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MG TD TF 1500 - Unbalanceable carburetors

Yesterday I ran into a strange set of problems while trying to tune up Lazarus' carburetors.
Lazarus, my '52 TD #10855, has been getting a lot of tweaking lately for the sake of vapor lock issues. A couple of days ago I found that I needed to have the choke pulled out a bit to have him run properly. Yesterday I pulled off the air intake manifold to do the classic 'lift the piston' tuning. Ran into a puzzler. Any lifting of the front carburetor piston would cause the engine to die. No matter how many flats the adjusting nut was turned. The rear carburetor has just the opposite condition - No matter how high I lifted the piston it had virtually no effect on the engine speed.
I decided that it was time to apply a couple of rebuilding kits and ordered them. While doing this I was attempting to retrieve a thread about the intake manifold's balancing device, the intake restrictor. It looks as if that may be my problem.
A number of years ago I installed an MGB power brake system into Lazarus. I tapped the intake manifold to get the vacuum. Lately I've noticed some odd braking effects. I'll let you know what I find. I may not need the rebuilding kits after all. Thank you to Erik who led me to that old thread. Bud
Bud Krueger

sounds like your rear carb has an air leak or no fuel to me
William Revit

William, you hit it on the nose. The rear carburetor is doing nothing. With the engine running I removed the rear chamber assembly, including the piston. Had no effect on the engine's running. Now to figure out how that happens.
My first move was to remove my vacuum tap from the intake manifold. The screw for the restrictor is still in place. A plug in the tap hole accomplished nothing.

Carburetor rebuilding kits should be here in a couple of days.

Thanks for your diagnosis. Bud
Bud Krueger

Just finished rebuilding both carburetors and shut down the power brake vacuum. Symptoms are still there. Rear carburetor has no effect except to provide air through the manifold. Front carburetor powers the engine and dies immediately from lifting the piston.
Not sure where to go next. Stealing too much family time. Have a house that we just bought a few months ago. Frustrating. Bud
Bud Krueger

I had that problem and it turned out to be the float valve. It seemed to be fine on examination, but apparently it was not. Just replace. Also check the fuel intake screen. They can get blocked by very fine material that can be hard to see. Hold up to the light to check.

Jim
J Barry

Jim, I have to say that the screen(s) looked finer than I recall they did in the past. May be worth giving a shot at trying sans filter. Bud
Bud Krueger

Tried it, Jim. Removed filter screens from both covers. No effect.
Both jet/needle systems were replaced with new parts.
May try swapping both chamber assemblies.
After that I'll swap positions of the carburetors.

So frustrating. Committed to be in Veteran's Day parade on 11/11. Bud
Bud Krueger

Bud, what about a leak in the booster brake? That would cause a constant vacuum on the carb that is near the vacuum fitting.

It also could account for the odd braking.
Bruce Cunha

You’ve made a lot of changes chasing vapor lock and tuning for the parade. Probably need to return to a normal base line and adjust for best performance.

Can you borrow an intake manifold from a club member? That will eliminate the possibility of a faulty baffle. Install it with new gaskets.

Go ahead and rebuild the carbs to be safe. If not, clean, inspect, and adjust floats, and initial jet setup to WSM specs.

Shine a light through the throttle opening to see if the throttle disks aren’t closing, leaking around the sides, or sticking. You may be able to improve by adjusting the springs on the ends of the shafts. The springs can move the shafts and throttle disks if too tight or too close to the throttle bodies. Doubt that you can get new shafts fitted before the parade.

Install carbs with new carb gaskets at manifold.

Check and adjust plugs, points, timing, and valve lash. Finally balance and adjust idle mixture.

Hope this eliminates the gremlins and lets you and Lazarus be the best team in the parade.

Lonnie
TF681
TF7211
LM Cook

Blocked jet?
J Barry

Hi guys, Bruce, I removed the vacuum tap from the manifold and plugged the opening. That was my first step. Probing into the manifold seems to indicate that the baffle is still in place. The screw is still there on the underside. Intake manifolds are as rare as birddoo in a coo-coo clock. Moss shows one on their cylinder head (page 8), but doesn't even show a price tag.

Lonnie, the Moss rebuild kit replaces all of the internal pieces of the carburetor. I even replaced my Grose jets with Vitons. Odd that this happened while doing the anti-vapor lock settings from Len's postings.

I'll try swapping the carburetors, but the float bowls have to stay where they are. I'm order new throttle disks. The present ones show some daylight.

Not looking forward to having to walk 3 miles in the parade (and back). This'll be Lazarus' third missed parade since 1995.

Bud
Bud Krueger

Have you checked the passageway from the bottom of the float chamber to the carb body is clear and not blocked/partially blocked?
B W Wood

Hi Bud,
when you take out the discs measure the carb bore at the spindle level.
It the carbs have been over tightened on a thicker gasket the bore can go oval, If only slight it can be pressed true.Ant light seen will make carbs impossible to tune,
Many years ago I helped club members with distorted Triple M zinc bodied carbs,They tended to crack when squeezed.
I bored 0.010" and made oversize discs, even 50 years ago parts were hard to find.
Ray TD 2884
Ray Lee

Check the vent pipe to the rear carb is free. I once had the same issue, and it turned out a mudwasp had deposited a lump of mud about 1/2" from the end of the breather pipe - took a poke with a wire and blow with an airline to clear it.

Tony
The Classic Workshop
A L SLATTERY

Agreed, Tony - those bar-stewards get into every crevice known to man : air lines, pressure washer nozzles, spare parts in the shed ... You name it. John.
J P Hall

Blown head gasket?
Mark Strang

I FOUND THE CAUSE --- Idiocy on my part. Throughout this battle I have been on the carburetor side of the car. Except for the time that I spent on a bench watching Lawrie Alexander's video while rebuilding both carburetors. Yesterday I closed the right side of the bonnet to get a tool from the left side of the tool box. Staring at me was the naked top of the rear spark plug. Just before all of this happened I was checking the spark plugs. Looks as if I FORGOT TO RECONNECT THE SPARK PLUG CONNECTOR TO PLUG #4.

I should have realized that it was something like that since it came on so abruptly. It came on abruptly while Lazarus was sitting in the garage and I was messing around with Len Fanelli's solutions for beating vapor issues. In fact, I had just ordered some components that I think might possibly be a solution to the vapor/starting problem. The money that I spent under the right side of the bonnet could have solved that issue.

Thanks for all of the concerns that you folks showed for me. I really appreciate it. Now I can get Lazarus ready for next Saturday's Veteran's Day Parade. Bud
Bud Krueger

WRONG!!! It's not from the lack of spark on #4. The car will run on just the front carburetor. It gets the absolute smoothest idle when I cover the intake of the rear carburetor with a piece of duct tape. Next step is to swap the carburetor bodies. Have to leave the float bowls in place. I'll try swapping pistons and chambers first. Getting very frustrating. Anybody have a spare intake manifold I can borrow for a test?
Lazarus would probably run adequately on 1 1/2 carburetors for Saturday's parade if I have to do that. Only missed 3 Veterans Day parades since 1995. Bud

Bud Krueger

Bud,

Assuming airflow over the jet, the issue has to be fuel supply. Swapping bodies is a lot of work, and I don’t see what it will accomplish. Airflow would be compromised by loose linkage (been there, done that). Check that fuel shows in the jet and work backwards.

Jim
J Barry

Suggestions: Loosen the throttle coupling and start. Open throttle on each carb separately. Engine should rev about the same. While revving, quickly put palm of hand over carb inlet. That should make engine slow way down, and should there be gunk in the jet or a passageway may suck it through. If one carb doesn't rev, check plugs/spark and firing order. I ahve seen many plugs that looked perfect that would not spark under compression so maybe just change all 4. Check spark with a real or home-made spark tester. If you have a compression gauge, check compression to make sure not a burned valve or something. Carefully spray carb cleaner around the manifold-carb gasket and manifold to head gasket areas. There should be no change in idle speed. I also don't think switching carbs would tell much and is a lot of work. Wish I was closer and could come over and help. George
George Butz III

Grasplng at straws,do you possibly have a non-vented or a blocked vent hole in the damper cap,
Ray TF2884
Ray Lee

Bud--
Back to basics-
Covers off, pistons out--
Are both jets set to the same depth-.065" down from the bridge ---
If that's ok then turn the key on and let the pump fill the bowls---can you see fuel down in the jets, pull the choke on a bit if you have to to get a look--As long as there's fuel there and roughly equal level---ok
Turn the key off-
While the pistons are out set the butterflys by poking your finger in the piston hole and hold it against the butterfly--let the idle scew off until you can't feel the butterfly moving(fully closed) then gradually screw the screw in until you 'just' feel the butterfly start to move--same on the other carb then give them both3/4 of a turn--now check that the throttle operates both carbs equally right at the start from rest--adjust the linkage until they both start off dead equal-
Then double check that you've got the shoulder on the needles dead level with the bottom face of the pistons-
Refit the pistons/covers without the damper pistons and check that both can be lifted and dropped freely with a clunk
Refit the damper pistons(top up the oil) and recheck that they both lift freely/equally (with some resistance from the oil/piston)and drop freely right down-clunk
start it up and get to operating temp, if the idle speed needs adjusting then, adjust both carb's idle speed screws exactly the same amount until you get to the desired speed

willy
William Revit

LAZARUS HAS RISEN!!! My idea of swapping carburetors was an attempt to differentiate the cause between the carburetors and the location. It wasn't necessary, but it was a good start. My first move was to try swapping the pistons and chambers. No effect. Discovered that smoothest idle was accomplished by putting a piece of duct tape across the rear carburetor's throat. Classic tuning technique of lifting the piston 1/32" for effect was useless. It would just stall the engine. Decided to reset both carburetors to the state of just having been rebuilt following Lawrie Alexander's video, i.e., 12 flats down. Had tried my best to set the Viton needles at something between 3/8" and 7/16". Checked the choke cable linkages to match. Pulled the choke out about 1/2", turned the ignition on, and pulled the starter knob. In a moment the engine started. In about a minute I was able to push the choke in for a reasonable, though a bit lumpy, idle. Realized that I had not set both throttles to an initial position. Shut it off and set each to a piece of paper plus one turn. Started it up again and cussed myself for not having bought that BK Air Flow Meter. My Synchronizer is useless. Balanced both, best I could for an idle around 900 rpm. Both carburetors were doing their part.
Tried some tweaking with the jet nut, but they seemed happy around the 12 flat setting. Locked the throttle shafts and did some revving. All went well. Went for a short spin around the neighborhood and shut it off to cool off enough to do some measurements.
Two measurement machines, counting flats and reading the projection on a digital vernier:
Rear carburetor was down 12 flats, jet measured .054" below bridge. Fuel measured 0.223" below bridge.
Front carburetor was down 11 flats, jet at .066" below bridge. Fuel was at 0.230" below bridge.

These values virtually match Len Fanelli's settings for the optimum values.

That's where all of this started some three weeks ago. Lazarus will be ready for tomorrow's Veterans Day Parade, and for today's Marine Corps 248th birthday, Semper Fidelis. Bud
Bud Krueger

Just in time!! Congrats.

Post some pix of you and Lazarus in the parades.

Lonnie
TF681
TF7211
LM Cook

This thread was discussed between 24/10/2023 and 10/11/2023

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