I want to get more rpms out of my engine. I ordered the reed kit and have an expansion chamber. im going to lengthen the header about 5 inches, it is the stubby bgf pipe. I havent seen the engine pulling harder than 5k rpm under load. Im trying to read about balancing the crank, ive heard the engine cant rev up because of crank imbalance but im wondering, is this it? what else needs to be done.
balanced crank hi rpm
- Thread starter graydog8josh
- Start date
How do you know what RPM your motor is running? If you have a 44T rear sprocket, and 26" wheels, 7000 rpm is only going to be about 28 mph.
All my china girl motors easily rev to 7000+ rpm with a 36T rear sprocket, and I weigh 230. Even my 48cc would rev to 7000 with a 40T.
All I had to do to get this performance was slightly open the stock baffle, and rejet the carb.
I've had BGF, and Flying Horse motors.
I used the calculator and put in the known values :
#1 36T 27.95" wheel and got my recorded top speed by inputting 5k rpm. (28mph)
( calc says i should be getting ~30mph)
#2 Then i switched to a 25.75" wheel with the 44T sprocket and input 5k rpm and got the top speed i was able to achieve on that run (22mph).
( calc says i should be getting ~40mph)
so i deduced that my motor isnt revving higher than ~5000 rpm.
am i doing somethign wrong? do i not include the 2nd tire number in the tire size?
Since ive not yet been able to reach 30-35 WOT on flats yet i think im not getting up to 7k
im currently running 24" rear wheel with 415 chain 1.75 tire on the back
ngk b7 iridium plug
z shaped offset intake
nt carb 66 jet clip in 2nd position
bgf expansion chamber
ive put a couple hundred miles on this motor and it has no leaks.
biknut: from what you're saying it should be much easier for me to achieve the targeted speed i want (30-35 mph) if you did it all near stock.
#1 36T 27.95" wheel and got my recorded top speed by inputting 5k rpm. (28mph)
( calc says i should be getting ~30mph)
#2 Then i switched to a 25.75" wheel with the 44T sprocket and input 5k rpm and got the top speed i was able to achieve on that run (22mph).
( calc says i should be getting ~40mph)
so i deduced that my motor isnt revving higher than ~5000 rpm.
am i doing somethign wrong? do i not include the 2nd tire number in the tire size?
Since ive not yet been able to reach 30-35 WOT on flats yet i think im not getting up to 7k
im currently running 24" rear wheel with 415 chain 1.75 tire on the back
ngk b7 iridium plug
z shaped offset intake
nt carb 66 jet clip in 2nd position
bgf expansion chamber
ive put a couple hundred miles on this motor and it has no leaks.
biknut: from what you're saying it should be much easier for me to achieve the targeted speed i want (30-35 mph) if you did it all near stock.
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If you do 35 mph you'll be over reving a few thousand rpm. Well that's based on my belief that china girl motors won't last long if run above 7000 rpm for extended time.
This is the calculation I did for my bike. I'm running a 24" wheel too, but my tire is a little bigger than yours. Take away 1 or 2 mph for your bike. I've already hit 37 mph once, and I'm still breaking it in, but almost done. With this gearing I think my upper cruising speed will be about 31-33 mph. Yours probably about 28-30 if you consider 6800-7000 max, with occasional rips above 7000 rpm.
36T sprocket
24x3 tire 76.625 circumference
The overall ratio is 14.76
RPM MPH
1000 4.9
1200 5.9
1400 6.9
1600 7.8
1800 8.8
2000 9.8
2200 10.8
2400 11.8
2600 12.7
2800 13.7
3000 14.7
3200 15.7
3400 16.7
3600 17.6
3800 18.6
4000 19.6
4200 20.6
4400 21.6
4600 22.6
4800 23.5
5000 24.5
5200 25.5
5400 26.5
5600 27.5
5800 28.4
6000 29.4
6200 30.4
6400 31.4
6600 32.4
6800 33.3
7000 34.3
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I can see what rpms i need to reach to reach those speeds, my question is more of I want to open the throttle while im on the bike and it rev up to 7k rpm on a flat. but i wanted tips on how i can get the speed on the flat to increase.
You should be able to do that now if your bike is tuned right.
You'll need at least a 36T sprocket. Probably a 66 or 68 jet. Your needle is already in the right spot.
I have a BGF expansion chamber that I ran for 2 days, and tossed it on the pile. It didn't work as well as my modified stock pipe. I thought I lost about 2 mph on the top end, and it 4 stroked more.
An SBP pipe works a lot better.
I think it was my spark plug wire, i switched it out for the sbp one i ordered and it was a lot smoother. It still seemed to only want to rev up to 5k then after a minute it reved out and i got to 27mph the stock one kind of wiggled around on the screws.
27 is probably about 6000 on your bike. With that kind of gearing it takes a while to get from 6000 to 7000.
You might actually find your crank is bent when you spin it on some vee blocks.
I blew mine up running at redline for an extended period of time. I think the next bike with a china will have a tach, all of my gas bikes will have tachs from since that incident, so I can stay just shy of the outer limits.
I blew mine up running at redline for an extended period of time. I think the next bike with a china will have a tach, all of my gas bikes will have tachs from since that incident, so I can stay just shy of the outer limits.
Re: Waterproofing a mag?
Hi, like was said earlier, if ur carb is tuned properly, (I cant coment on ur pipe, I havent used), u should be able to get it to rev out to 7000 no problem with ur gearing. Tune carb by plug color. Small things like port matching r good to do. An aftermarket or home made CDI will help. The stock CDI kinda acts like a rev limiter as it doesnt retard at higher rpm, which a 2 stroke CDI should. See thread "modified CDI & CR80 coil". That 1 will run smoother everywhere & let it rev higher. The reed kit will require piston porting to get the intended effect, which is increased torque, especially low & mid range, not really any increased top end, which requires a change of port timing to properly achieve. These things arent designed for hi rpm. I have a pretty worked over, reeded & piston ported/boost ported motor. They're all about torque. Good luck with. Cheers
Hi, like was said earlier, if ur carb is tuned properly, (I cant coment on ur pipe, I havent used), u should be able to get it to rev out to 7000 no problem with ur gearing. Tune carb by plug color. Small things like port matching r good to do. An aftermarket or home made CDI will help. The stock CDI kinda acts like a rev limiter as it doesnt retard at higher rpm, which a 2 stroke CDI should. See thread "modified CDI & CR80 coil". That 1 will run smoother everywhere & let it rev higher. The reed kit will require piston porting to get the intended effect, which is increased torque, especially low & mid range, not really any increased top end, which requires a change of port timing to properly achieve. These things arent designed for hi rpm. I have a pretty worked over, reeded & piston ported/boost ported motor. They're all about torque. Good luck with. Cheers
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LOL the title of this thread is balanced crank hi rpm, and yes it has been done and works to make these less of a craptastic motor.
