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Author Topic: flat spot 4000-5000 rpm  (Read 99 times)

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Offline evinrude7

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flat spot 4000-5000 rpm
« on: October 09, 2018, 06:47:12 pm »
this is something the bike has shown since running well.  so at least a year and a few months.  it's not a major issue for me but something i'd like to address during these colder months ahead.  between 4-5k rpm the bike has a flat-ish spot.  if i'm cruising along at 45-60 mph or so, usually 3rd or 4th gear, i notice a break up like it's not getting gas, but the bike doesn't lose speed.  i can give it more throttle and it generally starts to go away after about 4700-4800 rpm.  my guess on throttle is i'm at 1/2 or less.  stock airbox, 110 mains, 40 slow, jet needle at the 4th notch.  keihin jets and oem 4into4 exhaust.  no inline filters and only non-ethanol 87 gasoline.  floats were set last year at 26mm with the tang just touching the float needle but not compressing it.  3000 mile maintenance was done probably 1500 miles ago but again this is an issue the bike has always had since i got it running well.  i ride the bike often, usually a good hour or two straight on the weekend and few shorter trips during the week.   
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Offline madmtnmotors

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Re: flat spot 4000-5000 rpm
« Reply #1 on: October 09, 2018, 07:41:07 pm »
At 4000 to 5000 rpm you are in the transition zone to "full on" main jet. With stock intake and exhaust I would expect the "4th notch" is too far. 4th from the top (lowest notch) might be too rich, 4th from the bottom (highest notch) might be too lean. I had similar results with main jets that were too large. It would stumble approaching 5000 rpm and would even out once "over the hump".
« Last Edit: October 10, 2018, 05:47:20 am by madmtnmotors »
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Offline evinrude7

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Re: flat spot 4000-5000 rpm
« Reply #2 on: October 10, 2018, 05:40:36 am »
At 4000 to 5000 rpm you are in the transition zone to "full on" main jet. With stock intake and exhaust I would expect the "4th notch" is too far. 4th from the top (lowest notch) might be too rich, 4th from the bottom (highest notch) might be too lean. I had similar results with main jets that were too large. It would stumble approaching 5000 rpm and would even out once "over the hump.

that's the stock recommendation.  4th from the bottom i believe.  i verified it when i serviced the carbs well over a year ago.  maybe i'll move it down a notch and see what happens. 
« Last Edit: October 10, 2018, 05:42:30 am by evinrude7 »
cb750 k6 - ugly

 

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