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How to tune a carb?

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Markwheeler Avatar
Markwheeler Mark Wheeler
Heflin, Alabama, USA   USA
1977 MG MGB "Bertha"
So umm.. How do I tune my carb? Im running a 175- ZS... Shes been pretty faithful, but after doing some engine clean up, I need to tune her, and bad.. Someone feel like explaining?

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Dave Braun Avatar
Webster, Minnesota, USA   USA
1952 MG TD "Tommy"
1970 MG MGB "Maggie"
1974 Triumph Spitfire 1500 "Sammy"


without knowing what 'Bertha' is doing, it is hard to tell if you need to do anything. What percipitated this feeling that you needed to do something, besides a clean-up?

But here are some recomendations. First, run a unlit, slightly hissing propane torch around the throttle shafts while the engine is running. If the engine speed picks up, you have a leaky throttle shaft. Then check the areas where the manifold gasket is, and finally look for loose booster hoses and anything else that might be a source of a vacuum leak.

Then, open up the top of the carburetor and inspect the diaphram. Sometimes they are cracked or torn. Finally pull out the airvalve and see if the needle is scored or scraped, indicating it has worn the jet opening.

To check the float, you need to remove the carb from the engine but it should float, and the needle should seal off against fuel.

Then reassemble, set the jet height, the idle speed and the mixture.

hope this helps,

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Markwheeler Avatar
Markwheeler Mark Wheeler
Heflin, Alabama, USA   USA
1977 MG MGB "Bertha"
Well, the engine is idling way to high without the breather on, and after I put it on, it idles down a little bit, but it still is about 2500 and its cutting out (with the breather on) this leads me to believe that I should go less fuel and more air..... Though I could be wrong.. Ideas?

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Fairfield, CA, USA   USA
look for a couple of University Motors videos about the ZS on youtube

there is also a special took which makes it much easier to adjust a screw set inside of a nut (i.e. turning the screw without turning the nut, and vice verse)

1973 Pale Primrose Roadster. A nice 10-footer!
SUs, Datsun 5-speed

spikemichael Avatar
spikemichael Platinum Member Michael Caputo
Aberdeen, WA, USA   USA
1973 MG MGB "Freebie"
1973 MG MGB
1974 MG MGB "Spike"
1979 MG MGB "MegaBeanie"    & more
The Zenith must have the air filter housing in place to make adjustments if you intend on using the housing afterwards.

Decide how you will filter air then adjust with that in place.

Michael J. Caputo
'79 RBB and '73 CBB owner with extensive experience in 12v Audio System design and installation.
Vendor of Regalia and Promotional Products. Forum Member with a warped sense of humor.

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Markwheeler Avatar
Markwheeler Mark Wheeler
Heflin, Alabama, USA   USA
1977 MG MGB "Bertha"
I am planning on using this carb yes, my issue is that I dont know which nuts to turn, or what to adjust to tune a carb.. Never done it before on any car.

curtis7420 Avatar
curtis7420 Curtis Wright
University City, MO, USA   USA
it's actually pretty easy. the hardest part is getting it off the manifold. i would suggest ordering a carb rebuild kit, which will run you about $25-$30 (the ZS kit is a lot cheaper than older carb kits). then watch the videos and get a shop manual and go to work. it took me all of about 3.5 - 4 hours total, and i've never done ANYTHING like that before. i have heard it takes most people about half that much time. i was pleased i had done it myself and learned a great deal about the carb. if you don't want to purchase a manual, i could probably just scan mine for you. if interested, PM me. good luck

philipE philip williams
Oregon, USA   USA
this is the John Twist artical below link

there are two Fine idle screws on the side of the carb facing the driver.
one is a large hex shapped and there is a small screw right in the middle of it.
turn the small screw out a couple of turns.
then turn the large one till it bottoms snug, then back it out 2 full turns.
now turn the small scres in till it bottoms out, then back it out 1 1/2 turns.

that will set the Fine idle at Neutral position, its very seldom you need to adjust these further.

read that John Twist artical and follow his directions.

you will need the SPECIAL STROMBERG TOOL in order to acjust the mixture .. no way around that !!
Moss Motors sells the tool.

good luck .. its actually a simple job. ... depending on if you have the Water Choke .. i hear thats a bit to mess with. i have changed to a Manual Choke conversion.

Don't use Champion plugs !
make sure you check the oil in the carb damper.
re-check that vacumn line on the top back-left-side going up to the valve.
check the hose from the engine crankcase to the carb -- when this is off or lose or cracked it will affect the idle drasticlly.

Edited 2 time(s). Last edit at 2011-10-14 12:24 AM by philipE.

Markwheeler Avatar
Markwheeler Mark Wheeler
Heflin, Alabama, USA   USA
1977 MG MGB "Bertha"
No I don't have the water choke, and have heard that they were much more aggravating to use in comparison to a hand choke. I have considered going back to water choke, but not sure if it is even worth my time. I have a normal 1/8 allen wrench, will this not work to serve as a carb. tuning tool????

feathercraft1952 Avatar
feathercraft1952 ric boles
kentucky, USA   USA
1974 MG MGB
1974 MG MGB "Junkyard Dog"
1976 MG MGB "Penellope"
reg allen wrench will NOT work - it fits in but you will tear the diaphram when you twist it if there is any resistance - and there should be
the carbs are really easy after you get into them

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