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MG TD TF 1500 - Carb puzzle, rough (terrible) idle

'53 TD. Symptom: Rough idle, lack of acceleration.

If I lift the rear carb piston about 1/8" the idle smoothes out to a purr and rev-up to 4K is rock solid. If I don't, it feels like it's running on 2 1/2 cylinders.

The float level is dead-nuts on in each carb. Both pistons seem to move at about the same rate as you raise the RPMs. Light oil in each.

It doesn't matter what I do with the mixture nut.

Should I replace the jets and pins or look for something else?

.........
MAndrus

Also.... the front carb - makes no difference if I lift the piston.

.....
MAndrus

Sounds like the piston/bell area may be worn. Did you perform the drop test?

https://www.youtube.com/watch?v=IfU47Oqq9wA
kmclemore

Check for vacuum leak. Run a propane torch around the inake manifold (propane only! not flame) with it idling, or spray starter fluid... see if it perks up.

A small air leak can ruin idle but still run decent.

Your battery has healthy voltage, right?

You can also strap a timing light to individual plug wires and observe ignition. Maybe check your plugs if you have nothing better to do.
JRN JIM

When did these symptoms start to occur?

Generally carb problems don't suddenly appear. The old adage "90%of all carb problems are electrical" is a good one.

If you have done systematic troubleshooting and have narrow the trouble to the carbs...Where is your fuel level in relation to the bridge in the carb throat? Regards, Tom
tm peterson

TM, partly my fault. I was noting a lack of expected ooomph (as much as can be expected from 54 hp). The PO had practically eliminated the rear carb from use via lopp-sided balancing. Now that the carb draws air, it's flaws are showing.

So.... I gotta fix it (them).

I had a MGB and E-Type many years ago. I've never had a carb so out of whack.

.......
MAndrus

Ahhh...I understand. So I go back to the question of fuel level in relation to the bridge? Are both carbs the same? Just curious, did you do the timed drop test with piston and dome?
Best of luck. Regards, Tom
tm peterson

You might check and ensure the needle is fully up into the piston. Marv,ct
Marvin Stuart

TM. Drop test.. about the same at 3.5 and 4 seconds. Close enough, and the first thing I checked was float levels.

Marvin. Needles are even at the flange, but I'm going to see if pushing them in a bit will help.

.....
MAndrus

Check that the jets are returning to the full up position. Disconnect the choke linkage between the carbs as it may be part of the cause. Loosen the spring clamps on the throttle shaft between the carbs and then do your testing. This way the the carbs afre acting on their own.
I would also remover the dash pots and pistons. then check the fuel level in the jets. I usually set it just below the bridge.
Some times improper thickness of seals between float chambers and carb bodies will create those issues.
Sandy
SANDY

You might take a couple of minutes and reveiw Dave Braun's instructions on SUs.
http://www.dbraun99.com/Setting_SU_Carburetors­_Rev_C.pdf

Note the technique for adjusting float bowl fuel level.
JRN JIM

JRN, very helpful.

I've got it pretty much licked. The carbs were not balanced, the choke mechanism (who designs this stuff?) was not allowing the jets to seat evenly at relaxed position, the floats were a bit off, but not too bad.

It'll do for a while, I'll check the plugs once in a while to see what's really going on and adjust accordingly.

Next Fall, I think a valve and ring job is in order though. And maybe a ring/pinion to boost the highway speed.

....
MAndrus

Jim, Thanks for sharing my write-up. MAndrus, I'm glad my instructions helped you get the balance and the choke mechanism right. Often, centering the jet properly (with no spring load on the jet and the lever disconnected) will help the choke return better.

Warmly,
dave
Dave Braun

Dave,
Thak you for providing such a wealth of information. In fact, when I turned the computer on a minute ago, it was to go to your site for a quick overview of breaking down the trans.
Jim
JRN JIM

This thread was discussed between 19/03/2015 and 21/03/2015

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