Zenith 40TIN Carb Tuning questions

[69BahamaYellow]

Does anybody have experience tuning these things. I have a 1971 2.2 911T with these carbs. The engine and carbs have been rebuilt and runs strong and gets good fuel economy. Only problem is that I have a backfire @ idle that I can't get rid of. The backfire comes back through the carbs and through the exhaust on all cylinders. It acts like it runs rich @ idle (with mixture jets @ 2.5 turns open), but I have no CO meeter to verify. Closing mixture screws a bit helps some, but I turned a few needles completely closed, and those cylinders still backfired. Opening them up doesn't do anything either. Acceleration, and any engine speed above idle is excellent (but still a little backfire under no load conditions). No matter what I do to the auxiliary enrichment devices, it still doesn't change. With all the backfiring, it's hard to hold a steady 900RPM idle. Unless I occasionally pump the throttle, the engine will eventually die, and sometimes after doing this, it takes a while for RPM to come back around 900 (throttle linkage works fine). Cam timing (original T Cams), ignition timing, valve clearance, etc. is all perfect. The carbs are absolutely spotless and it doesn't appear that they have ever been modified. I thought I could fix anything, but this one has me stumped. Anybody got any suggestions?

[BK911]

See if this helps:

http://www.gary.suggate.dsl.pipex.com/new_site/zenith.htm

I found it while searching for "Zeniths" a while back. Do a search, there was also some other good info out there.

Have fun!
BK

[Allenk]

Float heights and fuel pressure should also be checked since these can make proper adjustment difficult if off.

[RoninLB]

member A Quiet Boom has posted mucho info on Zeniths

[C4 Pazzo]

The auxiliary enrichment only kicks in when you are in overrun (rpm >1350 and throttle at idle stops). The link that BK911 has a good description of the aux enrichment circuit and how to set it. An apparently common problem is that the rpm transducer stops working (do a search on rpm transducer -- lots of info on how to diagnose and fix -- much of it relates to MFI cars, but the device is the same for our Ts). None of this helps with your question about backfires when idling -- an issue I'm currently trying to address myself on my recently rebuilt Zeniths.

[69BahamaYellow]

All excellent ideas; I'll have to test some of them tomorrow. I didn't initially suspect the fuel pump or float levels, as the car runs so good above idle. Full throttle gives plenty of power, with no flat spots. I suppose the problem could very well be too much fuel pressure or too high fuel level in the bowls. This kind of makes sense, as the backfiring and slightly black smoky exhaust makes me think it's too rich at idle. The schematics, tech data and pictures in the Zenith link are great (Thanks for providing this Gary?) I checked out the links by A Quiet Boom, and there does indeed appear to be a lot of good stuff here. A couple more questions for those "in the know"

1) What should the fuel pressure be for Zenith carbs? My fuel pump is not factory, so perhaps it is too much.
2) My idle jets are absolutely clean, but perhaps a previous owner drilled them out too big and I didn't notice. What size hole should be in these things? One of the threads suggested .65mm, was I reading that right?
3) This 2.2 T engine gets 31mpg on long trips @ a steady 65-70MPH. It gets about 12MPG when I drive it really hard. Has anybody else had similar experience with the highway driving? From what I can tell, these engines average about 15MPG; am I just lucky to be getting such good highway mileage?

[MaquinaVerde]

A Quiet Boom's guide is great, between that and the factory manual I was able to get my zeniths to run really well.

You have checked your ignition timing, but what is the condition of the distributor? I experienced symptoms very similar to yours with my Marelli distributor and worn (original?) cap/rotor. I swapped in a used Bosch unit with new cap/rotor/points and my problems cleared right up. It really was a night and day difference. Make sure your dwell is in spec too, setting it correctly helped the backfiring a lot. Before this I was almost ready to throw in the towel and take the car in for tuning.

My '69-71 spec book suggests an idle jet size of 47.5, but my car isn't available right now to check. I don't have my overrun circuit connected and don't really have any problems. I occasionally get a slight burble getting off the freeway in fifth, but nothing bothersome.

For mileage I get about 16-18 in town without really flogging it, and high 20s for long drives on the open road.

[911s55]

Does your distributor have a vacuum "retard"? Mine does and idles much smoother and consistant with it blocked and vac line plugged, although I'm running Webers.
You need to get as much, and balanced vacuum draw from each carb especially while running on idle circuit, which is most of the time.

[Kobus Brits]

I had the same problem and after replacing the Marelli distributor my car is running as smooth as anything without any backfires. Initially we also thought the carburettors were the problem.

It is a problem to get a replacement distributor and a distributor remanufacturing company made one up for me with electronic ignition. The details were posted on this board about a year ago.

[rw7810]

69bahamayellow - send me your email address and i'll send you the tune-up / adjustment procedure.

rw7810@att.com

[69BahamaYellow]

Just an update on my carb tuning progress (since it seems like there are a lot of us with backfiring @ idle with these carbs:

I haven't been able to check the float level or fuel pressure yet, but I installed an RPM Transducer on my car and started adjusting the auxiliary enrichment circuit, and to my surprise, much of the backfiring has disappeared (not completely, but definitely a lot less than it was). Turns out my RPM transducer was present, but not wired correctly (probably because the stock ignition was replaced with a Perma-Tune. After sorting out the wiring on the RPM transducer. It appears to work as it's suppose to (enriching mixture on overrun above 1350 RPM. Problem is, whenever I snap open the throttle, the RPM's fall normally back to idle, but seem to get stuck @ about 2000RPM for about 20 seconds, then it will drop back down to 900 and idle just fine. Seems like when you seem to be fixing 1 problem, another one crops up. Ill keep fiddling with them and let everyone know what works and what doesn't.

Thanks,
69BahamaYellow

[C4 Pazzo]

I think the Zenith manual posted by Gary Sugate describes the procedure to address the aux enrichment hanging at about 1800 rpm. I think turning in the aux enrichment flow adjusting screw???

[RoninLB]

if you have air corrector jets I'd confirm passages are clean. Even a quick squirt of carb cleaner usually helps.

[69BahamaYellow]

Looks like bad ignition wires may have something to do with my backfiring problem, but I'll have to get some new ones before I'll know for sure. I initially did not suspect them, because they were a new set a few years ago, but I pulled the spark plugs, and 2 of them looked completely clean, while 4 were heavily sooted with carbon. This led me to check resistance values of individual plug wires. While all the wires are stranded coper core with 0 resistance, 3 of the spark plug end connectors read exactly 2.9K Ohms between the plug end and the wire end, where it screws into the plug wire, 1 reads 14.7K Ohms, and 2 appear to be open circuit. This made me wonder how it could even be running at all, but I guess high voltage ignition systems can overcome a lot. I assume the 2.9K Ohm connectors are good, but at least one of them was connected to a cylinder that is backfiring badly. Anybody had any experience with the plug connectors?

Once I get the ignition wires sorted out, I resume the carb tuning.

69BahamaYellow

[69BahamaYellow]

Turned out to be the plug wire connectors. Tech support @ Perma-tune suggested resistorless wires with their ignition system, so I took my spark plug connectors apart to find the small resistors and brass pieces heavily corroded (odd for a 2 year old set of wires). I replaced the resistors with a single piece of 3/16" brass rod and put them all back together with high temp epoxy, effectively making them resistorless. I also opened up my spark plug gaps to .065", also @ Perma-tune's advisement. When I fired it up, the backfires were gone. Now I just have to go through the usual syncronizing process, and the whole thing should idle perfectly. Currently it idles a little lumpy, but at least there's no backfires. Since I don't have a CO meeter to get the idle mixture just right, has anyone ever used the Colortune devise that Eastwood sells? This thing supposedly has some sort of clear ceramic sparkplug, instead of the usual white, with a viewing tube that lets you actually see the combustion process. Blue flame means perfect mixture, yellow means rich, and white means lean. At $65 it sounds interesting. Anybody got experience here or other methods? I've seen a few threads on this, but nothing from someone who actually used one.

Thanks,
69BahamaYellow

[rw7810]

for that price, just get the flow meter tool.

[69BahamaYellow]

I've got the syncrometer (flow meter) to balance the airflow between carbs, but I don't have a way to adjust the mixture between cylinders. That's why the color-tune devise sounds interesting. The manual suggests using a CO meeter in the tailpipe and adjusting all cylinders by the same amount to get the desired result. This assumes the mixture settings for all cylinders will be the same (maybe they are???), but a CO meeter costs $200 while the color-tune devise is $65.

69BahamaYellow

[deversi]

Hi Bob,
I'm getting my 70T going after a long sit. Lots of popping and backfire.
If you could sent the carb set up procedure I'd appreciate it too.
Thanks
Adam
adam.a@comcast.net

Originally Posted by rw7810

69bahamayellow - send me your email address and i'll send you the tune-up / adjustment procedure.

rw7810@att.com

[1QuickS]

I've used Colortunes since 1979. Tedious but very nice for individual mixture setting plus you can diagnose sparking vs. mixture issues. CO meter averages readings, Colortune lets you see individual cylinder operation. I provide user's info specific for 911s on my web site (scroll down until you find it):

Performance Oriented

[Mike-777]

Originally Posted by BK911

See if this helps:

http://www.gary.suggate.dsl.pipex.com/new_site/zenith.htm

I found it while searching for "Zeniths" a while back. Do a search, there was also some other good info out there.

Have fun!
BK

Zenith 40 TIN Carburettor Manual

The updated link to Mr. Suggate's Zenith carb manual.