Re: Hey, new guy here. Planning builds for 8 bikes on MTB frames. Recommend some pa
Definitely got a solid plan for your builds... i know mountainbikes do have their challenges with space limitations and all (I learned this one first hand) but not too bad once you get one done and can repeat what you did on the others to get things to fit.
For your questions... Here's my thoughts and answers...
Bike... Your choice sounds good as long as the frame is 20" or more, anything smaller and you'll need to pull the engine to change a spark plug like me. Carb clearance is an issue on mine too.
Motor... You're right again, they're really 66 or 70cc depending on wich crank they come with, they're marked and sold as 80cc because these manufacturers count the combustion chamber volume along with the bore and stroke so yes, 80cc, but really 66 to 70cc when you measure with conventional ways. There are a few of these engines that are noticeably better than the rest... Dax engines are really good quality with 1 piece cranks, 40mm stroke, upgraded bearings and run very smoothly, whether or not they're balanced I'm not sure what they do but they do run very smooth compared to others. Neil (MotorBicycleRacing in this forum) also sells a Very good engine that has all the goodies mentioned about Dax's as well as very smooth running. Both excellent engines to start with, especially if you plan on adding more power to them.
Exhaust... The SBP pipe is one of the best, but definitely the easiest to tune and they make excellent power, arrow's Snake pipe is also very powerful. Other than modified dirtbike pipes, the SBP or Snake pipe is best.
Carb... the stock NT carb will do just fine and I got one pushing mine up to 40mph easily, unless you plan on laying down some money for a Mikuni vm18 or Dellorto phbg 19mm + manifolds, the stock NT will do very well. another really good carb is the dellorto Sha 14-14 carb and it's clone the RT carb sold on Dax's site... The RT is a bit more responsive than the NT and easy to tune, they're priced very nicely too.
Head... The Fred head, the Diamond head, or a Hi Hi head from a 70cc Puch all work great for better compression, performance, and keep the engine nice and cool, even after a hard run. Stock heads are ok for stock engines but with low compression and they tend to get too hot too fast.
Mounting, you definitely want to upgrade here, the stock mounts aren't very rigid to the frame so lots of vibrations are transferred into the frame and handlebars. using solid billet mounts or welding brackets to solid mount the engine to the frame will give the smoothest ride. For the rear mount I used 2 rear mount blocks and did away with the mount strap, made things way more solid and rigid. The rule of thumb for mounting is that the engine should not move if you push it hard by hand, it should be as rigid to the frame as possible.
Chain... The 415HD chain will do well if you get a good namebrand chain or the 41 chain will also work really well. try to get good quality chains if you can.
Case matching... If you can do it I'd recommend it, it's pretty sloppy fit and not hard to clean up, there's also a nasty casting ridge at the top of the transfer openings, just cleaning up this ridge and smoothing it will make a noticeable difference, also cutting 5mm off the bottom of the piston skirt on the intake side will help with more port duration and better port timing, going past 5mm can make the engines boggy at low speed so don't go past 5mm here... There's a lot of porting how to's in the high performance section here if you want to go further than just the matching, cleanup, and piston cutting and ramping, but just by matching the transfers, cleaning up that casting ridge, chamferring the ports, and ramping the piston 1mm down at the transfer openings and exhaust port with the 5mm cut on the intake side of the skirt will give you much better performance without sacrificing low speed performance.
Seat... The Cloud 9 seats are very nice and comfortable, a layback seat post really helps if you're taller. There's a member in here that makes layback seat posts out of solid bar stock and they're very nice, no flexing or bending, they also help absorb any remaining vibration keeping it off the seat area. You can also fill your bars with lead shot if they feel buzzy from engine vibes, possibly not necessary if you got good well balanced engines, but that's a fix if there is vibration discomfort at the seat or bars.
Tank... The 2L tanks will provide plenty of ride time per tank since these things get very good milage, if you still need more fuel, used moped tanks or replica moped tanks can be found on ebay, you can also mount one of those Honda CRF50 clone tanks if you like the plastic rugged dirtbike look... There are also a few who make custom tanks, but they can get pricy. One cool looking one is the behind the seat mounted mini keg tanks, they hold at least 2L and mount to the seat post out of the way and look great.
Others will have similar or different opinions about all this, but most of them will be good suggestions and opinions, a lot of it is about how you want them to run and how you want the bikes to look. For the engines and performance, I just mentioned the basics, but a few others and myself can help you build something really fast if interested, there's a lot of talent in here so enjoy...
Hope some of this helps and enjoy the builds...