Carb/airbox mod issues

[J-May]

just received my boom 125cc bike. decided the do the air box mod. involves taking the air box out and removing the inner snorkel and reducer piece. very restrictive to air flow. anyways put it all back together and go for a run and have poor acceleration, i use the choke to richen up the mixture and it gets better. anyways i reinstalled the restrictor in the air box and go for a run no issues.

Is the carb that small??? i mean the restrictor brings 1.5" tubing down to .75" or about the size of a penny. that big of a difference causes a lean condition?
seems quite useless running this tiny carb. anyone else experience these issues? my stock carb only has idle adjustment.

[JerryHawk250]

You will need to change out to a larger main jet with the increased air flow.

[J-May]

I'll likely just change it to a larger carb that isn't Chinesium then play with the jetting to get actual flow and better top end, likely play around with it when messing with the exhaust. Just didn't realize how such a little change would throw the afr so out of whack it makes it near impossible to accelerate.

[Megadan]

Originally Posted by J-May

Is the carb that small??? i mean the restrictor brings 1.5" tubing down to .75" or about the size of a penny. that big of a difference causes a lean condition?
seems quite useless running this tiny carb.

So I am going to touch on a few things since it seems like you aren't overly experienced with carb tuning.

First, restricters are often used on a "larger than necessary" carburetor to increase charge veliocity/port vacuum that helps restore low speed throttle response or to alter the power curve.

The reason why taking that restriction out of the intake tube caused you to experience what you did has a few facets to it.
1. A 1.5" diameter tube has a 1.77 square inch area for its cross section while the .75" diameter tube has a .44 square inch cross section. While the smaller pipe may be half the diameter, it has 4 times less area in the cross section. Saving a lot of math, lets just say that the flow rate over a similar length and pressure differential is roughly 3 to 5 times higher with the tube that is twice as large in diameter. This is leaving out a point I will bring up later.

2. As Jerry mentioned, by removing the restriction and thus increasing the airflow, you need to compensate for the added airflow with larger jetting. Since gasoline requires an optimum range to make the best power and fuel economy, any change in one side of the equation means a change is needed on the opposite side. You added more air, now you need more fuel. If you get the fueling dialed in you may find much of your acceleration will return.

3. The point I left out on number 1... With the increase in area, there is a reduction in velocity. There is a lot of variables at play, but in simple terms, too large of a throttle orifice (carb, throttle body, etc) causes a reduction in intake velocity. Your engine is trying to pull in X amount of air, and during the intake stroke there is a pressure differential created. In a fluid system *(air included) in order to get the same volume move through a smaller system the molecules must move at a higher velocity.

Now, with a too large carburetor like I mentioned earlier, one side effect is very slow/poor low rpm throttle response and often a "twitchy" feeling throttle. This is due to the larger area of the throttle not creating a high enough port velocity at lower engine speeds. The "twitchy" nature of the throttle is due to the sudden and immediate change in said pressure differential. The inverse of this is that the airflow is increased at higher RPM, which is why most "race" builds use much larger carburetors than their street counterparts. They optimize the carb size for where the bike is used most of the time.

Another aspect of a larger carburetor and that lower velocity is often a non-linear increase in jet sizes in both pilot and main. This is because the lower velocity of the intake air means there is less draw on the fuel circuits which causes decreased fueling. As such, an even larger than usual jet will need to be used to introduce the same amount of fuel to the same volume of air with a lower velocity.

There is a lot more involved in this than I really touched on, but it should at least give you a good idea of what is involved.

The real question is, what size is your stock carburetor bore?

[J-May]

Okay that makes sense, around the same physics as porting and polishing, intake runners, you need volume but you also need velocity to move air, velocity is even more important with no fuel injected engines, as for the stock carb size, i am not sure how to go about measuring it for sizing info.

[JerryHawk250]

Model # should be marked on the carb. Like PZ26, PZ28... or just measure the throat.

[J-May]

The Carb is different from the one pictured but says it's a Sheng Wey pz20 didn't measure anything since I didn't have the tools atm to take it apart.

[JerryHawk250]

Those pictures for examples. A PZ20 has a 20mm throat. Some of the Vader and boom guys have put larger carbs. One of them should chime in as they are more familiar with your bike.

[J-May]

yeah i think i was going to upgrade to a Mikuni 26mm carb or some other more popular carb. I plan on doing an exhaust and some slight porting/polishing to the head and cylinder chamber. the benefits would be probably unnoticeable but i like to take stuff apart and modify it for fun/hobby/learning. What better of a bike to do that on then a cheap china thumper.

[Megadan]

Originally Posted by J-May

yeah i think i was going to upgrade to a Mikuni 26mm carb or some other more popular carb. I plan on doing an exhaust and some slight porting/polishing to the head and cylinder chamber. the benefits would be probably unnoticeable but i like to take stuff apart and modify it for fun/hobby/learning. What better of a bike to do that on then a cheap china thumper.

You and I are kindred spirits lol. The better part about the cylinder head is, if you screw up, another new head is pretty darn cheap.

I can definitely rule out the "large" carb issue. A 20mm bore is a little wimpy for a 125cc engine. I would say with a matching intake setup, the PZ26 or a 26mm bore mikuni should really wake up that little motor, especially up top.

[J-May]

Exactly! tinkering is quite fun, ill be sure to post stuff up at chinariders for others to benefit from!