Hello fellow K riders,

I am having trouble with my 84 K100 RS not running properly. It starts and runs but very rough and blurry. Theres also backfiring from the exhaust, even when idling.

The spark plugs looks pretty sooty/oily. And you can tell that they are a little wet. I have ordered new ones tho, because when i checked the spark quality one or two of them looked pretty weak, even though they are almost brand new. The bike have been sitting for a year since it last ran. But then it was running just fine.

I also checked the leads and they look ok. I dont suspect the coils because i had them replaced last year due to errors on the old ones. They are second hand tho.

Pulled the fuel rail to see if the injectors were working ( also new ) and they did.

Anyhow, when testing to see if i had the correct fuel pressure it looked like this ( see video ) If you look closely you can see that it spikes up to 50psi, sometimes 60. And when i increase the rpm, the pressure also increases. Is it supposed to be like that?

When i turn the ignition off, it stays on about 36 psi as it should. At least for an hour or so. Could it be a faulty fuel pressure regulator?

Any input appriciated!

Check your vacuum lines and vacuum caps first.

Just beside your throttle position sensor at no 4 there is a vacuum line, the original one has a spring on it, it goes to the fuel pressure regulator. If this line is cracked it will cause the fuel pressure regulator to not work and give you excessively high fuel pressure as you have found.

Those vacuum pipes and caps and Z pipe should be replaced max 5 years in my book.

Thanks for the tip, I have replaced all the vacuum lines and caps with new ones. And ther is no kinks anywhere that i can see.

But you do agree that its something wrong with the pressure?

Swedish.brick wrote:Thanks for the tip, I have replaced all the vacuum lines and caps with new ones. And ther is no kinks anywhere that i can see.

But you do agree that its something wrong with the pressure?

It does sound like it and I am actually replacing the FPR on one of my Ks for this reason. I have had a few go faulty over the years.

There is a diaphragm inside it that can go wrong, age, etc etc. Its worth checking to see if you find any petrol in the vacuum line at no 4- perforated diaphragm if you do. It will suck extra fuel into no 4 and seriously enrich 1 2 and 3.

Remove the return line to the tank an route it into a bottle.  Start the engine and check the fuel pressure. 

I suspect that you may have a stuck or blocked check valve in the spigot where the return line connects to the tank.  It will restrict the flow from the fuel pressure regulator to the tank forcing the pump to it's maximum output pressure which is approximately 60-65 psi.

Looks about right 2.2 to 2.5 bar 

You could tske the vacuume line off and see what it is ... perhaps 2.5 bar c or higher constant 

Rel issues are when its below 2 bar

Point-Seven-five wrote:Remove the return line to the tank an route it into a bottle.  Start the engine and check the fuel pressure. 

I suspect that you may have a stuck or blocked check valve in the spigot where the return line connects to the tank.  It will restrict the flow from the fuel pressure regulator to the tank forcing the pump to it's maximum output pressure which is approximately 60-65 psi.

That’s a good suggestion, I’ll try that and see what happens!

And be sure to use only Bosch X5DC spark plugs, NGK will not work.

slowandlow wrote:And be sure to use only Bosch X5DC spark plugs, NGK will not work.

NGK plugs of the same relative type should work if they have terminal nuts installed on them.

Laitch wrote:

slowandlow wrote:And be sure to use only Bosch X5DC spark plugs, NGK will not work.

NGK plugs of the same relative type should work if they have terminal nuts installed on them.

That’s right. I use NGK D7EA with the terminal nuts. Worked just fine last year.

I tried NGK years ago, they went charcoal black almost at once, went for Bosch instead, they lasted 10 years without problems. Someone here wrote it has to do with internal resistance: the Bosch plugs are adapted to the Beru original ignition cables/ caps, the NGK are not. I guess you have to try, if NGK works then its fine, if they goe black then try Bosch instead.

Also make sure that whatever plug you use, it must not have an R in the code. R codes are for the 16v engines.

A little update,

As suggested above I bypassed the fuel return valve in the tank with an extended hose from the fuel pressure regulator directly back in the tank through the fuel cap.

This did the trick! The pressure is much more stable and doesn’t spike or increase with the rpm. 

I removed the return valve from the tank, but was confused when there was nothing blocking it inside. But, when I blew through the hole to see if there was any resistance, there was a pressure build up inside the tank! (Fuel cap was shut)

The blockage was actually in the ventilation that goes from the top of the tank and out on the bottom through a small hose. This resulted in a pressurized tank and not letting the fuel flow normally back to the tank.

Ah. I've seen that happen in one tank and it was a bugger to unblock (I think it was Brick 2's tank). I got there eventually.