Can't Rev past 4500

[ricoled]

After a complete tune-up, my '68 280SL now runs very smoothly, with a lower operating temp (90C, used to be 100C) and pulls strongly even on high gear. As I attempted some spirited driving, I realized that I cannot rev past 4500rpm - feels like hitting a sponge wall.  No misfire although a slight pulsing rhythm as it tries to break past 4500rpm.  I've  changed coil and tried a smaller point gap but no difference, which leads me to conclude that this may be a fuel delivery issue?

Browsing through the Haynes manual, it states that the fuel inj has an adjustment for idle speed (up to 1700rpm) and 2 other screws - medium load and full load. Am I looking at the right areas for adjustment?  How do I proceed from here?  

Heeding all warnings, I will not attempt to take apart the fuel inj unit. It runs pretty well anyway and I'm hoping this can be 'tweaked' into better shape.  There is no one out here who can calibrate these machines.    

Thanks, Martin

[Benz Dr.]

This could be a fuel problem alright but something says ignition to me. If the car seems to run out of gas and even slows down a bit that's more like fuel but if it just hits a wall and won't go higher no matter how hard you push the pedal or how long , it's more likely an ignition timing problem.
Try moving the rotor inside the distributor and see if it has free movement. It should spring back to its' original position every time and have enough spring force to do it without any help. If not the springs are worn out or the fly weight system could be sticky or siezed up. The vacuum portion starts to advance right off idle and really begins before the mechanical part does. I've seen where someone used a screw that was too long to hold the condensor on and it hits the arms that hold the springs on. This makes them pop off and the mechanical advance is gone. The distributor will need to be rebuilt if this is the case.

Don't play around with injection pump. There are places that can rebuild or calibrate the pump. I have a whole set of tests I use to determine if the pump is faulty and only remove one when nothing else will work. This is a bit of work to R&R so you don't want to just guess about it.

regards,
Dan

Daniel G Caron

[rwmastel]

Martin,

If you want to check anything on the fuel side, you should start with the electric fuel pump.  Check your manual for flow rate and pressure.  When checking, do it at the return line after the fuel has moved through the main filter (and inj. pump?) in the engine bay.  This ensures the pump is providing all it should through the whole system.

For anyone who doesn't know, the extra fuel not used by the engine is returned to the tank.  The fuel system has a complete cycle of lines to the engine and back.

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[Malc]

Martin,
I know that you have just had a "complete" tune up but in order to check everything did you change the distributor cap, rotor arm, plus the plug and coil high tension leads? They do fail after a while. I once had a rough running car and happened to open the bonnet at night only to see beautiful blue sparks flickering around the plug leads which were about 4 years old, a new set cured that. Ran a lot better too!

Other than that does it rev past 4.5k when in neutral (no load) if yes I would agree that you should check the fuel delivery etc as it sounds like it just isn't getting the fuel.

Others may know but my experience with mechanical fuel injection is that they tend to be very reliable unless unused for a long long time or the fuel is contaminated in some way, water dirt etc

Hope this helps
MAlc
Scotland