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.
 
#11
Back in the day The first RUPP I bought had the end of the crank broken off. I used a bottle jack and shoe horned a 7hp Tech into it. That was a really big difference over the stock engine. No body that I knew had a fast one. So any big HP up grade will make a big difference in how it runs.
 

GustoGuy

Active Member
#12
Back in the day The first RUPP I bought had the end of the crank broken off. I used a bottle jack and shoe horned a 7hp Tech into it. That was a really big difference over the stock engine. No body that I knew had a fast one. So any big HP up grade will make a big difference in how it runs.
Plus a Non Hemi Predator 212cc with ARC billet rod and Billet flywheel with a Mukuni 22mm and a low restriction exhaust should make at least a solid 10hp. I may pony up for a 308 camshaft if I find it to be lacking in HP. However I am 5 11 and 188lbs and I have ridden upto 250cc dirt bikes and in my family there are fairly novice riders so even without the camshaft it going to have a ton more power than a HS40 ever did and people seemed to like them enough stock.

A neighbor when I was young had a Rupp Roadster 2 and it was a fairly decent about 35mph to 38mph stock bike but it did not hold a candle to the YZ80 which destroyed it in every performance category including off the line acceleration or even the Harley Davidson 90cc especially once the Harley could get up a head of steam since it needed to be shifted. Both of these bikes would blow past the Rupp every time. I am looking for decent 50+mph performance with good hill climbing abilities and decent acceleration and several members have re-powered Rupps with clones and said it was a huge performance increase over the 4hp HS40 Tecumseh which was the stock engine in these bikes.
 
#14
I was 180 to 195 back then and I am 5' 11" and was in my 30s with a stock 7hp it would do wheelies and stay up a ways too. And my boys just loved the power it had too.
My 23 year old son is disappointed that I haven't finished the Rupp. He loves riding the minibikes. Also it is pretty hard to put in a lot of time to finish it when you are working every day except for a few days off here and there. I have this weekend off but I will be doing things with the family and then next week on Monday through Sunday I will be working all week again. I am looking forward to just finishing the bike. Today I am painting it and I am such a stickler for details so I have to keep reminding myself that this is going to be a toy and a rider so who cares if it has a slight imperfection in the paint.
 
#16
Built a rupp restomod like yours a few years ago. TC with a warmed over predator. big valve head, black mamba JR and Chikuni. Plenty of power everywhere and if not careful it can get away from you!
Ole, I've been wanting to get a new carb. Do you happen to have a link for the Chikuni you like to use, and a jet kit for it?
 
#17
Here is one unfortunately someone on here had bought one of them and it had different jets than mine. I have bought a lot of them and they all used the same jets and pilots. The stock ones I get have a 15 pilot and that will work on most applications. If you have to open the adjustment screw out more than 2.5-3 turns out to get it to idle then go to a 17. For a main jet 130 will get you in the ballpark and be safe from too lean as stock jet is a bit on the lean side. Here is a link to one with the honda style manifold which I modify to fit the clones but you need a tig to do it.
https://www.ebay.com/sch/i.html?_from=R40&_nkw=mikuni+26mm+carburetor&_sacat=0&_sop=15

This rebuild kit show the type of jets and pilot mine use
http://rover.ebay.com/rover/1/711-5...0001&campid=5335845805&icep_item=202774916364
Pics of the ones I use and a pick of the modded manifold
 

Attachments

Last edited:
#18
Ole, I've been wanting to get a new carb. Do you happen to have a link for the Chikuni you like to use, and a jet kit for it?
Here are some pics of the manifold you are talking about and the carb. They work well but need the abrupt transition from round to dport blended a little. I use jb weld and make the transition about a inch long.
 

Attachments

Top