Cylinder 4 not firing- HELP! Options

[tornik550]

Over the past three years, I have been working on my 73 914. The majority of the car has been rebuilt (except he body). I recently got the engine working well enough to drive. I have been driving the car to work the past two days. The engine seemed to run great. I am still fine tuning the carbs though. The engine is freshly rebuilt with about 200miles on it. It is a 2.2.7 l. The only thing that wasn't rebuilt was the heads (long story why).

Tonight, I made a few carb changes, drove the car around for about 15 minutes.. The car seemed to run great but idled a slightly rough. When I got back to my garage, I decided to re-adjust the idle mixture screws. I started with cylinder 4. I didn't notice much of a difference when playing with the screw. I noticed that there was a lot of cold condensation on the manifold for that cylinder. With the engine running, I put my hand over the trumpet on each cylinder one at a time. 1,2,3 caused the engine to almost stall, 4 made no change. I checked to make sure that I was getting spark to cylinder 4- I was. I checked to make sure that I was getting fuel to cyl 4- I was. I took out the spark plug and it had a bunch of black soot. I checked the others and they were a nice brown color. I felt the exhaust pipe for cyl 4 and it was about the same temp as the other pipes. I did a compression test and it read 90 on cyl 4. In my panicked state, I did not check the other cylinders.

I had previously gotten compression tests on cyl 4 at around 150. The problem is that I think that I may have screwed up tonight's test. When I did get the 150 on previous tests, I did the comp test properly. This time, I took the spark plug out, engine hot, put in the compression gauge, turned the ignition for about 4 seconds. Read the gauge- 90. Repeated- 90 again. I was panicking and tried to think of what the problem could be. Decided in my haste that I should check valve lash. I got the cover off and decided that I should stop and think about things overnight.

Any guesses?

Problems that I can see- compression test done improperly but would this make that much of a difference? Also, I did not check the other cylinders. Even if the compression was good- why wouldn't the cylinder fire or is it firing but just not well? Could this all be because of a tight exhaust valve? Or is all of this just wishful thinking. I suppose that bad rings are an option but they are brand new so why? I also supposed that I could have a bad exhaust valve.

Argh- since I just got my engine were I like it- I really hope that I just did something stupid and that there is not a big problem that will take the car off the road for a while.

[type2man]

I'd check the idle jet on the 4 cylinder.....probably clogged. Any immediate changes in running condition, I blame on carbs. I'd slap a set of 44 on that puppy if I owned it. It's a bigger motor and should be able to breathe better with 44's on it...

[pilothyer]

When you say you had good spark to cylinder (4) did you actually remove the plug and ground it and see the spark on the plug itself while cranking the engine ?

[tornik550]

When you say you had good spark to cylinder (4) did you actually remove the plug and ground it and see the spark on the plug itself while cranking the engine ?

No, I plugged in one of those inline spark checker things. Similar to the one in the picture.

[phatnine11]

i would swap a good spark plug into #4. Just one question, when you did the impromptu compression test, did you have the throttle wide open?

[tornik550]

i would swap a good spark plug into #4. Just one question, when you did the impromptu compression test, did you have the throttle wide open?

No I didn't. I didn't because I did not unhook anything other that the cylinder 4 wire. I didn't want the other 3 cylinders to rev high.

This looks like another issue with my impromptu compression testing.

Does it sound reasonable to say that the compression test that I did was completely worthless? (that would be great news, I feel like a real idiot)

[pilothyer]

I'm not real familiar with that device, but if you remove the plug from the head and reconnect the wire to that plug then ground the plug by holding it to a good ground on the engine case while you have someone crank the engine you should see a good spark from the plug...if it is weak which it probably will be due to the black foul clean it well and try again. better still a new plug....You seem to have an extremely rich port on #4

[phatnine11]

i would swap a good spark plug into #4. Just one question, when you did the impromptu compression test, did you have the throttle wide open?

No I didn't. I didn't because I did not unhook anything other that the cylinder 4 wire. I didn't want the other 3 cylinders to rev high.

This looks like another issue with my impromptu compression testing.

Does it sound reasonable to say that the compression test that I did was completely worthless? (that would be great news, I feel like a real idiot)
You need to get the throttle wide open for a compression test. Also, you need to remove all four plugs to allow the engine to crank faster (no resistance from the other three cylinders).

[VaccaRabite]

When you put your hand over the horn, did it try to suck your hand in?

My guess is that you have a clogged idle jet. It happens, especially during tuning since you are running the engine with the air cleaners off.

Clean the plug, clean the idle jet, put it back together and see what you get. If it is still not firing, check the valves. If it is in spec at the valves and shit is still not right do a proper compression test. if you have a leak down tester, do that too (or rent/borrow one from a FLAPS).

Don't panic. Be methodical.

Zach

[tornik550]

When you put your hand over the horn, did it try to suck your hand in?

My guess is that you have a clogged idle jet. It happens, especially during tuning since you are running the engine with the air cleaners off.

Clean the plug, clean the idle jet, put it back together and see what you get. If it is still not firing, check the valves. If it is in spec at the valves and shit is still not right do a proper compression test. if you have a leak down tester, do that too (or rent/borrow one from a FLAPS).

Don't panic. Be methodical.

Zach

When I put my hand over cyl 4 trumpet, I could feel it sucking. I also put the snail on it and it was reading the same as the other cylinders.

I haven't checked the idle jet. It would seem that I could have a clogged jet. I had recently bored out the idle jet that I am using to a larger size. It is possible that some metal shavings got caught in the jet.

My concern about this is why would I have the black soot on the plug if the cylinder isn't firing? Wouldn't I have no soot because the cylinder would essentially be flooded? Is it possible that the cylinder isn't firing at idle because of a plugged idle jet but is firing at higher rpm when the main takes over? That could explain some soot and also why the h.e. from cyl 4 is hot?

[VaccaRabite]

plugged idle jet does not mean that you are not getting fuel through the carb. It means that you are not getting the correct mix of air and gas to fire. instead you might be getting drips of gas, and that does not burn well (uber rich). This leads to black fouled plugs.

If its reading the same on the synchometer as the other barrels, don't worry about your compression. Just clean the fouled jet and plug and I bet you are golden.

Zach

[tornik550]

plugged idle jet does not mean that you are not getting fuel through the carb. It means that you are not getting the correct mix of air and gas to fire. instead you might be getting drips of gas, and that does not burn well (uber rich). This leads to black fouled plugs.

If its reading the same on the synchometer as the other barrels, don't worry about your compression. Just clean the fouled jet and plug and I bet you are golden.

Zach

I went in the garage and pulled the idle jet. It looked clean but that probably doesn't mean much considering the hole is so small. I cleaned it. I also cleaned the spark plug?

If this doesn't pan out to be the idle jet (which it sound like it is the problem), is there a way that I could be getting a spark however a weak spark? I checked the spark plug wires, all the connections seem tight. The wires fairly new 8mm magnecor wires. I use a 050 dizzy with pertronix. New cap and rotor. MSD6al ign with blaster 2 coil. The spark plugs are all fairly new with about 30 miles on them- NGK. The plugs are all gapped the same. Has anyone ever gotten a bad NGK plug? If the idle jet doesn't pan out, I will try to switch the plug to a different cylinder and see if the problem moves with it.

[VaccaRabite]

Yes. And the fouling on the plug will also give weak spark. The problem can build on itself. If you are just using copper plugs, you can take a mild wire wheel to them to knock the fouling off. Fancy plugs will be destroyed by doing this, though.

Another possibility, since you tinned the jets and them bored them, is that you have a slight burr in the jet which would knock the fuel/air mix out of suspension. You would see it though - I'd think.

Zach

plugged idle jet does not mean that you are not getting fuel through the carb. It means that you are not getting the correct mix of air and gas to fire. instead you might be getting drips of gas, and that does not burn well (uber rich). This leads to black fouled plugs.

If its reading the same on the synchometer as the other barrels, don't worry about your compression. Just clean the fouled jet and plug and I bet you are golden.

Zach

I went in the garage and pulled the idle jet. It looked clean but that probably doesn't mean much considering the hole is so small. I cleaned it. I also cleaned the spark plug?

If this doesn't pan out to be the idle jet (which it sound like it is the problem), is there a way that I could be getting a spark however a weak spark? I checked the spark plug wires, all the connections seem tight. The wires fairly new 8mm magnecor wires. I use a 050 dizzy with pertronix. New cap and rotor. MSD6al ign with blaster 2 coil. The spark plugs are all fairly new with about 30 miles on them- NGK. The plugs are all gapped the same. Has anyone ever gotten a bad NGK plug? If the idle jet doesn't pan out, I will try to switch the plug to a different cylinder and see if the problem moves with it.

Thanks to everybody for their help!!!!

[tornik550]

[quote name='Vacca Rabite' date='Jul 29 2010, 08:51 AM' post='1349504']
Yes. And the fouling on the plug will also give weak spark. The problem can build on itself. If you are just using copper plugs, you can take a mild wire wheel to them to knock the fouling off. Fancy plugs will be destroyed by doing this, though.

Another possibility, since you tinned the jets and them bored them, is that you have a slight burr in the jet which would knock the fuel/air mix out of suspension. You would see it though - I'd think.

Zach

[quote name='tornik550' post='1349423' date='Jul 28 2010, 10:52 PM']

Is there anything else on the carb that I should check? I have that carb off and now would be a good time to look into things. I am going to check the idle mixture seat. I recently put in a new set.

[VaccaRabite]

Not that I can think of.

Every time I have tuned in a motor I have gone through what you are going through. 1 cylinder getting fouled and then caked with something. it concerned me greatly the first time. Now I expect it. Just clean everything, and be liberal with the carb cleaner and compressed air. Once you get past the initial tune and balance, and you are able to get load on the engine and get it hot, you will notice the color of the plugs go from black and oily to a nice proper brown. After all, your rings still need to seat, you are sucking in all kinds of dust from running w/o air cleaners in place. its just the way it goes.

Clean clean clean. Get things set, and drive it like you stole it.

Zach

[tornik550]

Here is an update on my situation. I went through my entire passenger side carb and cleaned it with compressed air and carb cleaner. I also cleaned all my spark plugs. I hooked everything back up and the engine started. Idle was a little rough. I noticed right away that the manifold for cyl 3,4 was cold with condensation and the other side was warm and dry. Once the engine was warm then I placed my hand on each trumpet- no change with cyl 3 AND 4, engine almost stalled with cyl 1,2. I played around a little more with the carbs but no change. I then checked the spark plugs and cyl 1, 2 were dark brown, cyl 3,4 were off white and wet with gas.
I felt the H.E. pipes from each cylinder- 1,2 were extremely hot (normal temp)- 3,4 were warm to hot.

Since the plugs were out, I decided to do a compression test. Due to my neighbors, it didn't work out as planned. Here are the results-

cyl 1- 150
cyl 2- 150
cyl 3- 110
cyl 4- 140

It looks bad but I think that I can explain the numbers (tell me if you think I am wrong). Cyl 1,2,4 were checked when the engine was warm. Cyl 1,2 were very hot and the passenger side head was warm to hot. I think this would explain why cyl 4 was slightly lower. Unfortunately, my neighbors stopped by to talk prior to me checking cyl 3. By the time we were done talking the engine was completely cool which I think would explain the 110.

I am not very worried about the compression numbers (or should I be?). I think that I am dumping fuel into cyl 3,4 for some reason. I think this is a carb problem. Any ideas?

[VaccaRabite]

HA! Should have told the neighbor to step off for just a moment while you did #3. I'd do a warm compression test on #3 just to be sure, but I agree that it is likely just fine.

Did you pull the idle jets out and clean them, or just spray carb cleaner into the carb?

When were these carbs last taken apart and rebuilt?

If 3 and four are not working right now, that is improvement. Could be bad balance and the idle stops are closed too far.
Zach

[tornik550]

HA! Should have told the neighbor to step off for just a moment while you did #3. I'd do a warm compression test on #3 just to be sure, but I agree that it is likely just fine.

Did you pull the idle jets out and clean them, or just spray carb cleaner into the carb?

When were these carbs last taken apart and rebuilt?

If 3 and four are not working right now, that is improvement. Could be bad balance and the idle stops are closed too far.
Zach

I did clean the idle jets with air and carb cleaner. They look brand new. The carbs are freshly rebuilt. All jets, seats etc are brand new.

One interesting thing that I did not mention was that last week then engine was running well. I was still in the process of tuning the carbs. They were running lean. I had 55 idles, 150 main and 220 airs (I only had 220 and 190). I changed the airs to 190 and the the carbs were still running a little lean (but better). I decided to bore out an old set of mains but I didn't have a 1.55 bit so I used a 1.60.

Is it possible that the mixture is so rich and maybe the balance is off slightly that the engine doesn't fire for the most part? Tonight, I will try to go back to the old settings to see what happens.

[SGB]

My simple brain wonders if the intake manifold on that side is tight (at the carb or maybe even the head)...


Also, what about the air bypass screws on the carbs?

1.60 mains is BIG. I tried 1.55 but went back to 1.50. I think I have 210 correctors.

Are you sure the dizzy is in right?

[tornik550]

I was frustrated so I decided to leave town for the weekend. I cam back and made several adjustments. Now the engine runs on all four cylinders however it does not run well. I am a bit confused but now that I am running on 4 cyl, I think I can work things out.

Since the plugs looked super rich and it had seemed that cyl 3 and 4 were not firing because they were getting flooded, I reduced the size of the main and turned down the idle mixture screw on all cyl. The setup that I tried (mainly because these jet sizes were available to me) main 140, air 190, idle 55. I had the idle mixture screws turned out 1 full turn. The engine ran however cyl 3 did not seem to fire constantly and I was shooting out a huge amount of blue smoke (I had shot about 1 oz of mystery marvel oil in the cyl as an experiment a few days ago- the engine hasn't been run since), once the blue smoke cleared, the smoked turned dark black. I was also getting occasional backfiring. I then changed the air correction jets to 220 to see what would happen. The engine ran better, still a little rich. Getting less backfiring but still occasional.

I should note that all cylinders suck air equally. I took cyl 3 plug out and it was black and covered in gas (didn't feel or smell like oil).
I really don't understand why I am getting this problem now. I have tried these settings previously and didn't have these problems. I am fairly certain that I am not having an exhaust leak.

What else could be causing my backfiring? Could I be having a valve lash issue in addition to a jetting issue?