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MG TD TF 1500 - Carburetor - Fuel level from 'bridge'

What am I doing wrong? Seems like I have read every thread in all years of the archives, but I am still confused.

I'm having a hard time setting the float level in my TF. I set the floats to 3/8" above the inverted top of the float bowl when I rebuilt the carbs using SU Master Rebuild kits from Moss. I lowered the jet adjusting nut two turns (12 flats). The engine started the first time.

Since then I replaced the Facet 4-7 psi fuel pump with a 1.5-4 psi fuel pump. Once again, the engine started the first time.

Now I am tweeking the carb settings using Dave Braum's method of adjusting the floats and jet by measuring the fuel level in the jet.

I could not see any fuel in the jets with the floats adjusted at 3/8".

I bent the float fork a few times and still could not see fuel. I finally removed the fork and needle completely, but left the float in the bowl. I lowered the jet 1/8" below the bridge and added fuel to the float bowl until I could see fuel in the jet. At that point the top of the float was above the top of the bowl.

When I move the jet up and down, it seems that the level of fuel in the hole in the jet which surrounds the needle remains constant.

I used posts in a couple of threads in the archives to clarify the process ...
> Lew Palmer's post and Sandy Sanders's post in the thread "Help, It won't idle!"
> Dave Braun's post in the thread "Need Help Setting Float Level" (This thread describes a similar problem as mine.)

Should just I give up and return the floats to 3/8" (or 7/16" as some have suggested)? Then move the jet adjusting nut to position the jet .072" below the bridge as Len Fanelli posted in the thread "SU jets"?

And to add to the confusion. Here is a video of the exhaust of my cold engine (no choke) with the floats at 3/8" and the jet adjusting nut open 12 flats. The black specks may be oil blowing past the pistons or may be unburned gas, or some other malady (???)

My head is spinning.

Thanks for your help,

LM Cook

I used to have those black specs when I first got the car. Took it out for about a 30 mile high speed drive. Specs gone forever. Just a thought.
efh Haskell

Lonnie, I had that same jet fuel problem for a long time. Finally found the solution on a page from Barney Gaylord's 'MGA With an Attitude' page. Seems that nobody bothers to tell you to lower the jet. Don't try to see the fuel through the small hole that the needle goes through. Uncouple the jet return spring on the side of the carburetor and lower the jet until it's at the same height as the fuel. Then take an instrument and measure the distance from the bridge down to the top of the jet. It works that way. I had spent hours trying to peer through that little hole. Bud
Bud Krueger

I used Dave Braun's method to set the fuel level in the jets and it is not that difficult to see the fuel level from above with a flashlight. You will see when the fuel moved up and down by bending the forks in the float chamber a small amount at a time, and as he says "it may not be where the WSM states" 3/8" bar dia. [for a TD]
Although that is a good place to start.
I Measured the depth of the Jet in the housing below the bridge with a electronic caliper depth gauge, and set it to his 0.160" +/- 0.040" just make sure both carbs are set the same here, within 0.010" of each other for fuel height. Adjust the fuel height to match this.
Then - set the Jet height to 0.070" below the bridge and start the engine & the balancing procedure.

The black spots that come out of my exhaust on my TD I am sure is soot in the silencer and pipes. - [from where it was running too rich]. That will continue to come out for quite a while even after you set everything up correctly. The condensation in the exhaust system will pull it through.

R D Jones

Dave usually responds to these, but his method was easy to do, takes a bit of fiddling to get the two floats exactly at the same level, but it is much more precise than the bar method that is in my SU tune-up kit.
Bruce Cunha

I should have responded sooner, but first it was the flu on the couch, and then the aviation gig, and then the clutch and gearbox install in a friend's TB.

But... I use a 7/16th bar when I set up the H series. I like a little lower level of fuel in the jet. Then, carefully lower the jet until it is level with the fuel. Measure the jet height with a dial caliper. It is more important to have the heights close to each other than exactly at a certain level.

Caveats: The jet is similar to a capillary tube, as such, it is sensitive to fuel movements caused by the jet movement. You really have to be patient to allow it to settle in. Tapping on the float bowl helps. Second, the recent fuels available up here in the Midwest seem to favor a lower fuel level. I've been getting better running on the lower side of the reading. Third, there are an incredible variety in the mating of the float bowls to the carb bodies, and that is why this method works a bit better than simply sticking with the bar. However, the bar can work very well on a lot of cars, especially with the stock fiber washers.

Hope this helps,
Dave Braun

I am no nearer to setting my float bowl level at .16" below the bridge than I was two days ago. I don't know if I misunderstand the proceedure, if my carbs have bastard parts, or if I am just a klutz.

Any additional suggestions of where I should start to look for problems (including looking at my technique)?

The rear carb is especally frustrating. I have bent the float fork just about as far as I can, but I can only raise the level in the bridge to about 0.275" below. I can get the level of the front carb to the target 0.160". Even so, I am using a lot of SWAG to determine the level of fuel in the front bridge.

The fuel level in the bridge of the rear carb does not fluctuate as easily as the front carb.

I waited thirty minutes for the levels of the bowl and bridge to equalize. And I pounded on everything that I could find, as Dave suggested. FYI - I use 90-octane non-ethanol gas blended for central Florida weather.

1) Freshly rebuilt carbs installed
2) Float fork near max adjustment - rear carb (it looks like the tips of the fork touch the lid, but they don't)
3) Float bowl and bridge - rear carb, with jet lowered.

I must get the car running before this Saturday so that I can attend our club's "Lift Day" at a member's shop about 40 miles from my house.

Thanks for your continued understanding and help,


LM Cook

My 1932 MG originally had a bottom feed float chamber that simply fell to pieces. These are extremely rare so I had to replace it with one from a Morris 8 which is similar to the TF's. Being a non standard set up I had horrendous problems in setting the fuel level. Eventually I lashed up a bar above the float chamber, without the lid, and used a bolt to push the float down until I got the level at the jet correct. I measured the depth of the float and compared this with the the lid/fork. On my "bitsa" carb I had to put a washer under the needle valve body. I also found it difficult to actually see the fuel meniscus in the jet and found a good method was to use a sliver of a paper tissue as a test strip. It was easy to see if the fuel was above the jet by a wet mark on the tissue.

Jan T
J Targosz

I agree it is a little difficult to get this right but please persevere - it can be done.

As Dave Braun said above Quote "I like a little lower level of fuel in the jet. Then, carefully lower the jet until it is level with the fuel. Measure the jet height with a dial caliper. It is more important to have the heights close to each other than exactly at a certain level." Unquote.

Let the fuel settle in the carb once you have the ignition on - then turn off the ignition and remove the carb top & piston. Using the screw nut and looking down from the top at the jet - lower the jet till you see the fuel level appear. You can then measure the depth of the jet from the bridge with the depth gauge of your caliper. If it is too deep then you would bend the forks up and we are talking thou here, so it is not much. If the forks seem to be hitting the lid of the bowl top - straighten them out a tad. To lower the tips. The important part is where it contacts the float.

You can keep doing this check till the depth of the jet from the bridge is correct and make the other carb jet the same. Within 0.010" of each other.

As Dave said - give the fuel and the pump a little time to settle before taking the measurement and repeat it several times to ensure it is as it should be.

It does work and the only trouble I found was it made my back ache bending over the bloody carbs for a long time. :)

Note how strait the forks are in this picture from the WSM - They should not contact the top in any way.

R D Jones

I may have a mis-match problem like Jan T.

So, what do I need to change to get back on the road?

I removed both lids to the float bowls and removed the brass floats. Both floats look exactly the same. No liquid inside either of them. I didn't weigh them because I don't have a scale or balance that will measure that low weight.

I lowered both jets to 0.16" below the bridge. Then I added gasoline to each bowl until the fuel in the bridge was roughly level with the jets.

With the fuel levels at the bridge equal on both carburetors, I measured the level of fuel in the float bowls ---
> The fuel level in the rear float bowl is 0.549" below the top rim of the bowl.
> The fuel level in the front float bowl is 0.741" below the rim.

That's 3/16" difference. I can't bend the fork for the rear float enough to raise the float to produce the 0.549" level in the bowl.

What are my alternatives?

1) Add weight to the rear float to sink it lower so that the fuel level will be higher without bending the fork? I could set some fender washers on top of the float to test the theory. If it works, then I could solder some brass washers to the bottom of the float, or just leave the loose washers sitting on top and hope that they don't bind againt the center float guide. (I may have just solved my problem!)

2) Find a taller float? Dream on.

3) Find a different rear float bowl? Another dream.

4) Lower the fuel level at the bridge of the front carb to match the level at the rear carb?

Seems like #1 and #4 are the least difficult.

With option #4, will the engine starve for fuel at higher RPMs and will it require a higher idle speed?

Jan - I touch a pick to the jet to see if fuel covers it. The pick breaks the surface tension and changes the pattern of the reflected light.

RD - I bent my rear fork as in your diagram. I originally adjusted it for 3/8" drop as in the diagram. But 3/8" is way to large for my rear carb. As noted I believe that the rear float bowl needs to be about 3/16" higher than normal. The forks don't touch the lid of the float bowl. I can straighten them, but that won't raise the fuel level. And yes, my back is killing me, too!!

I'm wide open to any suggestions to help me fire up this beast and hit the road.

LM Cook

Are both your carbs the same model SU?
They should be a matched pair. Do you have the float bowls on the carbs so that the shaft in the middle of the float is vertical? They mount at an angle to make that happen. Left and right handed.
I can't think what would cause your issue unless the carb bowls are not the same model.
Both float bowls need to be identical but the base part that connects to the carb body is set for left and right to get the float bowl in the vertical plain.


R D Jones

Rod -

I may have become a "DPO" with my brainstorm to fix my fuel level problem. Trying to cut corners with duct tape and bailing wire to get back on the road!

But if I don't find the problem, then I may go ahead and try the DPO fix. It's easily reversable. Then continue to research the real problem.

I'll double check the fuel levels in the bowls required to produce the desired 0.16" level below the bridge before I proceed.

I may have made a mistake when I reassembled the carbs after I rebuilt them but I don't know where. I followed Lawrie Alexander's video step by step as I reassembled, plus confirmed with the WSM. I believe that I put all gaskets and glans in the right place. The rear jet has a little more resistance to movement than the front. Joe Curto rebushed the bodies for throttle shafts and installed the shafts. I don't think that could have changed the flow between the rear float and the jet.

Today, I sat a level across the throttle bodies - they are very nearly level with each other. I sat a level across the float bowls and they are very nearly level, too. I don't know a way to check the relative positions of the ports between the float bowls and throttle bodies while the carbs are on the engine.

I removed the rear float bowl and shot carb cleaner through the port that goes to the throttle body. It is open and clean. Reattached and confirmed no leaks.

Yes, the shafts in the float bowls are vertical. The bowls are in the correct front / rear position, mirror images of each other. See my pics above.

Carb numbers:
AUC 6020 = Casting number on both throttle bodies
6030 = Engraved number after casting on front throttle body
6021 = Engraved number after casting on rear throttle body
3495H = Casting on front float bowl
3496 = Casting on rear float bowl
No numbers on floats.

New jets from the SU kits.

Still searching for answers.

LM Cook

Do you have the segmented washers under the float bowl vent pipes banjo, or the float bowl banjo upside down? If not, you will not vent your float bowl and there will be no atmospheric pressure to run the fuel to the bridge.

The banjo larger hole has to be down, towards the float bowl.

Dave Braun

I don't think the weight of the float affects the fuel level since when the fuel is turned off the fork is actually pushing the float down. I originally didn't appreciate this and tried to lower the float by weighting it will solder. This reduced the pressure on the fork and the carb flooded.

Jan T
J Targosz

This thread was discussed between 16/03/2014 and 19/03/2014

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