Mike Cyanide
Well-Known Member
When do step up to a bigger carb? The phantom come with a regular bofeng so that's kinda my bench mark. In not one of those guys that just buys stuff. I wanna know when it's appropriate. Plz and ty
When is it time for a bigger carb?
- Thread starter Mike Cyanide
- Start date
I have seen zero reason to "step up" to another carb with either of my Phantoms...It already goes faster than it needs to be stock straight outta the box, your strapping a motor onto a bicycle, just how much faster over 50 to 55 MPH do we really need to go???
If you kept going that fast on a regular bicycle, it will not stay in one piece very long at all.
If you kept going that fast on a regular bicycle, it will not stay in one piece very long at all.
Gordy
Well-Known Member
When or why, I don't know, but I've heard a tillitson saw carb is a good option.
Mike Cyanide
Well-Known Member
Agreed,however I see bigger carbs alot.just curious why and what brought them there. Further more what are the benefits? Ii would enjoy more intricate tuning features but I'm not out to throw away money. Lol I guess I'm asking why ppl do it.
Mike Cyanide
Well-Known Member
My priority is 100$ functionality can anyone point me to a guy with a bigger carb and his reasoning?
Are we talking the Phantom 85 specifically, or just in general?
Mike Cyanide
Well-Known Member
Just in general. U wanna know why. I understand the concept when it comes to larger 4 strokes but not sure here.
It will depend a lot on the port timing numbers and the rest of the setup. There are some basic carb size ranges that work best for different displacements and the VE of the engine. For an 85cc engine, generally a 18-22mm carb is considered the most ideal for output, but the intake and port itself on the Phantom is smaller than it is on a Stihl 046 saw, which is what the Phantom is based off of. The Stihl carb is 20mm, the Bofeng on the Phantom is 14mm. The Stihl makes 7hp and the Phantom makes 5. If you open up that intake port and have an intake capable of the airflow, it's reasonable to assume you could pick up a couple of HP with some simple port work and said bigger carb.
The other end the carb size spectrum has to do with "ride ability" and how the engine behaves. On a race engine, idle and part throttle performance take a back seat to all out power. You can put a 22mm carb on a Phantom, and in theory it would be great for all out power but would be a pain to live with day to day compared to an 18mm carb that may leave a half a HP or so on the table, but will have great idle and part throttle behavior while also still delivering excellent power up top.
As far as how big to go. With a dyno of the engines power output you could calculate the VE of the engine, and with the engine displacement and RPM capability figure out the maximum CFM the engine is capable of, and then match a carb to it. This would be the "best of both worlds" in terms of performance. It's a lot of work and math with a ton of variables involved.
One thing is for sure, when you get "too big" you will know. The idle will never be steady and the jetting becomes super sensitive to Density Altitude changes. You will also experience what essentially feels like a dead spot at the end of the throttle sweep past 3/4 throttle. Aka, you open the throttle, but it doesn't feel any different.
As far as knowing "when", it will really depend on the engine. An engine like the Phantom that is purposely restricted - any time is a good time if power is what you desire. For a generic 66cc engine or something like an LD/YD100 it will really come down to what other upgrades are done. Their port figures are very tame, so going too much bigger than about 18mm is a bit of a waste on it's own. Now, do some port work and add a pipe to that same engine, then the increased airflow from the carb will help build off of and support those other upgrades.
Mike Cyanide
Well-Known Member
Thanks man, you answer my question perfectly. Essentially it is the same reasoning on a smaller and adjusted scale. Excellent answer. I'm believe the skill is still slight beyond my grasp here but quickly approaching. Again very well put now I just need to read alittle about the flow characteristics of these motors and theory it self but you certainly spelled it out for me.
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