W0T popping at 4400 RPM | It Is The Ride

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W0T popping at 4400 RPM by: toddmaples - Sept 24, 2018 17:56:22 GMT -5 via mobile

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Post by toddmaples on Sept 24, 2018 17:56:22 GMT -5

Ok guys, I've ran into something new and can't seem to remedy this problem...

The setup:
-1P39QMB engine (pair system removed).
-Gravity-fed tunable carb, no petcock/shutoff
-

main jet (went up to

and

during testing) 35 idle.
-50mm bbk
-Uni filter w/ about a 3" 40 degree elbow between filter and carb
-Stock exhaust header w/ straight-through muffler
-A9 cam (swapped to stock during testing, no change at higher rpms)
-Orange coil and cdi
Transmission setup is irrelevant because the motors not right yet.

I can take off around 2500 rpm, and it pulls strong, up to about 4400 rpm, then it starts popping, sounds almost like a backfire or combustion process pushing thru intake. Also the power band seems to "level off" here where as usually it leveled off somewhere around 6K for this setup.

Jetting didn't help, up or down, cam swap between a9 and stock didn't help. Timing and valve lash have been checked, re-checked, and checked again, looks fine.

I'm beginning to think the valve springs are weak and I'm getting float. Ideas/suggestions?

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W0T popping at 4400 RPM by: hillbillybob - Oct 3, 2018 5:40:56 GMT -5

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Post by hillbillybob on Oct 3, 2018 5:40:56 GMT -5

Even though you say it's not the jetting, that's what I'm going with. I had to go to this thread, to figure out which jets you have tried......(what a pain).

I have a bike that will pop/fart through the carb when giving it gas....but only if I take off before engine has had sufficient warm up time. Once it has warmed up, or if I open the enrichers, there is no popping. This is a clear sign of a lean condition. Since I'm getting this when coming off idle, or at lower throttle settings, come winter I will be replacing this bike's 40 pilots with 42's. Your problem is at the upper end, when that free flow filter is really doing it's thing, so I'd be increasing that main jet size.

2011 Jonway 150cc......16,324 miles & counting

My other rides are:
1999 BMW, R1200C..(Dooley)
2000 BMW, R1200C..(Schultz)
2006 Ural Troyka..(Boris)

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W0T popping at 4400 RPM by: chewbaca - Oct 4, 2018 23:26:58 GMT -5

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Post by chewbaca on Oct 4, 2018 23:26:58 GMT -5

check the valve springs
if they are not counter coiled
that being the inside spring being twisted opposite the outside
then the two springs can bind up at certain frequency

I have found wrong springs in both stock motors and after market sets
your motor should be good up to 5000 rpm with only a single outside spring
but will start dumping at the exhaust above that
good enough for temporary fix
is also somewhat more low end power with one spring on the valve

ty valve spring retainers and rocker arm grind and polish
can push the valve float up another thousand rpm if you wanted

Last Edit: Oct 4, 2018 23:40:12 GMT -5 by chewbaca

W0T popping at 4400 RPM By toddmaples (Read 517 times)

Post by toddmaples on Sept 24, 2018 17:56:22 GMT -5

Post by hillbillybob on Oct 3, 2018 5:40:56 GMT -5

Post by chewbaca on Oct 4, 2018 23:26:58 GMT -5

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