So I have a early 71 240z that I bought not to long ago, and it runs but like crap. I have never been able to get it right. First I had an issue with the SUs and the choke being out of adjustment so I fixed that and started to tune the carbs and noticed a huge vacuum leak out of the air tube, one of the ports had a large hose hooked to it that was sucking in tons of air, so I plugged it. As soon as I plugged the port, the motor started running smoother but then started backfiring constantly out of  the tail pipe and out of the carbs back and forth, puffs of fuel would come out of the carbs every so often. It was almost like the motor is choking and builds up pressure and then pop! something opens up in the motor and the rpms die down for a few seconds and then back up with back firing. Also when you rev up the car there is a slight spudder through out the RPM range. its not a smooth rev.

 

Things I have done to fix this issue that has not worked:

 

Bought and watched the Just Su's Video.

Removed, Cleaned and rebuilt the SUs, Adjusted the Jet needles like the videos covers, Adjusted floats levels in both carbs, replaced needle valves (same issue), and gaskets.

Tuned carbs like video covers

Replaced the PCV valve

Checked for leaks around the throttle shafts while engine is running at low rpm, checked for leaks all over engine with carb cleaner, rpms never raised even checked brake booster hoses.

While engine running Moved Distributer completely advanced and then completely retarded and got back firing in all positions

Did a compression test with warm engine  165,162,170,165,165,168  PSI

New Spark plugs, Plug wires, cap and rotor..

 

I feel like I'm running out of options, could this be an electrical issue? I remember I couldn't start the car once unless I held the key in the start position, as soon as I let the key go it would die, I think I made some better connections on the ballast resistor and the problem went away, I wonder if the resistor is bad or ignition switch, not sure if it could cause backfiring.   What about a anti backfiring valve do you 240 guys use those?  HELP!

8 minutes ago, 5 Star Rising said:

So I have a early 71 240z

huge vacuum leak out of the air tube, one of the ports had a large hose hooked to it that was sucking in tons of air, so I plugged it.

While engine running Moved Distributer completely advanced and then completely retarded and got back firing in all positions

Did a compression test with warm engine  165,162,170,165,165,168  PSI

I think I made some better connections on the ballast resistor and the problem went away, I wonder if the resistor is bad or ignition switch, not sure if it could cause backfiring.  

What about a anti backfiring valve do you 240 guys use those?  HELP!

Try to get more methodical.  I don't see valve lash adjustment on your list.  That's the first item in the later years Engine Tune Up chapters.  Borrow a timing light and actually set the timing to where it should be so that you don't fix the problem but still have the symptoms because the timing is wrong.  Use a meter and measure voltage at the ballast.  Measure resistance of the resistor.  Find out what the other end of that hose was connected to.  Better to know than guess.  Seriously.  

I have a timing light, actually 3 ..I can't set the timing properly cause I can't get the car to run stable, the rpms drop due to the back fire constantly. Jumped into a car that I don't know the history of. I have yet to do a valve lash adjustment though but I will.. Will this cause the backfire symptoms if they are out of adjustment?

The unplugged hose was hanging in the wind, have a feeling it went to the air cleaner some where, which is missing from the car.

 

Do you know off hand what the voltage should be at the balast? And resistance tolerances for the resistor?

What do the new plugs look like now, first three compared to the last three?    A new rotor button and cap are cheap and will get that out of the way.

The only large hose I can think of on the balance tube is the brake booster hose towards the rear and then the PCV fitting and hose in the middle.

Good luck, hopefully it's something small. 

 

Since you don't know the history, and it doesn't run well enough to tune, your best path is to verify that the various rotating parts are timed correctly.  Hatepotholez is going through the same situation and just found that his damper pulley might have slipped, for example.  

I would start with the ignition timing mark on the damper pulley at zero and verify that the cam shaft is in the proper orientation, the rotor in the distributor is pointing at the correct electrode, and the plug wires are properly placed. 1-5-3-6-2-4, counterclockwise, like site's picture.  Then rotate the engine by hand and check valve lash, correcting any that are off.  The older engines are known for worn valve seats due to unleaded fuel.  That could cause some backfires, or popping back through the intake (some people call that front-fire, I think).  They're also known for throwing lash pads, which would cause the valve not to open fully.

Anyway, you've spent enough time on it to justify starting from zero.  Set all of those things once, and you won't have to wonder about them.

Reminds me of a story of my first Z . Traveling across country with a fresh motor and it started acting up. Tore apart my SU's in a parking lot in 20 degree weather and FINALLY gave up and had it towed.
Turned out it was a loose connection on the dizzy
Felt like a fool. The garage owner was so impressed with my car , probably not my troubleshooting skills though-
Good advice to stay methodical . Could be ignition/ electrical .

Since you've never had it right- time to pull the dizzy and make sure your ignition to valve timing is correct. All that dizzy adjusting isn't going to help if it's a tooth off.



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I have a timing light, actually 3 ..I can't set the timing properly cause I can't get the car to run stable, the rpms drop due to the back fire constantly. Jumped into a car that I don't know the history of. I have yet to do a valve lash adjustment though but I will.. Will this cause the backfire symptoms if they are out of adjustment?

The unplugged hose was hanging in the wind, have a feeling it went to the air cleaner some where, which is missing from the car.

 

Do you know off hand what the voltage should be at the balast? And resistance tolerances for the resistor?

Yeah really want to figure it out. I forgot to mention there is headers and a straight pipe out welded to a wide open muffler this thing is loud as hell, but thinking about it, there is no back pressure. I wonder if they had that hose unhooked up front on purpose to offset the backfiring, cause as soon as I plugged it the backfiring started..  Wow can't believe I been a member here since 2006. should probably cruise through this forum more often. Had 9 Zs since then haha.  Thanks for the input fellas.

+1 for starting at the begining. Check all the mechanical settings. If you still have breaker points in the ignition system you can set the point gap and timing without running the engine. The ballast resistor should be 1.6 ohms. The voltage at the ballast should be 12 volts on the battery side and about 9 volts on the coils side at idle.

21 hours ago, 5 Star Rising said:

Yeah really want to figure it out. I forgot to mention there is headers and a straight pipe out welded to a wide open muffler this thing is loud as hell, but thinking about it, there is no back pressure. I wonder if they had that hose unhooked up front on purpose to offset the backfiring, cause as soon as I plugged it the backfiring started..  Wow can't believe I been a member here since 2006. should probably cruise through this forum more often. Had 9 Zs since then haha.  Thanks for the input fellas.

A picture tells a thousand words. Can you post a picture of what hose was disconnected?

 

I have a timing light, actually 3 ..I can't set the timing properly cause I can't get the car to run stable, the rpms drop due to the back fire constantly. Jumped into a car that I don't know the history of. I have yet to do a valve lash adjustment though but I will.. Will this cause the backfire symptoms if they are out of adjustment?

The unplugged hose was hanging in the wind, have a feeling it went to the air cleaner some where, which is missing from the car.

 

Do you know off hand what the voltage should be at the balast? And resistance tolerances for the resistor?

Come to think of it I had a few vacuum leaks with the small hoses detached on the top picture. I forgot that I took a picture of this a while ago. I had unplugged the pcv valve hose on purpose on the top pic but the other hoses were just chilling.

 

The bottom pics shows everything plugged and that's when the backfiring started. Well the car still ran like crap with those leaks but didn't back fire haha