I just put a Pirhana 125 4 speed manual clutch in my 70..
I am no expert, but I will try to give you some good reading here for some good tips..
1st of all..Do you want it to be auto clutch or full manual?
If you want the auto clutch, then a Lifan 125 would be best, but I have heard that the trannies dont hold up well in a auto clutch engine above 110cc.
I used the 125 Pirhana engine with a clutch, specifically becuase I wanted something that had balls and clutch action, since I have a stock 70 to putt on.
So you need to make your decision. I would advise nothing bigger than a 125 either way..140 and 160 engines in a 70 chassis will wear you out in minutes. Unless you are a big boy..
2nd.. Only go with Pirhana, Pitster Pro or Lifan..They seem to have the best durabilty.
Now as long as you stay with the 125 engine or smaller, they almost bolt right in. You may have to grind about 1/8 inch off your frame at the rear upper motormount for the case to clear the frame..No biggie..Hell a file will work.
Get the complete set-up with the carb, and electrical..They hook up easy and are tuned right outta the box for the most part. You can even wire your stock thumb throttle killswitch to ground the coil and kill the motor.
You can use the stock cable, but it has to be modified for the bigger carb..A pain in the butt, for what a proper cable will cost, and be simple I would suggest the proper cable.
You will have to get a pipe. Either a 70 bigbore pipe from Sterdy, or you can use a cheap pitbike pipe and silencer and tweak the rear tube to clear your fenders. But it has to be done right or you will melt your fenders.
The 2 biggest problems are these.
1)The bigger carb will not clear the gas tank..There are many ways around this.
Some guys notch the intake bolt holes at the head, and rotate the carb to the inner frame rail, then dent the back side of the gas tank so the carb clears.
Others buy a shorter intake, that allows the carb to clear the tank, and others will cut the intake and reweld it so the carb clears the tank.
Some guys just adapt a different tank..
2)The starting..
You can either push start it...PITA
Or you can shave your fenders, and widen your axle and use the kickstarter like me...
Or you can buy a pullstart conversion kit...
Now here is some links for you...
This is a large thread of highly modded 70's, that has some great tips on how to make these engines fit..But you have to read the whole thread..They answer alot of questions..
http://www.3wheelerworld.com/showthr...-70s-In-action!!
Here is a link to my build, which is still currently under construction, but it is rideable now..
http://www.3wheelerworld.com/showthr...pdate.-9-23-10
Now I bought my engine from these guys..But they only deal Piranha engines..But they had it all in stock, great prices and 30 minutes from my house.
http://stores.wholesale-cycle.com/StoreFront.bok
Sterdy has a crap ton of parts for the 70 mods, and they are the ones that have the throttle cables you will need..
http://www.sterdyproducts.com/
And these guys have some things too. Dr ATV..They have the pullstart kits..
http://www.dratv.com/newreen.html
As far as the rear brakes, you either have to figure something custom out and adapt newer stuff, or add some front brakes..
Someone else can chime in on that.
It isnt that hard to do this conversion..And it is cheap..
Sorry for the long post, but it should give you some good ideas and a start point.
Here is where my long useless list of stuff nobody cares about should go...
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