My '71 CB750K1 is quite slow...

[Nurse Julie]

It sounds like the plug type may not be helping. You are using resisted spark plugs (the R denotes that) and if you have resisted plug caps fitted, which by your avatar looks like you are, you will be getting too much resistance. Change the plugs to NGK D8EA'S or the DENSO equivalent as Dennis suggested. You only need one element resisted, either the plug caps or the plugs, not both.

[Spitfire]

I'm with Julie, I made the same mistake with my bike with resisted plugs and caps.
Here's a discussion form the US site
http://forums.sohc4.net/index.php?topic=77832.msg869100#msg869100


Cheers

Dennis

[PaulS]

Thanks philward, yes, standard (original) intake, the gaiters and clips are in good condition, and the exhaust is standard HM340 except one replacement pipe to pattern. I'll adjust the needle to get the mix a little leaner.
In the meantime I will take the medicine offered by Nurse Julie  and confirmed by spitfire - I'll replace the plugs!

[Andrew-S]

Thanks philward, yes, standard (original) intake, the gaiters and clips are in good condition, and the exhaust is standard HM340 except one replacement pipe to pattern. I'll adjust the needle to get the mix a little leaner.
In the meantime I will take the medicine offered by Nurse Julie  and confirmed by spitfire - I'll replace the plugs!

Original K1 exhausts are HM300, the K2 was fitted with HM340 with extra baffling so maybe they are restricting the carbs/gas flow - not sure if you have to change the main jet when fitting HM340s but someone will know? 

Cheers,
Andrew

[PaulS]

Apologies Andrew-S; I have just checked and the pipes are HM300 (apart from one pattern replacement), all with O/E baffles.
Will let you all know how the 'tinkering' goes. I have ordered up the Denso X24ES-U plugs...

[PaulS]

Managed to finally track down a firm that has the Nippon Denso plugs in stock (buycarparts.co.uk) and they arrived today: from Germany!
Will get those into the bike in the next few days and let you know if I am on the way to solving my problems...
Fingers crossed.

[K2-K6]

If you aren't able to find significant progress with above suggestions,  try running it with air filter removed but airbox in tact. It should be leaner in that config and may serve to show you a direction without initially changing jetting.

In other words a quick and dirty "check see" to give low-no cost assessment and help with decision about mixture.

[PaulS]

Thanks K2-K; as a born and bred Yorkshireman* I like "low-no cost" suggestions.
And thanks to Oddjob for your comments. I checked the compression last year and that was really good. I also replaced the plug caps, leads and coils some time ago. I'm banking on the plugs being the issue, if not, then carburation, but will let you all know....

(*Yorkshire, where "tin tin tin" is a proper sentence)

[PaulS]

"WoooooHoooo!!!" 
Just tried the bike after installing the (non-resistor) Nippon Denso plugs recommended by Nurse Julie and....
- the bike started first time AND on all 4-cylinders (that's a first!) - I tried it a few times just in case it was a fluke - and
- it rides with the free flowing urgency I was hoping for.
So far I have only taken it on a 20 mile test ride, but all is well. The spark plugs are showing a tiny bit of soot but I'll give the bike a much longer run before making any fine tweaks to the carburation.
Looks like 7 years of miserable performance has been cured by 4 spark plugs.
Thanks each!

[Arch stanton]

Looks like your problem is sorted.
Thank you for letting us know the result. So many times people ask advice on forums. But then don't come back on, to tell others what worked.
Thanks again.
Sean.

[Spitfire]

Glad to hear that it worked, nice when a plan comes together relatively cheaply.

Cheers

Dennis

[McCabe-Thiele (Ted)]

"WoooooHoooo!!!" 
Just tried the bike after installing the (non-resistor) Nippon Denso plugs recommended by Nurse Julie and....
- the bike started first time AND on all 4-cylinders (that's a first!) - I tried it a few times just in case it was a fluke - and
- it rides with the free flowing urgency I was hoping for.
So far I have only taken it on a 20 mile test ride, but all is well. The spark plugs are showing a tiny bit of soot but I'll give the bike a much longer run before making any fine tweaks to the carburation.
Looks like 7 years of miserable performance has been cured by 4 spark plugs.
Thanks each!

Yes it's easy to forget what a difference a change of plugs can make a few year back we had our SL serviced at the main dealer where I worked part time as I drove it away I thought to myself this seems slow and restrained. I took it staight back - they found the old plugs & the parts department had given them the wrong ones - same reach but different plug.