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Rich running

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sparkydave

sparkydave

Jedi Knight
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I've been trying to figure out why my Midget seems to do its best to run as rich as possible, despite finding John Twist's info on adjusting the Zenith Stromberg, but it still produces a distinct smell at the tailpipe. Following what I've managed to find, I have the idle air bypass closed, and I've got the main adjustment set so that when I lift the air piston, the idle speed stays constant briefly and starts falling. On the one hand I don't have to pass emissions, but on the other hand it stinks up my garage, and my pants if I happen to walk behind it. I haven't looked at the plugs to see if they indicate that it's really running rich, but is this just as good as the Zenith gets? The car seems to run fine otherwise. I've determined that there are no vacuum leaks, the carburetor was rebuilt about 2 years ago, and the valve and ignition timing are what they are supposed to be. The automatic choke works as it should, so it's not sticking on. The one thing I didn't do when I rebuilt the carburetor was replace the metering needle. Does this really erode that much that it needs routine replacement? As far as emissions gear, the catalytic converter is gone and the air pump belt is disconnected, but everything else is still present. Any thoughts?
 
Dave,
When I replaced my needle it was eroded pretty visibly. I've never been able to completely tune out the "richness" on my 1500 and it seems to load up after a few minutes of idling. Once I clear the carb it idles fine again for a few minutes. You might try and replace the needle but I've heard that the seat can oval out with time as well. If you are certain that you have tried everything else I would go ahead, bite the bullet, and order a new needle.
JC
 
Thanks JC, I think I found that the choke wasn't quite turning off all the way, since I readjusted it and suddenly it seems to be running a little better. Still might keep that new needle in mind, or try the '78 Spitfire needle from the spare carb I picked up.
 
Sparky Dave, as I said on another thread. Throw the water choke away and get a manual choke from VB or Moss. I did and never looked back. The manual choke is the only way to go if your sick of messing with the auto choke.
Don
 
I had this same problem with my ZS and I went through everything you did. Finally, I retarded my timing a bit. It now hits around 8-9 BTC. This seems to have helped a great deal with the "rich smell."

I don't know if there is any real science behind this. But my exhaust definitely stinks less than it did.

morris
 
Okay, please disregard that previous message. I took Dennis (that's my Midget's name) out for a long drive today and stunk up the whole highway. I think what really happened is I shocked myself while setting the timing thus temporarily dammaging the part of my brain that processes exhaust stink.

I should stick to getting advice, and stop giving it.

morris
 
Yeah, I wasn't sure how retarding the timing just a little could lean up your fuel mixture. But hey, we all try crazy things from time to time. Some work and some don't. Thanks for sharing.
JC
 
Actually, timing and A/R ratio do effect one another. For a richer mixture you would want to advance the timing a bit to start the burn earlier and thus burn more of the fuel. A lean mixture would need to be retarded a bit to avoid pinging. Lean is worse than rich.

You could always replace your ZS in a spriget or spit with an SU of equal size. I just put an SU HS6 on my MGB (a Volvo 1800 carb) and there is a big difference in the way the car runs and the way the choke works. I will never use a ZS again. Runs great with a 'random' needle (I got lucky?) and a little tuning with some needles I have laying about, and this swap is one of the best things I've done for the car yet. For a 1500, an MGB carb would work well. Shouldn't be too much modification and the carbs are easy to find.

-D.
 
Advancing the timing to correct an over rich mixture will indeed help to a degree. But by the time you can smell it at the tailpipe I don't think it would help that much. Typically over rich conditions are caused by weak spark or excessive fuel. Kicking the timing up would increase the vacuum a bit as well as increase the idle speed so it would seem to make a slight rich condition a little better but I don't think it is actually correcting the problem but more masking it??? I don't know for sure, this conversation might have gotten out of my league...lol Congrats on the new SU. Sounds like it is working out!
JC
 
Advancing the timing won't cure an overly rich problem. As you stated, the spark/carb needs sorting for that. I just wanted to point out the relationship between A/F ratio and Timing because one does exist.
 
I have wondered whether an SU would work or not, but I wasn't aware that an SU would bolt up to the same manifold. I haven't played with the timing, so it's still running at what the book says. Might keep my eyes out for an HS4 and see if they have the same mounting.
-Dave
 
I double checked my timing, and I had misread the marks, it is hitting at about 12 BTC. Duh... Runs much better though.

Anyway, I am about a quarter way through a fuel injection conversion for my 79 Midget. It will eliminate all this carb adustment BS forever. Phew.

I hope to offer it as a kit eventually, but I will need to install it on several test bed cars before doing so. I plan to be ready for this phase early next year. If you are interested in being a beta tester for me, please let me know.

My project hit a little a speed bump this weekend when I discoverd my exhaust manifold was broken clean in two. Ugh!

Morris
 
YES!

I love that conversion. The system I am working on will be throttle body injection style, and it will use the original carb as the throttle body. In the future I would like to make a coustom plenum style multi-point set up for racing applications. But I worry that this will put most LBC racers in a class they cannot compete in.

The system I am working on will be a lot like this conversion https://topshamautoparts.com/tr6/tbi_updateSep04.htm
In fact, I am working with the guy who came up with this brilliant idea to create some of the parts for my system.
 
What are you using as your controller for your set-up? I'm curious about how you're triggering fuel/spark. I've been too busy to work on my own 1500 injection project yet in any serious way, but it's likely to be a sequential injection, MAP based system with spark control (a little more than megasquirt currently offers). However, it's not going to take any less time as a result of the additional complexity ...
 
I am using Megasquirt. There are mod's to MS that give you the ability to contorl spark. I will undertake that project after I have the fuel working. For now I, intend to trigger from the negative terminal of the coil.
 
Looking at the manifold in the first FI link, an HS4 R (right) should bolt right up the manifold. This puts the float bowl on the right and the linkage on the left. In the Apendex of 'SU Carbs Tuning Tip and Techniques' the book show triump running a single HS4 on a 1500 engine using an AAK needle and also an ABG needle.

I always thought that the SU's would make good throttle bodies for an FI system.

-D.
 
Well, for a racing setup, I think the best throttle body is in the 2" range, assuming you have a longish runner intake manifold (custom) which is 4 into 1 TB with a plenum in between the actual runners and the throttle body. I currently run 2X Weber 40DCOM's, and they're a bit overkill. Plus, when you consider that each cylinder intakes at a different time, you can get away with a smaller bore when using the 4-1 configuration. Another advantage to a single throttle body is the fact that it doesn't need to be synchronized! However, for optimum runner selection for our trusty 1500, the runner lengths need to be absurdly long, and so some things just aren't realistic. Only so much will fit under the hood of a midget, after all ... /ubbthreads/images/graemlins/smile.gif If we were looking for peak torque around 8000 RPM, it'd be much easier to achieve theoretically optimum runner length. However, friction losses through the pipe also become a concern when we're trying to get a peak of around 4000 RPM, so compromises must be made.
 
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