3 Cylinders not firing

[Patjamas]

Early this year I bought a 72 240z to be a project car. It was pretty far from top shape, which wasn't a problem for me because I hoped to use it to learn alot about gasoline engines. For a short period I was using it as a daily driver, in the fall. It never ran amazing, but it seemed to have good days and bad days, which I chalked up to weather conditions and the carb setup.

 

One day I was heading home from work and it just died when I was rolling up to a stop sign. Previously that day it HAD been running noticably rough. So I get it towed back to the house and I start checking it out some.

 

I can smell an unburnt gasoline smell in the exxhaust, so I am still assuming the carb's are working fine. I manage to get the car to stay running by holding the linkage that controls the fuel so that it is above whatever its set at for idle.

 

While holding this, I check what I think is the next easiest problem to diagnose, spark. One-by-one I pull off a spark plug wire, listen for a change in how the engine is running, put the wire back on. Assuming from the nose of the car, the first 3 cylinders seem to be firing. when I pull off the last 3 wires I hear no change in the engine.

 

So this leads me to believe compression or the valves to be the culprit. And I'm hoping an easy way can be suggested to determine if the valves are bad or not? I'm gonna find a compression gauge when I can and try that out as well, but in the mean time was wondering if theres some old mechanics trick to test the valves?

 

Also, when I was fidling with the car I noticed the air cleaner (brand new BTW) was more dark on the side with the cylinders that seem to be not firing. And the car seemed to backfire, but more so when the engine was on the colder side when I was driving, but sometimes it would just randomly backfire at operating temp when going down the highway at steady RPM or in town as well.

 

Thanks in advance.

[nismopick]

Have you sync'd the carbs? Start with the easy... no need to jump right to compression probs.

[g9m3c]

I had the same problem with my 260, and it turned out to be bent valves. The carbs being out of sync isn't going to make three cylinders just not fire altogether. Is there a spark being generated to those three cylinders? As in, do you see a blue spark between the wire and the plug when you pull them off? (probably better to do this in the dark) Also, check your fuel supply to that carb and make sure the suction piston is rising as you give it throttle. Check the oil in your dampeners as well. If all that checks out, then it's probably time to pull the valve cover off and run a compression check.

[nismopick]

One day I was heading home from work and it just died when I was rolling up to a stop sign.

 

The carbs being out of sync isn't going to make three cylinders just not fire altogether.

 

Okay... rephrase for simplicity's sake: Maybe the back carb is WAY out of tune... thus DUMPING TOO MUCH FUEL INTO THE CYLINDERS. Unless there was a huge Clunk! noise, and now the car barely cranks, I doubt the valves are bent (also quite hard to do with a chain driven cam).

 

But what do I know...

[g9m3c]

I agree that it'd be hard for it to be valve related, but it was in my case, and my engine was running EXACTLY like he just described, except it was the front 3 cylinders not firing. I tore it down, and intake valves #1,2, and 3 were not sealing, and had left nice little impact dimples on the pistons.

[blue72]

I've had problems in the past with not having 3 cylinders firing, but mine have been self inflicted because of improper float adjustment. If there's no fuel going into the float then there isn't any fuel going into one carburetor. If there's no fuel in one carburetor there is no fuel for three cylinders, and no firing on those cylinders. That might not be Patjamas predicament, but it's something easy enough to check.

[Zmanco]

Before you go too far tearing into the engine, there's a simpler thing to check. If the carb that feeds the 3 "dead" cylinders ran too rich for a while, it may have fouled the spark plugs and with the stock ignition, it may not be getting enough spark to ignite the fuel.

 

Pull the plugs and see if they're covered in soot. If so, get some new plugs and verify you can start the engine. Then tune the carbs (discussed many times on this site) before you foul the plugs again.

 

If this solves the problem, you might want to check that the plug wires are ok, and replace the points as well (or better yet, upgrade to electronic ignition). A weak spark will make this more likely to occur again.

[rags]

I'm going to guess a stuck float and NO gas getting through the back carb. If the plugs are dry, give the back bowl a love tap with the handle of a screwdriver. Sometimes it will unstick a stuck float.

 

Joe

[kakapoopie]

I'm having a similar problem on my 71 roundtop carbed Z. Instead, the first 3 cylinders were not firing. The plugs were definitely fouled and wet with fuel. Also there is a tiny pool of fuel sitting at the front cab air inlet.

 

I replaced the plugs and now they all are firing except for #1, which released white/blue smoke when I took out the sparkplug after running for a few minutes

[g9m3c]

I'm having a similar problem on my 71 roundtop carbed Z. Instead, the first 3 cylinders were not firing. The plugs were definitely fouled and wet with fuel. Also there is a tiny pool of fuel sitting at the front cab air inlet.

 

I replaced the plugs and now they all are firing except for #1, which released white/blue smoke when I took out the sparkplug after running for a few minutes

 

 

 

 

Make sure the bottom of your mixture needle assembly (black plastic thing with a small line going to the float bowls) on the bottom of your carbs is pushed all the way up. Sometimes they stick after you release the choke.

[kolonelklink87]

Pull the plugs and see if they're covered in soot. If so, get some new plugs and verify you can start the engine. Then tune the carbs (discussed many times on this site) before you foul the plugs again....

 

I agree; pull the plugs; pull the plugs; pull the plugs......

 

SU's are notorious without correct maintainence & tuning... I've have similar problems many times when i used to run flattops. Eventually i isolated my problems to a combination of points ignition and misalligned needles (the pointless dizzy make troubleshoot half as difficult because it's reliable:flamedevi)

 

if they haven't been touched in a while a rebuild could even be in order but... get more info on the problem before you throw time and money into it.

 

if you aren't sure about reading the plugs take a photo of all six and post... people on here are junkies for figures, photos and data - so you'll get the best response that way

[kakapoopie]

Make sure the bottom of your mixture needle assembly (black plastic thing with a small line going to the float bowls) on the bottom of your carbs is pushed all the way up. Sometimes they stick after you release the choke.

 

Thanks for the tip, sure enough it was super gummy stuck on the front carb. I worked it free with a bit of carb cleaner. Still running rough though. I'm waiting on a flow meter to come in so I can attempt to sync the carbs.

[cockerstar]

Also, check your firing order. When I first got my car running I had the wires for 5 and 6 swapped and it caused the back three cylinders to not fire. I don't know why it effected number four too, but there was no change when I pulled the wires at the back three cylinders to check spark.

Maybe 5 and 6 not firing resulted in very low vacuum on the back three cylinders, which added to the issue of my untuned carbs, causing the back three to be very lean?

 

FWIW, I've found that when something is screwy and you're not quite sure what's wrong, verifying the firing order is so easy and quick that it's always worth doing.