Author Topic: MIssfire after warmup  (Read 6124 times)

rogerh113

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MIssfire after warmup
« on: March 17, 2011, 13:31:37 »
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
1966 230SL black 4 speed (250 low compression engine)

J. Huber

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Re: MIssfire after warmup
« Reply #1 on: March 17, 2011, 15:18:41 »
Hey Roger -- have you checked the dwell reading for the points? and maybe next time try BP5ES since your engine is a bit older...
James
63 230SL

rogerh113

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Re: MIssfire after warmup
« Reply #2 on: March 17, 2011, 20:17:35 »
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
1966 230SL black 4 speed (250 low compression engine)

J. Huber

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Re: MIssfire after warmup
« Reply #3 on: March 17, 2011, 20:24:37 »
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!
James
63 230SL

Larry & Norma

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Re: MIssfire after warmup
« Reply #4 on: March 17, 2011, 20:39:03 »
Could the Pertronics be faulty, did this suddenly occur or is it gradually becoming worse.
Do you have the series resistor, check for cracks.
Larry Hall (Gnuface)
2023 Ioniq6
2005 C230
1970 280SL

merl

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Re: MIssfire after warmup
« Reply #5 on: March 17, 2011, 21:22:08 »
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

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Re: MIssfire after warmup
« Reply #6 on: March 18, 2011, 15:13:30 »
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
1966 230SL black 4 speed (250 low compression engine)

rogerh113

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Re: MIssfire after warmup
« Reply #7 on: March 18, 2011, 18:20:40 »
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
1966 230SL black 4 speed (250 low compression engine)

hands_aus

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Re: MIssfire after warmup
« Reply #8 on: March 20, 2011, 01:07:42 »
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.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL #114, auto, ps , 717,717
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