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

[motorman7]

It’s been great to follow this thread as I am going through similar issues here as well. My bright yellow '71 Z-Car was running great till I swapped my leaky fuel tank for a rusty one (not a good a idea). After numerous fuel issues, adjustments, replacements, etc and putting the repaired once leaky fuel tank back in, I now have a good tank, but am experiencing the exact same issue that Zedyone is having. At first I thought it was electrical but it is definitely fuel related (for the most part). I tightened the fuel mixture screws in ½ a turn and immediately fixed most of the problem. Went from bogging at 4K on short runs to revving all the way up to 6.5K with no issue in first and second (doing close to 60 at point). I am running the SM needles in round tops with a ZX dizzy. The mixture screws are at 7/8 turn from open….nowhere near the ‘standard’ two turns. Not sure why this is, but the more I open them up the worse they run.

So, this AM, I had a chance to open it up a bit. Car ran very well in short bursts revving easily to 6.5 k. Pushed it up to 100 MPH which is 4K RPM for me and noticed the power loss and bogging at this speed (Although, I have not ruled out accelerator foot hesitation as the fear of CHP comes into play during this test) . Same as Zedyone, I will need to experiment with the fuel mixture and vacuum advance. I currently have the vacuum attached but will try this again with vacuum off. Will update when further data is available.

[Zedyone_kenobi]

motorman, glad to know there is somebody else chasing this issue. Were you popping out of your exhaust?

Can you tell us your stats?

What initial static timing are you running? Max advance?

Are you seven eights turn form full lean? Or are you 7 to 8 turns from full lean? You lost me at your mixture screw setting?

Also is your motor stock?

The more data points the better.

OH, I forgot to mention, I only run 93 octane Chevron in my Z.

[motorman7]

I am pretty much stock. I have a 260 motor with MSA 6-1 headers, round tops with the SM needles, and the ZX dizzy, no smog stuff.

I will have to get back to you on the timing. I was going to check last night, but was happy with how it was running so l didn’t check. Will try and get that tonight.

The 7/8 is less than 1 but more than ¾. It’s a fraction.

I am running 91 octane Mobil. 91 is as high as it gets at the So Cal stations.

[Zedyone_kenobi]

wow, less than one turn!

My car would not even run like that... amazing how different two mostly stock cars are..

[motorman7]

wow, less than one turn!

My car would not even run like that... amazing how different two mostly stock cars are..

I think it means that my float bowl levels are off (level too high). Really trashed my whole fuel system putting on the bad tank. Learned lesson the hard way.

Edited December 22, 2011 by motorman7

[240260280z]

All my choke cables do return to the right spot.

Needles rise and fall with no binding what so ever

Is it possible that your jets are at different depths?

Try removing choke cable connections and manually verify that each jet rises and drops without binding when you activate the choke attachment on the carb by hand. This mechanism should move freely.

You can also pull the domes and measure the jet depth with a caliper's depth gauge attachment. Set both by tweaking the knurled knob to the same depth by measurement at your specified number of turns down (i.e. Set one to X number of turns, measure the depth then set the other jet to the same depth using the caliper. (I assume your needles are set correctly so that the needle's shoulder is flush with the bottom of the piston...not flush with the base of the channel it sits in).

Edited December 22, 2011 by Blue

[madkaw]

That's what I was saying Stephen, these cars can be way different in running characteristics even with the same engine or year. Motormans engine is using a ZX dizzy which has a different curve than yours Stephen. Enough that the two motors would run different. I hope you can compare and find something, but if something is working for you keep going that direction.

Stephen, did you check your dizzy cap for cracks?

wow, less than one turn!

My car would not even run like that... amazing how different two mostly stock cars are..

[Zedyone_kenobi]

yes, brand new cap and rotor. Well less than 4 month old and 400 miles.

I advanced my timing to 17 degrees static now which should yield me about 36 degrees total advance. Which I think is plenty.

I did order new float bowl gaskets today and some OEM factory needles. We will see how they act if I need them.

Today I experimented with my mixture. It still pops profusely at part throttle,

Here is an interesting tidbit.

As long as the rpm is increasing it seems to barely notice anything, but when I slowly creep up on about 4000 rpm, it spits and sputters if I hold it there.

How about some SU help here.

During acceleration the pistons rise, but when I reach the tail off of the acceleration, I would think the bowls would drop. Could my bowls be dropping way to fast, causing lean spot?

OH I run standard issue SU oil that MSA sells. No idea what weight it might be. It came in a clear bottle called 'SU oil'

Edited December 22, 2011 by Zedyone_kenobi

[Zedyone_kenobi]

tomorrow morning first thing I am going to do a compression test...

I did one 3 years ago, but we have to check it off the list.

[madkaw]

Stephen,

just out of curiousity, what is the model # on your dizzy. Should be engraved somewhere on the side.

[Zedyone_kenobi]

I will check for you, but remember I had it recurved, cleaned and had new bushings/bearings installed. , so it is nothing like stock any more.

Still I wish I had an extra dizzy just to see what a stock one would run like. I need another data point.

Just for kicks I installed a brand new in the box set of NGK plug wires and a new cap. Still does the same thing. These plug wires have a 5 kOhm reading, were as my taylor wires had a 0.740 kOhm reading. Car ran exactly the same.

Last thing I did was advance my timing to 17 degrees static and I will see how that runs tomorrow. I have yet to hear any pinging so I will do that. If that fails I will order a new valve cover gasket and take a look at my cam shaft.

It is possible that I may have a worn lobe, but I honestly think that is highly unlikely.

[madkaw]

Usually plug wires give you problems under heavy loading- like climbing a hill in forth gear.

As far as cam- more likely out of adjustment than anything .

Well you get this figured out you will definitely know your car better

The reason I asked what dizzy was to see how much vacuum advance your dizzy adds. At this point during you troubleshooting with the timing I would leave it disconnected.