Zenith Carb issues, off idol

[Porscheman912]

I have a 69 911 T with a 70 2.2 T engine in it. The car idols great, accels good, but starts popping when you try and hold at any certain speed. The big thing I have noticed, is the stock ignition is gone, and there is a MSD coil on there with a balast resistor. The timing is right, and the carbs have been set, and reset but I cannot get the popping to stop off idol

[pcar9119]

Could be a plugged or incorrect idle jet. The Idle jets are used all the way to like 3000 0r 4000 rpm. I don't remember exactly right now, but if your still under that rpm, the car will surge and pop...

[Green 912]

Excel pump and advancement circuit should be checked /cleaned as well.

[Trackrash]

Are you running a fuel filter? Very important that you have a good clean filter.

[Porscheman912]

pumps good, ilter is good.
The car runs great if you are building speed, it does not run good when you are trying to hold it off Idol. I even replaced and cleaned the main jets from another set of Zenith carbs I have, and it did not change either. The engine runs great at idol, or as long as you have your foot into it, but if you try and hold it somewhere, pow, it starts popping. I am wondering if it might be an ignition problem, since the stock CDI is gone, and it is using that MSD coil.

[Porscheman912]

pcar9119, I will check that out. I have an extra set of carbs, so it lets me clean, check and swop parts. I have never messed with these carbs.

[James Brown]

Also try to increase the main jets anout a 1/4 turn on all 6 and check that. The idle circut is dependant on the main circut (if that makes any sence to you) So, to increase idle fuel, main fuel must be increased and then idle jets reduced. Kind of give and take but they both work togeather. I wouldn't worry about the ignition if timing is correct. sounds like classic Zenith tuning.

[Porscheman912]

I just put another set of Idol jets in, I will wait until morning to start, but I did notice that the ones that can out were smaller. I ran my reamer through, and even though they have the same number, the hole size is different, and one set, had 3 large holes, instead of 4 like my other 2 sets have. I think I might have forgot to add that when I replaced the plugs, the car was running lean. The Idol jets maybe the problem, we'll see in the morning.

[Porscheman912]

Pcar9119, you were right, it turned out to be the idle jets. I have 3 sets sitting here, even though they have the same number, my mentor always told me to check the size. The ones that were in the car were very small, I could only get #2 reamer through. The set I put in, and another set, both took the #3 reamer. Sorry, using torch tip reamers. Anyways, started the car this morning, and it is running very smooth. The idle dropped off, and now I will have to reset everything, but at least it tell me that it has gotten richer. No more lean popping, and it runs good through the range now. I am good at setting others carbs, but for some reason, everytime I do mine, I will get mad, and over look things. Thank you everyone, for your time and help.

[pcar9119]

Glad we could help out, enjoy the drive!

[Porscheman912]

I am amzed at how good the throttle responce is, compared to my other one with webers. Thanks again

[James Brown]

Mine really scream! love them Zenith!!!

[pcar9119]

I love thr Z's. They are incredible. I'm working on seeing if porting out the Z's is possible to use on larger engines. I think they would be a great alternative to $3000.00 PMO. We'll see...

[Porscheman912]

They have impressed me. For as small as the chokes are, I am very supprised at how fast they will let this 2.2 rev. I swear, this 2.2 T engine feels better, and quicker than my 2.4 with webers, and my 2.7 CIS engines do, and it is suppose to be, what a whole 125Bhp. I have a machine shop, and might start making so other parts for these things. I have a few sets of them, and figured I would keep them as a novelty, but now that I have used them, I am happy with them.
Does anyone know if there is any documents on these carbs, books, or anything?

[Porsche-O-Phile]

I am setting up a set now for my 2.7 but due to how slowly this project is going I probably won't be able to give a report for a while. There are a couple of others who have done this conversion successfully; I'm using their recommendations for jet sizes and so-on. If you dig around in this forum you can probably find my inquiries about this (and the responses) which might be of help to you. Good luck!

[Porscheman912]

That is good to know, thank you. I was making custom chokes for the solex PII, for race cars, but the person I was making them for passed away, so I stopped. Making them is easy, once you know what you are doing. These carbs might be something I will start playing with, and making parts for.

[pcar9119]

I CNC'd machined my own vents, opened up my own jets, had the stock intakes port matched, then tuned the carbs myself. Really wasn't that much work really. I still have the program for the vents. They were 34mm for my 3.0. They worked great. I have sold a few on here. James Brown actually has a set I believe. I'm have to make a few more in a couple of weeks for a few people yet. But, that was the funnest part of doing these carbs, figuring it out on my own, and then having great rewards from it...

[Porscheman912]

That is very cool. That is the coolest thing about these cars, is what you can do to fix them, and take original parts to be used somewhere else and have them work great, Thanks Porsche for good engineering.

[pcar9119]

Don't forget the amazing sound that comes from them at wide open throttle. That impressive roart from the throats, it's intoxicating!