Best Carb to Use

[goat67]

I am thinking about changing from the stock carb on my Brozz

I seen the discussion on the pumper carb vs power jet and seems the pumper is the better of the two for our application.

Another carb that has been talked about is this:

https://www.amazon.com/gp/product/B0...f_=as_li_ss_tl

and this:

https://www.amazon.com/Carburetor-PE...f_=as_li_ss_tl

These are pretty fairly priced and I think the jets I have will work with this anybody care to offer an opinion

Thanks

[JerryHawk250]

That's the carb I'm using. It comes with extra main and pilot jets. I think it uses the same jets as your stock carb, But don't quote me on it. What size are you planning to go with? I'm running a 30mm with the ported head.

[goat67]

Thanks for the reply,

Size:
• 21 mm: 50cc to 100cc engine
• 24 mm: 50cc to 100cc engine
• 26 mm: 75cc to 125cc engine
• 28 mm: 75cc to 125cc engine
• 30 mm: 100cc to 150cc engine
• 32 mm: 125cc to 200cc engine
• 34 mm: 125cc to 250cc engine

From the info on the web site, I was thinking on going with the 30mm but maybe could go bigger?

I wish I could get a head to port but I am not sure on what one to order and I have never done it before so kinda leary on doing the port job anyway.

[JerryHawk250]

I think those recommendations are based on 2 stroke engines. 30mm would be the max I would go with but would probably perform better with the 28mm on the stock head. 28mm is the exact size of the bore. unlike the PZ carbs.

[goat67]

Originally Posted by JerryHawk250

I think those recommendations are based on 2 stroke engines. 30mm would be the max I would go with but would probably perform better with the 28mm on the stock head.

What issue would I see with the 30mm on the stock head?

[JerryHawk250]

Originally Posted by goat67

What issue would I see with the 30mm on the stock head?

The power jet works off the amount of air that flows through the carb. If the bore is to large, the stock head may not flow enough to create enough vacuum for the power jet to function properly. The carb was originally design for 2 stroke engine which flow a higher volume of air per CC. I know my stock PZ30 carb which is 28mm was a restriction once I ported the head.

[Megadan]

A 26mm carburetor - aka our stock carbs, the Mikuni, and the PZ30 Pumper carb is the carb size that offers the best balance of charectoristics - total airflow vs. velocity with the best fuel atomization and intake signal (draw) on the jets. It will make the best overall power across the full RPM range, but will sacrifice top end performance for better low RPM performance, and have a very strong mid range.

A 28mm Carb, in theory, will be the best size for shifting that power curve potential up the Rev range, giving less resistance to airflow at the top end of the RPM range, but losing a little bit of the intake signal at low RPM. Since our bikes don't really have much to offer down low, most people won't even notice or feel the slight change below the 3000rpm mark, but the added airflow at the top of the rev range should give the bike a little more pep.

The 30mm carb just follows suit, but increases the same effect. It can be run on a stock engine, but with the increase in throttle area combined with the reduced signal on the jets, throttle response and lower rpm power can be noticeably affected. This is why it is much better suited to an engine with a ported head, as they will flow more air and be able to take advantage of the larger size without much penalty.

To also clarify, the "power jet" carb is the same PWK carb. The power jet is just an add-on to the PWK carb that was designed mainly for 2 stroke use.

Also, to note, the PWK carbs being a half round flat slide, their venturi draw is different, offering a shorter run and higher intake signal at the jets with less turbulence to the air charge, thus giving better throttle response relative to throttle area.

The stock style round slide carbs, including the pumper carb, while they have a 26mm style venturi, also feature a raised roof/lowered floor venturi design, so it allows them to flow better than a standard 26mm venturi. This style of carburetor is actually more than enough to support upwards of 20hp, and will offer the best overall performance.

If I had to make a list.
Otherwise stock bike, or bolt ons = PZ20 or VM26,
"performance" oriented stock engine with bolt ons = PWK28 (power jet optional) or PZ30 Pumper.
Ported Head and bolt ons = VM30, PWK 30, but the above carbs will also work fine, but will hold the engine back a little at the top of the rev range.

As far as your question of a 32mm carb. Definitely only for a ported head. This is about as large of a carb as I would want to run, and I actually do have a 32mm carb in the wings for mine once I get done with break in. Without a different cam profile, this carb is likely too large and will suffer in the low rpm range. Throttle response will definitely be affected. I do know that CCW ran a VM32 on their land speed record bike, so it can definitely be run, but they also had a custom cam grind and head work and also revved the engine out past 9000rpm.

[Ariel Red Hunter]

Originally Posted by goat67

Thanks for the reply,

Size:
• 21 mm: 50cc to 100cc engine
• 24 mm: 50cc to 100cc engine
• 26 mm: 75cc to 125cc engine
• 28 mm: 75cc to 125cc engine
• 30 mm: 100cc to 150cc engine
• 32 mm: 125cc to 200cc engine
• 34 mm: 125cc to 250cc engine

From the info on the web site, I was thinking on going with the 30mm but maybe could go bigger?

I wish I could get a head to port but I am not sure on what one to order and I have never done it before so kinda leary on doing the port job anyway.

It is pretty easy to confuse apples with oranges based on those recommendations. Mikunis, no matter where made, diameter is measured at the venturi. Kiehins, and copies diameter is measured at the air cleaner end mouth. So, a VM26 Mikuni is equilevent to a 30 mm Kiehin...ARH

[Ariel Red Hunter]

Originally Posted by Megadan

A 26mm carburetor - aka our stock carbs, the Mikuni, and the PZ30 Pumper carb is the carb size that offers the best balance of charectoristics - total airflow vs. velocity with the best fuel atomization and intake signal (draw) on the jets. It will make the best overall power across the full RPM range, but will sacrifice top end performance for better low RPM performance, and have a very strong mid range.

A 28mm Carb, in theory, will be the best size for shifting that power curve potential up the Rev range, giving less resistance to airflow at the top end of the RPM range, but losing a little bit of the intake signal at low RPM. Since our bikes don't really have much to offer down low, most people won't even notice or feel the slight change below the 3000rpm mark, but the added airflow at the top of the rev range should give the bike a little more pep.

The 30mm carb just follows suit, but increases the same effect. It can be run on a stock engine, but with the increase in throttle area combined with the reduced signal on the jets, throttle response and lower rpm power can be noticeably affected. This is why it is much better suited to an engine with a ported head, as they will flow more air and be able to take advantage of the larger size without much penalty.

To also clarify, the "power jet" carb is the same PWK carb. The power jet is just an add-on to the PWK carb that was designed mainly for 2 stroke use.

Also, to note, the PWK carbs being a half round flat slide, their venturi draw is different, offering a shorter run and higher intake signal at the jets with less turbulence to the air charge, thus giving better throttle response relative to throttle area.

The stock style round slide carbs, including the pumper carb, while they have a 26mm style venturi, also feature a raised roof/lowered floor venturi design, so it allows them to flow better than a standard 26mm venturi. This style of carburetor is actually more than enough to support upwards of 20hp, and will offer the best overall performance.

If I had to make a list.
Otherwise stock bike, or bolt ons = PZ20 or VM26,
"performance" oriented stock engine with bolt ons = PWK28 (power jet optional) or PZ30 Pumper.
Ported Head and bolt ons = VM30, PWK 30, but the above carbs will also work fine, but will hold the engine back a little at the top of the rev range.

As far as your question of a 32mm carb. Definitely only for a ported head. This is about as large of a carb as I would want to run, and I actually do have a 32mm carb in the wings for mine once I get done with break in. Without a different cam profile, this carb is likely too large and will suffer in the low rpm range. Throttle response will definitely be affected. I do know that CCW ran a VM32 on their land speed record bike, so it can definitely be run, but they also had a custom cam grind and head work and also revved the engine out past 9000rpm.

Yes, you make the important points here. The Harley-Davidson Sprint/Aermacchi road racers used 32 mm carbs on their 10,000 rpm singles. These were five speed bikes, so they had to have a decent power range in order to be as successful as they were....ARH

[Megadan]

Originally Posted by Ariel Red Hunter

Yes, you make the important points here. The Harley-Davidson Sprint/Aermacchi road racers used 32 mm carbs on their 10,000 rpm singles. These were five speed bikes, so they had to have a decent power range in order to be as successful as they were....ARH

Or that in the days of old when race bikes ran carburetors, they would sometimes switch between larger and smaller carb bores to alter the power curve and responsiveness of the engine based on the type of track they were running, favoring smaller bore carbs for tracks with more corners and curves with short straights, and vice versa

[roundhouse]

Great info here fellas.

[goat67]

Thanks alot for the input, I think I may just order a 28mm and a 30mm total will be 44 bucks not a big deal.

[Cravin01]

My biking background is 90% off road and mostly 2 strokes, Bikes like that have the instant pop to maybe bounce over a log or if you feel the bike bog a bit as to where a 4 stroke will die when trying to react to a hard twist on the throttle when its already loaded at low rpm.

I chose the pumper carb to help in that area, at least I'm hoping it will. Where as some may choose a different style to affect their top end or overall performance more.

My goal is to improve response to maintain off road ability while I attempt to go slightly down on my rear sprocket to improve top end....we shall see!

A video on how a pump accelerator works on all motors.