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I have to admit this is a first for me until yesterday I had never heard of ZS Carbs. With that said I just purchased a 1976 MG Midget for my wife the car has been sitting in a garage for 8 years I did the tune up thiings and switched the ignition to an optical electronic version (Crane and it is really good and easy to install) I had the car started and gas is pouring out of the carb I checked the diaphram and it is ok. There is (on the right side of the carb, looking from air cleaner side) a plastic knob with a slotted screw in the center. What is this and what does it do? Also I would like the basic starting point adjustment position for this to get started setting the carb to run enough to get into tuning. Right now it just floods the engine and will not run enough to do any tuning. Thanks in advance for any and all help it is greatly appreciated.
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ZS Carb settings
- Thread starter randyc
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Start with the "basics." Is the fuel pump a proper one or has it been swapped out for an aftermarket one? The carbs on these cars want NO more than 3~4 LB/P.S.I. fuel delivery pressure. The float needle is at the bottom of the unit, so R & R is the best way to check it... If the car's been sitting that long, I would HIGHLY recommend you "bite th' bullet" and get a carb rebuild kit. The time it'll take you to go thru it will be well invested: you'll know the unit "inside-out" and know what is NOT the problem.
IIRC the "knob" you've referred to is an air bypass adjustment. If the thing is "passin' gas" as heavily as you indicate, that adjuster wouldn't do a thing to change it. Most likely the float needle is gummed in the open position or the float is "cemented" to the bowl by dessicated "dinosaur whizz."
HTH.
IIRC the "knob" you've referred to is an air bypass adjustment. If the thing is "passin' gas" as heavily as you indicate, that adjuster wouldn't do a thing to change it. Most likely the float needle is gummed in the open position or the float is "cemented" to the bowl by dessicated "dinosaur whizz."
HTH.
Nunyas
Yoda
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The Heynes Weber/Zenith Stromberg/SU Carburetor manual referes to the knob as the "idle air mixture adjustment" knob, a.k.a. air by-pass as the doc called it. The knob won't gain you anything useful if the car isn't running. The fuel mixture adjustments are done though the top of the dome (dash pot) with a special tool. If the fuel is pouring into the manifolds then the float is either stuck and not poinchin off the flow correctly, the float bowl's gross jet/needle valve is stuck open, or the pump is pushing too much pressure to allow the float to close properly.
Doc mentions rebuilding the carb. This is a good idea, particularly with these old ZS carbs. There's a "plug" in the bottom of the float bowl that only has a rubber o-ring to prevent the fuel from leaking out onto the ground/exhaust. After 8 years of sitting, potentially without use and potentially with no fuel in the car what so ever, I don't expect all the rubber and paper bits to be at 100% anymore.
Also, you mentioned the diaphram. I assume this is the diaphram in the dahpot. While these may appear to be ok on visual inspection they can sometimes have pin holes in them or the material could be too thin/weak. A thin/weak diaphram would cause the diaphram to stretch instead of allowing the piston to move. But these problems will cause lean running at speed becuase they will prevent the piston from rising and allowing more fuel to mix in with the greater amount of air rushing into the engine.
Doc, has the best suggestion: Maks sure the pump isn't forcing too much pressure, and rebuild the carb. Get rid of the unknowns and you'll have a better starting point.
Doc mentions rebuilding the carb. This is a good idea, particularly with these old ZS carbs. There's a "plug" in the bottom of the float bowl that only has a rubber o-ring to prevent the fuel from leaking out onto the ground/exhaust. After 8 years of sitting, potentially without use and potentially with no fuel in the car what so ever, I don't expect all the rubber and paper bits to be at 100% anymore.
Also, you mentioned the diaphram. I assume this is the diaphram in the dahpot. While these may appear to be ok on visual inspection they can sometimes have pin holes in them or the material could be too thin/weak. A thin/weak diaphram would cause the diaphram to stretch instead of allowing the piston to move. But these problems will cause lean running at speed becuase they will prevent the piston from rising and allowing more fuel to mix in with the greater amount of air rushing into the engine.
Doc, has the best suggestion: Maks sure the pump isn't forcing too much pressure, and rebuild the carb. Get rid of the unknowns and you'll have a better starting point.
sparkydave
Jedi Knight
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I agree, a rebuild kit would be helpful. My car actually ran decently after sitting for 18 years, but was much better after a rebuild kit. Don't forget to order the adjusting tool, you'll need it.
BritBox
Jedi Hopeful
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Hi,
I don't know if it's strictly necessary, but a Gunson "Colortune" kit can make it really easy to set the mixture on rebuilt carbs. It's the device that temporarily replaces a sparkplug but allows you to view the actual combustion taking place. You adjust your mixture until the color changes to what you want. It's pretty neat. On dual carb cars you do it in turn, one cylinder in the front and one in the rear. I used it on rebuilt dual ZS carbs on my '68 Triumph TR250 and it worked very well. Have not had to make another adjustment since 2001.
https://www.gunson.co.uk/item.aspx?item=1822&cat=439
Available from the usual suppliers like Moss, LBCarCo, etc.
Now, my leaky throttle shafts, that's another story...
Best regards,
I don't know if it's strictly necessary, but a Gunson "Colortune" kit can make it really easy to set the mixture on rebuilt carbs. It's the device that temporarily replaces a sparkplug but allows you to view the actual combustion taking place. You adjust your mixture until the color changes to what you want. It's pretty neat. On dual carb cars you do it in turn, one cylinder in the front and one in the rear. I used it on rebuilt dual ZS carbs on my '68 Triumph TR250 and it worked very well. Have not had to make another adjustment since 2001.
https://www.gunson.co.uk/item.aspx?item=1822&cat=439
Available from the usual suppliers like Moss, LBCarCo, etc.
Now, my leaky throttle shafts, that's another story...
Best regards,
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The ColourTune thingie is great. I've a pair of 'em so's I can do two carb setups in one go. Jag's are a bit problematic tho...
and a UniSyn is another good tool to have as well. But unless ya have four arms, two of those doesn't give much advantage, IYSWIM. /ubbthreads/images/graemlins/wink.gif
and a UniSyn is another good tool to have as well. But unless ya have four arms, two of those doesn't give much advantage, IYSWIM. /ubbthreads/images/graemlins/wink.gif
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Is your float floating? Think of the ball in the toilet - if it doesn't rise far enough, the water never shuts off...
Mickey
Mickey
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erm...pressure is right, float height, sticking needle valve, float cracked and full of fuel... those are about all the common causes of excessive fuel. Proper oil level in the piston slide? Choke actuating lever not engaged with the bi-metal "spring" properly?
This innernet thingie is both blessing and curse! GIMME that thing an' lemme sort it (IYSWIM)!
EDIT: You said "before cranking"... rules out oil/slide as a cause. Sorry.
This innernet thingie is both blessing and curse! GIMME that thing an' lemme sort it (IYSWIM)!
EDIT: You said "before cranking"... rules out oil/slide as a cause. Sorry.
What is thecorrect way to set float height? I feelpretty confident the problem is in the fuel inlet. I did change the inlet valve and after reading other posts on here I bent the tab ion the float lever to make it close more with float travel. Took out the float put it in a bucket of water, it floats and no fuel inside. am I overlooking something simple? I try to use the K.I.S.S. rule. (Keep it simple stupid) seems to save me alot of time ha ha,.when it does fire uop it will run at high idle but lots of smoke from overfueling. when I bring it to an Idle it floods out.If I crank it and it does not fire fuel runs outta th port on carb. By the way thanks guys for taking the time to reply. It is truely appreciated.
A Caterpillar 3516 Natural Gas Engine is soooo much easier to work on than this.........ha ha ha
A Caterpillar 3516 Natural Gas Engine is soooo much easier to work on than this.........ha ha ha
Nunyas
Yoda
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setting the float height... When I rebuilt my ZS carb I used this site to help me figure out how to set the float height:
https://www.chicagolandmgclub.com/photos/zs_rebuild/
I found it slightly annoying to do with ZS carbs becasue I had to pull the carb off of the car, drain the bowl, remove the bottom of the carb and perch the thing upside down somehow. It's been a while since I set the float height on my ZS so I've forgotten what the proper height should be. The pictures one the site don't show it very well, but they do describe on method for measuring the float height. They recommenct laying a 1/8" diameter rod across the float arms to make the height adjustment.
If I'm not mistaken the height of the float should be from the body of the carb. I used the body part that the float bowl seals against nearest the valve to make my height measurement. Also, try not to twist the float assy, and bend only the arm that touches the valve. Oh yeah, IIRC the part that contacts the valve should also be perpendicular to the valve (parrallel to the rest of the float arm)... You don't want the float arm contacting the valve at wierd angles. Finally, if you're finding that you have to bend the arm "too much" (I think personal judgement is the call maker there), try a different spacer between the carb and valve. I had a couple of spacers and a new grosse jet to choose from when I got my rebuild kit.
https://www.chicagolandmgclub.com/photos/zs_rebuild/
I found it slightly annoying to do with ZS carbs becasue I had to pull the carb off of the car, drain the bowl, remove the bottom of the carb and perch the thing upside down somehow. It's been a while since I set the float height on my ZS so I've forgotten what the proper height should be. The pictures one the site don't show it very well, but they do describe on method for measuring the float height. They recommenct laying a 1/8" diameter rod across the float arms to make the height adjustment.
If I'm not mistaken the height of the float should be from the body of the carb. I used the body part that the float bowl seals against nearest the valve to make my height measurement. Also, try not to twist the float assy, and bend only the arm that touches the valve. Oh yeah, IIRC the part that contacts the valve should also be perpendicular to the valve (parrallel to the rest of the float arm)... You don't want the float arm contacting the valve at wierd angles. Finally, if you're finding that you have to bend the arm "too much" (I think personal judgement is the call maker there), try a different spacer between the carb and valve. I had a couple of spacers and a new grosse jet to choose from when I got my rebuild kit.
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Randy -
To add to the above, here are the instructions found in the TR8 shop manual I have. (TR8 has two ZS 175's - twice the fun!)
"Check and adjust float levels
With the carburetter in the inverted position, check the distance between the gasket face on the carburetter body to the highest point of the float. Measurement should be .625-.627 in. (16-17mm)
To adjust, bend the tab that contacts the needle valve, but ensure that it sits at right angles to the valve to prevent the possibility of sticking."
Hope this helps - and be sure to let us know what you find.
Mickey
To add to the above, here are the instructions found in the TR8 shop manual I have. (TR8 has two ZS 175's - twice the fun!)
"Check and adjust float levels
With the carburetter in the inverted position, check the distance between the gasket face on the carburetter body to the highest point of the float. Measurement should be .625-.627 in. (16-17mm)
To adjust, bend the tab that contacts the needle valve, but ensure that it sits at right angles to the valve to prevent the possibility of sticking."
Hope this helps - and be sure to let us know what you find.
Mickey