K75 Troubleshooting Running Issues

Well, I rebuilt the K75 with a replacement second hand Engine, now begineth the fettling to get it running 100%

The feeling of pride getting an Engine to run, building a Bike or taking a set of parts and putting them into a mechanical masterpiece is second to none in my opinion. In fact I tell all young people “you should build a motorcycle, once in your life” and I truly believe that. The satisfaction, knowledge and payback you get is huge.

However, as you’ll know if you’re here reading this, they don’t always start, run or go like you’d expect and quite often many hours, days and even months have to be spent tracking down all the gremlins to get things right. While you should be systematic and go after each issue with research and science, I tend to use online resources and ask questions to get answers from those with specific model knowledge.

My K75 which now looks and rides great, still has issues and so it starts… Below then is a collection of things to check first, second third on your Bike, to maybe save you an hour or two. Hope they help.

Thanks to all those on Motorbrick.com to get me through some of these!

My Bike started almost first time and runs well when Cold but has a slight rumble/misfire when warm and won’t hold revs at idle… many K bikes from the forum get some form of poor running from research so I thought I’d try and capture a few quick checks and easy things to try to remedy.

First thing with K Bikes is the Earths – they need to be clean and by that very clean and strong. Both the one from the Battery to the Frame, take it off and clean it, plus the main ones 5/6 Brown wires that meet up in the middle under the Tank.

Earths – Frame and Under Tank

Remove Tank, remove all Brown Wires, clean, clean and protect, then re-tighten.

I bet that deals with 50% of running issues!

Beyond that, a long few weeks of troubleshooting a K75 Café Racer in Dubai led me to another culprit for none starting / poor running, namely the Ignition Switch

Ignition Switches and Cut-Out Switches

Remove and dismantle the Ignition switch, check, clean and protect all parts – if suspect, just bin it and get another as they are all 20 plus years old…

Next things in my humble experience is….

Petrol Tank Connector – 4 Pin

These affect the running of the Fuel Pump and affect starting – again, disconnect, examine and use a contact cleaner or simple sandpaper until 100% check and replace dodgy connectors or wiring, ensure it sparkles and listen for the Pump running after cranking – it should go for a couple of seconds if all is well.

All connections under the Tank are in the same boat – until you’ve had each one apart, cleaned and reconnected you are on a hiding to nothing troubleshooting.

Vacuum Leaks

These cause all sorts of trouble – the little Z Pipe that is the Crankcase Breather cause 100’s of K owners trouble as they crack, break down and leak – replace it as a matter of course.

The other culprits are the Throttle Body Rubbers that connect the fuelling system to the Head, again they age, and allow Air in, which causes running issues. One way to find out is to spray Brake cleaner around each Rubber while the bike is running, if the revs change you have a leak. Other areas on Vacuum to check are the Airbox Pipes and all places where they meet.

 

Beyond that the old fashioned signs of mechanical togetherness should be used if you have some Car knowledge.

Brown plugs are Good

The colour and smell of a plug when run is a good indicator of Engine and Fuelling health –

Black = Running Rich, White = Running Lean, Brown = Just about right.

That doesn’t get you to the solution, but it does point to a problem at that cylinder – could be fuel, could be electrical… more later, as I’m still only part way through this.

Throttle Position Sensor

The Black thing at the end of the Throttle Bodies near the Injectors. My bike when first taken out Backfired quite a bit – this is kinda normal on Ks as they are a little bit on off from a Fueling point of view like most EFI Engines – but the TPS – Throttle Position Sensor, is there to switch fuel off when you’re not twisting the handle. It should click when returning to 0 – if not, google it and adjust, fairly straightforward, but eliminates overfueling and some backfires, though not all…

Starter Motors

Again a troublesome area as they provide a path to Earth for various circuits and fill with dust and crap over 20 years. Take it off, dismantle with care, inspect, clean and refit.

Fuses and Relays

Kinda obvious but if the Fuel Pump isn’t running has the Fuse blown – they do! Check under the Seat – replace bingo, solved. Relays – remove each one, listen for clicks, the Starter relay sometimes welds itself inside and needs a thonk… anyhow, remove each one and clean, wobble and replace… check with a multimeter for operation if you’re Electrically minded.

Charging and Batteries

The Battery should be good, measure it, replace it if suspect and keep it charged should be giving 14V when running and over 12V when stationary – several charts online, but critical to get things moving, ditto the Alternator which whirrs along and provides charging – if the Lights are dull or yellow, you have an issue with Ziggys or Voltage before you start anywhere else.

Hope that helps – more soon… check back