Ru-p Roadster 2 engine replacement
- Thread starter Woodstock
- Start date
This should explain the upside/downside to slide carbs on a Tecumseh.
http://www.pinrepair.com/minibikes/tecumsehmb.htm#carb2
http://www.pinrepair.com/minibikes/tecumsehmb.htm#carb2
Rebuilding an hs40 isn't hard but it usually requires an oversized piston to do it correctly. The reproduction Rupp parts website called black widow motorsports is working on making some and should have some soon. If you can find a piston just send it with the block to any good kart engine shop and they can rebuild it for you. I have used dyno cams and checkered flag racing in the past. (both do good work)
If you want the look of the old hs40 without rebuilding it you can throw the shroud flywheel lighting coil and points system into a newer hs50 and it will look just like the older hs40 but have a bit more power. as far as a carb I would think a slide carb around 19mm would work good because the optional Dellorto carb that Rupp used to sell for this bike was that size. I have herd of people who put a mikuni vm22 on this bike with success. Just remember any aftermarket carb will need an intake just as large as it to get the best performance. you will have to either make that yourself or have it made for you. If you use a slide carb it will disable the governor so first open the block and add a billet rod, without it you could snap the stock rod and that can do quite a bit of damage to the motor and possibly you as well.
arc racing makes a billet rod for the ohh55 and it fits this motor. also remove the governor gear while your in there.
If you don't decide to use the old motor you should consider selling it. There are plenty of people looking for this engine and they are hard to come by.
If you want the look of the old hs40 without rebuilding it you can throw the shroud flywheel lighting coil and points system into a newer hs50 and it will look just like the older hs40 but have a bit more power. as far as a carb I would think a slide carb around 19mm would work good because the optional Dellorto carb that Rupp used to sell for this bike was that size. I have herd of people who put a mikuni vm22 on this bike with success. Just remember any aftermarket carb will need an intake just as large as it to get the best performance. you will have to either make that yourself or have it made for you. If you use a slide carb it will disable the governor so first open the block and add a billet rod, without it you could snap the stock rod and that can do quite a bit of damage to the motor and possibly you as well.
arc racing makes a billet rod for the ohh55 and it fits this motor. also remove the governor gear while your in there.
If you don't decide to use the old motor you should consider selling it. There are plenty of people looking for this engine and they are hard to come by.
Thanks for the advice. I’d like to keep this engine. I kind of looked around for a hs50 motor but didn’t find any in good condition. Besides eBay and Craigslist, where would a good place to look for one in good condition? I will look at those shops you mentioned. Lots of information out there! I had no idea.
if you live in a snow area, a nice HS50 are generally available from snowblower. low hours, no dust/dirt, cheap (if not free), generally pretty darn nice. and you can convert it to a mini bike application. again lots of info on this at www.pinrepair.com/minibikes/tecumsehmb.htm
quick tip for some easy power gains: where the governor spring attaches to the governor arm there are multiple sets of holes, move the spring up to the top one for some increased rpms. Also some Tecumseh 4 and 5hp carbs in stock form only open 3/4 of the way, grind at the spot the armature hits until it can fully open (with the carb off the motor looking at the throttle plate to determine this) and instantly get more torque. got to love free power!
Need to be careful since old Tecumseh HS40 engines were originally governed at 3600rpms. I would not operate one at over 4000rpm. Connecting rods can be replaced with Billet connecting rods but if it is an old engine with significant wear fitting the crankshaft can be difficult. One reason why Rupp only built the Black Widow bikes for just a short time plus gave them no warranty was connecting rod failure since the Dellorto carburetor effectively bypassed the govenor on the engine.
