High RPM little problem..

DanielMaia

DanielMaia

New Member
I guys,

I have my motorized bike, with expansion chamber exhaust, intake port-matched, and a main jet size #60 that i re-jetted..

The acceleration is very good i think, but in high rpm, 30mph or so, the motor starts to fail, like when you are out of gas, and then 1 or 2 seconds later she starts to rev again higher, and then the fails again, go to 26 or 27mph, 1 or 2 seconds, again, reving higher..

I have a TACH, and this problem occurs @ 7200, 7300RPM,

Its weird because this not happens ever i go to high rpms..sometimes it revs very good to 32, 33mph..

I thought it was because the motor sucks all the gas from the carburetor (when the gas drops, im not remember the name of it) at high rpms....and..its not because of tank pad, because with the tank pad off, the problem persists,... :\
 
Last edited by a moderator:
Sounds like it starving for fuel at wide open throttle.
You could check what the float setting is, float should be level when up side down or close to that.
Make sure the fuel valve is all the way on.
Make sure the in tank fuel filter and if it has one, and if it has one, the fuel filter is not getting close to being plugged.
Check the fuel line make sure its not collapsed inside.
You could take the fuel line off at the carb let the fuel run into a glass bottle or jar see if your getting a good fuel flow how much should it? I'm not sure but you should get a small stream kind of like when you take a whizz only it won't be that fast?
NO SMOKING while doing this or NO open flame or HOT engine you don't want to go up in a ball of flames.
As for your jet size? I run a 0.65mm I'm 3400 feet above sea level. Some guys run 0.70mm that are close to sea level .
Those sizes are the metric drill bit sizes. You say you are running a #60 is that in metric? It maybe too small and causing the cutting out or surging at wide open.
You might want to go a little larger on the jet size. The main jet affects wide open so that may be whats wrong, if so it is a simple fix by going to a larger jet.
Norman
I'd try changing the jet all the rest is if you can't get it to work after changing to a larger jet.
 
I had this same problem, just above 30 the motor would start to vibrate badly and I'd lose power, even on a good downhill grade it would not brake 32 mph. I figured the problem was because the gas in the float bowl was foaming. I balanced my crank a bit and now it'll cruise along at 33 on flat ground no problem and even made it to 40 on a downhill grade.
 
With all the extras you added, did you replace the pixie straw clear kit gas line?
Get automotive fat gas line if not.
You might want to pull the tank and carb filters out too and just put a clear plastic filter in-line with the new gas line.

If you experience the problem again then look at it, if it has gas and no other filters in the way, the problem is elsewhere.
 
Norman, even with #65 (0.65mm) it 4-strokes @ high rpms...a little...

Im 40 or 50m above sea level...

With the #70 it 4-strokes a lot..

So, what i understood...can be the low jet size or the fuel line..yes i have a filter, i will unplug it to see if it's better ;)
 
grenade? what that means?

I can go to 7800, 7900RPM..my max rpm it was about 7950RPM...8000RPM is a little over redline for this engine,

And the engine is very good...is pulling me up hill @ 30mph, sometimes 31mph (50km/h)

I only have this little problem, starving for gas @ high rpm's..
 
From assorted member's feedback, it seems that the "normal" redline for these engines is about 6000... granted there's a LOT of variance from engine to engine & you prolly lucked out and got one that's a lil better balanced than the others - but I figured I'd mention yer defo in the "red zone" lol

Still, I'd follow Norm's advice startin' with the float settings, I had a similar problem and that's all it was...
 
Last edited:
Yes, it revs very good at 7000RPM, i think its a good engine,

I took a pic of the spark plug, its a NGK BPR6HIX Iridium:

PICT0134.jpg
 
You have an exceptional engine if it will turn 8000 rpm.

By "grenade time" I meant most of them will start throwing parts at 7500 rpm if held there for a long period.
 
Thanks guys ;)

The engine is with 1500km on it, with a oil ratio 20:1, and its very good right now

The motor is kite inclinated to the front, and the carburetor is not 100% horizontal, can be a problem?

What do you guys think about the spark plug color i posted few posts ago?

thanks
 
You must log in or register to reply here.
Back
Top