popping through the exhaust at 4500 rpm or so.. HELP!

[madkaw]

I checked my carb balance and verified it was good at both idle and 3000 rpm.

Next I set the A/F mixture knob to two turns form full lean.

Third, I started it up and found the highest rpm at 1.5 turns out form full lean, so I leaned it out a half turn.

Sounds like your at 1 turn only-that's probably not enough-I would richen her up.

Steve-maybe I shouldn't have bashed the SM needles so much, it probably is not the issue here. I just know that the stock needles are more than ample for the L24.

Your car definitely has the pcv system. Just make sure the pcv valve that is attached to your balance tube moves freely and none of your lines are obstructed

Edited December 21, 2011 by madkaw

[LeonV]

WHat PVC system? I do not think my car has one?

All great advice Leon.

I am running SM needles and I adjusted my carbs per the SU video, finding the highest idle possible. 1.5 turns form full lean is not out of the ball park is it?

I get a nice pretty blue flame from my gimmicky colortune with that setting. Still, I can always check for vacuum leaks, easy enough, and look at my valves ( which I did actually adjust about 2000 miles ago! haha)

All my original tubes are hooked up to the air cleaner as the factory had them.

I will take her out for a drive and try to fatten her up a bit. I have exactly 2 turns clearly marked on my car so that will be a easy spot to reference.

More news in about an hour.

All cars sold in the USA since about 1968 had to have a PCV system. Refer to page EC-2 of the 240Z FSM.

You've figured out the problem with the "colortune", but here's the thing with round top SUs: you can't adjust the idle mixture without changing the AFR in the rest of its dynamic range. By leaning out the mixture to look good at idle, you have leaned it out off-idle as well. We'll see if making the mixture richer will help, and if turning the knob doesn't work I would take the carb pistons out and inspect needle installation.

A wideband O2 sensor is the only good method for finding out what is really going on in your engine.

[Zedyone_kenobi]

There is nothing at all on my balance tube. It was all taken off when I got my total Z therapy treatment.

Another piece of the puzzle. I went out for a drive today after work and fiddled a bit more. Set the carbs at 2.5 turns out which is where they were at, NOT 1.5 turns out, I misspoke earlier. As I wanted to be sure I checked them and set them to the 1.5 turns out from all the way up and the car would not idle. I knew that was wrong as my car always had an awesome idle. So I put them back to 2.5 turns and BAMMO, rock solid idle again.

Also, the popping only occurs at part throttle at around 4500 rpm. At full throttle it pulls hard to 6500 rpm no issues. Interesting problem!

[LeonV]

There is nothing at all on my balance tube. It was all taken off when I got my total Z therapy treatment.

Another piece of the puzzle. I went out for a drive today after work and fiddled a bit more. Set the carbs at 2.5 turns out which is where they were at, NOT 1.5 turns out, I misspoke earlier. As I wanted to be sure I checked them and set them to the 1.5 turns out from all the way up and the car would not idle. I knew that was wrong as my car always had an awesome idle. So I put them back to 2.5 turns and BAMMO, rock solid idle again.

Also, the popping only occurs at part throttle at around 4500 rpm. At full throttle it pulls hard to 6500 rpm no issues. Interesting problem!

Ok, so the problem from the original post has been partially fixed, correct?

However, once you get it up to the 4500 rpm range I start getting pops from the exhaust. I get these whether I ease slowly up to 4500 rpm or I nail it, something always happens at 4500 rpm and up.

I really doubt that ZT would plug the PCV hole (why?), are you 100% sure that there is no hose coming from the block breather that attaches to the bottom of your balance tube as the FSM depicts? If there isn't then check to make sure that you don't have a huge, gaping hole or a PCV valve sitting all by itself in the bottom of your balance tube.

If everything checks out, I would move on to valve lash. Check it hot.

If that checks out, then make your way to the carbs and make sure the needles are installed properly. If they are, then put in a good set of stock needles and see if anything changes.

[LeonV]

I just took a look at ZTherapy's site.

Here is what they say about their balance tube:

Smooth and clean 240Z Intake/Balance Tube, polished and plated--no smog, no control valves, no vac taps. Water heating still works, if desired. Crankcase breather and Valve cover breather fittings intact. Includes firewall linkage, carb mounting nuts, washers, lock washers. Cleaned up insulator with new gaskets. Your choice of intakes (240Z water, 240Z no water, 260Z, with a 240Z balance tube. Extraneous holes are plugged. Basically all the junk on the balance tube is deleted (Smog equipment available at additional cost)

Here is their photo (PCV valve front and center):

You must have the crankcase breather hose attached to the PCV valve. Otherwise, you have a giant vacuum leak in your intake making you run very lean.

[Zedyone_kenobi]

Ah, I never really looked that hard into that. Yup, That goes to my crank case ventilation, check, PCV valve is new when I got the carbs.

I removed it and it rattles when I shake it...

I will check to see if there are some diagnostic checks I can do on it.

Edited December 21, 2011 by Zedyone_kenobi

[Zedyone_kenobi]

I can do a valve adjustment Friday. I am taking off that day.

Can anybody with a stock 71 tell me what kind of max advance they are running with the stock cam at say 3000 rpm?

I did not touch the timing today. I am still at about 15 degrees static. Max around 32. As I was yesterday.

By the way folks, thanks VERY much for the help. I usually try to be the one helping others, but now I really need the wisdom of the Z gurus.

[madkaw]

I wouldn't worry about what others are running, just what works well with your engine. All these engines are different and hard to compare. 40 years and many owners and too many swaps and changes makes it hard to compare for BEST performance. Biggest thing to worrry about is detonation.

I'm running 38 total, but that would apply to your engine more likely

[steve91tt]

Here's another twist for you...I have a balancer that has TDC marked about 8 degrees off of actual TDC. I suspect that the rubber in between the two parts of the balancer has given way and allowed the outer ring to slip slightly relative to the inner. In other words, have you checked that zero degrees of timing is actually at TDC? Mine was not.

[Zedyone_kenobi]

nope, I have not done that...But the engine does not act like it is that far out of time. When I did my valve job 2 years ago I noted in my journal that the timing mark appeared to closely coincide with the TDC. But I can always double check.

A wide band will have to come into play eventually. So man variables to consider. I have to be able to concretely nail down some of them with data.

[Bonzi Lon]

PCV can be checked by blowing through each end, one way is open the other way it closes.

Bonzi Lon

[Zedyone_kenobi]

copy that, I checked mine out. Mine flows in the direction it is suppose to flow. I cleaned it up and reinstalled it.

Today I may be grounded. It is pouring rain out side all day and I may not get a chance to go for as spin. Sort of irresponsible to do wide open pulls on a wet road!

So here is where we are at.

Carbs are at 2.5 turns from full lean.

All my choke cables do return to the right spot.

Needles rise and fall with no binding what so ever

I have good fuel pressure

Timing is at ~15 BTDC

Vacuum Advance is installed but not hooked up

PCV valve is clean and operational

NO vacuum leaks of any kind

These are my symptoms:

Car starts idles and drives perfectly up to about 4000-4500 rpm

Under part throttle of any kind that takes me above 4000-4500 rpm I get popping and misfiring through the exhaust

Under wide open throttle in second gear anyway I can get the car to pull all the way to redline without hesitation

With the vacuum advance hooked up The part throttle symptoms are still there and may be worse

What I have tried:

I balanced the carbs 2 different times, they are spot on

I checked PCV valve

I installed new plugs

I installed a new fuel pump

I totally rebuilt my entire fuel system

I had my distributor rebuilt and recurved to european specifications

New Z therapy carbs with SM needles

I have run with the vacuum advance and without it

I have tried timing from 8 degrees to 15 degrees

I have run a jumper from the battery to the coil bypassing the tach to see if the old tach is messing up my power to the coil

What I have not tried yet:

I have not tried to advance the timing more

I have not swapped out my distributor or pertronix to see if that is at fault

I have not done a valve adjustment

I have not run more than 2.5 turns out on my carbs

I have not looked at my float levels since I bought the carbs

I have not bought a wideband to get real answers once and for all (after christmas I promise!)

What I am planning to do!

When it dries up, I am going to make a hard full throttle run on a long stretch of road, and then stop and pull the plugs immediately before the evidence is lost

Analyze the results and make changes

I may go more rich/lean by a quarter turn

I will place an order today for stock SU needles from Z therapy and a set of float bowl gaskets.

I will buy a wideband O2. NGK makes a nice one for less than 300 bucks.

If I get desperate and I am convinced my fuel A/F mixture is close, I may play with timing more.

Am I missing anything guys? Can you give me any more advice? I certainly appreciate all the ideas. This is truly a tricky problem.