What RPM should a A9 cam have?

[larryhobman]

Hi All,
I just added an A9 cam to my 72cc set up that I had already running good.  My question is what RPM should I be at?  Before the A9 I was running between 7900 and 8300 @ 42 to 44 mph on my gps.  Now after the A9 I am at 9000 to 9300 @ 46 to 48 mph on GPS.  I had also changed the tach so I do not know if my new cheap tach is better or worse than what I started with.  If the new tach is right is 9000 too much and should I change sliders.  It does run real good the way it is but, I don't want to blow it up.

The cam is a improvement with the 72cc kit, just wish I did it before the cvt tuning I did.  Looks like time to start over, let em know what you think.

Thank you

current set up
72cc with a 60cc head cleaned up casting imperfections in the ports and port matched the spacer.
stock carb 18mm with #90 main and #31 idle needle it the middle setting 1.5 turns on the af screw pod filter
6 gram koso sliders 1k koso contra
timing mod with stock cdi and coil
gates 669 belt

[Availien]

Welcome to the forum!

"7900 and 8300 @ 42 to 44 mph on my gps. Now after the A9 I am at 9000 to 9300 @ 46 to 48 mph on GPS."

I'm no expert, but it seems like the extra RPM is due to the extra speed. You may want to consider changing your final drive gears to keep it on the lower end of RPM. A cheaper, faster fix might be new roller weights, but once the variator expands it will probably still do the same RPM at top speed. 9k will probably cause your crank to fail if you didn't get a better one. The extra weight of the piston is going to have some negative consequences when it comes to the crank shaft.

Here are the most commonly used final drive gears when upgrading a 4t: www.ebay.com/itm/49-17t-Performance-Final-Drive-Gears-GY6-139QMB-Chinese-Taiwanese-Scooters-/171862877609?hash=item2803d365a9&vxp=mtr

Here are a few crankshaft options that will actually increase your displacement and withstand the extra weight from a bigger piston: www.ebay.com/itm/Performance-44mm-STROKER-CRANKSHAFT-139QMB-GY6-Engines-Scooter-Racing-Crank-/181703003861?hash=item2a4e57ced5&vxp=mtr

www.ebay.com/itm/HOCA-STROKER-CRANKSHAFT-ASSEMBLY-FOR-SCOOTERS-WITH-4-STROKE-QMB139-MOTORS-/171844901733?hash=item2802c11b65&vxp=mtr

You might have to get a modified oil pump to run either of those (I learned the hard way) and you will also need extra base gaskets to raise the cylinder so the valves don't hit the piston.