Engine Sputter at High RPM?

cory151

cory151

Member
Hello, been a lurker on here for a bit, Im having a problem with a sputter at high RPM. The bike fires up fine and idles well, it:-|| just lost its top end somehow. A few days ago I installed a high flow air filter, NGK plug and an inline fuel filter (from Sick Bike Parts), took it for a testdrive and all was well. Road 4 miles to the post office today and it developed this problem. Any ideas?

Thanks, Cory

Bikes a 50cc Starfire nothing special
 
Last edited:
I concur... it's probably 4-stroking... (Just a hair too rich.)
What plug did you put in it , & what did you set the gap to??
 
Venice Motor Bikes said:
I concur... it's probably 4-stroking... (Just a hair too rich.)
What plug did you put in it , & what did you set the gap to??
Click to expand...

Im running the NGK B6HS I believe and it said its pre gapped. The loss of power is noticeable. Turning the needle valve on the carb doesnt seem to do much at all. Im at a loss, I could throw the old plug back in and see what happens. It wouldn't be leaking vacuum some place would it?
 
Nougat said:
Cory, "Turning the needle valve on the carb"? You sure you're not talking about the idle screw? And I bet you just need to snug the spark plug up, you're probably losing compression past a loose plug.
Click to expand...

Thats actually exactly what it was doing, adjusting the idle and nothing else, I take it these arent really adjustable other than changing out a jet? Ill re-snug the plug and report back.

Thanks again, to all contributors.
 
cory151 said:
Im running the NGK B6HS I believe and it said its pre gapped. The loss of power is noticeable. Turning the needle valve on the carb doesnt seem to do much at all. Im at a loss, I could throw the old plug back in and see what happens. It wouldn't be leaking vacuum some place would it?
Click to expand...

they may be pre gapped for a car which is usually 35 thou.on up to 80 or so...............
The air filter causes less vaccum in the carb to pull gas up the jet,I havent desided that there is any venturi action working for these carbs........
and the idle screw won't help the main jet as stated above....I would start w/gapping the plug at the correct gap xct2
 
cory151 said:
Im running the NGK B6HS I believe and it said its pre gapped. The loss of power is noticeable. Turning the needle valve on the carb doesnt seem to do much at all. Im at a loss, I could throw the old plug back in and see what happens. It wouldn't be leaking vacuum some place would it?
Click to expand...

they may be pre gapped for a car which is usually 35 thou.on up to 80 or so...............
The air filter causes less vaccum in the carb to pull gas up the jet,I havent desided that there is any venturi action working for these carbs........
and the idle screw won't help the main jet as stated above....I would start w/gapping the plug at the correct gap xct2
 
Just wanted to thank everyone for the input, I finally found a solution to the problem of the hi speed sputter. It turns out the phillips head screw on the muffler had backed out and was rattling 1/2" in out of the housing. Who knew that the muffler could have caused a loss of power on the top RPM range.

Another weird aspect of it was that my midrange power was greatly increased. Go figure, I may be in the market for a better muffler thats still somewhat compact.
 
zus800.jpg


Heres the plug
 
Yeah that plug was from when the muffler was coming loose. That explains why it had so much more midrange and low end power, leaners meaner.
 
You must log in or register to reply here.
Back
Top