71 280SL Engine skips on cylinders #3 and #6

[CoreyJ]

We have a misfire on cylinders # and #6. Background: Just installed new 123 distributor, new plugs, wire set, fuel injectors sent out cleaned and flow tested, MFI pump rebuilt.
On #3 and #6 there's not much change when pulling off an ignition wire. We swapped injectors around and plugs around - no change. Compression is 148-150 PSI on all cylinders. Of note the other item noticed is the idle adjustment screw in the intake can only be adjust up to about 800 RPM and then it starts to falter. Any thoughts? Thanks!

[mrfatboy]

While you are waiting for the pros to comment can you give us more info?

- year and model?
- FIP model number?
- where was the FIP rebuilt?
- were all the changes you made done at once?
- Did you do all the changes to rectify the problem you describe or just upgrades over time?

[JamesL]

Have you any emissions control stuff in place?

[Pawel66]

Is it not that if it all worked before and you did something with distributor, leads, plugs and injector and you wrote you checked only injectors, the problem may be in the other changed parts? Do you have nice strong spark on 3and 6?

[CoreyJ]

It is a 1971 280SL, Injection Pump R-24 rebuilt at Buds Benz, all parts changed at the same time.

[hkollan]

Hi,

Could be clogged injector lines on those two cylinders.

Hans K

[ja17]

Crack open injector line for #3 and #6 while the engine is running one at a time to see if you notice a change. How was the engine running before the work?

[CoreyJ]

Cracking injector lines #3 and #6 one at a time while it's running made no difference, just like pulling the ignition wires one at a time. Compression is the same on all cylinders at 148 - 150 psi. What else could cause a misfire like this? Thanks!

[mrfatboy]

Just to clarify, did you see fuel leaking from each injector when you cracked them open? You want to make sure fuel is getting to the injector to verify the lines are not plugged.

[ja17]

Make double sure that your plug wires are in the correct firing order.

[Shvegel]

I second what Joe said especially since 3 and 6 are adjacent wires.  Due to the small size of the cap even with a modern coil etc these systems are sensitive to the type of ignition wire.  Our cars prefer non resistor spark plugs and metal core wires as opposed to carbon cores.  The increased firing voltage of a high resistance carbon wire can easily case sparkover between two adjacent terminals (Like 3 and 6). 

An earlier poster mentioned actual fuel flow out to the injector.  Did you verify fuel was flowing when you cracked the line at the injector?  This is especially important if the lines were plated during the refurbishment as they have a tendency to rust shut after plating.