Engine cuts out at speed aargh!

K so I'm starting to loose my mind...

I have a "80cc" 'Black Stallion' (NGK plug & nicely colored, 32/1 Amsoil*, no mods) kit on me bike and it's been a bit of a chore to get it running right. Four stroking for the longest time, now it's all broke in nice and running like a champ - mostly.

You may remember my "fuel starvation/hose clamp" post, maybe not - but the problem is the bike runs great until just about top speed (30mph) and then it just shuts down. Like off! Back down the throttle, it'll idle even - but unless you give it a few moments to recover from what ever is ailing it all that happens with a throttle twist is sputtering popping BS.

Give it a few (like 30 seconds or so) and she's fine - unless you try to get to top speed again

If I maintain speed at less than WOT, everything is fine. Give it to her, everything's fine. Try and hold top speed/high rpm & WOT = same as if I hit the kill switch.

The symptoms seem exactly the same as fuel starvation - but I've seriously been through the system. I've tried w/o the gas cap on in case it wasn't venting = no joy. I've pulled the line to check flow = it's fine, even down to the last drop. I went ahead and replaced the petcock, line, & filter = it's no better. I've even run about w/o my air filter in as it isn't stock and I wondered if it was too restrictive = nope, still not it... Floats? no - that'd be too easy ofc. Best yet - this is not an intermittent problem, it does this every time, but it has to be wide open throttle and at top speed/high rpm.

So at this point I'm out of ideas. I've got a Grubee comin in from the boys o'er at Spooky - I'm gonna swipe the carb off of it as the Grubee kits do have slightly nicer ones than mine, but I really don't think that's gonna help, I'm really starting to wonder if it's a fuel or air prob at all...

Could it be the CDI? I really can't figure this 'un out - it's driving me insane! The bike runs so well other than this little problem... grrr


*32/1 Amsoil is with their 50/1 injector flavor btw, great stuff

It seems as though the fuel is bubbling in the fuel bowl in the carburetor. The vibration of the engine causes the fuel to bubble, thus starving the engine of fuel until it settles back down. This is a common problem, don't worry about it.

o_O

Really? Ya think it's just vibration? It's an awful sharp cutout, like I hit the kill switch - I would think it would run rough with frothed fuel...

*shrug* I'll see what I can do to secure the carb and try and reduce vibration, heh - I can't just "not worry about it" it's killing my engine at speed o_O

Mopeds wanna race me sometimes, I can't let them get away with that

Yep - everything is tight and snug, at speed she doesn't 4stroke anymore, so there isn't really much in the way of vibration...

Just for the heck of it, I just rode and reached down and held the carb in hopes that it might steady it.

There was no difference and it really seemed to be less vibration at max speed than any other.

This is buggin the **** outa me

Did you take the carb apart and check the float, your float level may be a little low and it can't keep up with the fuel demand at full throttle, this tends to be a problem if the carb is sitting at an angle. All you would have to do is drop the fuel bowl and bend the little tabs up some so the float can come up higher.

Yep, I've done this too - but I wonder about the primer button pin. It seems awful long and prevents me from bending the tabs to allow for maximum fill as the float will hit the primer before closing the fill port.

Yet wouldn't this still be a flow problem? It's true my carb is at a slight angle, but wouldn't that just effectively make the bowl a touch smaller in volume? It seems to me that if it IS a starvation issue having just a drop or two more wouldn't make any difference - it'd still run out.

and I don't seem to have any flow restriction o_O



edit: Well, with yer thoughts in mind, I just took the primer pin assembly out and cut about 1/16" off the end of the pin and readjusted the float level to match. As my plug was a touch darker than usual (I just installed SBP's expansion pipe, prolly what did that) I also moved the needle clip up a notch to compensate.

Now I've only 15 minutes to get ready for work O.O

I'll get back to ya with whether this made any difference *crosses fingers* but I'm dubious because as I mentioned, I've run the carb at a steeper angle w/o probs before...

I stand humbled yet joyous

Despite all my denial and nattering on - as it turns out the 1/16" off the primer pin and rebending the float tabs did the trick!

I just clocked 34.7mph max with a sustained 33mph on my GPS without even a hiccup! I suspect the engine just wasn't running well enough before to reach the point of starvation, now that it's truly broken in and what with the small upgrades I've been doing (gaskets, some manifold porting & polishing, ign, pipe etc.) it's now running well enough to need the fuel at speed & WOT. As I've managed to beg, borrow & steal an additional 10mph since I first built it (and ran w/a more crooked carb) I guess I underestimated it's current fuel greed o_O

Thanks for all your help guys, I'm jus a stubborn ol' bastid that can't see the obvious unless it's stuffed in my face lol


WHOOHOO!!1! I <3 my MB

That's why I love this place!! I just learned something new!
I'm glad you got it going!!!

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Yeah me too Venice

Dang primer pin but btw - it still works fine even though it's 1/16" shorter, which is good cause my bike starts like a champ w/o choke and just 2 taps on the primer when she's cold.

I'd hate to not have that anymore lol

Living in Florida is a blessing as far as cold starts go.....mine fires right up after two pedals of the crank....and just like you BAW....pump that tickle button twice ...pedal twice with no choke and she fires right up !....every time too......down here we get to ride year round....it CAN get a little cold.....like 45 degrees sometimes but I can still ride I'm sure.....( it's not winter yet ).....actually it's NEVER winter here !..... LOL....Christmas shopping in shorts!


Laters,
Salty