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SOHC.co.uk Forums => CB750 => Topic started by: 750rideruk on February 22, 2015, 12:20:41 PM

Title: Cb750 misfire
Post by: 750rideruk 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?

Title: Re: Cb750 misfire
Post by: Trigger on February 22, 2015, 01:23:43 PM

Is the popping coming from the airbox ?

Title: Re: Cb750 misfire
Post by: 750rideruk on February 22, 2015, 01:43:02 PM

Thanks for the reply Trigger
No the backfire is through the Zorst mainly

Title: Re: Cb750 misfire
Post by: Bitsa (Ralph Wright - RIP) 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

Title: Re: Cb750 misfire
Post by: 750rideruk on February 22, 2015, 01:59:48 PM

air screws 1 1/2 turns out.
Carbs partially  stripped cleaned rebuilt and balanced

Title: Re: Cb750 misfire
Post by: 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 ?

Title: Re: Cb750 misfire
Post by: 750rideruk on February 22, 2015, 03:19:01 PM

Actually is spitting back through airbox, Any thoughts?

Title: Re: Cb750 misfire
Post by: Trigger 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 ?

Title: Re: Cb750 misfire
Post by: 750rideruk on February 22, 2015, 03:38:19 PM

Its a K1 with std pipes.
Thanks for the assistance trigger

Title: Re: Cb750 misfire
Post by: Trigger 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.

Title: Re: Cb750 misfire
Post by: 750rideruk 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

Title: Re: Cb750 misfire
Post by: Bitsa (Ralph Wright - RIP) 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

Title: Re: Cb750 misfire
Post by: Bryanj on February 25, 2015, 08:00:33 AM

You had better mean the marks BEFORE the F mark