I have been fighting this for over a year, and now I must surrender to the Z Gods on this board. I am at a loss of what to do next.

What I am running?

1971 240Z with 71000 miles.

Stock engine with MSA header and 2.5" exhaust

Z therapy rebuilt carbs (about 1000 miles ago installed), ran great when bolted up

Advance distributors Rebuild OEM dizzy recurved to Eurpoean specs (static timing at idle 13 Deg BTDC, at 3000 rpm I am 32 deg BTDC) I do run a vacuum advance on top of that.

New NGK resistor style plugs, but car had problem with new NGK non resistor plugs

New Pertonix installed in dizzy, and new flamethrower coil.

I have rebalanced the carbs and I am currently almost at 3 turns from the top on the mixture nuts

All the fuel lines and filters are new

I installed a new fuel pump and I am getting 3.6 to 4 psi at all rpm ranges

Running Taylor pro 8mm wires.

Car starts immediately even after sitting for a whle and has a rock solid idle at 750 rpm. Engine barely moves. Valves are quiet and in good adjustment as it was dodone 2000 miles ago.

Car has super quick throttle response and is a complete joy to drive normally with ZERO issues ever. 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. Seems I have had this issue since I owned the car in some form of severity or another. When I first bought the car, it would simply NOT rev past 4500 rpm at all. Now I can get it to rev to 6500 rpm thanks to the Z therapy carbs, but now it is popping pretty bad on the top end.

My most recent change was to put the timing to where it is right now (13 BTDC) and it definitely improved, as this is the setting the distributer guru told me to run. Go figure. Now the car pops in first, and a little in second, but will pull to 6500 rpm rather smoothly in second. I do not have a road safe enough to get it to 5000 in third near my house.

I keep thinking this is fuel related and not timing related. I am not sure if my M/R is right or not. Popping through the exhaust means to me it is too rich.

Before I adjust the carbs again, I would love to get more opinions on this.

Edited November 21, 201113 yr by Zedyone_kenobi

Well I tried moving the M/R knobs a half turn to the lean side, and the same thing happens. Wondering if I should keep going more lean or not..

By "popping" do you mean just a noise or a noise accompanied by the engine running rough / misfiring?

If the former, it could be excess fuel igniting in the exhaust indicating you are running too rich.

If the latter, could be running too lean.

You mentioned rebalancing the carbs. Forgive me if this is obvious to you, but are you aware that setting up the carbs is a three-step process? Conceptually three steps, in practice, sometimes more.

First: Balance airflow at idle.

Second: Adjust fuel mixture at idle.

Third: Balance airflow at 2000 rpm (or 2500 or 3000 rpm -- depending on your source).

After which, if you have been eating your vegetables and being kind to small animals, when you return to idle the airflow will still be balanced.

The only other thing I can come up with at the moment is that one of your new or rebuild pieces is defective. No clue as to which one, though.

Chris

Have you verified the timing with a light at 3000 or are you just stating what the specs are set . The pertronix was installed after the rebuilt dizzy was installed? Any problems before installing the pertronix ?

One last thing. Reach under there and feel the fuel hoses between the float bowls and carbs. Pay special attention for any kinking of the hose.

Check the hot water plumbing to the carbs as well, mine does this when it is not up to temp.

Great inputs guys. Um in no particular order

Yes, at 3000 rpm with an digital timing light verified timing at 32 +/- 1 Deg BTDC

OH one think I forgot to mention is that Z therapy put SM needles in my carbs...I think they tend to run richer than the needles standard in the US models.

Roger wilco on the carb balance procedures. Those are the exact steps I took. All is still right with the world after I came down from the high rpm trim setting.

THe car stumbles at 4500 rpm. You can feel it hesitate as it misses.

One thing I have not done in a while is a compression check. But a year ago they were all within 5% of each other.

I guess the next logical thing is to keep leaning it out until the symptoms exasperate the problem. Just do not want to play too much with detonation as bad things can happen.

I do not have any hot water plumbing 5th horseman. Removed when I went with a polished manifold from Z therapy.

Granted what I gained in a clean engine bay, I lost in time to warm up the cars. But being as how I live in Houston, being too warm is not normally an issue!

Ambient temp makes no difference in my car, if the temp. gauge isn't in the warm range, I get exhaust popping and can feel the car larch a bit no matter where the choke knob is. I don't know what a PITA it would be to add hot water to you're manifolds, maybe contact ZT and see what they recommend

Make no mistake, my car does this popping when the car is fully warmed up as well. Drives me batty. As it did not use to do this...

Have you tried playing with the choke when the car is in that rpm range? Might be interesting to see (on the fly) if the problem gets any better or worse.

Also, it was stated above that the fuel lines (tubes) between float bowl and jet can kink. When I had my roadster carbs rebuilt, I noticed that the new tubes kinked a bit-perhaps due to a thinner wall? I put the old ones on and have not had any problems.

I will certainly look at that... thank so much.

I cannot help but think all of this could be solved with a wide band O2 sensor.