Misfire when hot under load

[Steve.k]

Hi all,

I’ve had my 280SL for just over a month now.  I’ve been a full member here for over a year and have read the technical manual and hundreds of posts.  The education I received here helped me select my car and since I got it I’ve fixed a few things by reading the posts here.  I’ve repaired the glove box spring, did the dash light jumper fix, replaced all exterior bulbs with LED’s, adjusted the clock, repaired the washer pump, taken the door panel apart and replaced window felts.   
I’ve never done any of these things before but I’ve gained confidence thanks to the shared knowledge in this group for which I’m grateful.
I’ve got one nagging problem that I haven’t been able to resolve even after reading up on it here.   When I drive my car it starts up immediately, idles ok and pulls away smoothly.  The engine hums very nicely and drives well.  However, after about 10-15 minutes of driving, it will misfire above about 3000 under load.  Sometimes I can hit the kick down and it will run to about 4,500 before sputtering.  If I feather the throttle I can relieve the misfire but it’s always there once it’s hot.   
The spark plugs are textbook light rust color and I’ve replaced the fuel filter, air filter, coil and coil wire.  I bought NGK BP5ES plugs based on recommendations here but the original and replacement lead wires I got are for screw on terminals so I can’t swap them out yet. 
I’m not experienced enough to try fiddling with the fuel injection pump adjustments but I’m at a bit of a loss to resolve this.  It doesn’t seem to be an ignition issue and feels like fuel starvation.   It has about 75,000 miles and was well maintained by its previous owners.   It’s a’71 and appears to have all the emission equipment intact. 
I’d appreciate any suggestions or advice.

[mrtzmd]

Hi Stephen,
Did you replace the points? From my research this is the Achilles tendon of the ignition system. Look at the points section under the tech manual. Im currently looking at this problem myself. Good luck.
Mauricio

[Steve.k]

Solved! 

I got a 123 Distributor and installed it.  Problem solved.  Purrs like a kitten.  It was probably the points, but I won't have to deal with them again.  I'm not terribly experienced, but I was able to install the 123 in about an hour or so.  Started right up. 

[Tyler S]

I bought NGK BP5ES plugs based on recommendations here but the original and replacement lead wires I got are for screw on terminals so I can’t swap them out yet. 

Stephen, the terminal ends on the NGK plugs unscrew. Use a small pair of pliers to break them loose. Unscrew them and discard. Wires should fit just fine.

[Steve.k]

Stephen, the terminal ends on the NGK plugs unscrew. Use a small pair of pliers to break them loose. Unscrew them and discard. Wires should fit just fine.

Thank you!  Works like a charm.  I tried that before but I didn’t want to force it too much. 

Now I’ll try swapping out the old Bosch plugs for the hotter NGK’s.  After installing the 123, I doubt know how it could run any better

[Tom in seattle]

I know the "purists" here would like to continue to file their ignition points and set dwell angle but the rest should buy a light triggered replacement and eliminate the consequence of aging.  Note that what you described could also be caused by a coil which decreases its output as it gets hot.  The original coil in your car can be replaced with one which looks the same and adds a much hotter spark.  Drag racers even use multiple spark discharge (MSD) systems to ignite very high compression ratio motors at high RPMs.  I suggest that your problem is not spark plugs but improper fuel/air timing and ignition timing.