My '71 CB750K1 is quite slow...

[PaulS]

Hi.
My first post, so I hope you can help.
I bought my bike 7yrs ago in UK, recently imported from USA. It's done around 25k miles in total to date but has been sluggish since I bought it. Whilst I don't want to thrash the bike, I'm disappointed that it struggles to get much beyond 80mph and wheezes to get there. Also it sometimes only starts on 3 (sometimes 2) cylinders - but rarely the same ones! That starting issue seems to cure itself when warmed up.
So far I have dismantled, cleaned and rebuilt the carbs (using a refurb jets, needles, etc kit from David Silver Spares) and checked the rubber gaiters. I balanced the carbs with a vacuum set. The fuel lines and tap seem to be OK. I have replaced the battery, all the plugs, leads and coils, installed electronic ignition and checked the compression (OK).
And still it's sluggish.
I am probably at the extent of my mechanic skills now, so unless there is some magic answer (??) I probably need to get a specialist to breathe on it.
I had Rising Sun Restorations of Tockwith, York, carry out an initial checkover/service when I first bought the bike, but their telephone number seems to no longer be working. Are they still around?
So,
A. Any magic answers to get the bike free revving again? and
B. Where do I find the best mechanic to diagnose the problems and sort this out?
I am based near Bristol, but happy to travel to find the best person/people.

[Bryanj]

As a US import what size rear wheel and tyre and what size sprokets are on there, they do love pissing about with Mr Hondas design specs over there

[PaulS]

Thanks Bryanj.
Ill check against Spec.
However, the engine is really wheezing, so I suspect its the power plant at fault.

[McCabe-Thiele (Ted)]

Thanks Bryanj.
Ill check against Spec.
However, the engine is really wheezing, so I suspect its the power plant at fault.

If it was a two stroke I would ask are the exhuasts blocked up ?

Obviously you might have checked the ignition timing - but is the advance & retard unit working as it should - can make a big difference to any engine if it's not advancing as it should..

[philward]

Are you running standard air filter box and exhaust? What colour are the plugs after a longish run?
Welcome to the forum by the way!

[flatfour]

I understand that John Wyatt (who owned Rising Sun Restorations) has died recently, so that's probably why the telephone is out of action.

I am not aware (others might know better) of anyone taking on the business.

[Arch stanton]

I'm sorry to ask but..
What bikes do you normally ride?
If you usually ride modern bikes, then the k1 will appear to be a slug.
50 yrs ago they was comparably fast. But now vastly over weight & woefully Under powered. US spec bars make things even worse.
Sean.

[philward]

Although comparisons to modern bikes, should still breeze past 80 mph so there is an issue we need to sort. Do agree though that if you jump off a relatively new bike to our 70's 'superbikes' will show up their deficiencies. When I step off my 'modern' 1995 ZZR1100' my 750k2 seems very sedate! But enjoyable!

[commando1954]

A rolling road would tell you what sort of power it is making all through the rev range.

[Nurse Julie]

Let me ride it, I'll soon tell you if it's slow or not 😂😂😂😂😂😂 If I can't blow the engine up whilst flat out, you'll know its just fine 🤣🤣🤣
Joking aside, it can only really be carb or timing set up that's giving you the issue.

[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?