Project Bike - Trying to Light it Off

[Texas K75]

I have a 1988 K75C that I purchased a couple of years ago.  It did run when I purchased it but was very difficult to start when cold but ran fine when it warmed up.  Eventually (after 6 months or so) it got to the point where it would not start at all.  When it would not start, I attempted using starter fluid (since it had been very hard to start when cold).  I could get it fire up but only run until the starter fluid was consumed.  This pointed me to the fuel system.  I removed the fuel cap to access tank interior and discovered that all of the rubber in the tank was dissolving into chunks in the gas tank.  I began removing the tank internal components. The fuel pump isolator was turning to a gooey substance, the fuel pump screen was almost dissolved, fuel lines were  brittle. and fuel filter was plugged.  Actions taken:

1 - Replaced the fuel pump (Bosch)

2 - Replaced the fuel pump isolator

3 - Replaced the fuel pump screen

4 - Replaced all of the fuel lines inside the tank but did not use "official" BMW fuel line.  I purchased fuel injection grade hose from O'Reilly's  for the fuel delivery hose inside the tank.

5 - I purchased a clear fuel line similar to what I use for the fuel line on my weedeater for the hose that connects t the screen.  I did not remember how that line was routed but I can't see where it would make much difference so long as the open end of that line is submerged.

6 - I had the injectors rebuilt since the cost was rather cheap and I had concerns that they may be damaged from debris based on what I had found in the tank.

7 - My original tank was dented and had a small leak at the low spot on the left side of the tank.  I found a very good replacement from another 1988 K75C that was very clean, not corroded at all and had no dents.  At this point the only thing left in he original tank was the sending unit so that has now been transferred to the new tank.

8 - I replaced spark plugs.

9 - Changed oil and all other lubricants except fork oil.

I am now able to get it start again.  It was difficult to start initially but that may have been air in the fuel system and low fuel level.  I was able to get it to run at idle for a minute or so at a time but I cannot apply any throttle without it stalling out.  Battery is now dead and I am awaiting a recharge.  When the bike idled, it seemed to run OK but a little unstable.  In other words, it seemed to be running on all three cylinders but I could not apply any throttle to run up the RPM's.

This is my fist K-Bake and it was a borderline basket case when I purchased it.

Any ideas??

[Scott_]

With the injectors connected to the delivery rail, but yet not installed in the engine, how is the spray pattern when you press the starter button.

A few things come to my mind.

Fuel pressure regulator,
electrical connections for the fuel pump circuitry,
electrical connections for the engine controller,
engine temp sensor,
vacuum leaks.

Check for mechanical blockage with the air flow meter.

As far as the fuel line in the tank, it not only needs to be capable of high pressure of an injection system, it also needs to be rated for submersion in fuel(not all fuel line is rated for this) or it will start to degrade.

[bocutter Ed]

You didn't say so, but I'll assume you replaced the fuel filter?

[Texas K75]

Thanks for the prompt reply and good suggestions.

I neglected to mention that I replaced the fuel filter and the air filter as well.

I will check out the items that you have referenced.  I went back out to the garage and tried again after the bike had been sitting for a couple of hours and it did fire back up as before with idle only but it did so right away. 

Unfortunately, the tank is leaking through the sending unit gasket so it looks like I have an O-Ring that I need to purchase.  I had tried using the old one since I had forgotten to purchase one beforehand.

[Texas K75]

I may have found the problem.  The large angled rubber air hose from the top of the Air Filter Box was not connected to the plenum on the left side of the bike.  Since I'm assuming this would  sabotage any efforts the bike is making to meter air flow, I think I should correct this before trying to start it again.

[Texas K75]

That was it.  Fired right up.  Looks like I have the challenging cold start issue resolved as well.

Just need to order that O-Ring to fix the tank leak and I shall be in business.  Another one brought back to life.

[Grim]

The FI on these bikes is really not all that different then on Many cars. The first thing you will find in most manuals is check for vacuum leaks in any Bosch based injections systems. As you found out it has to be absolutely airtight between the airflow meter and engine.  The airflow meter is what adds the fuel so the air bypassing the AFM did not allow the door in the AFM to shuck open to add fuel as you opened the throttle.