Zenith setup idle problems

[924CarreraGTP]

I've been working on my '70 911 T trying to synchronize and tune my Zenith 40 TIN's. I'm not doing well with getting the car to idle.
I warmed the car up, and detached the accelerator linkage. I turned the mixture screws to 2.5 turns out. The air bypass screws were already all the way in. I cranked the car up and inceased the throttle stops to 1,500rpm and synchronized 1 and 4 using the synchrometer. Then I turned the idle mixture screws in to lean out the mixture, and then backed them out a slight turn at a time until the pops and fire went away. All 6 throats measured 10 on the meter so I didn't mess with the air bypass screws at all. So, I try to bring the throttle stops back down to 1000rpms and the car won't idle below 1500rpms without dieing. So I pulled the idle jets out and cleaned them. I redid the whole tune process again. This time the car will idle under 500rpms roughly for about 5 seconds before it dies. The car is in time and would idle fine before I started messing with the carbs. What am I doing wrong? The car actually runs better than before up into the high revs, but when I let off the throttle it kinda lingers on 2000rpms for a second, and then drops to 1,500, and finally down past 500 where it stalls.

[snbush67]

Try this:
Zenith 40 TIN Carburettor Manual

[snbush67]

Looking at the manual, you should be adjusting the idles at 1000rpm not 1500 as you posted above.

Also you missed a step, using the air screws synch one and four ( the left bank to the right bank).

now you have to synch 1-2-and 3 to each other so they are all pulling the same amount of air.

Then 4-5-and 6 to each other.

Now balance left to right again but using the main idle screws so that each side is pulling equal and RPMs are at 950-1000rpms. You should be pulling about 5 bars on the synch meter.

Now use the mix screws to the best lean positions.


Your symptoms sound like you may have float level or fuel pressure issues.

[924CarreraGTP]

I read all of the link above before starting the setup. I did all of that, but the car will not idle at 1000rpms. It won't really idle at all now unless the throttle stops are moved up to 1,500 running 10 bar on the synchro. When I attach the synchro on 1 or 4 and try to drop the idle the synchro jumps all over the place and I can't get a reading. Plus the car drops down and stalls. It starts back up fine, but it won't idle now. I almost wish I would have left it alone. It idled fine two days ago. How far out should I be adjusting the idle mixture screws? I'm maybe going out as far as 3.5 turns or more to get the popping to stop. Is that normal? All of the throats are running 10 bars at 1,500 rpms. I think I've got them balanced, I just can't get it to idle below 1,500 rpms without stalling. I can only drop the throttle stops to about 7 bars before the engine stalls. Trying to keep it running with the linkage detached is a real PITA.

[924CarreraGTP]

Snbush67, you said I use the air screws to sync 1 and 4 (right bank to left bank). The air bypass screws? I thought you did that using the throttle stops. How exactly do you set idle speed on these carbs? Maybe I'm missing that?

[VFR750]

Probably want to start over. What are your jet sizes?

When you manually open the throttle plate to reach 1500 rpm, you are uncovering the first transition port. You leaned out the idle jet with the first port flowing.

When you tried to close the throttle to reach 1000, you shut off the port and had too little flow from the idle fuel circuit.

[dfhtrhjn]

As idle mixture screws go, you do one cylinder at a time. Don't worry about idle speed until the end.

Screw it in a mixture screw until the cylinder turns off.
Unscrew it until the idle is at the highest rpm, You'll have to give throttle for about 5-10 seconds and let it settle each time you turn the screw in/out some.
Do this for each cylinder, then when you're done adjust the idle speed.

Then repeat the process again for good measure, you never get it right on the first pass.

Your ideal settings will be when each cylinder is firing at it's strongest hence the highest idle. If you're adjusting the mixture screws in or out and the rpm drops, it's because the cylinder is too lean or too rich.

Additionally, if you're driving and put it in neutral and brake really hard and your idle dips, or the car stalls, i.e. the idle doesn't always settle nicely back to ~1000 rpm then your idle circuit jet size may incorrect (there could be other things to check like the float bowl).

If you replace the idle circuit jets you'll also need to readjust the mixture. If your ideal mixture setting is less that 1/2 turn out, then the idle circuit jet is probably too large. If the ideal setting is greater than 3 turns out, then the idle circuit jet is probably too small.

If you get through all this and the car still stalls, you may want to double check your fuel pressure.

I know it's not very scientific advice, but I hope it helps.