Hi all. i've just about got the k100 (1984) running nice, however after about four miles or when warmed up she starts to run like a bag of poo any ideas, new (second hand injectors and coils) new plugs new leads new fuel filter and pump new crank case breather new air filter new vac and fuel pipes.

My guess:Air leaks (cracks) in the boots between the TBs and cylinder head.

Have you been through all the intake tract. These bikes are getting old and leaks will happen and when warm will run badly.
Check and make sure all the clamps are tight and the manifold stubs are sealed. Anywhere that air can leak into the inlet from head to air filter. Because the air is measured just after the filter anything that gets in after there will make a big difference to the mixture ratio. Also check the Z shaped rubber tube from the rear of the plenum chamber to the engine. It will leak a heap of air if perished.
If you have already done all of this you may need to check out the temp sensor for correct readings.

What Rick says. Spray WD40 or similar around the injector body rubbers when the engine's running. Any rise in revs indicates a leak. For the price or new ones (top and bottom), I wouldn't even consider buying secondhand because you have no idea how old they are, how they've been looked after and if they have unseen cracks in them. Help the lower rubbers to seal to the cylinder head properly with a bit of Threebond (expensive but when you've used it once you'll never go back to ordinary gasket cement again). I treated LFB to a new set of rubbers after I got just a bit p!ssed off when she dropped to 2.5 cylinders in the Welsh mountains on the first day of a four day run. Unbelievable difference...

One place to check is just above and below the clamps, the rubbers tend to split there and it's not till you remove the throttle bodies that it becomes evident. Also check the O-rings on the injectors they can leak air when old

Dai wrote:What Rick says. Spray WD40 or similar around the injector body rubbers when the engine's running. Any rise in revs indicates a leak. For the price or new ones (top and bottom), I wouldn't even consider buying secondhand because you have no idea how old they are, how they've been looked after and if they have unseen cracks in them. Help the lower rubbers to seal to the cylinder head properly with a bit of Threebond (expensive but when you've used it once you'll never go back to ordinary gasket cement again). I treated LFB to a new set of rubbers after I got just a bit p!ssed off when she dropped to 2.5 cylinders in the Welsh mountains on the first day of a four day run. Unbelievable difference...

Despite what the can says, WD40 is no longer flammable like it used to be. (Thank the safety Nazis for that one.)

I use starter fluid although I've heard of others using propane or other flammable gasses.

You can also spray some water mist and if the RPMs go down then that indicates you've got an air leak.