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TOPIC: Not perfect RPM range...

Not perfect RPM range... 10 Sep 2019 06:30 #810629

  • DGA
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Sort of sounds like your bike is jetted fine for power, for acceleration, but you do not like how it seems rich when held at certain throttle openings at steady state. This is totally normal for a carburetor. I always jet for power, for acceleration, so it pulls hardest through the rev range, and this same jet setting will then be rich for steady state riding, and there is no easy way around that, well there is, it's EFI. Carburetors have no good load sensing capabilities and they can't change fuel flow to adapt to different power settings. In theory the vacuum slide types are better for this, but it's been my experience that they fall short of this task. So, either jet for partial acceleration and accept that under higher loads the motor will be lean, but run smoother, or jet for hard acceleration and accept that it will be rich and not run so cleanly at part power settings.
"83 KZ1100 Spectre, '81 Yamaha XV750 Cafe, '74 Kawasaki H2B 750
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Not perfect RPM range... 10 Sep 2019 07:48 #810634

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DGA wrote: Sort of sounds like your bike is jetted fine for power, for acceleration, but you do not like how it seems rich when held at certain throttle openings at steady state. This is totally normal for a carburetor. I always jet for power, for acceleration, so it pulls hardest through the rev range, and this same jet setting will then be rich for steady state riding, and there is no easy way around that, well there is, it's EFI. Carburetors have no good load sensing capabilities and they can't change fuel flow to adapt to different power settings. In theory the vacuum slide types are better for this, but it's been my experience that they fall short of this task. So, either jet for partial acceleration and accept that under higher loads the motor will be lean, but run smoother, or jet for hard acceleration and accept that it will be rich and not run so cleanly at part power settings.


Thank you for the information, the bike is running fine, just a metter off make it perfect.

I`m not running constant in that speed we talking about either, so no big problem...

More that… is it possible to make it even better…. :)
1981 KZ650-D4, with 1981 z750L engine (Wiensco 810 big bore).

Project:
www.kzrider.com/forum/11-projects/607213...sr-1981-z750l-engine

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Not perfect RPM range... 10 Sep 2019 07:52 #810636

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No, I don't believe you will get 'perfect' with carburetors. We are all so used to EFI in cars and newer bikes, younger people often have no idea what a carb is these days. If you want perfect, consider going down the rabbit hole of EFI, I am doing just that right now with a Microsquirt system on a GPZ500R.
"83 KZ1100 Spectre, '81 Yamaha XV750 Cafe, '74 Kawasaki H2B 750

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Not perfect RPM range... 10 Sep 2019 15:41 #810656

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Really, yes you can get carbs to run perfect at all speeds and rpm ranges. Been tuning carbs for over 40 years and I seem to dial them in perfectly. You just need to know what throttle setting effects which carb circuit then tune each circuit till it's perfect. EFI is not the magic bullet.
If the engine surges under neutral throttle which sounds like your problem then it's lean on the needle. By neutral throttle I mean your not accelerating or slowing down but keeping the throttle steady.
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Last edit: by baldy110.

Not perfect RPM range... 10 Sep 2019 22:46 #810676

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baldy110 wrote: Really, yes you can get carbs to run perfect at all speeds and rpm ranges. Been tuning carbs for over 40 years and I seem to dial them in perfectly. You just need to know what throttle setting effects which carb circuit then tune each circuit till it's perfect. EFI is not the magic bullet.
If the engine surges under neutral throttle which sounds like your problem then it's lean on the needle. By neutral throttle I mean your not accelerating or slowing down but keeping the throttle steady.


Thank you, its only happens in high RPM, not 0-6k RPM, but that make sense when it comes to lean needle?
So the hootest thing I can test is to raise the needle one step?
1981 KZ650-D4, with 1981 z750L engine (Wiensco 810 big bore).

Project:
www.kzrider.com/forum/11-projects/607213...sr-1981-z750l-engine

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Not perfect RPM range... 11 Sep 2019 09:13 #810702

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Yes
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Not perfect RPM range... 13 Sep 2019 14:22 #810817

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Today when I started for driving to work I heard a new sound from the engine... «thinking...» I dident use sound protection today, so I was not sure if it was a new sound or not, but when I drive home I had, and the sound was there, so then I was sure it was new.

When breaked on the engine it also come some smell from the exhaust...

When I come home I checked the nuts on the exhaust and they was loose...

Tigheted all up and the sound disapear... and it looks like it removed the issue with high load and high RPM....

I need to fix the issue nuts get loose over time, but I’m happy that the issue was found :)
1981 KZ650-D4, with 1981 z750L engine (Wiensco 810 big bore).

Project:
www.kzrider.com/forum/11-projects/607213...sr-1981-z750l-engine

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Last edit: by gordone.

Not perfect RPM range... 14 Sep 2019 16:06 #810848

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That will do it.

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