Cylinder #6 doesn't fire

[todor]

I'm trying to sort out a '76 911S that I just bought. It's running rough, so I decided to check if pulling each spark plug wire one by one would make a difference... and sure enough, pulling the wire on one of the cylinders did not. I swapped the spark plugs with the adjacent cylinder, but there's no change - still the same cylinder (#6) doesn't seem to fire (as determined by pulling the plug wire and seeing that nothing changes in the RPMs or the way the engine runs). I used an in-line spark tester and verified that there is current flowing through the wire and the plug.

So what does that mean? No fuel? The car has been converted to Weber carburetors by a previous owner. I really don't know anything about carburetors - this is my first carbureted car. Is it possible that somehow fuel is not getting to that cylinder but is getting to the others? Can a carburetor get plugged somehow? I should mention the car had been sitting for 3 years, so the fuel in the tank was probably nasty. Luckily there wasn't much left and I just put in new gas, but still, I guess it may have plugged up something.

Or is it possible that there's something more major wrong with the internals or valves of that cylinder? I'm planning to do a compression test tomorrow, just to check.

But as far as troubleshooting the fuel goes, I really don't even know where to start. There must be fuel pressure, otherwise the rest of the cylinders wouldn't work either. The carb setup looks like this:





Any ideas what could be wrong? Thanks in advance.

[Dan J]

You probably have a plugged idle jet very common on Webers. Search around the web for a diagram. They're easy (and fun) to work on and since you own them you might as well dive in
Good Luck

[T77911S]

congratulations. you did not not just put one foot in the water when it came to your first carb'd car, you jumped in up to your nads.

verify compression/leak down. carbon can get lodged in the exhaust port.
if that is good, and you have good spark, then it could be that that carb adjustment for that cylinder is not right, yes, each cyclinder is adjustable, or you have a clogged port in the carb.

i would also put in new cap, rotor, plugs and wires....just to be sure.

[Grady Clay]

Remove the #6 sparkplug and make sure it isn’t fowled.
Replace if necessary.
Carefully inspect the plug connector for damage (crack, arcing) particularly at the sparkplug connection.
Is the sparkplug insulator very dirty?
Re-test for running on that cylinder.




Remove the idle jet holder (red arrow) and inspect for something in the jet.
Remove the jet from the holder and look through it for something.
Blow it out.

Reinstall jet and holder.

See if the engine fires on this cylinder.


If not, screw in the idle mixture screw (green arrow) until it stops counting the number of ¼ turns.
Remove the idle mixture screw.
Slightly loosen the lock-nut on the air bypass screw (violet arrow) but retain the nut position and orientation of the screwdriver slot.
Remove the air bypass screw retaining the nut in position.
Remove the idle jet holder (red arrow) again.

Use a bit of aerosol carb cleaner in each of these ports.
Not too much so it floods down in the cylinder.
Use compressed air in each of these ports.
Do NOT blow compressed air in the vent in the float bowel.

Reinstall all of the parts in the original orientation.

See if the engine fires on this cylinder.

Try screwing the idle mixture screw all the way in and back it out slowly seeing if the cylinder starts firing.


If still not running, do a cranking compression test and cylinder leak test on all the cylinders.
It is necessary to measure all the cylinders for comparison purposes.
Save these measurements as a baseline for future trouble-shooting.

Best,
Grady

[todor]

Wow, thanks for the detailed instructions Grady! I'll try those tonight.

So if we think it's the idle jet that's causing the problem, then does this mean that cylinder should be firing when NOT idling? Should I do the same test with unplugging the spark plug wire when the throttle is partly open? Or will the difference (between the cyl firing and not firing) not be noticeable at higher RPM anyway?

[todor]

And by the way, where's my fuel filter? I don't see anything like a filter in the engine compartment.

[todor]

I just tried everything that was suggested here, but still no luck. I don't have my compression tester here so I couldn't do a compression test, but everything related to the carb above was tried. Any other suggestions, until I find my compression tester?

[DanielDudley]

You are absolutely sure the spark is making it to the cylinder ? My dad used to have a saying that ''90% of all carburetor problems are electrical.''.

[69911e]

Spray your choice of flammable gas/liquid down the stack and see if idle speed responds. If it does, problem is in the carb or gaskets. If no change in idle, look at spark or compression.

[todor]

Update and more questions:

Found out cyl #3 is not firing either! I'm pretty sure last time I did this test, removing its spark plug wire made a difference... but it doesn't anymore! (and this is before I did any of the work listed below.) Tomorrow I'm going to clean the idle jet and ports on #3 like I did for #6.

Yes, spraying carb cleaner into the carb ports for the bad cylinders (#3 and #6) does bring up the RPMs. Spraying it into any of the good cylinders makes the RPM drop and the engine almost dies. Is this normal?

I replaced the spark plugs and distributor rotor. Distributor cap is on order, arriving tomorrow. Gapped new plugs at 0.030", which I figure is relatively conservative, given that I'm running an MSD ignition. The old plugs were at about 0.035". Btw, the old plugs looked fine - their tips were completely black, but not much different from other used plugs I've seen before.

Cyl 3 and 6 still not firing, even after the spark plug and rotor replacement. Actually, after this replacement, #6 doesn't even respond to carb cleaner sprayed in it anymore... (but #3 still does). So I may have messed up something with the spark. I'll pull the spark plug to check for anything weird, and I'll also retest with the inline spark plug tester.

I did a compression test and got surprisingly good results: all cylinders are between 160-170psi.

Maybe bad fuel is plugging up the carbs. Where is the fuel filter on this car? I want to replace it. It's not in the engine compartment.

[sobamaflyer]

There should be a cylindrical fuel filter (not the see through little paper ones) in the Upper Left side of your engine compartment?

I tend to agree w/ another suggestion about clogged jets, Weber rebuilding isn't hard or particularly expensive. I rather miss my carb'd cars (still haven't made good friends with my CIS)

[todor]

There's no fuel filter in the upper left side of my engine compartment, or anywhere in there. The fuel line comes from a hole in the firewall to a splitter with a pressure gauge (which doesn't work) and from there to each carburetor. No filter. I wonder if whoever converted the car to carbs removed the filter... is that common practice? Sounds dumb to me... Or maybe they relocated it under the car. Isn't there also supposed to be a fuel accumulator to smooth out pulsation from the fuel pump and maintain pressure when the pump is off? The other thing that's weird is that there's no fuel return line. Is that normal for carburetors?

Ok, so I guess I need to do a more major cleaning of the carbs then. I'll search for instructions. Hopefully I don't have to remove too many things.

[HarryD]

Originally Posted by todor

There's no fuel filter in the upper left side of my engine compartment, or anywhere in there. The fuel line comes from a hole in the firewall to a splitter with a pressure gauge (which doesn't work) and from there to each carburetor. No filter. I wonder if whoever converted the car to carbs removed the filter... is that common practice? Sounds dumb to me... Or maybe they relocated it under the car. Isn't there also supposed to be a fuel accumulator to smooth out pulsation from the fuel pump and maintain pressure when the pump is off? The other thing that's weird is that there's no fuel return line. Is that normal for carburetors?

Ok, so I guess I need to do a more major cleaning of the carbs then. I'll search for instructions. Hopefully I don't have to remove too many things.

Sounds weird. Check Grady Clay's comment on this thread: Fitted webers, fuel filters?.

[kodioneill]

are you certain that you have the firing order correct ?

You have to test the compression somehow to rule out a dead cylinder.

[Lufeco]

Nobody has suggested to measure the impedance on the spark plug wires. Sometimes they look good outside but totally worthless inside.

You'll need an ohmmeter. Take one of the good wires, measure it and then compare to the one without spark.

[todor]

Originally Posted by kodioneill

are you certain that you have the firing order correct ?

You have to test the compression somehow to rule out a dead cylinder.

I did test the compression and it's fine.
I haven't changed the wire routing... and each wire that goes to the plugs has a little number on it. That said, it doesn't mean they're plugged in correctly to the distributor cap... so I guess I'll check that when I get home.

Originally Posted by Lufeco

Nobody has suggested to measure the impedance on the spark plug wires. Sometimes they look good outside but totally worthless inside.

You'll need an ohmmeter. Take one of the good wires, measure it and then compare to the one without spark.

I have a multimeter, so yes, I'll check the impedance. Thanks for the idea. What's a normal value? I'll compare to the working cylinders of course, but just asking.

[todor]

Somehow cyl #3 is back to life... without me doing anything. So since 5 of the cylinders are working, the wire routing must be correct. But cyl #6 still isn't firing. I did a spark test with the inline spark tester tool and it did blink. I swapped plugs between #5 and #6 (even though the plugs are brand-new), still no change. The plug that came out of #5 (working cylinder) was already covered in black stuff (carbon?), after only about 10min of running, whereas the one from #6 was shiny clean. Spraying carb cleaner into cyl 6 still does nothing.

I forgot to check the resistance in the wires... will do that tomorrow. I also forgot to pick up the distributor cap from the parts store and will do that tomorrow too.

[stlrj]

Are you using the original copper core braided plug wires with the MSD?

[Lufeco]

Originally Posted by todor

Somehow cyl #3 is back to life... without me doing anything. So since 5 of the cylinders are working, the wire routing must be correct. But cyl #6 still isn't firing. I did a spark test with the inline spark tester tool and it did blink. I swapped plugs between #5 and #6 (even though the plugs are brand-new), still no change. The plug that came out of #5 (working cylinder) was already covered in black stuff (carbon?), after only about 10min of running, whereas the one from #6 was shiny clean. Spraying carb cleaner into cyl 6 still does nothing.

I forgot to check the resistance in the wires... will do that tomorrow. I also forgot to pick up the distributor cap from the parts store and will do that tomorrow too.

If your wire checks out, then you might have a valve issue. I had a similar situation back in high school with a VW that I was rebuilding. It turned out that the shop did not cut the valves correctly on one side and they were not sitting/closing correctly, hence there could be no ignition on that piston and the spark plug always came out clean as new.

I hope is the wire, otherwise remove the valve cover and look for anything abnormal.

[Grady Clay]

todor,

If you can’t find a fuel filter, you need to install one.

If there hasn’t been a fuel filter, the problem most likely is contamination of the carburetors with fuel system debris.
You may not have been able to clear the debris when you blew-out the jets and passages above.


Assuming you find continuity to #6 sparkplug, I would recommend cleaning the carburetors.

Our host sells a ‘kit’ of parts.

Make sure the kit has the (6) carb-base gaskets (green arrow).

You seem to have the proper phenolic insulating spacers between the intake manifolds and heads.
You need (12) new gaskets there (teal arrows).
Inspect the insulating spacers for cracks.







I see ‘NAPA’ (9.5 mm, red circle ) fuel hose needing replacement.
When you install the filter and clean the carbs, use the Factory original 7 mm hose (available from out host) and the Norma-Schellen (N-S) type hose clamps.
The N-S type clamps don’t damage the hose like the ‘screw-type’ (violet arrows).
The piece of hose between the fittings (green lines) should be straight and ‘relaxed’ (not too long or too short).
The two fittings (banjos) should be 'in-line'.
Reposition the breather hose so it doesn’t interfere with the end of the carburetor shaft (BIG red arrow)!
(The breather hose may be a bit too short, causing this.)




It is worth buying a large enough container of carb cleaner to totally immerse each carb body.
(Share with other Pelicans.)

With a good fuel filter, you should not have this issue for another decade.



When apart, you will be able to assess how much debris has come from the fuel tank.
This is a good occasion to drain and clean the fuel tank.
You have the late (CIS) fuel tank with the filter screen accessible from the bottom.
Inspecting the screen filter will also give you a sense of the fuel system contamination.


Please keep up the great reporting.
Your diagnosis and solutions will help many others.

Best,
Grady