SOHC.co.uk Forums > CB750

My '71 CB750K1 is quite slow...

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flatfour:
A few years ago, a 750 K6 that I owned exhibited similar symptoms. It would ride normally until reaching some 4000 rpm and then just flatten out.

After lots of time and trouble, I tried easing the choke on one day whilst in the problem rev band, and immediately it ran normally.

I changed the main jets for new items and all was cured. My friend subsequently bought the bike from me and for the years that he owned it, things were also fine.

I later found that the bike had stood for over a year at the selling dealer before I bought it, so perhaps stale fuel caused a restriction within the jets?

Spitfire:
Two things that might be a problem you mentioned rebuilding the carbs with a kit, a lot of people have problems with the carbs going via this route, it is better to re use the original brass.
The second one is plugs, what plugs are you using, I went through several types of plugs on mine when I first got it back on the road and had problems, as the old faithful D8ES-L is no longer made I tried several different NGK plugs all with no improvement so I switched to ND plugs and have not had a problem since.
Without going down to the garage I don't know which plugs I have in at the moment but they are either Nippon Denso ND X24ES-U or X22ES-U as recommended by Hondaman on the US SOHC site.

Cheers

Dennis

philward:
As per Dennis's comments, retaining the original brassware is the way to go. I bought an aftermarket kit for the 500k2 and mic'ed up the needles at various points on its length of the needle and they were miles out compared to the standard needles. Never fitted them and cleaned and put the originals back

PaulS:
Thanks all!
In summary it sounds as though the issue is either timing, carburration or perhaps plugs.
Timing; I replaced the points with an electronic system a few years ago, so assume that won't be the problem. Or am I fooling myself?
Carburration; I checked the plugs this afternoon (last run was around 40miles) and they are looking sooty, so the mix is too rich. I'll start by lowering the needle and check again. BTW, the carb refurb kits were genuine Kehin, so I'm hoping they are OK... before doing so I might try the 'choke trick' suggested by 'flatfour' to see what happens.
The plugs I am using are NGK DR8ES. Spitfire: should I change to ND plugs? If so, which ones?
As for the bikes I am used to riding, it's a real mix. My newest is a 1 year old BMW R1250RS that I use for touring. I previously had a Ducati 1200 MultiStrada Pikes Peak. However, I have been riding a '58 Triton for 10yrs (just sold it), a Ducati 916 for just over 20yrs and regularly ride my '56 BSA DBD34 Gold Star. In the past I have owned many old and new Triumphs and an old Norton Dominator. I am really convinced that the CB750 should have more 'urgency' than it displays. My very first bike, a '62 James Captain, had more 'get up and go'!

philward:
Definately sounds like mixture from your plug description (stating the obvious). Is the needle set on the middle notch? As per my previous post, are you running standard intake and exhaust?

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