tuning the zenith-stromberg 175-cd2 su carburetter

My friend has a 1978 MGB equipped with the Zenith-Stromberg 175-cd2 SU carb. As of this moment it runs rough, loses power when a load is placed on the engine, stalls on occasion, and idles rough. The spark plugs are covered with black soot and the interior tail pipe is covered in black soot. This tells me that the car is running with too rich of a mixture, well at least from past automotive repair experience anyway. I have attempted to locate a tech manual to help me troubleshoot the carb so I could adjust (?) it; alas no luck. I am familiar with SU carbs as I owned a Datsun 240Z and worked on the Hitachi SU carbs, but this Zenith carb is unfamiliar territory. For one thing, I've noticed it doesn't have the moving piston, but has a diaphram instead; thus much of the information I have found has to do with the carb that has the moveable piston. Has anyone out there had experience troubleshooting, tuning up, rebuilding this carb?Can it be adjusted to a leaner fuel mixture?

Thanks for any input.
.......Bruce Price

these are zs not su carbs. don't know where you're looking but these do have a piston. haynes manual isbn 1 85010 019 5 is a good reference

http://www.amazon.com/Haynes-Owners-Workshop-Manual-Carburetors/dp/1850100195/ref=sr_11_1/103-9484251-0825426?ie=UTF8&qid=1184191989&sr=11-1

or maybe on ebay

more than likely the auto choke.

This should help:
http://www.sterlingbritishmotoringsociety.org/files/zenith%20stromberg%20tech%20tips.pdf

Sounds like it needs a slight rebuild, the rubber diaphragm should be replaced as well as the o-ring in the damper oil reservoir http://www.mgexperience.net/article/zenith-oring.html . BTW the oil reservoir is also where one can adjust the idle mixture (similar to the SU) with the proper tool.

Oh and what Mick said!

Oh forgot about this article on the water choke adjustment: http://www.theautoist.com/zs_water_chokes.htm

Thanks, Mick for the info. See, I thought the Z was an su carb, thanks for steering me straight as that helps.

Bruce

Eric; The carb has been rebuilt (new diaphram, autochoke cleaned and recallibrated, carb cleaned). Where is the oil reservoir located? For lack of better term, the dipstick at the top of the carb housing is attached to the diaphram (I think) as it doesn't seem to come all of the way out. Not sure where to put oil. I know we have to break down and buy a manual.

Thanks,
Bruce

09191941 Wrote:

Bruce=
if all else fails, you being in Kentwood could run over to Ada and visit University Motors, he's located on M21 just west of the Amway campus. He literally wrote the book on ZS carbs

Mike

09191941 Wrote:

Thanks for the pics and info, Eric. The dashpot is drier than the Sahara...no oil at all! This leads me to suspect that the lack of oil in the dashpot is contributing to the rich air-fuel mix. Is that logical? I'm detaching the autochoke from the carburetor to see how the engine runs. I'm going to add oil to the dashpot first, however; how much oil do I put in? I imagine there is a fill mark on the damper, right? Thanks again,

Bruce

There is no fill mark and according to John Twist (University Motors) you can't overfill the dashpot. The oil in there will help with the idle and acceleration problems. It basically smooths out the operation of the piston in the carb which makes for better acceleration and smoother idle. It probably won't correct the rich running problem. I would suggest you check out the links I posted above on servicing the auto-choke.

dashpot oil slows the rise of the piston on accelleration giving a richer mixture while speed is increased. no oil aloows the piston to rise quicker making the mixture leaner on acceleration. piston should reach the same height with or without oil, just how fast it reacts is affected by the oi or lack thereofl.

I'd recommend checking and cleaning up the ignition system before tinkering with the carb. I played with my ZS for months and couldn't get it right. Then I installed a rebuilt dizzy from Jeff and all my problems went away--literally. Idle smoothed out, acceleration smoothed out. The difference was amazing.

Chuck

Chuck; I've wondered about the ignition, especially the timing. I'll check the distributor points as I have put in new plugs and plug wires. I'm now wishing I had a strobe to check the timing....dang! Anyway, thanks for the suggestion as I'll follow through on it.

Bruce

Eric; Well, the car runs much better. I found two stripped screws that held the mass cover to the choke body. I drilled and tapped those holes, got the appropriate sized screws, insterted them and snugged them down. The idle rpms suddenly rose when the car was started, it idled much smoother. However, taking it out on a test drive it still lugs a little bit when the engine is under a load and acceleration is sluggish, but much better than before the "repair". Your suggestions were very helpful and much appreciated....thanks. I had a suggestion about checking the ignition and I'm going that route to see what happens (the car already has new plugs and new plug wires). Incidently, I did pull out the damper and it did come out....I was afraid to pull too hard as to wreck something...I added oil (20-50 as you recommended) and all is well. Thanks for that too.

Bruce

Bruce, if it has the original distributor for the late model cars, it may only have mechanical advance, and the mechanism may be sticking. You may want to lubricate it good, or better yet, send it to Jeff Shlemmer at Advanced Distributors and have him add vacuum advance to it. I have a new carb that runs really well, but the down side is I can tell just how bad my original distributor is now! :) I'm talking to Jeff about a replacement, if he can help me figure out how to hook up the vacuum line.

John; I looked at the distributor and it looks okay as there is no scoring or pitting on the rotor; everything looks clean, but I can't find any points. I don't know if it is original or not. I don't have a vacuum advance I know, but I don't know where the mechanical advance is located. On a lark I thought I would check the spark at the spark plug and it is a yellowish-orange. I am of the opinion that it should be a bluish-white spark. Could the coil be going bad? It seems to me that a bad coil would have something to do with sluggish running. Am I wrong? I am running out of options as to why the engine lugs when under a load (accelerating, going up a hill,...). The tappets in this car seem awful loud to me, but then I am not familiar with the mgb as you can well guess. However, it is fun learning and doing. Thanks for your suggestion.

Bruce