Bully clutch stall speed

[Allenwrench]

Help, we are trying to determine the best RPM engagement for our kart. We are running 15 teeth on the clutch, 63 sprocket with a clone 196 engine and a Bully 2 plate 6 spring clutch. Our kid / driver weighs about 170lbs and we are questioning our clutch setup. Right now it is set at 2800rpms. It doesn't seem to Launch at the start. Long story shorter, with the weight of our driver and heavy kart, does it make sense to run the clutch RPMS up to about 3200 to 3600?

Thanks for any advice, you guys know a lot more then me.

 

[madprofessor]

That 4.2:1 gear ratio isn't an ideal drag race ratio (too tall a gear) to begin with. Assuming you're running a lightweight kart, that would be a good ratio for a 80 lb. driver. Total weight of yours exceed 300 lbs. with the driver? Way too tall a gear, just my opinion. How many rpm can you max out of that engine on the track?

 

[Allenwrench]

That 4.2:1 gear ratio isn't an ideal drag race ratio (too tall a gear) to begin with. Assuming you're running a lightweight kart, that would be a good ratio for a 80 lb. driver. Total weight of yours exceed 300 lbs. with the driver? Way too tall a gear, just my opinion. How many rpm can you max out of that engine on the track?

our total weight is up around 380lbs. Racing on a 1/8 mile oval. Max RPMs we get is around 4500 running a blue plate restrictor.

 

[madprofessor]

I know nothing about sanctioned kart racing rules, so no idea what a blue restrictor does. I do know what a governor does, and you only winding it up to 4500 rpm makes me wonder if you have a working governor that's just adjusted to 4500 rpm. Right or wrong?
If you're running completely ungoverned then I'd expect more rpm than 4500, but not running that 4.2:1 ratio trying to pull 380 lbs., that certainly could be keeping your rpm down. Here's the thing, that little clone makes it's best power at the highest rpm you can wring out of it.
A shorter ratio (Ex: 5:1) would give you a faster holeshot, and let that clone wind out more rpm in the straights for cutting off some lap time in the curve exits with the increased hp. of increased rpm.
I really think you wouldn't lose anything on the top end speed just because of the change to a shorter ratio, because it should wind up higher as a result, and you'd be cutting off some lap time with the better holeshot and curve exits.

 

[anderkart]

The Bully website recommends a 3500 rpm stall speed for your clutch model when racing in the blueplate class.
There's more info here on their site: https://www.buller.net/Category/bully_2d6su

It does sound like you also need to also start experimenting with different gear ratios. You could start by asking some of the top drivers in his class (with a similar total weight as yours) what gear ratio their running, then try that ratio as your baseline; and then if necessary ~ keep tweaking your gearing from that bassline until the engine simply wont rev any higher at the very end of the straightaways.

 

[madprofessor]

Here's a very close approximation of my own 1" double-split aluminum sprocket carrier hub, and the split sprockets for it...................
Aluminum Sprocket Hub - 1" Bore | 433692 | PPH1 | BMI Karts And Parts
#35 Split Sprocket | 6335** | BMI Karts And Parts
If not running a split sprocket already, definitely do change over to 5.25" 6-hole sprocket hub of your rear axle's diameter. You only have to remove a wheel, bearing, sprocket, maybe a brake disc, and put it all back on just one time to get the hub onto the axle (mine's $30). From then on you can swap out ($16) split sprockets with any tooth count you want, even skip-tooth, in about 60 seconds.
Note: You don't have to use a double-split hub like mine, I'm just a big fan of them due to being OCD about stuff. You can get a hub for just ($15) that works just fine................Unihub - Sprocket Hub | AZ2556 | Azusa 2556 | BMI Karts And Parts

 

[Allenwrench]

I know nothing about sanctioned kart racing rules, so no idea what a blue restrictor does. I do know what a governor does, and you only winding it up to 4500 rpm makes me wonder if you have a working governor that's just adjusted to 4500 rpm. Right or wrong?
If you're running completely ungoverned then I'd expect more rpm than 4500, but not running that 4.2:1 ratio trying to pull 380 lbs., that certainly could be keeping your rpm down. Here's the thing, that little clone makes it's best power at the highest rpm you can wring out of it.
A shorter ratio (Ex: 5:1) would give you a faster holeshot, and let that clone wind out more rpm in the straights for cutting off some lap time in the curve exits with the increased hp. of increased rpm.
I really think you wouldn't lose anything on the top end speed just because of the change to a shorter ratio, because it should wind up higher as a result, and you'd be cutting off some lap time with the better holeshot and curve exits.

thank you, good info. we may have to try a shorter ratio.

 

[Allenwrench]

The Bully website recommends a 3500 rpm stall speed for your clutch model when racing in the blueplate class.
There's more info here on their site: https://www.buller.net/Category/bully_2d6su

It does sound like you also need to also start experimenting with different gear ratios. You could start by asking some of the top drivers in his class (with a similar total weight as yours) what gear ratio their running, then try that ratio as your baseline; and then if necessary ~ keep tweaking your gearing from that bassline until the engine simply wont rev any higher at the very end of the straightaways.

thank you, i see the chart on Bully's website now and may adjust the springs.