MG MGB and MGB GT Tech Talk
I was following John Twist's instructions on adjusting the ZS carb, and when I got to the part where you "lift the piston slightly" I determined that the mixture was too lean. I tried to turn the allen screw clockwise to richen, and the screw is as far clockwise as it will go! I ripped the diaphram trying to go some more :(
I had another diaphram :) so I fixed that, but where do I go from here?
I've checked for vacuum leaks (none found)
HELP!?
I wish I could help.
I don't have a later car with one of those things. So I friggin don't know.
Sorry. But I sure hope someone who does know comes along pretty soon.
Why are messing with the carb? Have you got any problems that you were trying to fix?
When tuning my carb I always start with the base-line tune up of adjusting the valves, cleaning the distributor cap, setting the points and setting the timing. With that all squared away, center the course and fine idle adjustments in the middle of their range and leave them there. Only a gas analyzer will pick up the minute changes made here. Remove the air piston and set the needle assembly flush with the bottom face of the air piston. Upon reassembly, rotate the needle in the adjuster so that it is angled away from the manifold side of the carb. That should you get you very close to a good running condition. Set the idle and adjust the throttle cable if needed. Then drive the car and see how it's running. If it has a slight case of the spits (backfiring through the carb) give a quarter turn clockwise to enrich the mix. Drive and repeat until the spits are gone. If there are no spits, just drive and enjoy for while and check the plugs after 500 miles or so. You could also lean it out until is spits a little then work back to the rich side.
Not real scientific, but it's always worked for me. Hope that helps.
locolobo85 Wrote:
If you've got the tabs on the diaphram lined up with the grooves on the piston and carb body, you can tell how the piston will be situated in the carb. Once you've got that worked out you can pull the piston out and just rotate the needle around to where you want it. It sits a little crooked in its carraige. Just rotate it until it is leaning away from the manifold side of the carb.
Unfortunately, we're not going to get much zip out the ZS carb no matter what we do. It was fairly economical and allowed MG to tune the cars for US emission standards. From 74.5 on, the MGs were woefully slow. By this time the cars were merely tourers. If she's running smooth with no spits and not fowling the plugs, that's about all we can hope for.
I am working I my ZS carb as well. What do you mean by
"pull the piston out and just rotate the needle around to where you want it. It sits a little crooked in its carraige. Just rotate it until it is leaning away from the manifold side of the carb"
when you say needle are you talking the long pointed one? This has to be rotated? I didn't know that has to be in a certain position down in the hole.
paul
That's a little tip I picked up somewhere along the way. I'm no expert but have always followed that little tid-bit of advice to bias the needle away from the manifold. It may not amount to a hill of beans but it's simple enough to do.
Brad,
Thanks a Million for the advice, I removed the piston from the carb and found that I could not adjust the needle to be flush with the bottom of the piston. In fact it would not move period! It was sticking out about 1/8" from the bottom of the piston, and would not move.
Upon further investigation I found that some DPO had not assembled it correctly and there was a burr on the brass housing portion of the needle assembly. I smoothed that out and re-assembled the needle in the piston, works great now, I also found that the course "nut" for Idle mixture was stripped and the screw was too far out. Replaced the "nut", put both at the center of their range, and WOW it drives like a new car!!
I was able to enrich the mixture slightly, (which is what I was trying to do in the first place) I got the idle smoothed out, Badda bing!!
A million thanks again!
Tony
Glad to hear it! There's always that sick satisfaction in keeping the oft maligned ZS purring like the family cat.