My Predator 212cc running ARC billet rod and ARC billet flywheel and 22lb valve springs and 22mm Mikuni carburetor

GustoGuy

Active Member
#1
Here is what I will be putting on my resto-mod Rupp Roadster 2. It should be in the neighborhood of about 12 to 13hp. It still has the stock camshaft so I doubt it will turn much more than 6000rpms when it is attached to the 20 series TAV on the bike. Should I pony up another $75 or more dollars for an aftermarket camshaft or will the stock one be plenty good?

 

GustoGuy

Active Member
#5
Unless you put a cam in there, and change out that restrictive exhaust, you won't be anywhere near the HP you claim.
I do have a nice pipe but it doesn't fit between the seat and the fender on the Rupp Roadster 2. I am going to take my pipe to Exhaust Pros and see if they can bend it to fit. As to the cam how much more power would a Mod 2 cam add over the stock cam? If it has 11hp with the pipe I will be pleased and the bike will be plenty fast. I heard with stock internals and low restriction exhaust and re-jetted carburetor NR RACING dyno'd about 9hp with all the stock parts untouched in the crankcase.
 
#6
I do have a nice pipe but it doesn't fit between the seat and the fender on the Rupp Roadster 2. I am going to take my pipe to Exhaust Pros and see if they can bend it to fit. As to the cam how much more power would a Mod 2 cam add over the stock cam? If it has 11hp with the pipe I will be pleased and the bike will be plenty fast. I heard with stock internals and low restriction exhaust and re-jetted carburetor NR RACING dyno'd about 9hp with all the stock parts untouched in the crankcase.
I ran the MOD2 for a few months, and IMHO it is a bit too tame, with only .226 for lift, which isn't much more than stock. The main diff over stock cam is the longer lift duration.

I'd go with at least the .308. Although that said, I just put in the .356 and it's pretty impressive, but you have to spend a lot more on other parts to support that (valves, etc). I figure I'm easily over 15hp now.

Here is a chart I put together on various cam options from either OMB or Gopowersports.com:

1565704525366.png
 

GustoGuy

Active Member
#7
I ran the MOD2 for a few months, and IMHO it is a bit too tame, with only .226 for lift, which isn't much more than stock. The main diff over stock cam is the longer lift duration.

I'd go with at least the .308. Although that said, I just put in the .356 and it's pretty impressive, but you have to spend a lot more on other parts to support that (valves, etc). I figure I'm easily over 15hp now.

Here is a chart I put together on various cam options from either OMB or Gopowersports.com:

View attachment 248110
That 308 cam sounds nice. Easy on the valve train and a very wide power band. It should mean a nice easy to drive yet powerful minibike.
 

GustoGuy

Active Member
#10
stock cam is done by 4800, you can force it to rev higher but its on the downhill slide as far as makin power. I'm a 3rd for the 308 cam, should be exactly what you want.
Sounds good. I know I would like it but I will first see what the bike does with the stock cam since it should easily be putting out about 3 times the hp of the original HS40 with a low restriction exhaust pipe which I got to bend to fit the Roadster 2 frame. I used to ride 250cc dirt bikes when I was a teenager and I know I will like it but I am a bit concerned about pumping too many steroids into a bike which will he ridden by novice riders in my family like my wife and Stepdaughter. I am looking for a good running and easy to ride yet powerful enough to have fun Rupp Resto-mod. I know there are tons of go fast parts for this engine that can be added like a pinch of spice if it is gutless and if your not careful that you can easily turn it into an nearly unridable by novices overpowered bike. Plus the .308 cam shaft is another $105 and this build is starting to get expen$ive. I will definitely add the .308 cam if I am not satisfied with the bike's performance. Heck lots of people were satisfied with a stock 4hp Tecumseh let alone a modified OHV with a billet rod and flywheel and not to mention a Mikuni 22mm and low restriction exhast.
 

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