View Full Version : ZS tuning question

06-24-2004, 11:23 AM
okay, quick question to make sure I'm not loosing my mind. The carb was tuned to work at 9000' in Bailey CO. It now resides at about 4500' here in NM. When i shift, as I let of the gas, the engine backfires. It's not that violent kind of backfire, it's the pop and sputter stuff, know what I mean? Anyway, I'm no wiz at carb tuning to say the least, but I take this to mean it's running lean? I was going to give the idle mixture screw a quarter turn clockwise and see what happens....

Dave Russell
06-25-2004, 02:24 AM
Hi Rob,
You are correct - The engine will run leaner at lower altitude. Air density will change about 3% for each 1,000 ft elevation change. In your case this would be about 14% more air. You may need to do more than adjust idle mixture. If it WAS correct at 9,000 ft, it will need to be richer in all ranges. Keep an eye on sparkplug readings for a lean condition. The popping sounds more like an exhaust air leak though. Maybe a combination of the two is causing the problem. BTW - You should have noticeably more power at the "lower" altitude.
D /ubbthreads/images/graemlins/laugh.gif

06-25-2004, 06:33 AM

This is sort of a guess, since I haven't owned a car with a ZS carb in some while, but I do not believe that your carb has an "idle mixture screw". The ZS does have an "idle speed screw" and and "idle air screw" (sometimes called an idle air bypass). The idle speed screw will change idle RPM and nothing else. The idle air screw is useful for improving the emissions and will allow the car to run more cleanly and smooth at idle speeds. Most idle air screws on these carbs came with a plastic restrictor head that limited the rotation (adjustment) to about a half-turn. If the restrictor head has been removed, you can turn the idle air screw as much as you like. I think that the initial setting for this screw is to close it all the way (clockwise) and then unscrew it about 2 turns. In any event, the idle air screw does not affect the power and performance of the engine at speeds above about 1500 RPM.
Adjustment of the high speed fuel circuit on your car is fixed and not easily adjustable. This is the only real method that can change your engine performance (fuel-wise) for speeds above idle. "Adjustment" involves removing the dashpot piston needle and replacing it with a different one. A different discharge jet (the part that the needle fits into) is probably also avialable. To see the needle, remove your air filter and look in the throat of the carb. The needle is brass and is attached to the piston (the part that moves up and down when you rev the engine). There is also a "needle and seat" in your fuel bowl, but this serves a different function.
If your car was tuned for higher altitude, the piston needle was likely swapped for a leaner one.
Newer MGBs have these ZS carbs too, so you may want to post a note regarding tuning adjustments on the MG forum. Some of those folks may have some more recent experience with the ZS unit. The older-style SU carbs are much easier to fine-tune in my opinion. The ZS is basically a "fixed mixture" emissions-oriented carb.
By the way, in my experience, most of these cars backfire lightly during shifts. There is a device fitted called a "gulp valve" that is supposed to reduce this tendency, but it is really sort of a "band aid" solution.

06-25-2004, 02:11 PM
aeronca65t, I think you're right about the idle air screw and the restrictor. I couldn't turn the mixture in more clockwise (only about 1/16 of a turn), seemed to be all the way and there was little play in the adjustment. I've let it be for now.

In regards to the SU swap, what's a good donor vehicle to look out for? will the older pre-74 midgets have an SU that will bolt right up? what about mgbs? what's a fair price for a used unit?

06-25-2004, 02:27 PM
I think Euro-market Spitfires are the best donor for twin SUs

06-25-2004, 03:07 PM
Due to rules restrictions, I am limited to running a single SU HS4 on the 1500 that I race. It bolts right up to the existing ZS manifold, and seems to work well with the race engine. I don't see why it wouldn't work for the street.
The one I have on there now was originally the front carb from an MGB. (AUD 135)

06-25-2004, 03:54 PM
In regards to the SU swap, what's a good donor vehicle to look out for? will the older pre-74 midgets have an SU that will bolt right up? what about mgbs? what's a fair price for a used unit?...

The older twin-carb Midgets (1275 engine) have a completely different engine and manifolds from these cars will not fit. Your car has an engine originally developed for the Triumph Spitfire: as 78Z has said, the best bet is to look at the twin carb setups used on 1500 Euro-Spitfires (and some 1500 Canadian cars). There was actually a twin carb 1500 Midget sold in Europe (same as twin carb Spitfire engine).
MGB manifolds will not come close to fitting.
Honestly, I like Jeff's idea best. I drove a car with this setup and it ran great.

06-25-2004, 06:01 PM
understood. How easy is it to hook up the linkage on twin HS2s (right model?) vs a single HS4 on stock intake? what would be the performance difference between the two? I like the idea of the single HS4 since it would be easy to tune, and I gather these are easy to rebuild too...what would be a fair cost for something like this?
thanks, /ubbthreads/images/graemlins/thumbsup.gif