MIssfire after warmup

[rogerh113]

Hello,

The car is a 1966 with a 250 low compression engine and 4 speed.  The car fires up fine, and runs great until it starts to warm up.  Between when the idle drops and before the oil pressure drops, it slowly develops a miss-fire (particularly on takeoff).  At first it is just a barely audible burble, maybe only one.  Then as the car warms up it becomes more pronounced.  Then it get very noticeable, again mostly on takeoff, evolving into a pretty nasty miss-fire.

I have replaced the plugs (NGK BP6ES) - no impact.  Checked and rebuilt the warm-running unit on the IP (figuring the piston may have been seized, and the mixture gradually became too rich) - it was fine, and no impact.  I also checked around the engine compartment for loose wires and took a look at the distributor (and vacuum advance unit) which all seemed fine.  It's been a while since I replaced the dist cap and rotor, but they looked fine.

If anyone has any suggestions please let me know.  I am hoping this is a known problem with an obvious culprit.....

thanks again -- Roger

[J. Huber]

Hey Roger -- have you checked the dwell reading for the points? and maybe next time try BP5ES since your engine is a bit older...

[rogerh113]

I am running with the Pertronix electronic points, and did look to see if there was any wear or rubbing - there was not.  I think I have an old dwell meter outside, and will see if I can find it to check the dwell.  Points would have been one of my first suspects if I was not running the Pertronix.

Is it likely that the other plugs (BP5ES) would resolve the problem? 

Thanks -- Roger

[J. Huber]

Not at all sure that would resolve it. I just recall discussions about the 5 (a hotter plug) working well with some cars. I have them --my car is happy with them. I wouldn't go changing them just yet if the others are new. Meanwhile, I'd wait for the experts to chime in!

[Larry & Norma]

Could the Pertronics be faulty, did this suddenly occur or is it gradually becoming worse.
Do you have the series resistor, check for cracks.

[merl]

Hey Roger -- have you checked the dwell reading for the points? and maybe next time try BP5ES since your engine is a bit older...

i to have a petronix system love it 1970 280sl the first petronix lasted about a year and the car began to mis-fire when warm i had purchased an extra installed and the problem went away petronix replaced at no charge now again have a spare just in case merl

[rogerh113]

Thanks for the info on your Pertronix failure mode.  I put my Pertronix in about 11 years ago - it is just possible that it is finally getting tired..... 

I have contacted Pertronix for their take on standard failure modes, and for a replacement unit.  Hopefully this is the issue.  Love the Pertronix, and 11 years of trouble free use is pretty hard to moan about.

Regards -- Roger

[rogerh113]

The Pertronix folks (actually a distributor I got my unit from) got back to me fast and said that is probably not the Pertronix.  Since I said I had it for a long time (looooong past warranty) and was ready to buy a replacement, I am inclined to think they are being up-front.  They think it may be a bad ground, low voltage, or poor connection, so I am going through to check these things.  They offered to help troubleshoot if I provided more detail on the problem, which I have. 

If there are any other suggestions on problem areas you guys can think of, please let me know.  I will be proceeding on the above things and will report back progress.  It is rainy here, so I will not be going out for a test drive for a while.....

Thanks -- Roger

[hands_aus]

I had my 250sl Konk out while doing 100Km on the highway.
It hit a bump and back to life it came.
This went on and off with each bump of the road.

I stopped beside the road, found a screw driver from the boot and went over the electrical connections of the ignition system.

The second screw i tried was on the ballast resistor and it was the culprit.
The washer under the screw was missing.
After tightening the screw the engine fired right up and I was on my way.