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My '71 CB750K1 is quite slow...

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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):

--- Quote from: PaulS on August 20, 2021, 02:56:13 PM ---Thanks Bryanj.
Ill check against Spec.
However, the engine is really wheezing, so I suspect its the power plant at fault.

--- End quote ---

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!

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