carb stall

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FileStyle

FileStyle

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May 27, 2008
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what makes a NT carb poop out on the top end, but runs and idles good everywhere else? starts and runs good and at 38mph it just falls flat on its face at 38mph! then picks up again a few seconds later! I want to think that it's a fuel issue! it acts as if it runs out of fuel momentarily or lack of fire! any body have any suggestions! I have another carb that just keeps going, it hits 38 mph and still climbs til almost 41-42 mph! I ordered a couple of NT carbs but it seems that the round slides seem to not want to slide easily, I tried ea. carb with my own slide and neither helped only made it run worse! so whats the deal, they all look the same but what is the difference and what to look for when comparing carbs?
 
BarelyAWake

BarelyAWake

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FileStyle said:
what makes a NT carb poop out on the top end, but runs and idles good everywhere else? starts and runs good (then) it just falls flat on its face... then picks up again a few seconds later! I want to think that it's a fuel issue! it acts as if it runs out of fuel momentarily...
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I'd say it's a classic case of fuel starvation - usually from the float tabs being too low/excessive angle of carburetor, tho sometimes it can be as simple as insufficient venting (usually cap).

IF your speeds are accurate... then there's the remote possibility the your exceeding the fuel inlet port's ability to fill the fuel bowl - but I seriously doubt that's the case ;)
 
bairdco

bairdco

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Aug 18, 2009
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i believe that it is fuel starvation, not caused by the carb, but by the petcock. my bike used to do that all the time. the inlet of the NT carb is bigger than the outlet of the petcock, and i would have problems with my fuel line. i used 1/4" I.D. fuel line, and it would get pinched by the hoseclamp on the petcock. i finally re-tapped my tank to take a 1/8" pipe thread petcock, installed a better one from a tecumseh lawnmower that had
a 1/4" outlet, and the problem completely went away.

(edit) i tried everything else, too. float tabs did nothing. from my experiences with cars with multi carbs, it seemed to me to be exactly what B.A. is winking about. the fuel wouldn't flow fast enough from the tank to keep the carb full.
 
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BarelyAWake

BarelyAWake

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Oh right!

I'm glad ya mentioned that baird, I completely spaced that 'un as I don't run the stock petcock for that & it's habit of leaking :p

meh... I need mo' cawfee :D
 
FileStyle

FileStyle

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yeah , 38mph is nice but I know it can go faster. something with carb itself is wrong, I will look at the float arms. also could it be a jet issue? I have noticed that jets are different as I have checked with a micro drill bit that one jet is a little big compared to the other. would too much fuel cause this to happen also? the plug looks good tho, not too rich and not too lean.
 
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bairdco

bairdco

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Aug 18, 2009
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i don't think it's the carb. i think it's burning up the fuel in the bowl faster than the flow can replenish it. i had the exact same thing. my bike would rev up nice all the way, then, when running it at top speed for 15 minutes or so it would start to bog. i'd pull the clutch, let off the throttle for 20 seconds, then gas it again and it would work for another 10-15 minutes.

i changed the fuel line, clean out the carb, check for debris, floats, everything but re-jetting, and it'd work again for a week or two, then start all over again.

finally changed the petcock and it's never done it again.
 
circleball

circleball

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Aug 24, 2010
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grubee skyhawk 66cc + CNS Carb + micargi prado deluxe, was going down the loop to a friends, first "long haul" (lots of 5 min. n under rides) bout 10 min. in, doin' close to top, (i think). Bout 30~40mph ??? i dont have a speedometer yet, sorry. the limit is 55, and they look like they were doing about 15mph when they would pass me. i still had some (throttle to spare). it "bogged down"?? i left off the throtle and it kinda straightened out, but not really. that top speed is still unattainable. i lowered the needle to restrict the gas flow on the top end, by one notch. the low was not running right till i made the adjustment, but the top is still missing. Even on a cold start,.... or warm. the carb. is full, not a loss of gas. i still need one more notch to full drop. the front skrew(toward the engine is the idle. IN IS FASTER. the other is all the way in. (no one covered the overage, just the lack there of)
 
James912

James912

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Apr 12, 2011
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I wonder if that may be why my engine bogs and dies after running for about 15 minutes at any throttle level. I have to wait about 15 minute and then it starts again. xcept, how would u retap a stock tank?
 
James912

James912

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Apr 12, 2011
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I fixed my bike. The reason it would stop working after 15 minutes is the gas cap does not have a top vent, there are only air holes on the bottom of it, below the rubber seal. I drilled a hole in the top and she keeps running. Just get a waterbottle cap and hot glue it over the hole and drill a hole in the side of that if u want to ride in the rain.
 
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