple of people tell me the carb is going to be too big. 250cc's come with 30mm carbs.... However, I don't know stock jet sizes.... On a side note, I have pulled the float bowl off of my Hoca 30mm carb and I can not identify the size of the main Jet. The idle is a 40, but they also included a 38 and a 42. Has anyone else purchased this carb and do you know what size the main jet is? Hoca 30mm
on a side note, the 30mm carb is listed for 125/150cc's.... Not 250cc's.  |
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Post by ricardoguitars on Jan 30, 2015 9:18:17 GMT -5
My assumption is, bigger carb means more air, so you need more fuel to make it work properly, which requires a fuel pump with more pressure, I believe that if you have one 24mm carb and a 30mm carb, bot with the same fuel pump and the same main jet, you will be having troubles with the bigger carb because it would be running lean, you need more fuel pressure and/or a bigger jet.
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Post by rcq92130 on Jan 30, 2015 9:47:54 GMT -5
1. Forget the fuel pump being part of this. Fuel is not under pressure in the carb; the only task of the fuel pump is to keep the bowl filled. Fuel is SUCKED into the air stream, not pushed in.
2. All engines have a maximum amount of air/fuel they can inhale. Even with a mile wide opening this max. remains. The maximum can be increased with a different cam, bigger valves, bigger piston. But once you fix the configuration of the engine you have a new maximum that remains the same as long as the carb opening is big enough to not get in the way.
3. We do not know if a stock carb is big enough to satisfy all the fuel/air an improved 150cc GY6 demanded. Some experts claim "Yes; big enough", and become angry if anyone does not shut up. Others have experience they say implies a larger carb. opening can give more performance, which by definition means the smaller carb. had been not big enough. But we do not know, so a discussion like this is called for.
4. Imagine using the SAME engine configuration, and comparing a 24mm carb and a 30mm card (again, connected to identical engines under identical load conditions). Same conditions ===> the exact same volume of fuel/air is being processed thru the 2 engines and thus through the 2 carbs. The larger carb, with a larger area venturi section, will be passing that volume at lower airflow speed inside the carb. Fuel is SUCKED up the jet, past the needle, by the vacuum the airflow creates in the venturi. Slower speed ===> less vacuum ===> less fuel sucked into the airstream. THUS, IF THE SAME SIZE JET IS USED THE LARGER CARB WILL BY DEFINITION PRODUCE A MORE LEAN AIR/FUEL MIXTURE. This is why it's necessary to upjet when using a larger carb.
5. Using a properly selected bigger main jet of the right size in the bigger carb will produce exactly the same air/fuel mixture as a smaller jet in the smaller carb. There is no reason a larger carb, properly jetted, will run any richer or leaner. Period.
6. The idle jet satisfies airflow fuel needed at idle and should be properly selected to get an optimum idle. At higher RPMs the main should be adjusted for correct a/f mixture. Once the correct idle jet is selected, it should not greatly contribute to what is needed for a main jet.
7. A 30mm carb may, in fact, be capable of supplying more air/fuel than even a BBK'ed GY6 can utilize. Presuming the larger carb is set up properly, this simply means opening the throttle further will NOT DO ANYTHING. The engine is already gulping as much as it can gulp; a larger opening (butterfly opened more) will not produce results. HOWEVER, IT ALSO WILL NOT CHANGE THE AIR/FUEL RATIO THE PROPERLY JETTED 30MM CARB CREATES. This is because - after the engine is gulping all it can - further opening the throttle will NOT increase airflow thru the carb NOR THE SPEED OF AIR IN THE VENTURI (which does not change size just because the butterfly is opened), so same airflow rate ===> same vacuum in the venturi ===> same amount of fuel sucked into the airstream. A properly tuned "too big" carb will simply have a throttle above which you will see no engine increase. It should NOT cause any engine DECREASE if it is properly tuned. In other words - the larger carb - if TOO large - will simply have unusable reserve capacity.
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Post by geh3333 on Jan 30, 2015 10:44:00 GMT -5
That's a good point , about the throttle cutoff. I still have throttle response at the last 1/4 turn of the throttle even with a 32mm carb with a 125 main jet and I'm assuming a 40 pilot and possibly a 42.
I'm really interested in knowin if my theory about the crank rotation is true . I would imagine less restriction from less resistance on the crank as it is in down stroke " from the bigger carb ". Will cause the theory of more fuel from the main jet at top end to hold water . I really wish we could find a way to test this.
All I know is most if not all that I've talked to with a bigger carb has noticed gains in performance. So something has to be going on .
The only thing that bothers me is when one completely ignores the results of using a bigger carb , especially when they have not yet even tried any other carb then stock. Opinions are wanted and will be very well received , but hateful comment will not be tolerated.
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Post by geh3333 on Jan 30, 2015 11:03:15 GMT -5
What I'd like others to chime in on is about : will a bigger carb pull more fuel from a main jet then a smaller carb ? That is if the same size main jet is used in both and only at top end . This was touched on before but not in depth. I too have wondered about this... In fact, I think you and I have briefly touched on this subject in the past. I purchased a 30mm Hoca Carb to go on my 180cc power kit I bought from PFS. I figured worst case scenario I would decrease the Jet size but I've had a couple of people tell me the carb is going to be too big. 250cc's come with 30mm carbs.... However, I don't know stock jet sizes.... On a side note, I have pulled the float bowl off of my Hoca 30mm carb and I can not identify the size of the main Jet. The idle is a 40, but they also included a 38 and a 42. Has anyone else purchased this carb and do you know what size the main jet is? Hoca 30mm
on a side note, the 30mm carb is listed for 125/150cc's.... Not 250cc's. First off the carb u bought will not be too big . this is a proven fact. So let us know how it goes. I would start with a 125 main and go from there . also use the 40 pilot and do the same " see how it goes and adjust if necessary. |
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Post by geh3333 on Jan 30, 2015 11:08:32 GMT -5
This thread is not to argue whether a 28-32mm carb is too big or not. It is a proven fact they are not too big . I and many on here have proven that already by actually using them with great results. This thread it to try to see how a bigger carb works compared to a smaller carb with certain jets , and why we think it works in this fashion.
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Post by dmartin95 on Jan 30, 2015 11:55:21 GMT -5
First off the carb u bought will not be too big . this is a proven fact. So let us know how it goes. I would start with a 125 main and go from there . also use the 40 pilot and do the same " see how it goes and adjust if necessary. My thing is, I don't know what Jet I'm starting out with.... I just had a Chat with Dexter from Parts for scooters and he's gonna get the info for me... I can't believe they (Hoca) didn't stamp this jet. On a side note, Parts for scooters IMO is the BEST place to get parts from... They offered to send me any jet I wanted.... I declined of course because I don't know what I'm starting out with... |
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Post by JoeyBee on Jan 30, 2015 12:06:51 GMT -5
My thing is, I don't know what Jet I'm starting out with.... I just had a Chat with Dexter from Parts for scooters and he's gonna get the info for me... I can't believe they (Hoca) didn't stamp this jet. On a side note, Parts for scooters IMO is the BEST place to get parts from... They offered to send me any jet I wanted.... I declined of course because I don't know what I'm starting out with... Do you have a set of micro drill bits? I had the same problem with an unstamped main jet from a carburetor from Scrappy Dogs. Using those I was able to determine the jet was probably a 100. Interestingly enough, after testing and tuning with my wideband O2 sensor, my 150cc with unifilter consistently rides at 13.0 air/fuel ratio with a 104 main jet. But that's a write-up for another day. |
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Post by JerryScript on Jan 30, 2015 20:06:57 GMT -5
These carbs are not magical. They cannot force more fuel through the same size jet just because the carb's openings are larger. Fuel vapors compress, liquid fuel doesn't. You cannot draw more liquid fuel through the same size jet.
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Post by rcq92130 on Jan 30, 2015 20:40:00 GMT -5
If the old (smaller) carb was adequate the new (bigger) carb will not necessarily cause more air into the engine. The engine will only gulp the amount of air it wants.
but the SAME airflow in a bigger carb means lower velocity through the bigger venturi - the lower airflow speed will suck less fuel past the needle and cause a lean condition. So with a bigger carb you are almost guaranteed to need a bigger jet.
WITH the right sized jet the mixture will be the same as with the smaller carb. The theory a too-large carb MUST produce a too rich or too-lean mixture seems false.
The upside of a larger carb is if your engine really COULD inhale more air/fuel, the larger carb will satisfy the engine with less resistance.
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Post by jerseyboy on Jan 30, 2015 23:15:35 GMT -5
First of all Geh we have to see how much fuel and air these GY6 motors will handle before they choke,,with bigger valves,ported heads,cam and all there is no question,,but we have to find out if a stock motor will benefit from a bigger carb first,,and if that is true then yes a bigger carb is an upgrade..also would there be a limit to how big of a jet a smaller carb will take before it doesn't have enough air to mix correctly.
Lots of testing needs to be done on this subject...Im going to experiment with my stock carb and see how it performs compared when I get some time..
Plus if the smaller carb where to pull more fuel up the main it would need more air to burn the fuel,,so my thoughts are yes there will be a limit,,and Im sure its around the 125-130 range before it starts choking rich.Also wonder how much a 1" exhaust will take before it starts having an restrictive effect on the air coming in,,if the air cant get out fast enough,,more air wont come in...
You may be able to draw more fuel through the same size jet if you have more vacuum,,just like more pressure behind a garden hose causes it to squirt farther with the same size nozzle.
Very interesting thread bro...will be fun to follow..
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Post by JerryScript on Jan 31, 2015 1:16:33 GMT -5
....You may be able to draw more fuel through the same size jet if you have more vacuum,,just like more pressure behind a garden hose causes it to squirt farther with the same size nozzle. Very interesting thread bro...will be fun to follow.. Larger carbs on the same size intake port will create a smaller less powerful vacuum, I'm pretty sure we had a thread about that recently. It's the same amount of air being pulled by the engine, so a lager orifice results in a smaller vacuum effect. Looking forward to hearing about your results, I love being proven wrong when real world experiments work out, forces me to learn something!  |
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Post by geh3333 on Jan 31, 2015 8:16:08 GMT -5
I wish there was a way to measure the volume of fuel and air being sucked into the chamber to see if there is any change at certain rpms even if it is minimal.
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