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Discussion Starter #1
Ok, as the title says cylinder 8 in my 1974 450SLC is not firing properly, misses every so often and now doesnt seem to fire at all. The car runs ok and has not lost much power, its just wrong....

History, Car has always had a vibration as tho one cylinder wasnt working too well, and previous owner had a failed spark plug lead going to C8, changed that and it worked well again. Since i have had the car the engine has run well and strong with MSII controlling it.

Tested the sparks, checked for spark and there was, changed plug swaped leads with another cylinder and no change, Removed lead from dizzy while engine running to see if it changed, it didn't noticeably change, put it back on and pulled another lead and was a noticeable difference. Plug has spark.

Fuel, After doing my MSII conversion its very easy for me to pull one bank of injectors out to see if they are firing, all 4 were firing and all seemed to be firing in the same manner with the same amount of fuel been collected in each cup. Too make sure i replaced the injector that services that cylinder. Tested the signal from MSII and was the same as all the other injectors.

My father thinks it could be something physically wrong with the Valves for the cylinder because just before this it started the dreaded ticking noise, was planing to adjust the tapets this weekend :(.

What tests can anyone think of to find out why C8 is not working, and are there any common problems like this for the M117s?
 

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Look at the cam lobes for that cylinder. Look at the plug and see it it looks different.

Side note: I worked with a "mate" from near you last week. Been over here about a year working for an electronic engineering co. I ask him what was the hardest thing to get custom too over here, and he said Christmas. He said it was a summer time holiday down-under. Nice guy .... or is that mate. No problems mate.
 

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Discussion Starter #4
The compression test was fine, the mechanic who did the compression test thinks that it could be to much air getting into that cylinder either thru the injector seals or through the intake manifold gasket, he sprayed the area abit with carby cleaner but could not locate the exact location. But there was a noticable difference in the engine when he sprayed the area.

I ll do some work on it later hopefully get it working well again.
 

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Discussion Starter #5
I sealed all the extra air leaking into the cylinder, also adjusted all the Valves back to spec(was suggested that it could cause C8 to not work if it was along way out), engine is running well but C8 is still not firing.

I have double checked the spark and fuel supplies and they are still working. Compression is great, valves are adjusted, no extra air leaks......What else could be wrong
 

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This reminds me of a scene in Airplane 2, The Sequel where a subordinate says to William Shatner "Sir, these lights are flashing out of sequence". Shatner: "Make them flash in sequence!" "Yes, sir!"
 

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have you spare/old distributor cap? maybe #8 terminal is leaking spark to the ground making it not fire properly?

Also, it can be (theoretically) that triggering of spark for #8 is not good (breaker/induction breaker)
 

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Discussion Starter #8
Hmm could be leaking power to ground making the spark abit weaker i guess, but pulling the lead and sticking a screwdriver in it and holding it near a rocket cover produces the same looking spark as doing it on any of the other cylinders. But i ll find out if it is the spark soon as i will be finally ripping out the Dizzy and replacing it with coil packs direct fired from my MSII in wasted spark.

Hopefully that its something wrong with my sparks causing the problem because i ll be replacing EVERYTHING, kinda wacky way to solve problems :) If its something else causing problems i ll be pulling my hair out and you will be able to find me in a mental hospital.
 

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do not forget that the problem was, more or less, before MSII you have installed and is back again. If cap is not at fault, triggering can produce late or early spark for that cylindre only... jerky indecisive trigger pulse can produce weakened spark.

Where else can be the culprit as you repaired/checked everything else?
Gremlins!
 
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