Trying to tune my new 70cc engine. The stock carb would do this at about 34mph with the throttle open. Just cuts cold, no noises, no pinging, good acceleration then nothing it dies. Starts back immediately though. Now with the VM-18 it does this at about 45. Is my bowl running dry? Am I not getting enough fuel? Do I need to upjet?
70cc cuts cold at high speed
Re: 70cc cuts cold at high speed
I'm gonna try upping the jet, and see how it runs.
Re: 70cc cuts cold at high speed
VM-18? Man we gotta talk. I'm about to buy one of those. How have you got it set up?
For the record my vote at this point is yes, you're running the bowl dry. But give us more details.
Re: 70cc cuts cold at high speed
check that the vent hole in your gas cap is working
(turn fuel on with it on, check flow. then turn fuel on with cap off, check flow).
then make sure that your intank filter isnt plugged up.
haha this sounds like my emt classes
Re: 70cc cuts cold at high speed
Thank you for the help, I'll go check it out. I'll change the filter too, it needs to be done.
It's a QT50 with a YT60 cylinder bored out to take a 70cc piston. Biturbo exhaust, YT60 head, boysen reeds, ported as much of the intake manifold as much as I could reach, and the VM18 which is gorgeous.
Vm18 took a while to set up, and I still haven't adjusted the throttle cable enough to open the throttle fully. If anyone is trying to do the VM18 on a Yamaha QT I have some tips for ghetto rigging it:
1) If you can't get a delrin spacer made (my local machine and bike shops couldn't do it), uses your stock spacer and space it out using strips of aluminum between the intake manifold body and the spacer. Works great.
2) Making the right length throttle cable is easy. There is a 2 to 1 throttle divider that runs to the stock carb and to the oil injector. take both cables off and use the oil injector cable in reverse as your throttle cable. This is nice because the oil injector cable has an adjuster on it. I have mine adjusted all the way out. Also, keep the cable on the oil injector spot in the 2-1 divider, this position gives more throttle pull.
3) I'm working on making the choke adjustable from the handle bars, it looks do-able.
Re: 70cc cuts cold at high speed
My VM18 is going to feed a malossi kit on a motobecane 40t. 62cc. gorgeous, hunh? that sounds good ..
go on...
Re: 70cc cuts cold at high speed
Very easy to tune using the airscrew, moped has much more low end, mid range, and high end power and runs much smoother. I should have done this mod a long time ago, as the stock carb was a throw away piece. Comes with a nice tuning manual that I read cover to cover. I bought it from:
Re: 70cc cuts cold at high speed
shee-it, that's a lot cheaper than i was quoted! what are you jetting at?
Re: 70cc cuts cold at high speed
Running the float bowl dry or it's way too lean is all I can think of. Def. go up a few sizes and see how it does. See if you can go up until you 4stroke.
Re: 70cc cuts cold at high speed
this is why i'm looking for the biggest petcock i can find.
Re: 70cc cuts cold at high speed
if it just stops dead no pop or warning it is ignition not fuel. fuel problems the engine will starve for fuel or run lean a bit or whatever first before dieing so if it is just suddden silence it is ignition.
Re: 70cc cuts cold at high speed
no fair wari, you've had more sleep than me!
of course, she's right you know..
Re: 70cc cuts cold at high speed
My bike has had a similar problem for a while now. It only happens at high rpms. I've suspected the ignition for a long time beause it just dies with no warning and starts back up again when the engine revs down a little.
No pops, no cracks or hesitations, it just dies.
When it comes back on it comes back on all at once, just like nothing ever happened,
Re: 70cc cuts cold at high speed
Looks like it was soft seizing, jet was a 70. Upped it to an 80 and it's fine. BTW if anyone wants a 70cc QT. Just take the piston for a 70cc Zuma or Jog. I believe it's listed as 47.6 or 48mm piston, and bore your cylinder out. I'm not sure this works with the stock cylinder as I used the YT60 I had blown for this project. Make sure you get the one with the 10mm wrist pin as the piston comes in both 10 and 12mm sizes.
This mod really made the bike, as it had pretty good mid-range and high-end before with the 60cc, but now its got low end and much better mid/high acceleration.
Re: 70cc cuts cold at high speed
try cleaning air filter and clean fuel jet incarb thats all
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