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MG TD TF 1500 - Unisyn

Kind of a quirky question, but still trying to determine which of my carbs is the bad boy that is causing idle to be at 2,300 screws backed off.When I try to move the red floater from the bottom on the one carb, the unisyn wheel has to be almost closed hardly any airspace. On the other, to get the red float off the top, I have to open up the Unisyn wheel so I can see maybe 3/16 or quarter of inch space. QUESTION; which of these is closest to what you have experienced? It has been 25 years since I did it on a normal operating car - this one - and I forget.
Keith
K E MURPHY

Keith, it would appear to me that the first carburetor that you mention has way too much air going in and the latter one has virtually none. If you put the heel of your against the opening of the second carburetor, does the engine continue running? If you do the same with the first, does it continue running? I'm guessing that the answer to the first question is 'yes'. Loosen one of the couplings and take that carburetor off of the car. Look thru its bore at a bright source and find out where the air leak is. Then fix it. Bud
Bud Krueger

Check to see if one butterfly is sticking open...
Geoffrey M Baker

Did you rule out the butterfly's being installed backwards that was suggested in your other posts about this? If not, you really need to check that and I would suggest adding to the original post rather than starting new ones. I speak from experience on this one.
Ed
efh Haskell

Butterflys close ok. Bud - to be clear - the one carb which needs no air and I turn the uni all the way down - when I put my hand over it, it still runs unless I really block off all air. The second carb which wants to suck my hand in, yes it dies when I put my hand or the somewhat closed unisyn that the other runs on. So which carb should I be taking off, the one that is getting air and continues to run with my hand over it? I think that is the correct answer. fyi someone else said to tighten the manifold and carb nuts. I was able to get 1 or 2 flats tighter on all, but did not cure the issue - they weren't really that loose. Thanks to all who are trying to help me. I will be at the Gof and maybe can get some advice if I don't solve.
K E MURPHY

The one that needs no air is the problem carb.

Jim
James Neel

Thanks, I will remove it and look to see whats up.
K E MURPHY

Before you remove the carb...have a look at the number 3 and 4 spark plug wires and verify they haven't been reversed.
MG LaVerne

The carb that causes the engine to die when you put your hand over it is the bad one. That's the one that is leaking air through the butterfly valve. I had the same problem.

Jim
J Barry

Yes, the one that continues to run when you put your hand over it is the one to check out. (After you do LED's check). Bud
Bud Krueger

Thanks for the tip LaVerne, I switched the wires and it ran rougher than a cob as we used to say in Indiana, so the plug wires are ok. It doesn't run bad at rpms 3,000 and over and on the road, its just the ding danged idle. That carb has to be getting air somewhere - so I will remove it, look at it, fly to Vatican and have it blessed as a last resort.
K E MURPHY

I think SU stood for Satan's Union so taking it to the Pope is just going to make things worse...
Geoffrey M Baker

Still no one has mentioned a major vacuum leak, although it was suggested as a possibility. And I still want to know whether the intake manifold baffle is still in place.
Lew Palmer

In the other post didn't someone suggest the spray of starter fluid around the intake manifold trick? I had that problem as well. The trick showed it instantly! Easy fix too! I think this is what Lew above is referring to.
Ed
efh Haskell

Hi Keith -

Well, we are on your third post on this same problem - and yet still no real progress it seems.

I looked over your other posts and wonder about the following:

You mention that you can push down on your throttle and get the engine to slow down. I would disconnect your pedal shaft push rod from the rear carb throttle shaft - that removes that assembly as an issue.

I would double check both disks again and ensure that they are placed correctly in the shafts - you should not see a beveled edge at either top or bottom - it almost sounds as if one is in backwards. They seem to close when in backwards - please double check

Spray the starting fluid at the gasket between the head and intake - see what you get.

ensure that both disks rotate at the same degree as you move your throttle shaft up. Make sure your carb to filter gaskets are not blocking the air holes on the carb flanges.

Do you have good fuel flow in both carbs - you mentioned that you just put them back on the car with no work - is your one carb getting fuel as well as air?

I still think you have a throttle shaft/disk issue

DLD

Keith, have you done any of the things that have been suggested, e.g., testing with starter fluid or other flammable spray? Bud
Bud Krueger

Done most of the things mentioned - starter fluid etc and I was able to tighten manifold nuts a bit but to no avail. Now I am going to look at the butterflies again. Now the manifold baffle is something new to me!! I never heard of that, and will make sure I have one by the carb that is getting air and wont die. I saw an image from an old archive to even know what it looks like. Stay tuned.
Thanks
K E MURPHY

Keith. I would again use the suggested starter fluid test particularly around the throttle spindles. If they're sucking in air they will need to be rebushed. Then I'd lift the pistons & allow them to fall, listening for the desirable "clunk" sound. Both carby's should sound the same. I'd then remove the suction chamber & piston rod & check the movement of the piston rod in the suction chamber. Block the transfer hole. With the suction chamber inverted allow it to drop off the piston. The time taken to fall should be between 3 & 5 seconds for an 1 1/4" SU. If it were me I'd then remove the problematic carby, strip it down, clean it & replace all the gaskets. There are a couple of great books on SU's available at any library. They will guide you through these tests & also explain how to easily centre the jet. (Photocopy the relevant pages so you don't upset the librarian).

During the teardown I'd take particular note of the relative positions of the cork & brass jet gland (seal) washers. It is very easy to reverse their positions. The other item that is incredibly easy to install backwards is the throttle disc. (Just about everybody comes back to this & rightly so). The disc may well appear to be perfectly closed even when it is in backwards! I'd also take the flange end of the body, wipe it with chalk & pass it over some fine sandpaper on a flat piece of stone a few times & check to ensure that it IS perfectly flat. If not, then sand it until it is. Take care to reassemble everything in the correct order. Stick it back on the car. I would again use the suggested starter fluid test. If you're happy then don't stop there. Remove the other carby & give it the same treatment.
Cheers
Peter TD 5801
P Hehir

Thanks Peter, Have not done the fluid around the spindles and will look at the butterflies. As I understand it, when I look down the bore with the aircleaners off, I should NOT see the beveled edge - in other words the bevel is toward the back side - the side toward the engine when closed.
K E MURPHY

This thread was discussed between 06/06/2014 and 07/06/2014

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