Classic Bike Guide
Seven year itch
My wonderful wife asked me what I would like for my 60th birthday. “Let me think; a 1971 Honda CB750-4 K1 in metallic dark brown please?”
She bought me a lovely example from a kindly old gentleman over in far west
Wales. He had always wanted one, bought it – unseen and unregistered – from an importer. When the bike arrived, he sat astride, started it up and promptly suffered a heart attack. Under doctor’s orders he was told never to contemplate riding a motorbike, so it came on the market.
A deal was struck over a mug of strong Welsh tea, then
Lee, the owner of my local bike shop Kellaway Motors, went over to Aberystwyth to collect it. I managed to get the bike UK registered, insured, on the road and looked forward to many years of enjoyment.
It looked amazing and seemed to need only a little bit of TLC after languishing in a garage in Wisconsin, USA, for many years. Original everything and only 23k miles on the clock. New tyres and battery were installed, the seized rear wheel bearings and cush drive renewed, a NOS seat installed and the bike was ‘breathed on’ by the CB750 guru and author of the go-to guide, John Wyatt of Red Sun Restorations in Yorkshire. [John sadly passed away during 2021].
I rode it up to Yorkshire and back home to Bristol with no dramas, but it seemed more sluggish than I expected. John suggested I check the carb jetting. And so began seven years of fettling to try and get the wheezy old big H to act more like a puppy…
I dismantled the carbs and found that the jetting was fine, but I cleaned and refurbished them anyway. No change. Next I replaced both of the ignition coils and their leads on the assumption that after more than 40 years the originals might be past their best. But to no avail. It was then I noticed that while the bike would always start, it would only fire on three cylinders. But never the same three. And sometimes just two, never the same two. When it warmed up, all four cylinders would play, but was still sluggish. The bike would just about get to 80mph, but was wheezing and laboured. More head scratching, reading of guides and manuals, watching online videos and debating various opinions from chums.
Conclusions seemed to be either the carburation or the timing weren’t right.
So, off to David Silver
Spares again to purchase an electronic ignition set. Simple to fit, looks lovely... made not a jot of difference. Hmmmm. So, I dismantled and rebuilt the carbs again. And invested in an expensive set of vacuum gauges to make sure that the fuelling was spot on. And the difference was? Yes, you guessed, none.
I had already checked the fuel lines, the airbox; a new filter was bought and installed of course, the tank was clean and the exhaust system perfect. I had, back in 2018, even treated the bike to iridium spark plugs, at £17 each! They didn’t work at all, so new versions of the original plugs went back in. Was this bike destined to gasp and groan for the remainder of its life?
After seven years of trial, but mainly error, I joined the Honda SOHC Forum online – which is free – and set out my woes. Could anyone help?
Lots of suggestions were forthcoming but none worked or hadn’t already been tried.
I was starting to abandon hope. Then one of the members spotted the red/brown NGK plug caps in the bike’s tiny profile picture. He clearly has 20/20 vision. “Those are a resistor type” he said knowingly. I had to look that up. Resistor construction to minimise electromagnetic interference with electronic equipment by flattening and reducing the ignition spark – every day is a learning day! He also asked what plugs I was using. “The NGK ones with ‘R’ for racing” I said. Oh dear. The ‘R’ actually stands for resistor type; I had spark resistance not only in the caps but also the plugs. No wonder the firing was poor. I immediately ordered up a set of new Nippon Denso plugs – not resistor type – spun those in and…
…success! The bike starts first go on all four cylinders, is as urgent as a stag at a rut and screams along. Faster and easier than I would ever want to push it on those thin rubber boots, but a great joy.
And it only took seven years to sort out.
Paul, thank you! I’m now off to check the plug caps on my bikes, as I didn’t realise that either – Matt