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ZS Carb Auto Choke Idle Issue

Posted by 1980owner 
Jeff Grant
Central, MA, USA   usa
1975 MG Midget 1500 "Project Not Yet In Progress..."
1980 MG MGB Limited Edition (LE) "Daily Drivable"

This is for my 80 B LE. I bought a carb off someone a couple of years ago which was taken off their running 79 B (I think). They rebuild them. Fast forward two years of the engine running great and we are at today...

I depress the throttle to set the choke and release. Start it up as always. It does not start as easily as it usually did. Then, I go to the first stop sign in my neighborhood and the idle has usually dropped down. However, now it sits up around 3500rpm while at the stop sign. I blip the throttle and the idle goes up higher but does not continue to drop down.

In addition, another symptom is that after the car is warm and I let it idle down while coming to a stop, it will sometimes stall.

I am guessing that either the auto choke is a little frozen (is there anything I can lube?) or maybe there is an air pocket in the water system at the choke assembly on the carb.

Suggestions?

As a side note, I am not currently willing to install a Weber/SU/etc carburetor.



Below links are for my 80 B LE with factory 1800 and include pictures.
Blackstone Labs Oil Report #2 on 80 B LE (8/2011)
My MTX-L Wideband O2 Installation (8/2011)
M!ata Seat Install (5/2011)
Rear wheel cylinders, brake line bending, brake master cylinder replacement, pressure bleeding (9/2010)
1975 No-Cat Stainless Exhaust Installed (8/2010)
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Ogre1 Avatar
Allan Ogilvie
Melbourne Victoria, Australia   aus
1977 MG MGB "Greenb"

1. Follow john twists great video on the zenith carb also a good description somewhere on the net of rebuilding the choke.
2. Also check you thermostat as they can fail in the closed position and restrict the water flow round the head and to the carbs. Usually doesnt affect the choke though. For $6 I throw mine away and replace once a year when I do a service.
3. If your still not happy for about $100 you can get a nmanual choke conversion for the ZS.
rbluff Avatar
Rick L
oregon, USA   usa
1971 Datsun 510
1980 MG MGB

I seem to be having the same type of symptoms
things inside the choke seem to be operating correctly as in the Twist video
Hmm
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leenhodn Avatar
Dennis Leenhouts
League City, Texas, USA   usa
1977 MG MGB "Mr. George"

Lucky for me, mine came with the manual choke, and gives me very little trouble. But the fast idle could be a loose or cracked vacuum hose or a missing plastic nut on the idle air assembly. A number of other little problems could be the cause, not just the auto choke.



Edited 1 time(s). Last edit at 06/22/2011 01:36AM by leenhodn.
mgb922 Avatar
Tim Dowell
Solomons Maryland, USA   usa
1979 MG MGB
1979 MG MGB

The "choke" realy isn't a choke it's an enrichment valve. I had mucho problems with it and if it's not set up correctly can casue crack head, burned valves etc... Went to manual to get control back. The probelm I had with the manual is the valve pin was pushing the valves barrel back. Had to run saftey wire on the back side of the barrel cylander where the cap is. Worked then but have converted to HIF 4; much happer. If you want to keep the Zenith spend the money on a manual choke.
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Jeff Grant
Central, MA, USA   usa
1975 MG Midget 1500 "Project Not Yet In Progress..."
1980 MG MGB Limited Edition (LE) "Daily Drivable"

Based on my symptoms in the first post, can anyone suggest the problem? The carb is under 10k miles from a rebuild, stabilized gas every winter, has run great this year for just over 1k miles and now this has started up.

Can I remove the choke assembly from the side of the carb and spray it with some carb cleaner? I don't know if there is anything (seals etc) I would hurt in there with the carb cleaner.



Below links are for my 80 B LE with factory 1800 and include pictures.
Blackstone Labs Oil Report #2 on 80 B LE (8/2011)
My MTX-L Wideband O2 Installation (8/2011)
M!ata Seat Install (5/2011)
Rear wheel cylinders, brake line bending, brake master cylinder replacement, pressure bleeding (9/2010)
1975 No-Cat Stainless Exhaust Installed (8/2010)
Sinewave Avatar
T. Keith Vezina
Kenner, LA, USA   usa

Spraying carb cleaner on the outside of the choke won't do much, you will have to remove the heat mass assembly to get the cleaner on the moving parts. You could try removing the choke plunger cover (3/4" alum triangle held on with 3 screws) and spraying a little cleaner in there.

Did you try adjusting the idle? If so, it sounds like the wrong screw was adjusted. There are two adjustment screws near to each other and the most prominent one is the wrong one to adjust for warm idle. Some versions have a lock nut on this screw.

I suggest you get the Haynes manual and study the diagram.

Adjusting the cold and warm idle is a 3-step process.
First, adjust the gap between the brackets (3/32" IIRC) using the lower screw.

Second, remove the choke heat mass assembly and adjust the gap between the choke plunger and the fast idle cam (.015" IIRC) using the upper screw. You may find that the plunger is sticking, if so, this is the time to spray some carb cleaner.

Do not touch that screw again.

Third, set your warm idle with lower screw.



T. Keith Vezina
British Motoring Club New Orleans
1976 MGB, 1976 MGB Trailer & 1967 MGB MK. I
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Jeff Grant
Central, MA, USA   usa
1975 MG Midget 1500 "Project Not Yet In Progress..."
1980 MG MGB Limited Edition (LE) "Daily Drivable"

I have never adjusted anything on this carb from the day I received it. I suppose a screw could have wiggled loose though causing my idle to drop.

If the idle screw had wiggled out of adjustment, would that cause my rpm to stay up when I blip the throttle soon after starting? In the past it would have dropped after the first blip.

This was an almost isntant change in performance of the choke though from running great when cold to keeping the high idle and stalling when warm. That would lead me to think it was a mechanical item at fault.



Below links are for my 80 B LE with factory 1800 and include pictures.
Blackstone Labs Oil Report #2 on 80 B LE (8/2011)
My MTX-L Wideband O2 Installation (8/2011)
M!ata Seat Install (5/2011)
Rear wheel cylinders, brake line bending, brake master cylinder replacement, pressure bleeding (9/2010)
1975 No-Cat Stainless Exhaust Installed (8/2010)
Sinewave Avatar
T. Keith Vezina
Kenner, LA, USA   usa

I doubt that the screws vibrated out of adjustment. The more likely senario is that the choke plunger is sticking.



T. Keith Vezina
British Motoring Club New Orleans
1976 MGB, 1976 MGB Trailer & 1967 MGB MK. I
Jeff Grant
Central, MA, USA   usa
1975 MG Midget 1500 "Project Not Yet In Progress..."
1980 MG MGB Limited Edition (LE) "Daily Drivable"

In reply to a post by Sinewave I doubt that the screws vibrated out of adjustment. The more likely senario is that the choke plunger is sticking.

Can you describe that a little more? Will carb cleaner help and do you have a picture to show what the plunger loold like?



Below links are for my 80 B LE with factory 1800 and include pictures.
Blackstone Labs Oil Report #2 on 80 B LE (8/2011)
My MTX-L Wideband O2 Installation (8/2011)
M!ata Seat Install (5/2011)
Rear wheel cylinders, brake line bending, brake master cylinder replacement, pressure bleeding (9/2010)
1975 No-Cat Stainless Exhaust Installed (8/2010)
Brian Avatar
Brian Livingstone
Georgetown Ontario, Canada   can
1980 MG MGB

Jeff: I have also installed the manual choke and have had no more problems . Since then I am very pleased with the zenith carb
Brian



Brian
Ontario
80 MGB
Sinewave Avatar
T. Keith Vezina
Kenner, LA, USA   usa

Remove the heat mass assembly (parts 55 thru 60 in the diagram below) by removing the 3 screws(61). You will see the fast idle cam and the bottom of the plunger. Make sure that everything moves freely and that you have the proper gap between the fast idle cam and the plunger. You can spray carb cleaner here to clean things up.



T. Keith Vezina
British Motoring Club New Orleans
1976 MGB, 1976 MGB Trailer & 1967 MGB MK. I

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Jeff Grant
Central, MA, USA   usa
1975 MG Midget 1500 "Project Not Yet In Progress..."
1980 MG MGB Limited Edition (LE) "Daily Drivable"
Jeff Grant
Central, MA, USA   usa
1975 MG Midget 1500 "Project Not Yet In Progress..."
1980 MG MGB Limited Edition (LE) "Daily Drivable"

I had two of three screws off the rotating assembly when the entire choke assembly fell off the carb. I thought to myself, "OH NO, something must be broken!"

Nope, just 3 missing screws. I am somewhat amazed it ran at all, but it hadn't completely fallen off until I attacked the above screws with a screw driver.



Good thing I never sold my old ZS carb, sourced the screws from it.



Up and running again! I think it would be a good idea to put some lock tite on those screws now that I see this is a possibility. I'm thinking blue as red may be overkill, what do you think?



Below links are for my 80 B LE with factory 1800 and include pictures.
Blackstone Labs Oil Report #2 on 80 B LE (8/2011)
My MTX-L Wideband O2 Installation (8/2011)
M!ata Seat Install (5/2011)
Rear wheel cylinders, brake line bending, brake master cylinder replacement, pressure bleeding (9/2010)
1975 No-Cat Stainless Exhaust Installed (8/2010)
Sinewave Avatar
T. Keith Vezina
Kenner, LA, USA   usa

I would not use locktite on them. Just snug them up and they should be fine.

Glad to see it was an easy fix.



T. Keith Vezina
British Motoring Club New Orleans
1976 MGB, 1976 MGB Trailer & 1967 MGB MK. I
Jeff Grant
Central, MA, USA   usa
1975 MG Midget 1500 "Project Not Yet In Progress..."
1980 MG MGB Limited Edition (LE) "Daily Drivable"

So this then begs the question, how often should I have to regularly snug them up. That's a silly question. I'll eventually pull them out and put some blue locktite on as not knowing they have decided to loosen up till one (or more) is missing is not a good idea to me.

By the way, drove to a car show about 40 miles away today and the car ran awesome.



Below links are for my 80 B LE with factory 1800 and include pictures.
Blackstone Labs Oil Report #2 on 80 B LE (8/2011)
My MTX-L Wideband O2 Installation (8/2011)
M!ata Seat Install (5/2011)
Rear wheel cylinders, brake line bending, brake master cylinder replacement, pressure bleeding (9/2010)
1975 No-Cat Stainless Exhaust Installed (8/2010)
Sinewave Avatar
T. Keith Vezina
Kenner, LA, USA   usa

In the 18 years I've had my car, I've never had to tighten them. It could be that they were not tightened properly in the last rebuild.



T. Keith Vezina
British Motoring Club New Orleans
1976 MGB, 1976 MGB Trailer & 1967 MGB MK. I
Jeff Grant
Central, MA, USA   usa
1975 MG Midget 1500 "Project Not Yet In Progress..."
1980 MG MGB Limited Edition (LE) "Daily Drivable"

In reply to a post by Sinewave In the 18 years I've had my car, I've never had to tighten them. It could be that they were not tightened properly in the last rebuild.

Is there a torque spec? "tight enough"can be different for different people.



Below links are for my 80 B LE with factory 1800 and include pictures.
Blackstone Labs Oil Report #2 on 80 B LE (8/2011)
My MTX-L Wideband O2 Installation (8/2011)
M!ata Seat Install (5/2011)
Rear wheel cylinders, brake line bending, brake master cylinder replacement, pressure bleeding (9/2010)
1975 No-Cat Stainless Exhaust Installed (8/2010)
Sinewave Avatar
T. Keith Vezina
Kenner, LA, USA   usa

I'm sure there is one somewhere but I don't know what use it would be. The torque "feel" on a screwdriver handle would be much different than the "feel" on the end of a 12"-14" torque wrench.

I would never even consider tightening a slotted carberator screw with a torque wrench; WAY too easy to snap them off with all that extra leverage.

Try this: snug them up with a screwdriver and then check them once a week until you are satisfied that they are not backing out.



T. Keith Vezina
British Motoring Club New Orleans
1976 MGB, 1976 MGB Trailer & 1967 MGB MK. I



Edited 1 time(s). Last edit at 06/27/2011 01:37PM by Sinewave.
Jeff Grant
Central, MA, USA   usa
1975 MG Midget 1500 "Project Not Yet In Progress..."
1980 MG MGB Limited Edition (LE) "Daily Drivable"

In reply to a post by Sinewave I'm sure there is one somewhere but I don't know what use it would be. The torque "feel" on a screwdriver handle would be much different than the "feel" on the end of a 12"-14" torque wrench.

I would never even consider tightening a slotted carberator screw with a torque wrench; WAY too easy to snap them off with all that extra leverage.

Try this: snug them up with a screwdriver and then check them once a week until you are satisfied that they are not backing out.

Torque is torque, be it given by a little screwdriver handle or a BFL (big ... lever). Yes, there is this thing called leverage, but 12 in/lb is 12 in/lb and 12 ft/lb is 12 ft/lb no matter how you are creating that torque.

It seems no one has the actual answer other than "snug it" which is what I would expect the last rebuild guy did. For something that lasted 3 years, I wouldn't have gone 3 years looking at it to confirm it hadn't rattled out. If I did that, there are so many tiny bolts without official torque specs that I'd spend more time confirming bolt position than driving.

Anyway, thanks for everyone's input. I'd like to say I'm back to motoring, but my positive terminal end is in need for replacement so just on to the next project.



Below links are for my 80 B LE with factory 1800 and include pictures.
Blackstone Labs Oil Report #2 on 80 B LE (8/2011)
My MTX-L Wideband O2 Installation (8/2011)
M!ata Seat Install (5/2011)
Rear wheel cylinders, brake line bending, brake master cylinder replacement, pressure bleeding (9/2010)
1975 No-Cat Stainless Exhaust Installed (8/2010)
. You can hide this ad & support this site by upgrading to a Gold Membership ~ click here for more info

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