Choosing a carburetor for an open class without limits?

oneman

Member
My Chinese PZ30 left me in the last race. I am now planning to purchase a new carburetor. But I wanted to ask your advice. What new carburetor do I need?

My engine config: 70mm piston, 54mm crank, 18cc head, 27.5mm inlet 25.5 outlet valve. The camshaft is standard but its base is reduced to increase the lift, now the lift is + 1mm (0.270) to the standard camshaft. Springs 36lb, HD champions rockers. ARC connecting rod, flywheel aluminum PVL. And it is used in karting.

I think the PZ30 is too big, I had problems with starting and unstable idle. But during the races, he worked confidently up to 8300 rpm and I liked it. And yes, I used a carburetor with an accelerator pump that gives an extra jet of fuel.

Now I am wondering if I can use the PZ27 with a booster pump for improved starting and stable idle. Or Try the PWK28 Carburetor? But it doesn't have an accelerating pump. I am afraid that this will affect negatively during the race.
 

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Hello dear forum users! I didn't want to create a new topic, so I'll ask here. The technical specifications of my engine are written in the first post of this topic. For the next season, I want to install a 72mm piston. I have a question about the compression ratio, now my calculated compression ratio is 10.4: 1. For the next season, am I thinking of keeping it, or increasing it a little? Our race is an hour long endurance race, non-stop, I'm afraid that the higher compression ratio could damage my engine. it is also possible to use high-octane fuel 100 (RON).
 
I havent experimented much in carbs for my open motors but both of my open Predators run very well on big block alky carbs using adaptors. My open Animal uses a genuine Mikuni 28mm. All three of them have considerably bigger valves than you are using.
 
I havent experimented much in carbs for my open motors but both of my open Predators run very well on big block alky carbs using adaptors. My open Animal uses a genuine Mikuni 28mm. All three of them have considerably bigger valves than you are using.
thanks, but can you tell me about the compression ratio? do you have any ideas on this?
 
thanks, but can you tell me about the compression ratio? do you have any ideas on this?
I really dont know for sure but i would expect the Animal with the Mikuni and one of the Predators to be in the 11-1 rage, the other Predator more like 10.5.
 
How will increasing the length of the carburetor inlet affect? my new carburetor won't fit due to the proximity to the wheel. And I would like to make a pipe that will take the carburetor to the side.
 
Hello dear forum users! I didn't want to create a new topic, so I'll ask here. The technical specifications of my engine are written in the first post of this topic. For the next season, I want to install a 72mm piston. I have a question about the compression ratio, now my calculated compression ratio is 10.4: 1. For the next season, am I thinking of keeping it, or increasing it a little? Our race is an hour long endurance race, non-stop, I'm afraid that the higher compression ratio could damage my engine. it is also possible to use high-octane fuel 100 (RON).
Ideally for best power, the most compression with the lowest octane fuel.
Increasing octane slows the fuel burn rate. The reason this is important is we must control when the fuel ignites. We do not want the fuel to start burning before the spark starts the burn.
Increased compression also increases the possibility of preignition. We can do some things to lower this possibility, like colder spark plug, and smoothing sharp edges in combustion chamberto remove hot spots.
Although not a possibility here, a longer duration cam can also affect dynamic compression.

It becomes a balancing act of compression vs octane to get best power and reliability.
 
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How will increasing the length of the carburetor inlet affect? my new carburetor won't fit due to the proximity to the wheel. And I would like to make a pipe that will take the carburetor to the side.
A longer intake usually adds bottom end torque and loses upper RPM.
If you went with an accelerator pump that would be a plus .
Try to keep it short with minimum turns .
 
My engine config: 70mm piston, 54mm crank, 18cc
that is a stock 208cc motor. With an 18cc head (uncut?), stock thickness head gasket (not stated otherwise -4.7cc), stock style dish piston (not stated otherwise -1 cc)
that gives around 8.77:1. - which is the advertised C/R for that motor - stock.
swapping in a 72mm stock dish piston would bump it to 9.2
a 14cc w/70mm stock dish piston would bump it up to 10.5:1 - but the valves are smaller.
cut the 18cc head, drop the head gasket thickness and go to a flat top piston.

if your looking for a good, tunable carb with plenty of available parts, a Briggs Animal PZ22 would work very well. If you step up the engine with more piston, high compression (^11:1), cam (and proper springs / rockers), then the Briggs World Forumula PZ 26 would be nice.

ps -208 cc = 12.68 cubic inch
 
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One thing to consider in choosing a carburetor is that bigger isnt necessarily better size-wise. The bigger the venturi size the slower air and fuel will flow thru it at a cost of lower end throttle response. Since the beginning of aftermarket parts guys have wondered why the 800 Holly double pumper made their 305 Chevy daily driver run worse.
 
One thing to consider in choosing a carburetor is that bigger isnt necessarily better size-wise. The bigger the venturi size the slower air and fuel will flow thru it at a cost of lower end throttle response. Since the beginning of aftermarket parts guys have wondered why the 800 Holly double pumper made their 305 Chevy daily driver run worse.
Exactly true!
 
that is a stock 208cc motor. With an 18cc head (uncut?), stock thickness head gasket (not stated otherwise -4.7cc), stock style dish piston (not stated otherwise -1 cc)
that gives around 8.77:1. - which is the advertised C/R for that motor - stock.
swapping in a 72mm stock dish piston would bump it to 9.2
a 14cc w/70mm stock dish piston would bump it up to 10.5:1 - but the valves are smaller.
cut the 18cc head, drop the head gasket thickness and go to a flat top piston.

if your looking for a good, tunable carb with plenty of available parts, a Briggs Animal PZ22 would work very well. If you step up the engine with more piston, high compression (^11:1), cam (and proper springs / rockers), then the Briggs World Forumula PZ 26 would be nice.

ps -208 cc = 12.68 cubic inch
Thanks! I used a thin metal .010 cylinder head gasket.

I'll consider installing a 72mm piston, but the main problem is that I live in Russia and we don't have as many spare parts as in the USA. And buying abroad is expensive, and expensive shipping. Therefore, we use what we can find here.

One thing to consider in choosing a carburetor is that bigger isnt necessarily better size-wise. The bigger the venturi size the slower air and fuel will flow thru it at a cost of lower end throttle response. Since the beginning of aftermarket parts guys have wondered why the 800 Holly double pumper made their 305 Chevy daily driver run worse.
Yes, I understand you, I know it! But the Chinese manufacturer Lifan, who sells these clones to us, installs a PZ27 carburetor from the factory, sometimes with an accelerator pump and sometimes not, and they work pretty well.
 
Sorry for the offtopic, but I would like to show you our races, we use asphalt karts, we have an endurance race, it takes one hour to drive, there are no stops. I don't have a YouTube channel, I haven't bought a helmet camera yet, but here is a video from my teammate.

 
Thanks! I used a thin metal .010 cylinder head gasket.
using the .010 head gasket with the 70mm dished piston and 18cc head bumps the CR to 10.4:1
going to a 72mm dished piston with 18cc head and .010 head gasket will give you 10.97:1 C/R
a flat top 72mm piston with 18cc head and .010 head gasket will give you 11.55:1 C/R - nice!
 
I'm afraid that increasing the compression ratio will lead to increased wear on the connecting rod bearing.
I read in another post that you are using gasoline / petrol for your fuel?
I have an ignition advance of 28-30,
In the other thread post, you state that your ignition timing is 30 btdc. I think that is too much. On the Briggs Animal, on gasoline, we found best timing was around 26 or 27 btdc with 87 pump octane.
The piston under-stroke is 0.035
add that to the engine specs and your actual (calculated) C/R is 8.89
bore 70mm, sweep volume 208 cc, head 18cc, head gasket(.010 thk) 1cc, piston dish 1cc and piston clearance (down the bore)3.4cc
From what I read between the two posts is:
you do not have "too much" compression - a 72mm flat top piston & .010 head gasket would be a nice 9.71:1. with a .020 head gasket, 9.28
you have too much ignition timing
you are running at too high of sustained RPM. The rod might take it but the piston won't (see your picture with the detached piston top ring set)
you may need slightly thicker oil -10w40
 
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