Attempted to tune SU Carburetors but still backfiring on intake

[jalexquijano]

My 72´ 240z is still not getting the correct tune up! I have tried almost everything but its still backfiring at the intake on both carburetors. I have watch the Ztherapy video like 10 times and followed their instructions but cannot get the job done. The Carbs were recently overhauled by them on last November.

 

1. Warmed up the car for 5 minutes

 

2. Took both of the Domes out and rinsed them throughly with Carb Cleaner

 

3. Cleaned the 6 spark plugs (NGK BP6ES) with brass brush and contact cleaner

 

4. Put the domes back with their springs and filled the carbs with ATF fluid.

 

5. Piston falls and rises freely.

 

6. Set the RPM with the throttle screws on each carb to 1,000 RPM

 

7. Balanced both carbs: Fast idle screw to 2,000 RPM and did the balance with the right balance screw and synchronizer. Back off Fast Idle Screw

 

8. Turned both mixture knobs on the front and rear carb 4 turns clockwise.

 

 

 

Result:

 

Car is still sputtering and backfiring on the intake. I step on the gas, the car is not running smoothly and both carbs exploding on their intake.

 

Could the electric fuel pressure pump not sending enough pressure:

 

I have an Airtex 8016S which pushes 2.5 to 4.5 psi. Should i switch to a more powerful pump?

 

WHat could be the cause of this intake backfire? Cant believe i cannot manage to tune the carbs properly

Edited October 26, 2015 by jalexquijano

[siteunseen]

For the tenth time, turn the adjustment knobs all the way clockwise.  That will bring the nozzles all the way up.  THEN!!! turn them COUNTER CLOCKWISE 2.5 turns, lowering them which adds fuel.

 

You're welcome for the choke light help, Napoleon. 

[jalexquijano]

Did you meant the following:  open them up all the way clockwise looking from the top of the car and then anticlockwise 2.5 turns.

 

CLOCKWISE: Lowers the nozzle, allowing more fuel.

 

ANTICLOCKWISE: Raises the nozzle, restricts the fuel

 

Please clarify!!!

Edited October 26, 2015 by jalexquijano

[cbuczesk]

Since you know how to raise and lower it...raise it all the way to close off fuel. Then lower it 2.5 turns and you should be set.

 

Chuck

[jalexquijano]

I tried this at 2.5 turns and still backfires at the intake of both carbs. Even at 4.5 turns it backfires at the intake!

[Stanley]

Maybe it's better not to think of it as looking from the top of the engine, since that would be backwards. An ordinary nut or bolt (tightens) when it's turned clockwise; the mix nuts are the same.

 

Something wrong with the tune-up sequence. It's an iterative process. Don't get them balanced and then change the mix nut settings and think you're done. The Ztherapy video example shows how to do it, but for me it was easier to follow the tune-up procedure in the FSM.

The Porsche mechanic had it running perfect, right?

 

If it was running good but you want to practice the tune-up, you could do this:

1. turn idle screw on each carb all the way in and write down the number turns (to nearest 1/8 turn) for each. Then put them back where they were.

2. turn mix nuts all the way up and write down number of turns for each to nearest 1/8 turn. Put them back exactly where they were.

If your tune-up makes it worse just put everything back where it was.

 

Once they're balanced I tend to make small adjustments based on performance, plug color, etc. without re-balancing. But it's easy to get it messed up, and forget the current setting if I don't write them down.

Edited October 26, 2015 by Stanley

[cbuczesk]

Set it at 2.5 turns down. Have you checked timing? Have you checked for vacuum leaks? Are you running points or electronic ignition?

 

Chuck

[siteunseen]

He's turning something other than the mixture knobs. I bet he's trying to raise the rpms. FWIW

[Mark Maras]

 As i have always understood it, backfire thru the intake indicates the mixture is too lean, ASSUMING everything else is fine, valves, timing, etc.

 IF the mixture screw IS being adjusted & the backfire is still happening there is only two possibilities. Not enough fuel or too much air. Not enough fuel? I would check fuel pump volume & pressure. Float level, if you've changed it from ZT settings. In fact, review anything you have changed on the carbs since they came back from ZT & return to their settings.

 Too much air? Vacuum leaks, intake manifold leak, loose carbs, brake booster, anything that can pull air into the intake.

 Mixture adjustment mix-up. If you are looking down at the top of the carbs, the mixture screws turn anti-clockwise for less fuel. They are being screwed into the carb, leaning the fuel mixture but the manuals want you to adjust the m. screws as if you are looking at the bottom of the carbs. Now, clockwise is the direction for less fuel. 

[jalexquijano]

I guess the problem of not being able to tune them is that the car has an Airtex 8016s electric fuel pump (2.5 to 4 psi), not enough fuel pressure and even though i turn the mixture knob clockwise to 4 turns, its still lean.

 

Should i go back and replace it with Airtex 8012S fuel pump rated 5 to 9 psi??

 

 http://www.summitracing.com/int/parts/atx-e8012s/overview/

 

Is there a better option or will this suffice?

[Patcon]

The pressure on the 8016 is sufficient. The real question is flow. Nobody specifies how much that pump flows. You need to do a flow test and calculate how much fuel it flows. It may or may not be a fuel issue. Mark is right there are other possibilities.

[jalexquijano]

I am taking it to the Porsche Mechanic in Panama. Maybe ill make a video of him tuning the carbs. He told me the 8012S pump was not working correctly and the 8016S needs more.

 

The Airtex 8012S has the following specs:

 

Free Flow Rate: 30 gph

Maximum Pressure (psi): 9 psi

 

Any other suggestion on how to tune them correctly?