New cheap carb Nibbi 26mm PWK copy

ole4

ole4

#1
#1
I just can't resist these cheap flatslide copies. First I got the OKO copy then I saw this one a Nibbi which is similar but the other ones as well as the mikuni TM come in either 24 or 28mm. This one was a 26mm. Just got it late today so I have not taken it apart yet but it is very similar to the OKO with the exception that the slide looks a lot better. I will take some pics disassembled tomorrow if I can. I have a built hemi predator that has the 24mm OKO on it which I have not run yet but I will set it up on a stand to test both carbs. I will also put this one on the flowbench to see what it flows. The 24mm OKO flows around 96CFM at 28"H2o so I would expect this one to be slightly north of 100. Now that I can cut and install the 32mm valves I wanted to try a slightly bigger carb and a straight manifold.
 
ole4

ole4

#4
#4
I sure have and it is kind of a mystery as to why. I flowed the head and it had a max of arount 73CFM. I had a carb and curved manifold I made with not steps or anything I forget what the carb and manifold, then both together flowed but it was a lot more than the head, high 80's maybe. anyway with the head, manifold and carb it was around 66CFM. I may be able to find the data and post it.
 
ole4

ole4

#5
#5
I put the nibbi carb on the bench and when I set the depression to 28" it sucked all the water out of the manometer. 100% of the scale equals 110CFM so after getting manometer back with fluid again I ran the depression until I was a inch past 100% and it was 26" of H20 so adjusting for the 2 cfm (multiplying 115 x 1.05) it flowed around 127CFM about 30 more than the 24mm Pwk. I was very supprised at that much of a increase. I am going to calculate the area of a 24mm and 26mm circle then see if the math matches the increase.
 
KMC3420

KMC3420

Member
#6
#6
ole4 said:
I sure have and it is kind of a mystery as to why. I flowed the head and it had a max of arount 73CFM. I had a carb and curved manifold I made with not steps or anything I forget what the carb and manifold, then both together flowed but it was a lot more than the head, high 80's maybe. anyway with the head, manifold and carb it was around 66CFM. I may be able to find the data and post it.
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Thanks Ole, just so I'm reading correct you said a straight manifold flows better than a curved?
 
ole4

ole4

#7
#7
I don't remember a straight one flowing better as all the ones I tested I fabricated and curved around to fit better. But looking at the excel file of when I had this issue one of the better flowing combos was a Tillotson HL 375 which had the smallest loss and I think may have been a straight manifold but it was a long time ago and I am not sure. I don't know how to post a excel file but when I can comvert to a picture I will post it. It does not state what manifold.
 
KMC3420

KMC3420

Member
#8
#8
ole4 said:
I don't remember a straight one flowing better as all the ones I tested I fabricated and curved around to fit better. But looking at the excel file of when I had this issue one of the better flowing combos was a Tillotson HL 375 which had the smallest loss and I think may have been a straight manifold but it was a long time ago and I am not sure. I don't know how to post a excel file but when I can comvert to a picture I will post it. It does not state what manifold.
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Oh okay, gotcha. I guess I might get both and give each one a go. I don't have a way to flow a carb/intake combo. I did receive my genuine Mikuni and still have to get jets for it to test for best fuel-air.
 
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ole4

ole4

#9
#9
I just finished a 32/25 head and bought 8 feet of 1 1/8th aluminum tubing. I am going to make a 10" long manifold taper bored from 26mm to 24 then to the intake d shape. I will post flow results vs head flow when I am done. I also have a short straight manifold that I can try on that head now. You have peaked my interest in this!!
 
KMC3420

KMC3420

Member
#10
#10
ole4 said:
I just finished a 32/25 head and bought 8 feet of 1 1/8th aluminum tubing. I am going to make a 10" long manifold taper bored from 26mm to 24 then to the intake d shape. I will post flow results vs head flow when I am done. I also have a short straight manifold that I can try on that head now. You have peaked my interest in this!!
Click to expand...
Sweet, you are the right man for the job. Thanks for doing this. That new carb appears to be a very good flowing carb, I'm going to look for this carb on eBay.
 
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Prowler08

Prowler08

New Member
#11
#11
Wow, I am impressed with the flow of that carb as well...
I just switched over to a Yz250f motor on my Kart for UAS racing...it did not come with a carb, so I used a Mikuni SBN 38mm carb I had...

I flowed the carb and best I could get out of the SBN was 100-105 cfm...
It looks like from your tests... that smaller slide carb flows better and flows enough for the 250f... I like the smaller venture for throttle response off the corners...

Where did you get the carb from? Any other sizes available? Thanx
 
ole4

ole4

#12
#12
Couple of things here, First I did the math using the flow thru orfice and set it up to come close to the reading for the 24mm copy as both carbs are more or less the same design and using a cd that would calculate 94CFM (what the PWK flowed I changed the size to 26mm (1.02) and it came out to 110 which is more realistic. I did not recalibrate my bench after it sucked out the fluid and I need to go back and do that as my caclulations were very optimistic (125CFM) I am sure this is wrong. Your numbers for the 38mm SBN see way way too low. Are you testing at 28" H20? That carb is 1.5 inches in diameter and even a sharp edged orfice sized at 38mm flows north of 150 that carb should be closer to 200+. I also have a shifter with a YZ250! PICT0291.JPG
 
Prowler08

Prowler08

New Member
#14
#14
We are using 14" WC... with a 200cfm orfice... I thought it should flow more than 100cfm...
I wish I could have found a 250 2 stroke instead of the 250 4stroke...
Nice Kart... I kept the tranny and added a shifter and clutch lever to the kart...
Just got done tuning the carb... it was a bit finicky at first... had to raise the pop off to 18lbs go a bigger low jet and smaller main jet to get it to idle and take off clean and crisp...
 
ole4

ole4

#15
#15
My conversion chart only shows test depression of 15 (no 14) but to convert to 28 you multiply by 1.37 so at 14" H20 probably be around 140CFM but that still seems to low for a 38MM carb unless it has a much smaller venturi. I do not know anything about those. My 250 YZ engine came with a flatside Mikuni.
 
ole4

ole4

#17
#17
So that is 1.338 inches so I entered that in the orifice flow calculator with a CD of 75 which is probably low and it would pass 153.5 cfm if cd = 85 which is more like it it would flow 174 CFM

Calculate Flow
Gravitational constant 32.2 ft/s²
Density of air 0.07484 lb/ft³
Density of water 62.28 lb/ft³
Orifice diameter 1.33 inches
Pressure difference 28.0 WC in inches
Discharge coefficient 0.85
Combined coefficients 23.66831863
Orifice area 1.38929406 in²
Flow 174.0 cfm
 
ole4

ole4

#18
#18
I just realized I can change the depression in the calculator so here it is for 14" H20

Calculate Flow
Gravitational constant 32.2 ft/s²
Density of air 0.07484 lb/ft³
Density of water 62.28 lb/ft³
Orifice diameter 1.33 inches
Pressure difference 14.0 WC in inches
Discharge coefficient 0.85
Combined coefficients 23.66831863
Orifice area 1.38929406 in²
Flow 123.0 cfm
 
Prowler08

Prowler08

New Member
#19
#19
Thx ole4.... great info... appreciate the help!!!
I know the atomizer hurts flow a bit... but it helps for overall balance and low end torque as well as tuneability...
 
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