Won't rev past 5000 RPM - New build - Carb?

Discussion in 'General Tuning' started by TeeZer, May 18, 2016.

  1. TeeZer

    TeeZer Member

    R1, DNA 50

    I finally managed to get it my Gilera DNA out for the first run around the block since assembly over the winter. Unfortunately, I was a little disappointed as the engine will not go past 5000 RPM on road nor stand. So this rules rollers out, and comes down to carb I would assume (he says trying to ignore the possibility of the newly fitted crank seals being a possibility).

    The build is a completely new motor (including crank cases):
    • Stage 6 Racing MKII cylinder
    • Yasuni C16 City pipe
    • Stage6 R/T 35mm Intake and Spacer
    • V-force reeds
    • Dellorto VHST 24mm carb
    • 108 main jet

    Some background:
    Originally I had a problem with fuel blasting out the exhaust, which I solved by adjusting the float level. The float level can't be adjusted any further as .5 mm more, the needle closes and no fuel comes through. Symptom: was the bike would idle and then very abruptly cutout and battled to restart. Once restarted and throttle held wide open, it would hesitate sharply and dump fuel straight out the pipe.

    Seems similar problem but the bike idles perfectly, revs under 5000 RPM nicely from idle considering I have not adjusted anything yet. The problem is when I open the throttle, the bike just will not rev above 5000 RPM. It is smooth getting up there and then give a very sharp constant hesitation. I removed the carb and looked down the intake, and seems to be quite damp with fuel which I fear may be a float problem again.

    Research Possible problems
    • Crankshaft seals, but mine are new and fitted competently
    • Floats, been there
    • Main Jet way too large, will have to try this but 108 does not seem massively excessive

    Combined with the initial problem, I thought I had a carb angle problem, as it is at quite an angle. But looking at builds with the same VHST carb and intake, they seem to have no problems. Another thing is Stage 6 sell a VHST carb which I suspect matches their R/T intake I have. As mentioned, I will try a way smaller main when I get another chance.

    Any ideas out there?

    Here is a sound clip https://www.dropbox.com/s/apfxp43w06smkq2/out_crop.mp3?dl=0

    Last edited: May 18, 2016
  2. Stevep

    Stevep Well-Known Member

    Drop the jet size to 90. See what happens.
    I would think 108 is way too big and its flooding
  3. TeeZer

    TeeZer Member

    R1, DNA 50
    Thanks @Stevep, will give it a go this weekend weather permitting.

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