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

[Captain Obvious]

Fine. Then I'm not stumped.

I just don't have any suggestions at this time.

If you were anywhere near me, I'd loan you another pair carbs to try. Sure, it would be a shotgun approach, but it would help narrow the field so much that it would be worth it.

[Bonzi Lon]

I thought maybe worn throttle shafts that at part throttle that allowed a leak (unmetered air) into the system at half throttle, but not at WOT. But the bearings Z therapy puts in should have eliminated that as a possibility. I need to get on the ball with the O2 sensor and stop messing around chasing my tail.

A quick but messy trick. Get some very thick high temp grease and run a good size bead around each of the throttle shafts where air COULD enter. The grease should close off any air or suck it in.

Bonzi Lon

[Zedyone_kenobi]

A glimmer of HOPE>>>

I put my carbs back together (SM needles back in) with the only difference being that I put in some 10w-30, and she ran VERY well this morning. I was able to push her at part throttle easily to 4500 rpm, without the slightest hint of a pop. When I pushed it past that, I got some minor pops, but 10w30 did make an appreciable difference.

I am taking her in to the mufler shop today to get the O2 sensor bung welded on and I will finally know where I am running. I am also going to do a full test on my brake booster check valve tonight. I will report back later...

Having good O2 readings will be very informative and hopefully helpful.

[doradox]

Just remember that an O2 sensor senses O2 in the exhaust and not the F/A mixture entering the engine. Incomplete combustion can give readings that are not reliable from a mixture standpoint.

Steve

[Zedyone_kenobi]

Understand completely. Like any data you have to know its limitations.

[Zedyone_kenobi]

O2 sensor bung installed.

now to wire up my Innovative sensor and we can start to get some readings!

[motorman7]

The suspense is killing me :paranoid:

[het976]

Not to hi-jack the post, but..

What you are dealing with is very much the same thing I am experiencing, except, I only notice it in 2nd gear. As I bring it up to 4500 it runs well, but when I tip in a little more throttle, it just runs flat and burbles and pops and makes no power...lift off the throttle a little and it clears up and runs fine...nail it and it pulls fine to the redline, although the tach does some pretty wild bouncing as I reach 7K.

It seems like SU issues, except when it is running flat, the tach bounces from 4K to 5K then 3.5K to 5.5k and so on...if I hold it there eventually it will bounce from 0 to 7K... This makes me think it is electrical or some strange combo. I have a ZX dist on a 2.8L with a mild cam and header, running ZT rebuilt carbs with SM needles. The tach is the stock 240Z and I am thinking this may be part of my issue. The engine has about 1,000 miles on it and has run more or less like this from the start.

It is drivable, but just not clean.

I have a lot of troubleshooting to do, based on what I read here before I ask for help, but I thought I would post to add in my experience.

I am going to switch to an old set of stock SUs and see if it clears up the problem, then change to a 280 tach, then drop in an LS3, in that order…

[Zedyone_kenobi]

If you think it is your tach, run a jumper from your 12v source to your + coil terminal and unplug your tach. This takes the tach out of the loop. Did not work for me but good luck

[beermanpete]

Not to hi-jack the post, but..

What you are dealing with is very much the same thing I am experiencing, except, I only notice it in 2nd gear. As I bring it up to 4500 it runs well, but when I tip in a little more throttle, it just runs flat and burbles and pops and makes no power...lift off the throttle a little and it clears up and runs fine...nail it and it pulls fine to the redline, although the tach does some pretty wild bouncing as I reach 7K.

It seems like SU issues, except when it is running flat, the tach bounces from 4K to 5K then 3.5K to 5.5k and so on...if I hold it there eventually it will bounce from 0 to 7K... This makes me think it is electrical or some strange combo. I have a ZX dist on a 2.8L with a mild cam and header, running ZT rebuilt carbs with SM needles. The tach is the stock 240Z and I am thinking this may be part of my issue. The engine has about 1,000 miles on it and has run more or less like this from the start.

It is drivable, but just not clean.

I have a lot of troubleshooting to do, based on what I read here before I ask for help, but I thought I would post to add in my experience.

I am going to switch to an old set of stock SUs and see if it clears up the problem, then change to a 280 tach, then drop in an LS3, in that order…

In the early Z the tach is inductively driven (4-wire tach) so it is not likely a tach failure will have any affect on engine performance. It is more likely that your tach is responding to an ignition problem such as timing bounce/drift or a bad component (points, condenser, etc.)

[het976]

Thanks all...I have brand new points dist that I will plug in and see what happens...Still leaning towards an LS3.

[Zedyone_kenobi]

UPDATE:

My innovative wideband O2 sensor came to life today after work. I have it hidden in my glovebox. And it seems to come to life and read very quickly. I have not put my center console back together yet, but I will this weekend, and then I can take it for a drive.

Updates to follow.