Black smoke at full throttle

zorin · 09-14-2011, 12:39 PM

I have a '79 911sc with 3.2 engine and Zenith Carburetors . When I floor the gas pedal to accelerate, I see black smoke (runs very rich) which goes away eventually once the speed increases enough. Any idea what needs adjusting?

Thanks

Winter · 09-14-2011, 01:05 PM

You have determined that the oil isn't overfilled, right? I'm assuming you know how to properly check this. car fully warmed up, level ground, running.

Let's start with the easy stuff and go from there.

zorin · 09-14-2011, 01:38 PM

Yes. That I am sure of. Thanks

BKC · 09-14-2011, 01:50 PM

My 3.0 does the same thing and from all that I've read its pretty much, sorta, kind of par for the course. My Zeniths are, again from what I've read, properly set up.

Side affects are cooler running engine and unburned fuel getting into the oil, which means changing it every 2500 miles. Also, fouled plugs are an issue but I've found that occasional revs to redline alleviate that issue.

Oh yeah, don't even talk to me about mpg. It's not good.

zorin · 09-14-2011, 02:08 PM

Thanks. Brian. I am guessing we may have to adjust the accelerator pumps. I don't know if I should mess with those though.

BKC · 09-14-2011, 02:14 PM

I'm a newb but my car was given a PPI by a highly respected wrench and he wasn't concerned other than suggesting the accelerated oil change interval.

daniel911T · daniel911T's Garage

Only thing I would suggest to check is the float levels.

If it runs good, makes smooth power, and isn't fouling plugs, then you're probably all right, and the smoke is just a quirk.

IF, big IF, you're getting some plug fouling, or the power seems to kind of slack off when you're getting the smoke, AND your floats are perfect (try setting with the engine running, frequently makes a pretty big difference), then you can try stepping up the air correction jets.

I suggest contacting Paul Abbot of Performance Oriented, he has some great tuning procedures that will help you find the problem, if there is one.

Good luck! You're living my dream, Zeniths on a 3.2!

-Dan

zorin · 09-14-2011, 04:50 PM

Thanks Dan.
The engine runs great. I guess I would not worry about it then.
BTW, I have a set of 964 camshafts. Is it worth the trouble of replacing the 3.2 camshafts with 964's? Will I notice huge difference in engine performance?

john walker's workshop · 09-15-2011, 04:21 AM

what's the venturi size? too small and lots of fuel gets sucked up.

daniel911T · daniel911T's Garage

Quote:

Originally Posted by john walker's workshop

what's the venturi size? too small and lots of fuel gets sucked up.

He says he's running 36mm. That should be about right for his application. The black smoke is pretty clearly rich, but he's running pretty well all around, so his main jets are very likely right. I think the only thing he might be able to change that would help is the air jets. For a 2.2 they run at 185, and I've seen 2.7s running with 115s to enrichen the main circuit. He may need something 5 or 10 larger than he's already running.

Zorin,
If you want or need some air jets to play around with, I've got enough to hook you up. With a bit of solder and a good drill bit set you can work wonders. Once you work out the right size while drilling and soldering, you can just order a set that size and be lean dumb and happy!

One more time for emphasis: set/check your float levels with the car running, and rev it a bit to see if and how much the level changes. It really does make a significant difference. If you didn't already, do it now, you may simply have high fuel levels in the bowls.

One last request: could you please post your current jet setup?

-Dan

zorin · 09-17-2011, 07:56 AM

Hi Dan,
I checked the air correction jet and it has 795 stamped on it.
BTW, I have a set of 964 camshafts. Is it worth the trouble of replacing the 3.2 camshafts with 964's? Will I notice huge difference in engine performance? Does the engine have to come off to replace the camshaft?

daniel911T · daniel911T's Garage

Quote:

Originally Posted by zorin

Hi Dan,
I checked the air correction jet and it has 795 stamped on it.
BTW, I have a set of 964 camshafts. Is it worth the trouble of replacing the 3.2 camshafts with 964's? Will I notice huge difference in engine performance? Does the engine have to come off to replace the camshaft?

195 is already 10 bigger than stock 2.2 jets. The font they use looks funny, but that 7 is actually a 1, it's like that on all my jets.

How about I send you a set of 185s drilled out to 205ish. If you still have the black smoke, just drill them out a little bigger.

I don't really know enough about the 3.2 vs 964 cams other than the latter are considered a very well balanced setup for earlier cars. Once you start getting in to cams, the only knowledge I have is what I read here, and what's in Wayne's engine book. He seems to like 964 cams a lot, for what that's worth. You need to do more research.

Let me know if you want to experiment with larger air jets.

-Dan

zorin · 09-21-2011, 05:18 PM

I noticed that the bolts connecting the carburetors to the engine and the bolts connecting the vents to the carbs were loose. I tightened them and I also cleaned the jets. It seems that since then there has not been any black smoke at full throttle. :-)

dicklague · dicklague's Garage

get and AFR instrument to really know what you have.