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Post by dmartin95 on Nov 13, 2014 21:34:37 GMT -5
I've been considering doing a BBK and I'm way out of my depth.... I'm pretty handy with a wrench and have a full shop of mechanics tools. I have been trying to learn the in's and out's and now I'm on to carburetors.
My scooters engine is currently stock. It's a GY6 150cc. I believe I have a 24mm cvk carb. It's one of those "tamper resistant" so I'm going to have bust out the easy-outs.... However, when (or I should say if) I do a BBK, I would like to use my existing carb if possible. It's a high quality Deni (standard of Japan) and no reason to get a bigger carb if I can use my current carb. Plus i'm trying to cut cost wherever I can as this upgrade is going to be expensive.
So, this is my question(s):
What is the largest cc you can use a 24mm cvk on and what size jets would you use?
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Post by geh3333 on Nov 13, 2014 22:08:56 GMT -5
I've been considering doing a BBK and I'm way out of my depth.... I'm pretty handy with a wrench and have a full shop of mechanics tools. I have been trying to learn the in's and out's and now I'm on to carburetors. My scooters engine is currently stock. It's a GY6 150cc. I believe I have a 24mm cvk carb. It's one of those "tamper resistant" so I'm going to have bust out the easy-outs.... However, when (or I should say if) I do a BBK, I would like to use my existing carb if possible. It's a high quality Deni (standard of Japan) and no reason to get a bigger carb if I can use my current carb. Plus i'm trying to cut cost wherever I can as this upgrade is going to be expensive. So, this is my question(s): What is the largest cc you can use a 24mm cvk on and what size jets would you use?U should be able to use the stock carb with most of the bbk's . when it comes to the 67mm bbk The 24mm carb may be ineficient. I'm not sure if the carb would be able to pull enough air to match the fuel needed to not run to rich . however this all depends on many factors such as the head , cam,exhaust and type of air filter being used . If you have questions about carb sizes that can be used with the different size bbk's just let us know . for example I have a 58.5bbk with the other mods listed below . as u can see I use a 32mm carb with a 125 main jet . the reason some of us use a bigger carb is that it increases the HP and we are able to get the most out of are mods . that doesn't mean u have to use a bigger carb but if you are looking to get the most out of your engine and mods its a necessity. I could have went with a 30 mm carb and a 130 main jet and had the same results .There are many on here now that are using bigger carbs so u should be able to get a sense of what size carb and jets to use with whatever bbk u decide on . |
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Post by dmartin95 on Nov 13, 2014 22:55:23 GMT -5
Ok, can you explain that??? I thought the whole purpose of "upjetting" was to increase the air/fuel flow... Now, I realize there will be a limit to the amount you can increase a 24mm cvk, but until you reach that limit, how would a larger carb help?... I.E., If I did a bbk that brought the cc's up to 175, it would have a min/max of fuel/air it would use.. If my 24mm cvk was capable of delivering the required amount of fuel, how would a bigger carb help produce more HP? |
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Post by ccr on Nov 14, 2014 2:54:16 GMT -5
a smaller carb will provide a faster start with a good low speed throttle response due to its higher flow velocity.a bigger carb will provide a slower start with a good mid range and high speed throttle response and allow you to reach higher horsepower levels on a BBK or even some stock setups. however, going too big will destroy your off the line acceleration to the point where you would never reach your top speed. the flow velocity would not be able to pull enough fuel from the jet no matter how big it is, and the fuel that does mix with the air will not atomize with the air causing an uneven and unstable burn. choosing the carb will depend on 3 main factors. 1, how fast you want to go / how fast you want to reach that speed.
2, how high your rpm settings are and what your gear settings are.
and finally 3, what the ambient temperature of the intake air is. (in a hot country there is less air density and a bigger carb is needed to compensate for the lack of cold dense air) so if your driving in the city and you only get 2 blocks before you have to stop at the next set of lights, you should get a smaller carb, because your scooter needs to get up and go quickly. theres no point having a scooter that can go 200km/h but takes 5 minutes to reach that speed. however if you are driving longer distances and you have a lot of time between stops you can use a bigger carb to achievea higher top speed. how long are you prepared to wait for it? another thing is engine rpm. if you install a bbk and use an original crank, that makes it more of a high rpm build. because the bore is bigger than the stroke the piston does not have to travel so far between revolutions. this means a higher rpm can be achieved and it can accelerate through that rpm faster. if you lighten the variator weights and use a bigger carb, this will allow you to rev higher and the time required for the engine to get up to its ideal flow velocity will be reduced. for example my stock 100cc yamaha engine came with a cvk24it was pretty slow to reach a top speed and not very fast off the line. topped out around 95km/hso i changed to a cvk30, big mistake on my part. i got an even slower start and only a little gain in top speed. 105km/hthen i upgraded the engine to 128cc. and it was faster off the line and had a much better top speed. something like 108km/h. after that my friend (ex racing champion and racing bike builder) suggested i use a smaller carb and i was reluctant to at first. but after trying it on and a quick rip around the block..  it was like the beast had been set free finally and all the other mods i had made seemed to have finally paid off. i took it out of the city and hit it hard! KEHIN CVK26it went off the clocks at 120km/h and kept going i can only guess it topped out at 125km/hso the moral of the story is, start small and work your way up. test as many sizes as you can before you buy one. i'm sure if you know someone with a keen interest in bikes they will have a few laying around that you can try out. if not then maybe a local junk yard can help you out |
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Post by ccr on Nov 14, 2014 2:58:46 GMT -5
oh and as a straight answer to your question, about 125cc would give the maximum high speed performance gains from cvk24.
but a cvk could give a lot better and more balanced performance.
if your manifold won't take a cvk 26 you can have your carb machined to fit like mine was.
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Post by ccr on Nov 14, 2014 3:03:19 GMT -5
I.E., If I did a bbk that brought the cc's up to 175, it would have a min/max of fuel/air it would use.. If my 24mm cvk was capable of delivering the required amount of fuel, how would a bigger carb help produce more HP? it would give you the horsepower sooner you wouldn't vacuum your house with a straw because it would take too long just like the air flow would be restricted on your small carb |
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Post by geh3333 on Nov 14, 2014 9:40:24 GMT -5
Ok, can you explain that??? I thought the whole purpose of "upjetting" was to increase the air/fuel flow... Now, I realize there will be a limit to the amount you can increase a 24mm cvk, but until you reach that limit, how would a larger carb help?... I.E., If I did a bbk that brought the cc's up to 175, it would have a min/max of fuel/air it would use.. If my 24mm cvk was capable of delivering the required amount of fuel, how would a bigger carb help produce more HP? When u upjet you are increasing the amount of fuel not air . if one up jets they normally should use a less restrictive air filter and a high clflow exhaust to allow for more airflow . many with a 172cc bbk have used a 30mm carb with a 125 main jet . the 30mm carb will pull more fuel through the 125 main jet then the 24mm carb is able to . at the same time the 30 mm carb is pulling more air then the 24mm . this all depends on how much air and fuel the engine can take . over the past few years I have come to find that these engines can take and burn more fuel and air then previously thought " without running to rich" . this is based on the trial an error of many members on here . you can slightly up jet a stock 150cc without upgrading the air filter or exhaust due to most are running lean at purchase . if I were to use a 24mm carb and try to get the same results I have with the 32mm I would find that first I would have to use something closer to a 140 main jet to be able to pull enough fuel to match the 125 in the 32mm carb , and then I would find that my scoot would bog and probably not even start due to the 24mm not being able to match the airflow of the 32mm carb and in turn be running way to rich . When it comes to the takeoff and top end with a bigger carb , I has not been any noticeable changes other then a much stronger running engine with noticeable gains through out the throttle range . it is true that the bigger carb will pull more fuel through mid and top range through the main jet then a smaller carb will , and when it comes to the bottom end many of the bigger carbs have an accelerator pump to allow for more fuel at takeoff . I hope this helps |
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Post by dmartin95 on Nov 14, 2014 12:20:55 GMT -5
Hey guys, thanks for the replies! A lot of informative stuff for me to digest.
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Post by geh3333 on Nov 14, 2014 13:07:56 GMT -5
Hey guys, thanks for the replies! A lot of informative stuff for me to digest. No prob , that's what we are here for . |
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Post by geh3333 on Nov 14, 2014 13:18:08 GMT -5
Many probably could have saved a lot of cash by staying with the stock bore and only purchasing a high flow exhaust , uni filter and a 28mm carb with a 117 main jet . also a little CVT tuning to reach a true 60+mph . that is with the 150's
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Post by tvnacman on Nov 14, 2014 18:52:27 GMT -5
well I have a stock 150cc with a pod air filter and free flow exhaust , I use it in the cold weather so to aid in cold start ups I installed a 40 pilot jet I get some engine break up in the upper rpm about 7k I have a 115 main jet I was thinking of dropping in the 120 or 125 main jet and see what happens , I'm using a Keihin 24mm CVK . What are your thoughts ?
John
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Post by geh3333 on Nov 14, 2014 20:01:09 GMT -5
well I have a stock 150cc with a pod air filter and free flow exhaust , I use it in the cold weather so to aid in cold start ups I installed a 40 pilot jet I get some engine break up in the upper rpm about 7k I have a 115 main jet I was thinking of dropping in the 120 or 125 main jet and see what happens , I'm using a Keihin 24mm CVK . What are your thoughts ? John I would like to see the results with a 26mm carb and the 115 main . the 26mm will pull more fuel through the 115 then the 24mm carb. I would stick with whatever pilot that the 26mm carb comes with . a 125 main with the 24 mm carb will prob be to rich for the stock bore and stock head . but the extra air flow the 26mm carb will allow should match the extra fuel it will pull with the 115. |
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Post by geh3333 on Nov 14, 2014 20:10:50 GMT -5
I'd love to be able to ride all year round . I'm really considering buying a heated jacket and gloves. They have some that use rechargeable battery packs . some are very expensive but I saw a kobalt jacket for around 100 bucks after shipping , and a cheap set of gloves on eBay .
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Post by tvnacman on Nov 14, 2014 23:15:12 GMT -5
I'd love to be able to ride all year round . I'm really considering buying a heated jacket and gloves. They have some that use rechargeable battery packs . some are very expensive but I saw a kobalt jacket for around 100 bucks after shipping , and a cheap set of gloves on eBay . I bought the gerbing heated gloves , when not using the electric heat they are still the best pair of gloves I have ever owned . John |
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Post by geh3333 on Nov 17, 2014 18:08:57 GMT -5
I went for a ride sat morning around 9:30 , it was around 36degrees out. I had a long sleeve thermal with a T-shirt then a hooded sweatshirt and then I had a wind breaker with an ecko vest that is also sort of a windbreaker on . I was surprised how warm I was , I also had a pair of neumans"football gloves" on with cheap cotton gloves under them , but it was actually pretty comfortable. I still like to have the heated jacket and gloves for when it gets in the teens.
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