45mph at ~7600rpms

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k.mah

New Member
Nov 22, 2012
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oceanside,ca
Went riding with a friend today, he was on a suzuki dualsport 600something.
Was finally able to verify my top speed on flat ground was 45mph, and the tiny tac read right about 7600rpms. Even hit 8500rpms downhill....I was HAULING!
Im running a 36t rear sprocket with 26", 1.75" tires.
Have ALOT of port work done (transfer ports fully opened, boost ports, piston ramping, piston skirt trimming, port matching), as well as puch hi hi head, race carb thats ported further, modified minibike exhaust (hopefully making a new one out of a mx expansion chamber to further my powerband up the rpm range).
Very happy with these speeds, cant wait till i can hit 50 and get it gps verified!
Oh, and i switched to amsoil saber professional, currently at about 60:1. It seems to be running VERY smooth, time will tell though!
 
maurtis

maurtis

New Member
Dec 14, 2011
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Kyle, TX
Grats!!! Your speed is completely believable. I hit a top speed of 39.7 MPH (GPS verified) on flat ground with my BGF RB80 on my Cranbrick with 36T rear and no internal mods. I REALLY wanted to hit 40 but never could tuck down quite enough (or stop shoving cheeseburgers in my face long enough) to quite make it.

Then my wrist pin bearing went kabloooie!

Grats again, and looking forward to you hitting 50! I thought going nearly 40 on an MB was exhilarating, I could only imagine what 50 is like. Hopefully you have some good brakes!
 
biknut

biknut

Well-Known Member
Sep 28, 2010
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Dallas
k.mah said:
Went riding with a friend today, he was on a suzuki dualsport 600something.
Was finally able to verify my top speed on flat ground was 45mph, and the tiny tac read right about 7600rpms. Even hit 8500rpms downhill....I was HAULING!
Im running a 36t rear sprocket with 26", 1.75" tires.
Have ALOT of port work done (transfer ports fully opened, boost ports, piston ramping, piston skirt trimming, port matching), as well as puch hi hi head, race carb thats ported further, modified minibike exhaust (hopefully making a new one out of a mx expansion chamber to further my powerband up the rpm range).
Very happy with these speeds, cant wait till i can hit 50 and get it gps verified!
Oh, and i switched to amsoil saber professional, currently at about 60:1. It seems to be running VERY smooth, time will tell though!
Click to expand...
Sorry but that does not compute. If you do the math you'll fall far short of 45 mph at 7600 rpm. Even 8500 would be less than 45. I'm not trying to say you didn't in fact get up to 45, I'm just saying if you did, you had to be north of 8500 RPM to do it. Your motor will have a very short lifespan at those engine speeds.

This is what the RPM to speed works out to with a 26x1.95 tire.

36T sprocket
26x1.95 tire 80.25" circumference

5.2 mph per 1000 rpm


RPM MPH
1000 5.2
1200 6.2
1400 7.2
1600 8.3
1800 9.3
2000 10.3
2200 11.4
2400 12.4
2600 13.4
2800 14.5
3000 15.5
3200 16.5
3400 17.6
3600 18.6
3800 19.6
4000 20.7
4200 21.7
4400 22.7
4600 23.8
4800 24.8
5000 25.8
5200 26.9
5400 27.9
5600 28.9
5800 30.0
6000 31.0
6200 32.0
6400 33.1
6600 34.1
6800 35.1
7000 36.2
 
K

k.mah

New Member
Nov 22, 2012
173
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oceanside,ca
Stock front sprocket, so 10t? I can't argue with math but I'll have to borrow an I phone and gps verify it. All I know is stock kit hit 30 with a 44t rear and without all the work. The bike HAULS, I'll update when I get it gps, don't want false claims flying around :)
 
K

k.mah

New Member
Nov 22, 2012
173
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oceanside,ca
And does anyone know if there is a way to verify the tac's accuracy? I'm thinking hook it to my car but the lines are under an intercooler, unless I borrow a car!
 
K

k.mah

New Member
Nov 22, 2012
173
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0
oceanside,ca
I just put a speedometer on the bike, got it calibrated to what they say is correct, ran it down the street real quickly...with a busted cdi (the screw that the spark plug wire attaches to snapped off while riding last night. i have it taped back in place for the time being, but i can hear the snap of the spark jumping every so often)
40.5mph at 7800rpms.
Biknut-you are absolutely correct!
 
biknut

biknut

Well-Known Member
Sep 28, 2010
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Dallas
Sounds like your bike is running real good though. 39 is the fastest I've been able to any of mine up to on level ground.
 
K

k.mah

New Member
Nov 22, 2012
173
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0
oceanside,ca
I still wonder how to get the tac. is reading correctly, because your math is totally sound.
I got VERY lucky with this grubee kit, its weighted near perfectly from the factory
 
biknut

biknut

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Sep 28, 2010
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Dallas
I haven't ever bothered to run a tach. I have always run an accurate electronic speedo though. I always mathematically calculate my speed to rpm, and after that I know my rpm by what speed I'm going. That's easy to do when you only have 1 gear.

Sometimes I run my motor over 7000 rpm, but I like to keep it a little under 7000. I've found that if you cruise over 7000 very much it really shortens the life of your motor. Above 7500 really is pushing it. At 8500 it could blow at any time.
 
biknut

biknut

Well-Known Member
Sep 28, 2010
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Dallas
Most of the china girl motors have such poor quality crankshafts, high rpm kills them pretty fast.
 
K

k.mah

New Member
Nov 22, 2012
173
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oceanside,ca
My engine is living on borrowed time.
I'm in the process of rebuilding a 1978 puch maxi i got for $60. Got it running today. Hopefully get it to cruise around 40mph when i'm done, then the poor china girl wont get beat up so badly!
 
KCvale

KCvale

Well-Known Member
Feb 28, 2010
3,966
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Phoenix,AZ
The_Aleman said:
You know what I mean - first time he's verified he's going over 40 on something he built!
I remember that time fondly, myself :D
Click to expand...
As do I Aleman, my first shifter build, half of my builds these days are shifters that don't even break a hard sweat at 40MPH and go faster than I care to ride.
 
maniac57

maniac57

Old, Fat, and still faster than you
Oct 8, 2011
4,484
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memphis Tn
Mine maxed at 34 with no internal mods, a generic pocketbike pipe, and stock gearing.
(I do have a lot of drag from the basket though...:))
 
maniac57

maniac57

Old, Fat, and still faster than you
Oct 8, 2011
4,484
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0
memphis Tn
biknut said:
I haven't ever bothered to run a tach. I have always run an accurate electronic speedo though. I always mathematically calculate my speed to rpm, and after that I know my rpm by what speed I'm going. That's easy to do when you only have 1 gear.

Sometimes I run my motor over 7000 rpm, but I like to keep it a little under 7000. I've found that if you cruise over 7000 very much it really shortens the life of your motor. Above 7500 really is pushing it. At 8500 it could blow at any time.
Click to expand...
These numbers sound about right to me. I very seldom spin it above 6-6.5k. This may explain why I get such trouble free service out of my kits. I know I've never seen one yet that was happy spinning anywhere near 8k. I've always wondered how a perfectly balanced china girl engine would feel.
 
KCvale

KCvale

Well-Known Member
Feb 28, 2010
3,966
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48
Phoenix,AZ
maniac57 said:
These numbers sound about right to me. I very seldom spin it above 6-6.5k. This may explain why I get such trouble free service out of my kits. I know I've never seen one yet that was happy spinning anywhere near 8k. I've always wondered how a perfectly balanced china girl engine would feel.
Click to expand...
2-strokers power better at low RPM's where 4-strokers like the high revs for the power band.

If you want to see perfectly balanced high performance 2-stroke motors look at large scale model airplane motors, not enough power for real speed on a MB without gears but pretty cool.

kerwin50, Jackshafting a motor kit with a shift kit so it uses the pedal gears really isn't something for a first time builder, it costs as much as the whole motor kit itself, and no picnic to attach right either.

If you can pull it off right however gears change everything, even just a 3-speed hub. If you have ever drove a manual shift car picture a 3-speed car but you are always in second gear. It sucks off the line and tops out too quick. That is a stock direct drive MB.
 
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