Bad misfire under loaded conditions.Please help..

[Brent Lineker]

I have a 75 911S with stock Bosch CDI with points and silver Bosche coil,webers and early style exhaust with SSI's.I developed a misfire at higher rpm's last weekend after the car sat in the rail for a few hours.I though at the time maybe my wires,dis cap ect was just wet and would go away after drying in the heated garage but no go.I went ahead and replaced cap.rotor,and points and cleaned up my plugs which were just a couple of months old.Their were a couple that were really sooted up.I checked for spark at each connector with a spare plug I had on hand.Every one had spark,not really sure if it was a lot of spark or not,nothing to compare to really.I put the old plugs back in after cleaning up with some carb cleaner and a stiff plastic brush but the car seemed to run worse than before.The plugs were Bosch W5's,I decided to purchase some new plugs and went with NGK BP7ES's,when I took out the previously cleaned Bosche's the same two plugs were really sooted up and only after a 1 mile of running.I have ordered some plug wires and will put them in tonight but they are only a couple of years old and when I checked the resistance of the wires they all came back 3.89 to 3.95k's.I am lost on what to check next if the plug wires don't do the trick.I did start the car with one plug out of the car,could I have knocked some carbon down into a valve seat maybe.I am lost,any suggestion....

[911quest]

Which two plugs were fouled out? Could have a carb float that has a hole in it causing it to run rich on those two cylinders.

[Brent Lineker]

I don't have the diagram in front of me but they were right side 1 in the front and 1 in the rear.

[Brent Lineker]

Sorry wrong picture.

[Netspeed]

You can use carb cleaner to initially clean the plugs but it'll leave a residue that could cause the plugs to foul. Good shops use a sandblaster-type devise to clean them. My suggestion would be to get really fine wet-sandpaper and give the electrode and the underside of the tip and little brushing to clean them. Not too much and watch out for the sandpaper bits getting into the plug.

I big on grounds so don't forget the body ground and the ignition system's ground. A good spark should be a nice light blue color.

[Brent Lineker]

Thanks netspeed but I actually took the cleaned up plugs out and replaced with brand new with no effect on runnability.

[Netspeed]

I forgot to type that I didn't want you to do carb-cleaning method on the new plugs.

[RoninLB]

I don't know much about Webers.

Wild guess

There is some kind of e-tube hole or something accessed from top of carb. Anyway, what's ever up there gets dirty and causes stinkie issues. You unscrew it, remove, and clean.

[Noel]

When was the last time you replaced your fuel filter?

[911quest]

Brent after you replaced the plugs did you drive the car or did you let it idle? These things are bad about plug fouling what I usually do it replace the plugs and take it down the highway for some good high rpm runs to kinda clear it out. I would also suggest checking the main jets to make sure there is no dirt in them.

[gigem75]

this all started after it rained? Try draining the float bowls. There are two bronze nuts on each carb at the bottom that will drain the each of the two bowls on each carb. Check for water when you drain them. You can also try removing the low speed jets and blow them out with air. Do a search on webers, there is a diagram that will show you what each one of dem bronze nutz in bolts are on your webers.

you can sometimes clear a fouled plug by disconecting the plug wire at the plug and holding it just off the plug tip, you'll hear the spark jump and it'll pick up that cylinder.

How about any moisture in the cdi unit itself. I see you have a cover on yours, might be some resitual moisture under that?

[Brent Lineker]

I replaced the plug wires with no better results,I then ran some combustion chamber cleaner through the carbs that is supposed to loosen any carbon buildup on the intake valves,and still no better.I did run the motor for 30 seconds or so with no spark plug in one of the holes,could this have possibly damaged/plugged an exhaust valve.The sound it makes is kinda of a pfft,pfft,pftt and only under high load conditions.this does not make the noise just reving the motor in neutral or at a gentle cruise.The noise goes away just coasting downhill so it is not an axle noise or such.I am not really sure the wet weather had anything to do with this or was coincidental.I am pretty well out of ideas,there are no P mechanics here at all so may have to trailer the car to Vancouver.

[Brent Lineker]

The filter is only a month old or so.

[KevinS]

Might want to check the timing. Chain could have jumped a tooth.

[Quicksilver]

I would suspect it is an ignition problem. A spark will jump easily in a vacuum. As the pressure increases it becomes harder and harder to get a spark to jump. If there is any weak points in your ignition system it will jump there instead of at the spark plug. A full throttle miss is a common ignition failure symptom.

The plug wires need to be good. They need to be very clean at the connections, around the boots and between the wires and the boots. The cap and rotor need to be perfectly clean. Any trace of dirt or hairline crack is a path to ground.

[Brent Lineker]

The plugs,cap,rotor,plug wires,points,fuel filter are all less than 20 miles old now,idle jets were blown clear with cleaner and then air.I started the car without a plug in one of the holes not sure if this caused damage or not.The sound is kind of a phtt sound and only under high load conditions(low rpm/high gear ect)it almost sound like an exhaust leak but couldn't find one.

[Netspeed]

Ok....let's keep this really simple. You drove the car and it ran fine...parked it in he rain and then afterwards it ran lousy....correct? This should rule out any mechanical problem. Focus on the simple stuff....water causes corrosion fairly quickly. Check all ignition grounds....take the ground connectors off and clean them. I really think that if it was water in the carbs (you had the covers on correct?), it would mis-fire all the time. Quicksilver is right. Under load, he ignition system has to work extra hard to make that spark.

As for removing the spark plug and running it, that sound you described just sounds like normal compression coing out of the spark plug hole. I doubt you damaged anything.

Keep it simple once again......it rained and it ran lousy underload immediately afterwards. Normally I'd have you work backwards (plug, plug wire, cap, coil wire, coil) if it mis-fired all the time but in this case....I think you should work from the ignition system's source....the ground. Then work towards the plugs checking for signs of corrosion. If anything looks remotely questionable, clean it or replace it. Make sure things fit snug....it doesn't take much to degrade the spark.

[Lacagnina]

My 82SC developed a misfire at about 4,000-5,000 rpm. The problem went away after I replaced the coil. Sounds like you have other issues, though.

[3.2 CAB]

I don't think the open plug hole did anything wrong to your engine. There used to be, and may still be, an inflater that uses on of the spark plug holes to pump air. You would remove the spark plug, then install the inflater plug in its place, fire up the engine, and you had an air supply. This seemed SO WRONG to me, actually wanting someone to air their tires up, with gasoline vapors. There are probably some others that remember seeing these things from years past.

Now to your problem at hand. It is not unheard of for a spark plug to be a dud, straight out of the box. I know that you have replaced the plug wires and a lot of other components, but, have you checked each new spark plug wire for spark? You can do this easily with a timing light, just go from plug to plug. That way you know that you are getting spark to each plug. It also sounds like it could be a vacuum related problem. I do not know a lot about the earlier P-car ignition systems, but I did not see you mention a condenser. Does your car use an ignition condenser? If you have access to a known good coil, you might want to see what it does with that. It is difficult at times to pinpoint some of the little things like this, without being able to do a hands on check. Good luck!! Tony.

[Brent Lineker]

Thanks all for the advice,I am at work right now but will go over all my previous work tonight.I am wondering if I mixed up the wires when I first changed the cap,I was on the side of the road broken down in a downpour so it may have happened then.I am wondering if that pfft sound is just the normal compression stroke and just is not firing.My #6 and # 4 plugs were really bad so I may have mixed up those two wires.I am hoping this might have been my mistake.