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Engine Sputter at High RPM?

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Old 10-28-2010, 06:03 PM
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Default Engine Sputter at High RPM?

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 by cory151; 10-28-2010 at 06:06 PM.
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Old 10-28-2010, 06:21 PM
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Default Re: Engine Sputter at High RPM?

Describe the sputter in more detail. If it's mild, with a barely noticeable drop in power, it's probably four stroking - which you should expect when it runs out of RPM at top speed.

With a better air cleaner, it might be running lean, though (more air, same amount of fuel). Check your plug color and adjust the needle clip accordingly.

May also be a clogging exhaust, unrelated to your changes, just coincidental. How long has this bike been running?
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Old 10-28-2010, 07:19 PM
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Default Re: Engine Sputter at High RPM?

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??
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Old 10-29-2010, 12:32 AM
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Default Re: Engine Sputter at High RPM?

I gap to .025-.027 How about the rest of you? Running a NGK B5HS
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Old 10-29-2010, 03:48 AM
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Default Re: Engine Sputter at High RPM?

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Originally Posted by Venice Motor Bikes View Post
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??
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?
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Old 10-29-2010, 07:08 AM
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Default Re: Engine Sputter at High RPM?

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.
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Old 10-29-2010, 11:24 AM
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Default Re: Engine Sputter at High RPM?

Quote:
Originally Posted by Nougat View Post
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.
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.
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Old 10-29-2010, 05:09 PM
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Default Re: Engine Sputter at High RPM?

Quote:
Originally Posted by cory151 View Post
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?
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
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Old 10-29-2010, 05:16 PM
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Default Re: Engine Sputter at High RPM?

Quote:
Originally Posted by cory151 View Post
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?
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
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Old 10-29-2010, 05:16 PM
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Default Re: Engine Sputter at High RPM?

please remove the second post
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