After exhaust mod...cutting out at top-end.

M

mrsaxman99

Guest
#1
This mostly has to do with fuel. This situation has made itself much more evident after opening up the exhaust a little (35% less back pressure or so).

So consider fuel setting A:

Engine has plenty of torque down low, midrange is good. Pulls hard till a certain (unknown) rpm, and then it just bogs. If you stay on the throttle, it'll bog till it gets back down to under the bad point, where it kicks back in and pulls strong to the "bog point" again, to repeat indefinitely.

Fuel setting B, where I've leaned it out a good bit:

Little less torque in the low range, about the same in the middle. Pulls the same up high, except the "bog point" is all but gone. It pulls under load to a MUCH higher rpm, and pretty much at redline, it finally reaches the similar conditions of the previously mentioned "bog point"


Here's what I think:

I think since I opened up the exhaust a bit, the engine is obviously flowing air much better. I think the intake is the limiting factor, and under fuel setup A, the intake simply isn't flowing enough air (really small intake), and it's sucking in way too much fuel, and bogging the engine. I think the richer fuel condition A is really the ideal fuel mixture, but when the intake restricts airflow it just starts sucking in too much fuel.

I know I know...typically an engine leans out under higher rpms...but then how come the early "bogging" symptoms disappear when I lean out the mixture.

The surefire way to test my theory would be to open up the intake, which I plan on doing.

What do you all think about this situation?
 


G

gone_fishin

Guest
#2
this is what i think...i've taken the end of my muffler off & played with the mixture settings. and i've been trying to find the right words to describe the exact same conditions!!

i know it's not a leak, because she runs too goldurned consistently, but i can surely feel the difference between under load and when inertia takes over.

i really look forward to seeing this one get worked out, because i can just feel it trying to go faster...

i wonder if the directhits thing would do it, i eagerly await feedback on that lil cutie.
 
C

chieflets

Guest
#3
simple

when you make the exhuast bigger you must make the intake bigger

the standard exhuash manifold and intake manifold are great for 48cc motor, but the chinese have used same exhuast and intake for 48cc which don't match up the cylinder ports

1. you must custom make new exhuast and intake flanges and larger diam pipes

Chieflets
 
G

gone_fishin

Guest
#4
well, the "simple" part is a given, plenty of fuel and the ability to use it, but...even if i dremel out the intake port and manage to make a bigger intake manifold, won't i still be limited by the carb inlet size?

so, since i have no fabrication facilities, and certainly can't afford to modify the carburation, i'm still gonna cross my fingers and hope the directhits will burn that extra rich high-end mixture, then i'll just use a bit more oil.

this is why we need to organize a real motosport, it would encourage and reward innovations that can eventually be used by all.
 
M

mrsaxman99

Guest
#5
I'm just curious why it is acting the way it does. Can the carb/intake not flow enough fuel or not flow enough air?

I would assume that with lessening the restriction in the exhaust, the engine would flow more air, which would lead me to believe that the carb ISN'T flowing enough air or fuel. But when I modded the exhaust and this problem started, I actually changed the needle to a LEANER setting, and it improved the situation.

Just seems kind of contradictory, doesn't it?
 
G

gone_fishin

Guest
#6
whenever you hit a slight incline or face the wind it breaks into the sweetest & strongest "buzz" eh?

doesn't it feel that once your weight/momentum takes over it's like a retarded timing setting? like, if you could advance the spark a bit it would take off like the proverbial bat?

problem i have is i keep trying to relate my 4-stroke knowledge to these things & i wind up at impasses like this one.

is it just that the 70cc needs more everything than the universal carb/intake can give it? what about if we do manage to increase our top-end, won't we just experience the same thing at the new higher speed?

just for giggles, i'm gonna go try all 4 notches right now...i'll pay close attention and give as full a report as i can.

EDIT: as soon as i posted this it started raining again, this may take a while...
 
M

mrsaxman99

Guest
#7
yeah lemme know what you find out with all 4 carb settings.

Do you have stock exhaust / exhaust restriction?
 
M

mrsaxman99

Guest
#10
I am at work and can't get to my bike, so you might try removing your air filter just for a quick spin to see if that helps. I'm kinda curious about that one
 
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