CT 70 Budget Build- YX140 with stock CDI &Coil70 mph Mark

Discussion in 'Modifications' started by Drew, Aug 5, 2018.

  1. Drew

    Drew Member

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    Most of you know about my CT 70 with a YX 140 motor totally stock with the original CDI and original coil and still using the cheap China carb-and hitting 70 mph top speed WOT. I recently changed my gearing to 18/35 that seems to be a really good gear for this engine.I know several people think ya so what but I think very few have actually done it on a cheap parts build,my wallet won't let me build a Honda stroker motor lol.I just wanna let people know it is possible to hit that 70 mph number and stay in a decent budget,I got $500.00 into my engine kit and oil cooler combined. I did go overboard on the exhaust,but I really like this one,$189.00 system.
     

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  3. kirrbby

    kirrbby Well-Known Member

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    That's awesome Drew. 70mph for only 500 bucks is hard to beat. If I ever want to try a Chinese engine in one of my bikes, I'll definitely consider that YX140. Sounds like you did really good, and it sounds like you are very happy with it too. Be safe, and ride the dog out of that thing :)
     
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  4. racerx

    racerx Administrator
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    This is where Chinese engines shine...dollars per horsepower and dollars per mph.

    Like kirrbby said, be careful. The stock rolling chassis is marginal above 50mph. Above 60mph, things start to change...rapidly & dramatically...it's basic physics. At 50mph, the dynamic forces will be ~75% greater than they are at 40mph and the curve is logarithmic(!). The brakes start losing effectiveness around 55mph, the stock shocks will bottom-out easily, as will the fork. Over anything but billiard-table smooth pavement, K0 pogo sticks are the stuff of white-knuckled nightmares, at high speed. Most tires available carry a "J" (62mph) rating. Knobbies, at freeway speeds, no thanks. I've experienced 80mph+ on a very well setup CT70. The last time I pushed that far was before this web board existed. Not trying to piss on your parade, only give you some advance info on what to expect. For the occasional short blast, the tires will live, engine braking can be used - quite effectively - to yank the bike below 60mph, before grabbing a handful of front brake and finding a stretch of smooth pavement, free of side streets + intersections, becomes far easier.

    I strongly suggest making sure that the brakes, bearings & tires are all in tip-top condition. The rear shocks should be upgraded to something with spring rates that match your weight. If they're bottoming-out, they're dangerous; eventually, you'll bend, or break, a shock mount. If your fork is K1-later, learn how to "oil tune" it; that makes a huge difference. And...when it comes time for new shoes, road tread improves handling & stability. Getting a passable 50mph cruiser doesn't have to cost much.

    IOW..."keep the shiny side up".:cool:
     
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  5. OLD CT

    OLD CT Well-Known Member

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    When I bought my complete 140 kit, it was only 375 shipped. 140's went up a little.
     
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  6. fatcaaat

    fatcaaat Well-Known Member

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    When people ask me about the bikes I build and the engines I choose, I don't have an issue recommending the Piranha 140's at all. From my experience, they are the stoutest versions. I have built them out to 157cc with bores and strokes. I have never had one fail. Stock, you should be able to pull 70mph...just run it 500 miles or so before you do it!

    With the TB roller rocker head, sd30cam, and a 146 high compression piston setup, you can up that top speed to just shy of 80mph and increase your gearing, and hence, your safe cruising speed to about 60mph. Def do an oil cooler on it. As a last item, the stock carb is ok, generally jetted lean from the factory, so going up 1 size jet on the main and messing with the needle is usually required. If you decide you want to update to the high compression piston and v2 head, you'll def want that vm26-606 carb.
     
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  7. Drew

    Drew Member

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    Yes already done most of your fore mentioned things.I put All Balls bearings in both front and rear hubs.Also in the steering tube.I bought aftermarket rear shocks that should be on a SL 100 or bigger,they were super stiff until I used them for several months on a old gravel road,then they got ALITTLE softer.And I bought front forks off a 1982 CT 70 that actually work,yes the original ones bottom out way to easy especially when coming down from a wheelie lol I didn't want to break the originals so I bought the way newer and nicer forks.I am still running both Trailwing tires,but I do have a ML 16 on another rim for the rear.Oh has anyone ever seen a 100% original front tire for a 1970 CT 70? They are NOT called Trailwing- they are called Trail 10 many year BEFORE they produced the Trailwing.My bike STILL has the original 1970 Trail 10 on the front.Looks just like the Trailwing,but it isn't.The production date on the tire is 1-69 Bet you can't match that.That was one of the first things I noticed when I pulled it apart and went through everything.I googled it,nothing about a Bridgestone Trail 10 exist.
     
    #6 Drew, Aug 12, 2018
    Last edited: Aug 12, 2018
  8. racerx

    racerx Administrator
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    I have seen a few...less than a handful... of those prehistoric Bridgestones. Never gave them a second thought. If yours is (are) in good condition, save them for display only...unsafe to use them, as functional tires, for sure.

    Overly stiff shocks are safer than pitifully weak ones. They're still rough on one's kidneys...and other parts. It's not mandatory that one has to endure the beating. Unfortunately, there are no inexpensive alternatives that I can recommend, at this time.
     
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  9. kirrbby

    kirrbby Well-Known Member

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    I have a few of the trail-10 tires in my garage. They aren't all that rare, but it's rare to find them without cracks/checks in them. If you can pinch them without seeing a bunch of tiny cracks, and they have good tread, they could be valuable. Same with old Nitto's. I actually think the Nitto's are older...but, I've been wrong before :)
     
  10. 79MAC70

    79MAC70 New Member

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    Drew
    I just purchased two 1978 CT 70's from the original owners in Palm Springs. The first thing I wanted to do to one of them was put a bigger engine in it, but I didn't want to have to change out the stock exhaust. Sounds like you are in the know, so can I put either a CT 90 or 125 cc engine on the bike without too much difficulty or changes?

    79MAC70
     

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