How to test fuel injector cold start concerns?

[Tom in seattle]

My nemesis is a 1967 250 SL, a Euro car.  Last fall it was starting from cold.  Usually a couple of Starter pulls and on the third will move into a fast idle and then slow down as water temp rises.  But this spring it will only start after many short stumbles and coaxing until the head warms.  I believe the cold start solenoid on the intake manifold is functioning as is the thermal time switch in the "pod".  I filed the contacts on two relays and they are feeding 12v to the cold start solenoid on the intake manifold and to the single solenoid on the FIP.  I tested this with a light bulb while activating the starter.  I also took the two wires off the FIP solenoid and jumped a 12v wire momentarily and heard the solenoid "click" as well.  But I believe that the intended movement of the FIP  rack during cold start may not be working to enrich the fuel mix.  I have not taken the solenoid off the FIP to examine it as I'm not sure if this is advisable, not sure what to expect.  I'm pretty sure my ignition is strong and properly advanced.  When warmed up the engine runs strong and idles smoothly at about 750.  I have read the Cold Starting Tour but don't find specifics regarding the enrichment function.
What advise can you give to test or resolve my concern about the FIP solenoid/enrichment ?

[Pawel66]

Is the WRD working?

[mrfatboy]

It is my understanding that fuel enrichment solenoid has very little effect if anything when the engine is cold.  I know for a fact it doesn't on my car.

The FES is always activated when the starter is engaged, hot or cold engine. But depending on the FIP rack position at the time (forward when engine cold,  back when engine warm) the solenoid pin may not move the FIP rack.

An easy test to see if the solenoid is moving the rack when the  starter is activated is screw in a ~3" long 5mm(?) bolt in rack port in the front of the FIP .  There usually is a rubber cap that needs to be taken off or earlier pumps have a cap that needs to be unscrewed. I can't remember exactly.

Place a mark on the bolt with a sharpie pen and note it's location relative to the pump.

Now when engine cold activate solenoid and see if FIP test bolt moved. I bet it doesn't.  That's normal on my car.

When engine is warm/hot you will notice the bolt and the "mark" have moved back into the pump. That's because the WRD (if working properly) has leaned out the pump and move the rack back to its warm position.   These are only small movements.

 Now,  when engine warm/hot activate the FES. I bet you will see your test bolt move forward. That means the FES pin was able to reach and affect the rack thus enriching warm/hot starts. I believe this to be  normal.

Long story short. I don't think this is your cold start problem.

[Benz Dr.]

Sounds like the slide valve is stuck in the WRD. Remove the small air filter and see if you have a strong vacuum when cold. If not then it's stuck closed.

[ja17]

Ditto with Dan on the slide valve.

[Tom in seattle]

With the motor cold I started the car after removing the air filter and noted vacuum at the threaded opening.  I then drove the car to 80 C which is the temp that the BBB says the air valve should be shut and again checked for vacuum and there was none.  So I'm guessing that the slide valve is working. 

I'm beginning to believe that my cold start problems are ignition advance influenced.  I'm using a Pertronix photo triggered spark and a hot coil, so no points to gap or set dwell.  I'm 6 degrees BTDC with vacuum connected.  It jumps to 30 degrees by 2500 maybe quicker.  But I'm confused by my reading of the tech manual which mentions several alternatives as to timing and with late model 280s having a different timing than the early cars.  Mine is a 250 so maybe I have it set to far advanced.  It runs really powerfully on the road this way.  Should I retard the advance which I'm guessing will make starting easier?

[Pawel66]

Are you sure your thermo time switch is ok? A simple test is to check if you have ground on W connector when the engine is cold. If no ground there with cold engine - TTS does not work. Do not go with grounded test wire to G with key turned!