mofo_911

My engine has started misfiring.
It does it from idle all the way up to around 3000rpm. I mostly hear it in the exhaust, but it also happens in the airbox (my pop off valve is mounted the wrong way, and it sometimes gets stuck).
I have newish plugs, new leads, new distributor cap, new rotor and a new coil.
Today I discovered something weird.
If I disconnect the lead to the #5 cylinder while idling, it makes no difference to the sound of the engine or the revs. I removed the sparkplug to see if it had spark, which it did (but maybe it was a little weak), so I cleaned and gapped it. I had the old ignition leads laying around, so I replaced the #5 lead with the old one, which did nothing. I checked the breaker point gap, and adjusted the ignition, which was already pretty close.
The last thing I did before calling it quits tonight was to remove the #5 lead and go for a 200m drive - that made the misfire go away...
So when the spark plug is connected, it misfires, when it is disconnected it just runs a little rough (on 5 cylinders)...

I am hoping it is a bad plug, but I am fearing it is valve related. Any advice?

Thanks,
Morten

mofo_911

So I switched the #5 plug with the #6 plug, with no effect. I also compression tested those two cylinders. The both tested 130psi in a cold engine. I am at a loss... I am thinking about just dropping it of at my indy to let him figure it out. Any thing I should try before giving up?

theiceman

possibly a bad injector or even the fuel distributor .. i would do lots before dropping it off but our comfort levels are all different..
the fuel injectors can be pulled out fairly easily on these cars ... you may be just running lean due to a bad injector.. id swap them over.. with another cylinder..
id also consider swapping the ports at the fuel distributor to see if the problem will move..

you may be pulling air in at the injector sleeve even .. there is a sleeve and two orings at play here .. one between the runner and the sleeve, and another between the sleeve and the injector.

be careful driving around with no ignition on a cylinder as fuel is dumping in , washing down your cylinder walls and diluting your oil ...

also make sure you don't have 2 ignition wires crossed , it can happen easily.

mofo_911

Thanks for the suggestions, I have not looked at the injectors yet - I actually have a set of O-rings for them, so I could pull them to check them out. I am not quite sure what to look for tho, but switching a couple around seems like a good plan. I have checked and rechecked the firing order of the cables, so it should not be that.
I have been thinking that it could be my cam timing that is off, I am no longer completely convinced that it is only cylinder 5 that has the problem, I think cyl 6 may have a slight misfire as well.
When I have the injectors out, I might as well check to see how their spray pattern is. I just can't think of a safe way to do it, without attaching them to some kind of test rig...

theiceman

pull all 6 ... get 6 baby food jars or something similar ... turn on ignition and let injectors spray into the jars... check levels against each other ... you can confirm levels and spray patterns if you have a helper lift the air plate for you ..

cam timing on these is pretty solid once its set..

mofo_911

I pulled the injectors, and I think these are at least part of the problem. With only raising the plunger slightly, one of the injectors delivered only half the amount of fuel of the others. The spray patterns are not great either, one just drips, and another does not mist at all. So I ordered a new set. Meanwhile I am having fun with the oil line that goes from the engine hardline to the thermostat, the PO jerryrigged some weird repair that damaged the flare nut connection on one end. So I'm not bored :-)

mofo_911

BTW when working with this stuff remember to work in a well ventilated area. I was working outside, and I still got loopy from the fumes....

theiceman

sounds like you are on your way .. remember it could still be the FD... if you don't get enough pressure out of it to open the injector fully ... thats why i suggested swapping at that end too... but looks like you are on your way to recovery.

r911

clean the old ones and keep them as spares

gggdds

I had to change my injectors soon after I had the engine rebuilt. (no, they didn't change them at the time of the rebuild). When we tested them they were all over the place in terms of spray patterns... The secret is to use the fuel injection cleaner every now and then. But another thing. Why are you using a breaker type ignition? A breakerless kit is less than $100 and it eliminates the points, and will work with the Bosch CDI. Get into at least the 1990's....

mofo_911

I have a Permatune CDI, it only uses the points for the pulse, so they never wear out, and they don't have to be set (which I only realized after I had set them :-)). When I get it running, cleaning the injectors will be a small winter project. I don't think mine have ever been changed, so I guess it about time. Permatune ignition module.

mofo_911

What would you use to clean them? Acetone, injector cleaner?....? It needs to not disolve the plastic screen inside them, but still be strong enough to disolve 40 year old gasoline residue...

theiceman

my buddy is a deisel truck mechanic and he has an old cleaner /tester in his shop, ill ask him what he uses.

mofo_911

I saw a guy online who experimented with something called diesel purge, but all these products are made to be put in the tank as an additive. I haven't seen a product that is meant to be used concentrated, directly on the injectors.

mofo_911

The new injectors arrived today, and I have mounted them on the lines (some of the old ones were a xxxxx to get off) the first 4 ones popped right back into their holes with a new seal on, but I just can get last two in (the two inner ones in the drivers side). One of them may be in there because I can't get it out anymore - I just never felt it pop, but inner most one I just can't get in. Does anyone know a trick to do this?