How much power can a stock PZ30 feed ??

[Bill Hilly]

My Hawk had the VM26 clone when I got it, and it was fine, but I never could get it's float needle to completely stop gas flow, and since I had my old PZ30 that was on my TBR7, that fortunately came stock with a 36 pilot jet. I had already been running the carb on my TBR7 with full aftermarket exhaust, and pod filter .I already had the needle shimmed, and it has a 115 main ( maybe a 120). It runs absolutely perfect on my Hawk, which also has a pod filter, and full exhaust, but is otherwise stock. I bought one of those 70mm cylinder kits, that are supposed to fit without machine work. I also plan to port the head, and probably use a longer duration cam that I bought from eBay about a year ago. I will probably also be using a 4 degree advance key as well. Will I need to go with a bigger carb ? , and if so, should I go with something bigger than the Nibbi PE28? Also I don't know if I should do the build on my 2016 Hawk that I have on the road, which has around 3,000 miles on it ( I bought it a year ago, and it had almost no miles), OR if I should do the build to the motor of my 2020 Hawk that I bought for potential parts. It only has 400 miles, and seems to run good, BUT it has been abused probably during all 400 miles. I put a rear tube in it, and the cush bushings are already worn out on it. It's just been used as a dirt bike. I took it down the road, and all the gears work, but I know some years of Hawks had transmission issues. Any advice I get is appreciated.

[Deckard_Cain]

I'd do the work to teh higher mileage engine that was treated better. 3k cared for miles beat 400 abused miles any day.

The pz30 will run on the engine with all the upgrades, but seriously choke it off. I wouldn't consider it a viable option.

My engine, with the cg300 cylinder, ported head, cam, thin head gasket, full exhaust, timing advance, the whole nine yards... I ran a pe28 and it was great. I have my pe30 on it right now, and it is snappier, and makes more peak power at the cost of smoother power delivery.

A choked down venturi bore PZ carb is going to neuter all your hard work on this engine.

[Bill Hilly]

Originally Posted by Deckard_Cain

I'd do the work to teh higher mileage engine that was treated better. 3k cared for miles beat 400 abused miles any day.

The pz30 will run on the engine with all the upgrades, but seriously choke it off. I wouldn't consider it a viable option.

My engine, with the cg300 cylinder, ported head, cam, thin head gasket, full exhaust, timing advance, the whole nine yards... I ran a pe28 and it was great. I have my pe30 on it right now, and it is snappier, and makes more peak power at the cost of smoother power delivery.

A choked down venturi bore PZ carb is going to neuter all your hard work on this engine.

Thanks. I put a Nibbi PE28FL, and pod filter on my TBR7, which at the time only had an aftermarket exhaust, and thought that it seemed like it made it run higher rpm more freely, but it seemed like my Hawk always responded to the cheaper version of the same mods , just as well . I now have a full aftermarket exhaust on the Hawk, because the parts bike had it , but before that, I had removed the cat, and baffle, and later drilled holes in the plate up inside the muffler, and ran it with the properly jetted VM26, as well as the properly jetted PZ30 that was stock on the TBR7. I had settled on 42/16 sprockets on the TBR7, and 46/16 on the Hawk, and both bikes ran close enough to one another that I couldn't tell you which ran better. I later home ported the head, and installed a 4 degree advance key on the TBR7 , and it was then the more powerful of the two, but I still liked my Hawk better, and it seemed like after the key, and porting on the TBR7, I started having little issues, like it seemed like my battery was no longer sufficient, and then I experienced some issue with the coil, or ground. I replaced the battery, and then the coil ,and it was fine, but I still ran the Hawk more, because it was better in gravel, and dirt. I later wrecked the TBR7, in gravel. I just didn't like it as much as the Hawk, and so when I made up my mind that I was going to get a new bike to handle road speed, and distance easier, I chose to sell the 7, and keep the Hawk. I am actually wondering why I am even doing these upcoming mods to the Hawk, because it runs good enough for the lion's share of what I need. My biggest compliant is that at road speed, it's already high enough in rpm that it's overtop of it's power band. I can hit a long gradual hill at 55, or so, and it will start loosing steam, until it gets to about 45, and then it's not loosing, or gaining, just flat, and full throttle. I had 17 front sprockets on both bikes, and with my weight, and fact that a lot of the fast roads around here are hilly, it was just too tall with the 46 on the Hawk, and 42 on the 7. I hope that these mods will allow me to re install the 17 front sprocket, and do better on the main roads. I really don't know what duration my cam that I plan to run is, but I think it looks like it's got a thickener lobe than from what I have seen of the stock ones on videos of people replacing them. I ordered it from eBay, along with the roller followers, but I am afraid of the potential problems that the low quality followers may cause. The cam is on a gear, but there's no timing mark on the gear, but I think I can just count teeth from the cam's pin, and make my own mark. The cam ended up being free, because it was evidently lost for such a long time that they refunded my money, and then ,like a month later the cam, and followers show up. I figure even if it's not a 292 duration, it's at least going to bring my power up some at higher rpm. I plan on sprucing the Hawk up some as well. I don't really mind the flat black, but after seeing all the things AliExpress sells, I decided to paint it medium to dark grey, and do a few things here, and there.

[Discoveror]

Carburetor air flow is designated in cfm (cubic feet per minute); rather than power.

Before getting carried away with 'more is better', be mindful (of the Bernoulli equation) - that air-flow speed at a given pressure etc. increases through a narrower carburetor venturi ... and visa versa; air flow speed decreases through a larger venturi ...

The result of this is that larger carburetor throats (e.g. 30 mm vs. 28 mm) experience slower air flow (cfm) albeit larger air volume flowed, resulting in slower throttle response (less snappy) and, likely, less torque, especially at lower rpm.

Presumably, the factory engineers know how many cfm their engine can flow, given valves size, number, camshaft profile, compression, rpm etc. etc. etc. And, factory engineers have access to air-flow and power measuring equipment that (most) riders lack. I wouldn't second-guess the factory carburetor throat size choices ... unless building an engine with far greater air flow capabilities.

Those narrower air filter box plastic pipes, which so many folks are fond of removing are there to SPEED up air flow and help low rpm torque (nice for low speed rock and rough terrain crawling etc.).

Otherwise, a lethargic throttle response is likely, especially at lower rpm.

[Bill Hilly]

Yeah, I know that many people tend to over carb the bikes, and that's the reason that I didn't ask this same question of the Facebook pages. I'm not painting everyone on the Facebook groups with the same brush, but there are a lot of them that seem to think that slapping a PE30 on a stock bike somehow unlocks incredible HP, and how the bike went from barely hitting 50 mph to popping power wheelies in 3rd gear. I will be gaining about 20 cc, and possibly a little compression, if the kit comes with a thin head gasket. I will also do some porting on the head. I don't know what the specs of the cam I plan on using are. I am not going to order a carb till I make sure the kit fits. They evidently sent a guy on one of the Facebook pages the wrong one, and it used a 18mm wrist pin. The kit I ordered is specifically for a 167FMM, and has a 16 mm wrist pin, and the lower skirt on the jug is 73mm O.D.

[Bill Hilly]

This is the cam that I plan on using, Like I say, I don't know the specs of it, but it looks a lot less pointy than the pictures,and videos of stock cams. I am even thinking about going ahead and using the roller followers. They seem pretty well made, and having some sort of brand on them, but I don't know yet. Motocult sells roller followers, and they look made differently, but they are expensive. http://

[XLsior]

That cam profile might be fine with the rollers. But I found the roller followers robbed top end power which I hypothesis as being a result of more reciprocating mass reacting to the cam shape/duration. So on a stock cam I went back to stock flat tappet followers. it might make for a less tappety engine, but i think they have a negative impact on power band.


That's just my experience with the rollers...

[Deckard_Cain]

Originally Posted by Discoveror

Carburetor air flow is designated in cfm (cubic feet per minute); rather than power.

Before getting carried away with 'more is better', be mindful (of the Bernoulli equation) - that air-flow speed at a given pressure etc. increases through a narrower carburetor venturi ... and visa versa; air flow speed decreases through a larger venturi ...

The result of this is that larger carburetor throats (e.g. 30 mm vs. 28 mm) experience slower air flow (cfm) albeit larger air volume flowed, resulting in slower throttle response (less snappy) and, likely, less torque, especially at lower rpm.

Presumably, the factory engineers know how many cfm their engine can flow, given valves size, number, camshaft profile, compression, rpm etc. etc. etc. And, factory engineers have access to air-flow and power measuring equipment that (most) riders lack. I wouldn't second-guess the factory carburetor throat size choices ... unless building an engine with far greater air flow capabilities.

Those narrower air filter box plastic pipes, which so many folks are fond of removing are there to SPEED up air flow and help low rpm torque (nice for low speed rock and rough terrain crawling etc.).

Otherwise, a lethargic throttle response is likely, especially at lower rpm.

Absolutely correct. Except... we know for a fact that the factory installed carburetor, at least the PZ carbs are barely sufficient for the stock engines, and are jetted very lean to pass US emissions requirements, many times to the detriment of the engine itself.

So while on more reputable manufacturers, I always defer to the design engineers, in this case, I think almost 10 years of having these bikes in our hands, we can safely and decidedly say that the stock carburetor is not the ideal choice.

Even just a lowly vm26 is a huge improvement with proper jetting.

Beyond that, I wouldn't change much unless the engine has been opened up for work.

[Deckard_Cain]

Originally Posted by XLsior

That cam profile might be fine with the rollers. But I found the roller followers robbed top end power which I hypothesis as being a result of more reciprocating mass reacting to the cam shape/duration. So on a stock cam I went back to stock flat tappet followers. it might make for a less tappety engine, but i think they have a negative impact on power band.


That's just my experience with the rollers...

On a stock cam, agreed. I wouldn't bother. I found on a fully worked over engine, with the improved cam, head porting, full exhaust and larger cylinder kit installed, the rollers were smoother, faster revving, and top end power was retained.

It's kind of a "don't put the cherry on top if you haven't scooped the ice cream yet" sort of thing.

[Bill Hilly]

Originally Posted by Deckard_Cain

On a stock cam, agreed. I wouldn't bother. I found on a fully worked over engine, with the improved cam, head porting, full exhaust and larger cylinder kit installed, the rollers were smoother, faster revving, and top end power was retained.

It's kind of a "don't put the cherry on top if you haven't scooped the ice cream yet" sort of thing.

The motor will have a home ported head, and a 70 mm piston,to give it 250 cc. ,and 4 degree ignition advance I am probably going to risk using the roller followers, because even though, I don't know the specs of the cam, it's a drastic difference between it and the stock one visually. So is it possible to adequately supply the motor ,if I buy a Nibbi PE28FL ? , or do I definitely need a 30? I do like the idea of being able to use my flange manifold, but I also want to get the benefit of my engine work. I am posting a picture of the cam's side by side. http://

[Discoveror]

Originally Posted by Deckard_Cain

... the PZ carbs are barely sufficient for the stock engines, and are jetted very lean to pass US emissions requirements, many times to the detriment of the engine itself.

Need for a richer air:fuel mixture doesn't mean that more air volume is required! Adjustable carburetors (e.g. Nibbi, Mikuni et al) can be used to correct the air:fuel ratio (at various rpm) - without increased air volume (and the slower air velocity of a larger carburetor throat). Higher air velocity, flowing through a narrower carburetor venturi, also enhances fuel atomization for the combustion chamber ... an advantage of fuel injection (more precise air:fuel metering - to achieve that ideal 14.7:1 stoichiometric combustion relationship).

An engine can only breath so much at various rpm in the absence of major design changes such as more valves (for more combustion chamber cross flow breathing), valves opening longer (via camshaft profile), higher compression, forced induction (turbo/super-charging) etc.

Think about blowing through a straw vs. through a larger diameter pipe. The pipe can flow more volume - from a given pressure (Bernoulli's equation, again), but the straw will offer higher air velocity - from that same pressure, which translates into more responsive (snappier) throttle response, especially at low rpm. When rock/obstacle crawling, do you want to try to feather the clutch at high rpm ... or have low rpm tractor-like torque?

I'm mindful that MILLIONS of these little engines have been produced - for worldwide sales in many makes/models. Though competitors try, it's pretty hard to elicit significant power or torque differences out of only ~15 cubic inches (250 cc).

The 'race tuned' engines (e.g. the KTM family) produce more power largely via higher rpm and compression plus 4-valves/combustion chamber - for more air-flow (breathing); but, do you REALLY want to change the oil at (nearly) every fuel stop? ... and rebuild the engine twice per year? ... hardly 'bullet-proof', like those low-compression, 2-valves per combustion chamber engines that run on nearly anything you can pour into them!

So, if the engine came with a PZ28 mm carburetor, a 28 mm Nibbi (or Mikuni etc.) may be all that's necessary - to provide variable idle air:fuel mixture adjustment and richer jetting for higher rpm etc. - to make optimal air:fuel mixture for torque and power out of the volume that that particular engine can breath.

[Bill Hilly]

I saw a video last night, where Motocult was installing a PE28 on one of those Janus bikes, and it had a stage 3 kit, so I think I will go that route. I notice that Nibbi now sells a a PEZ28, which from what I gather is the PE28FL, but with a 44mm O.D. in the breather side, instead of a 48mm like the PE28FL. They pitch it as a direct replacement of the stock PZ. I think it's just a a PE28FL that is made to accept the stock airbox tube. I would buy the PEZ28, IF it didn't harm performance when compared to the PE28FL, because not only do I already have a Nibbi Pod filter that would fit the PEZ, but I may end up hooking the airbox up one day. What do you all think? Would the smaller size on the breather side make a difference in performance if everything else is the same ?

[Discoveror]

Originally Posted by Bill Hilly

Nibbi now sells a a PEZ28, which from what I gather is the PE28FL, but with a 44mm O.D. in the breather side, instead of a 48mm like the PE28FL. They pitch it as a direct replacement of the stock PZ. I think it's just a a PE28FL that is made to accept the stock airbox tube. I would buy the PEZ28, IF it didn't harm performance when compared to the PE28FL, ...

The venturi size (the 28 mm) is the choke point, limiting air volume flow, rather than the flange size (the 44 mm vs. 48 mm).
I would go with the better/easier fitting flange size (if it's the 44 mm) - for the convenience of pulling the carburetor on/off during road testing.

I would NOT just go by someone else's jet choices, which probably weren't very thoroughly tested, other than "it runs"; so, they left it at that.

Jet testing is a fun game in itself ... for the patient.

I would take a spark plug socket along on rides. Kill the ignition (red switch) under various throttle openings and speeds. If you let the throttle close before killing the ignition, you'll get a different spark plug electrode color reading.

Then, with the clutch disengaged, pull over, stop and pull the plug. Look for a very LIGHT tan color on the center white electrode.
You'll be changing jets etc. frequently.

When you get it REALLY 'right', it's a beautiful thing.

[Deckard_Cain]

Originally Posted by Bill Hilly

The motor will have a home ported head, and a 70 mm piston,to give it 250 cc. ,and 4 degree ignition advance I am probably going to risk using the roller followers, because even though, I don't know the specs of the cam, it's a drastic difference between it and the stock one visually. So is it possible to adequately supply the motor ,if I buy a Nibbi PE28FL ? , or do I definitely need a 30? I do like the idea of being able to use my flange manifold, but I also want to get the benefit of my engine work. I am posting a picture of the cam's side by side. http://

I don't feel there is any "risk" in using roller followers. Use them or don't, it won't play a significant enough factor for you to get all worked up over it.

A PE28 would be just fine. You absolutely do not need a 30mm carb. It can be tuned for one, sure, i've done it on mine, as well as used the PE28. Overall performance was close, but the 28 was more "gentle" and easier to ride as a commuter. If you just wanted peak power to show off on a dyno graph, the 30 would do that if it were tuned as such.

Originally Posted by Discoveror

Need for a richer air:fuel mixture doesn't mean that more air volume is required! Adjustable carburetors (e.g. Nibbi, Mikuni et al) can be used to correct the air:fuel ratio (at various rpm) - without increased air volume (and the slower air velocity of a larger carburetor throat). Higher air velocity, flowing through a narrower carburetor venturi, also enhances fuel atomization for the combustion chamber ... an advantage of fuel injection (more precise air:fuel metering - to achieve that ideal 14.7:1 stoichiometric combustion relationship).

An engine can only breath so much at various rpm in the absence of major design changes such as more valves (for more combustion chamber cross flow breathing), valves opening longer (via camshaft profile), higher compression, forced induction (turbo/super-charging) etc.

Think about blowing through a straw vs. through a larger diameter pipe. The pipe can flow more volume - from a given pressure (Bernoulli's equation, again), but the straw will offer higher air velocity - from that same pressure, which translates into more responsive (snappier) throttle response, especially at low rpm. When rock/obstacle crawling, do you want to try to feather the clutch at high rpm ... or have low rpm tractor-like torque?

I'm mindful that MILLIONS of these little engines have been produced - for worldwide sales in many makes/models. Though competitors try, it's pretty hard to elicit significant power or torque differences out of only ~15 cubic inches (250 cc).

The 'race tuned' engines (e.g. the KTM family) produce more power largely via higher rpm and compression plus 4-valves/combustion chamber - for more air-flow (breathing); but, do you REALLY want to change the oil at (nearly) every fuel stop? ... and rebuild the engine twice per year? ... hardly 'bullet-proof', like those low-compression, 2-valves per combustion chamber engines that run on nearly anything you can pour into them!

So, if the engine came with a PZ28 mm carburetor, a 28 mm Nibbi (or Mikuni etc.) may be all that's necessary - to provide variable idle air:fuel mixture adjustment and richer jetting for higher rpm etc. - to make optimal air:fuel mixture for torque and power out of the volume that that particular engine can breath.

I am aware of all of that. Not my first time.

I only want to comment on your last statement, however. The engine did not come originally with a PZ28... it comes with a very tiny PZ19. It wasn't done just to keep AFR lean, obviously as you said, you can change that regardless of carb size, but also because it is cheap and plentiful, not because it was the ideal choice for the engine.

However, a 28mm is absolutely all that he will need. If they had come with a PZ28 to start with, that would have been an improvement for sure.

I've tuned my engine with all the work and upgrades/changes done to it via Wideband O2, both with a PE28 and PE30 Nibbi. Both are fine carburetors, however the 28mm I feel is a closer, better fit in most use cases.

If anyone is interested, the formula to calculate maximum air flow through an engine- it's CFM requirement, if you will, is as follows:

Engine size (in cu in) X Max Engine speed (in RPM) / 3456

Multiply this times the volumetric efficiency of the engine. Most carbureted engines have this value somewhere between 0.70 and 0.80.

So if we take a CG300 cylindered Hawk/Tbr7 engine at 15.249cu in, Max engine RPM (realistically) of 8000rpm, and assume VE of 0.75 (let's take the middle of the range)....

The engine, an air pump really, can only move 26.47CFM of air. Not a huge number.

I can't find published data for CFM of various small engine carburetors, however anecdotally I found one statement putting a 34mm Keihin at 110cfm. Seems perhaps high, but even at half that number, it's twice what a 250cc CG would be able to move.

Of course, air temperature and pressure, cylinder pressure and flow all figure into this as well.

Consider the original Honda CG125 came with a PZ26 carburetor with 28mm inlet and is half the size... This is why you see so many on this forum and on the FB group OVER-carb'ing their engines.