Honda-SOHC

SOHC.co.uk Forums => CB500/550 => Topic started by: Ashdowner on June 20, 2015, 09:58:41 AM

Title: CB 550K3 suddenly running very lean
Post by: Ashdowner on June 20, 2015, 09:58:41 AM

Bought my CB550K3 and it unbelievably had 3 different grades of NGK plug in it - DE7EA, DR8ES and DR8EA. Now has the recommended DR7ES but is running incredibly lean, with the plugs almost white. Do I need to increase the jet size or raise the needle and what is the difference in those two options?

Title: Re: CB 550K3 suddenly running very lean
Post by: deltarider on June 20, 2015, 11:16:42 AM

First things first. Sufficient fuelflow? Floatlevels OK? Original airbox and airfilter? Any airleaks?
On the K3s the idle jets are notorious for clogging. Wouldn't touch the needles.

Title: Re: CB 550K3 suddenly running very lean
Post by: Ashdowner on June 20, 2015, 02:12:04 PM

Floatlevels were done a month ago and had the carbs ultrasonic cleaned at the same time. Idle has always been poor but up till yesterday it was fantastic above 4000rpm with beautiful brown plugs. Now they're grey/white

Title: Re: CB 550K3 suddenly running very lean
Post by: Bryanj on June 20, 2015, 04:08:05 PM

I would suggest removing the push in pilot jets

Title: Re: CB 550K3 suddenly running very lean
Post by: Ashdowner on June 20, 2015, 04:55:03 PM

Gulp. That seems a bit drastic, and that's only affecting the idle. It's over 4000 rpm that it's running lean and that's on the needle jet

Title: Re: CB 550K3 suddenly running very lean
Post by: deltarider on June 20, 2015, 05:43:39 PM

Maybe first try running a tank with added an additive like Tunap or Forté (don't know what brands you have in the UK). I used it years ago and it did miracles. Might help and it's easy to do.

Title: Re: CB 550K3 suddenly running very lean
Post by: Bryanj on June 20, 2015, 07:53:51 PM

Believe me the pilot jets cause all sorts of agro and are a known arse

Title: Re: CB 550K3 suddenly running very lean
Post by: Ashdowner on June 20, 2015, 10:43:21 PM

Yeah I keep hearing that but nobody can say why except that they might clog up - which mine don't. I sometimes think that people are desperate to pin the whiplash from the primary chain when at idle on anything they can and the float levels and idle jests are usually the first choice

Title: Re: CB 550K3 suddenly running very lean
Post by: Bryanj on June 20, 2015, 11:16:34 PM

I had a K3 in a chop and having gotten it perfect for MOT it stood 4 weeks and the pilots blocked up again so i know theu can be a swine, also there are very small washers and "O" rings on the air screws which if missing or in bad condition cause trouble.

Its a myth that only certain parts of the carb work at certain revs--they may have MORE affect at certain revs but fuel and/or air is going through all the pathways all the time

Title: Re: CB 550K3 suddenly running very lean
Post by: deltarider on June 21, 2015, 07:54:32 AM

Quote

Its a myth that only certain parts of the carb work at certain revs--they may have MORE affect at certain revs but fuel and/or air is going through all the pathways all the time

True, that's why I always look at these diagrams that show from how much throttle what works, with scepsis.