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Author Topic: Cb750 misfire  (Read 2803 times)

Offline 750rideruk

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Cb750 misfire
« on: February 22, 2015, 12:20:41 PM »
OK
Engine top end rebuild clean carbs, new plugs, carbs balanced, timing spot on but have a misfire. Bogs down over half throttle and pops on overrun as if the exhaust is leaking.
The only thing I can find is that the rubber bellmouths are not a great fit into the air filter casing as they have shrunk with age.
Any of the experts out there with any clues?
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Offline Trigger

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Re: Cb750 misfire
« Reply #1 on: February 22, 2015, 01:23:43 PM »
Is the popping coming from the airbox ?
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Offline 750rideruk

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Re: Cb750 misfire
« Reply #2 on: February 22, 2015, 01:43:02 PM »
Thanks for the reply Trigger
No the backfire is through the Zorst mainly
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Offline Bitsa (Ralph Wright - RIP)

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Re: Cb750 misfire
« Reply #3 on: February 22, 2015, 01:45:57 PM »
What have you set the air screws at from fully closed?Did you change all the moving carb parts what else have you done? More info please? :D
Cheers
Bitsa
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Offline 750rideruk

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Re: Cb750 misfire
« Reply #4 on: February 22, 2015, 01:59:48 PM »
air screws 1 1/2 turns out.
Carbs partially  stripped cleaned rebuilt and balanced
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Offline Trigger

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Re: Cb750 misfire
« Reply #5 on: February 22, 2015, 02:29:19 PM »
Quote from: 750rideruk on February 22, 2015, 01:59:48 PM
air screws 1 1/2 turns out.
Carbs partially  stripped cleaned rebuilt and balanced

The bogging down could be that your air screws are too far out. Try them at 1 to 1 1/8. Have you checked that the air screws are not blocked ? If popping from the pipes, are you sure they are sealed up. I take it that you are running std pipes ?
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Offline 750rideruk

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Re: Cb750 misfire
« Reply #6 on: February 22, 2015, 03:19:01 PM »
Actually is spitting back through airbox, Any thoughts?
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Offline Trigger

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Re: Cb750 misfire
« Reply #7 on: February 22, 2015, 03:24:26 PM »
Quote from: Trigger on February 22, 2015, 02:29:19 PM
Quote from: 750rideruk on February 22, 2015, 01:59:48 PM
air screws 1 1/2 turns out.
Carbs partially  stripped cleaned rebuilt and balanced

The bogging down could be that your air screws are too far out. Try them at 1 to 1 1/8. Have you checked that the air screws are not blocked ? If popping from the pipes, are you sure they are sealed up. I take it that you are running std pipes ?

Did i not ask that in the first place ? Check all manifold bands are tight, no air leaks and no cracks in the air box. What 750 are we on about ?
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Offline 750rideruk

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Re: Cb750 misfire
« Reply #8 on: February 22, 2015, 03:38:19 PM »
Its a K1 with std pipes.
Thanks for the assistance trigger
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Offline Trigger

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Re: Cb750 misfire
« Reply #9 on: February 22, 2015, 04:04:57 PM »
Quote from: 750rideruk on February 22, 2015, 03:38:19 PM
Its a K1 with std pipes.
Thanks for the assistance trigger

Sorry but, we had a member asking about carbs a couple of weeks ago and he forgot to mention that he was running a big bore kit. It is going to be a air leak somewhere around the air box carb area.
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Offline 750rideruk

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Re: Cb750 misfire
« Reply #10 on: February 24, 2015, 08:31:18 PM »
Note to self
When timing engine with strobe, time to the marks after the f mark and not the f mark on full advance you dipstick!!!!
Bike running like a sewing machine now
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Offline Bitsa (Ralph Wright - RIP)

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Re: Cb750 misfire
« Reply #11 on: February 24, 2015, 08:33:31 PM »
That's how we all learn at some point doubt your the first or the last
Cheers
Bitsa
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Offline Bryanj

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Re: Cb750 misfire
« Reply #12 on: February 25, 2015, 08:00:33 AM »
You had better mean the marks BEFORE the F mark
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