-71 911T carb popping

[Forssling]

Hi All,
I'm at the final leg of my total restoration of a -71 911T. I'm currently breaking in the engine and have had a first run of 20 mins at 2000 rpm.

Today I started it for the second time to adjust the carbs. I did this according to the Zenith 40 Tin original instructions.

I have blocked the auxiliary enrichment system.

I run these jets:

Idle jet, (57)
main jet, (115)
air correction jet, (195)

The floats are adjusted correctly.

The distributor is 123-ignition set according to instructions.

So when I accelerate I have some occasional popping from the carb throats. It is hard to determine which one but I'll say its more than one.

I've read that this is a lean condition mostly...

1) Should I open the idle mixture control screw some more?
How much, a quarter of a turn? Working my way around the throats starting with the one I suspect popping?

2) Should I adjust the timing? Which way, clockwise or anti-clockwise? (It states 35 BTDC at 6000 rpms is a bit hard to achieve, I don't want to rev my fresh rebuild that hard.)

3) Anything else?

Just to get a running car so I can get on with the breaking in of the engine. I can do the fine tuning when I get passed this point.

Thanks for any input!

ps. Here's a couple of films from the first startup.
https://youtu.be/sCshFWC0o0o
https://youtu.be/Sjuh7BBcyxk

Jörgen Forssling
Gothenburg
SWEDEN

[jpnovak]

Yes, popping is a sign the engine is lean. This could be from clogged jets, wrong size jets, wrong adjustments on the mixture screws or linkage that opens one side more than the other.

What RPM is it popping?
Does it only pop when you accelerate?
has your linkage been synchronized? not just at idle but also at 1500 and 3k rpm?

[Hi_Fi_Guy]

I’ve been through this with a 2.2 T. Here are my thoughts.

Heat can be an issue with these engines. Especially the cylinder and carb barrels closest to the oil cooler. Have you checked the oil cooler to ensure it is clean and free from obstructions? Mine had nearly 50 years of dirt and grime built up on it. A good cleaning will help lower temps. Also, many states now mandate E10 fuels (10 % ethanol). This percolates at lower temps than non ethanol fuel and can cause a carb fire. The fuel delivery systems were not designed for ethanol and it can wreck havoc. Stabil ethanol treatment can help.

The Zenith carbs were a compromise for emissions over performance. Many people like them and they are easy to work on and upgrade but if any worn areas are leaking air it can be a challenge to stop them popping. Especially on decel or hot idle. Likewise some have mentioned air leaks at the intake manifolds so this is something to check for as well. Always smart to verify a rotary fuel pump is in place and delivering correct fuel volume and pressure.

Many will say only the black coil with a CDI ignition which is most likely correct. That said in my experience as long as the ignition timing is correct (you should verify with a timing light) you will most likely find the carbs/intakes the source of the popping.

When in doubt there is a proper process of elimination. Otherwise you may be chasing ghosts and making matters worse.
- Compression: always ensure compression is present. This includes correct valve adjustment.
- ignition: next be sure the spark plugs and gaps are correct, wires, ignition timing, etc.
- carbs: only after you verify the first two should you mess with the carbs. Too many people blame carbs for issues caused by poor valve adjustments or incorrect ignition timing.

Working through the process of elimination will help you diagnose which components are contributing to your popping issues and how to correct them. Just remember many parts are 50 years old and even rebuilding these old parts may not eliminate the issue. You might have to resort to some modern parts and solutions.

Let us know what you find and if you solve the popping issue.

[Forssling]

Thanks for your input! For starters I had the wrong curve selected on my 123-distributor. That helped a lot to set that right. Was going to check the timing but found that my strobe light was broken. Got a new one now but have not had the time to check yet. It is a bit intimidating checking the timing as it is set 35° at 6000 rpms. I feel that is a bit harsh on a rebuilt engine not properly broken in yet.

I'll let you know how it works out.

ps. I have rebuilt the carbs. So I hope that I have no clogged jets. The heads has been rebuilt with new valve guides etc so that area should be sorted as well.

KR/Jörgen

[Forssling]

Yesterday I started the engine again. It is really hard to start when its been sitting for a while. Checked the timing and it was set right.

It pops occasionally on acceleration. When turning the idle mixture control screw out it seems like it doesn't get better. The initial setting was 2.5 turns as per the original instructions.

1) How small increments should I go? I've been at 1/4 turns at the time.

Now we have the modern fuel, 5% etanol here in Sweden.

2) Should I look into the jetting?
Idle jet, (57)
main jet, (115)
air correction jet, (195)

It feels that I've lost track on where I'm at. I figure I would go with the original setting of 2.5 turns on the idle mixture control screw and the air bypass control screw fully closed and start over again getting them synchronized.
3) Anything else I should look at?

Thanks for helping me out,
Jörgen

[targa72e]

HI Jorgen,
Did this adjustment with Webers 6 months ago and just finished with Zeniths. Key thought is that you will be adjusting over and over again as you get things dialed in. Assuming all the other basic settings are correct, Basic process I followed that seemed to work is below.

Close all air bypass screws, set idle screw at 2.5 turns. Start engine and set idle stops so idle is about 1200RPM. Use Sync meter to measure airflow and find two highest air flow cylinders on each side.
Adjust both sides so highest air flow read the same. Adjust idle stop to about 1100RPM Idle
Adjust other cylinders with air bypass to match airflow of highest cylinders. RPM will go up. Turn Idle stop back down until idle back at 1100. Start over with adjusting air flow on each cylinder. Once you get idle down to 1000RPM and all cylinders flowing the same time to check they are opening the same (they probably will not be.
Open throttle to 2000 RPM check air flow on left and right cylinders that have air bypass totally closed, it will probably be off. Adjust linkage so airflow matches at 2000RPM. Start over checking at 1000RPM idle. Repeat for 3000RPM.
If all is good start checking idle mixture adjustment. Turn idle screw in 1/4 turn and see if RPM's drop. When drop detected turn back open 1/4 turn. Move to next cylinder and repeat for all. Readjust idle stop to get back to 1000RPM idle. Check that air flow for all cylinders still is the same. re-adjust if necessary. Re-check at 2000 adjust as necessary. repeat idle screw adjustment. When everything is adjusted correctly the airflow will be the same on all cylinders, the idle air bypass will be opened the least amount possible and turning any idle mixture screw in will cause idle to drop. I had to loosen the shaft couplers for one cylinder so that throttle could close more when on idle stop as Idle air bypass screws had to be out to far to match on other cylinders. After that adjustment, you guessed it repeat all steps above.
Once all the above is set you can adjust accel pumps to all squirt the same amount of fuel. Now you can see how the car drives. If any of these things are out of sync the car may not drive correctly. I spent several hours getting things adjusted.

I then used a wide band O2 sensor to log mixtures while driving. This will tell you if you need to adjust jets. I needed larger mains. Then repeat above.

Weber specific but basic concepts apply in tuning section of link below will apply to Zeniths there is a ton of detail.
Performance Oriented

john

[Forssling]

Thanks John, I'll dig into it again. You're alright with the jetting as a start?

//Jörgen

[Forssling]

Hi All!
I want to check the accelerator pumps working properly. Just to make sure they deliver fuel as expected, then I can calibrate them.

To check my understanding, with engine off and the bowls full with fuel I should get fuel from the acc pump discharge nozzles in each throat?

Or do I need to have the fuel pump running to have proper fuel pressure to have an accurate reading? With the engine not running that is.

Let me know if you have any pointers on this.

KR,
Jörgen

[jpnovak]

The Accel pumps are driven by the gas in the float bowl. Just need to turn key on and make sure float bowls are primed.

Then you can use the small burett on a stick to collect the fuel from the Accel pump jet.

If you are having popping while steady driving the accel pumps are not your problem.

Have you verified your sync from L to R and then across each bank? Has this been done at idle and also at 2k rpm?

Were the linkages replaced? I Have found that the modern replacement linkage will have a different length from L to R arms. This means the carbs will open at different rates and is especially bad off-idle and into the cruise RPMs where throttle openings are generally small. It takes some careful measurements to make sure the distance from the center of the cross-shaft arm to the ball stud is the same on both sides.

[Forssling]

Originally Posted by jpnovak

The Accel pumps are driven by the gas in the float bowl. Just need to turn key on and make sure float bowls are primed.

Then you can use the small burett on a stick to collect the fuel from the Accel pump jet.

If you are having popping while steady driving the accel pumps are not your problem.

Have you verified your sync from L to R and then across each bank? Has this been done at idle and also at 2k rpm?

Were the linkages replaced? I Have found that the modern replacement linkage will have a different length from L to R arms. This means the carbs will open at different rates and is especially bad off-idle and into the cruise RPMs where throttle openings are generally small. It takes some careful measurements to make sure the distance from the center of the cross-shaft arm to the ball stud is the same on both sides.

The popping is upon acceleration. I want to check the acc pumps just to eliminate them.

//Jörgen

[Forssling]

Hi All!
Now I've checked the accel pumps and can confirm that three out of six is not delivering.

Anyone have any troubleshooting tips?

I figure taking out the nozzles to see if they are plugged or not (I'm positive I did this rebuilding the carbs but you never know if something happened afterwards). Also check the ball working properly in the nozzles.

Is there an easy way to see if there is fuel in each accel pump chamber and if the inlet valve is working properly.

Any other "typical" areas to look at regarding the accel pumps?

Appreciate your help
Jörgen

[buster73]

I also had some issues with accelerator nozzles. In my case there were some leaks at the nozzle base and one leaky inlet nozzle valve (in the float bowl). Good luck finding a solution for you engine.

Weber-Zenith swap for 2.4T

[ed mayo]

Take each nozzle out and use aerosol carb cleaner with plastic spray tube and spray into each nozzle in same direction as gas flow, then blow out with compressed air, repeat until carb cleaner spray comes out of the nozzle. This may take several tries. Also, there are outlet check valves in each tube, they look like a brass screw without threads cut. They keep gas in the tube so there is a ready supply of gas at first pump. They are often lost during rebuild if you don't know they are there.

[Forssling]

Originally Posted by ed mayo

Take each nozzle out and use aerosol carb cleaner with plastic spray tube and spray into each nozzle in same direction as gas flow, then blow out with compressed air, repeat until carb cleaner spray comes out of the nozzle. This may take several tries. Also, there are outlet check valves in each tube, they look like a brass screw without threads cut. They keep gas in the tube so there is a ready supply of gas at first pump. They are often lost during rebuild if you don't know they are there.

Thanks! I will do this. I know I have the screw looking check valves.

KR
Jörgen

[Forssling]

Hi All!
A small update. I removed the faulty nozzles and made sure I had fuel delivery.

I cleaned out the acc pump nozzles. I used a very thin metal thread from a cable to insert in the small nozzle hole. Then I used some brake cleaner to blow through the nozzle with my compressor. When delivering a fine jet of brake cleaner coming out I put them back and checked the function in place.

Now I have all acc pump nozzles working.

I reset the carbs to base line spec. Now the engine is out of the car but I'll get back to report when I have it running again.

KR
Jörgen

[WMontald]

I had this problem months ago 911 T 1970 2.2
Could not get rid of it, whatever I tried.

Even Porsche dealer was not able to get it, right.

Then I realized that I installed a new Fuel pump as it was leaking when staying in winter storage.
This pump was giving too much pressure I assume Probably bought and installed one for a 911 injection.
I installed a Fuel pressure device on the internet and .....problem solved, no popping, smooth running engine.

See post
Show me your carb fuel pressure test setup