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MG TD TF 1500 - Carb fuel level

I am following the SU Carburetor Tuning instructions from Dave Braun.
On setting the fuellevel by measuring the depth below the bridge my range of measurements when I try to repaet them, is far wider than Daves suggested (step 6) 0.010 difference between the two carbs.
I set the float lever at a certain value, pump on and moving the jet up and down gently. I find it very hard to see a clear distinctive moment to define when the jet is at the top of the fuellevel. Repeating such measurements give me differences in mm.
Playing with light above and magnifying glasses did not really improve.
What I than tried is to put the jet in highest position, to dry it from fuel with a cloth and than to slowly move the jet down.
Not really satisfied with the results of the way I followed the instructions so far.
What am I doing wrong??
Here is a collection of pictures.

Huib Bruijstens

Danke, Huib. I am so happy to hear that I am not the only one who can't handle that technique. I've tried it a number of different ways, but I can not make this measurement to the fuel. Bud
Bud Krueger


I use a 6 inch vernier caliper depth gauge to set the jet position. Then disconnect the coil so I can leave the ignition on for a prolonged period of time. This protects the points and coil but has nothing to do with fuel level.

Next disconnect the fuel pump and adjust the float position. Then while looking down the jet touch the fuel pump feed wire to the pump and watch the fuel rise. Repeat and repeat until correct. When I am close I add or delete a washer from the float valve to get it just right.

Some times it takes an hour or so to do both carbs. Good luck

Jim Haskins

1953 TD
J. M. Haskins

Thanks for the confession Bud.
Jim, I like your idea of the pump feed wire to see the rising level. I will try that.
But when you say: "adjust the float position" what do you mean?
Why do you deal with washers under the float valve instead of bending the lever?
And the main question: what about your measurements with the gauge if you repeat this procedure for the same level? How much would they differ?
And can you reach the difference between the two carbs within +/- 0.010 as Dave Braun states?
thanks and greetings, Huib
Huib Bruijstens

Why is it that some fellows have the magic touch on tuning carbs and some of us, (me included), take forever to get them right? Float bowl settings are a pain, putting it nicely. For balance, I have air flow meters I use, that's the easy part. PJ
P Jennings

Huib, I think the reference to changing washers indicates that the writer is using GroseJet float valves. Bud
Bud Krueger

I'm sorry my instructions are driving people crazy. I didn't mean for that to be the case. Please try out the process and modify it if you find you are taking more than 10 minutes to take the reading.

The fuel does tend to bounce when you lower the jet to see what the fuel level is. It does become a bit of a feel thing.

I really believe that 3/8 of an inch down the jet for the fuel level (where the 7/16 float measurement tends to place it... or so depending on wear and washer thicknesses and other factors) is too low.

I once read something to the effect that if the cars never cornered ideal fuel level would be almost even with the jet head. I found the 1/6 inch down figure in an old manual and have been working on that basis since. Note that the fuel level I'm asking for is between .120 and .200 down; with about .160 about right. That's a pretty wide variance, and although I think .120 would be abit too high, it works, and both are a far cry from the .375 that is recommended with the regular float setting.

The important thing is that the carbs are both about the same so the same depth down on the jet for the mixture setting will work. I was probably too strict with the .010 variance, .020 might work as well.

So strive for something higher than .375... even a quarter inch would be better... and make them as even as possible.

1. allow the pump to fill the bowls.

2. looking down with the piston and needles removed, slowly lower the jet until you see the fuel. As a check you can wiggle your tickler pin on your float bowl lid or remove the lid and just bob the float. If you are in the right ball park, very little movement of the float should flood the jet above where it is placed. Measure that depth.

I hope this helps,
Dave Braun

Sorry, I should have explained myself more clearly. First, I do not claim to by an SU expert. This info is just the accumulated experience of countless weekends spent tinkering with the carbs.

First, by adjust the float position I mean bend the lever arm that actuates the float valve (see WSM instructions). Second, my approach is to eliminate as many variables between the carbs as possible. When I first disassembled my carbs it looked like the PO had let his dog chew on the float levers. I worked on them until they were as near to identical as possible. Regarding the lever be certain that the ears where the pivot pin passes are parallel so there is no binding when the lever moves up and down.

Also be certain that the float chamber is mounted to the carb body in a perfectly vertical position. The float chamber and mounting arm are designed to achieve this position.

Last, I have difficulty making the bend in the lever with enough precision to achieve 0.010 difference. I install the float valves with two washers to start. To get my final level I can remove one washer or add one to move the valve up or down slightly. Can I achieve 0.010, who knows? An LED light and my 71 year old eyes are my best gauge. I will say that when you get them just right you will not that the engine idles, pulls under load, and responds to throttle in a very pleasing way. Good luck.

Best regards,

Jim Haskins

P.S. Just saw Bud's comment. He is correct.
J. M. Haskins

One more thing. Regarding GroseJets. There has been much comment about recently manufactured GroseJets being of inferior quality. Last spring I encountered GroseJets stuck open by contamination. Almost like glue. I blame this on the 10% ethanol fuel sold here. I also had carb problems on six other small gas engines. All required carb cleaning. If you do not have access to pure gasoline you may be wise to stay with the all brass float valves. I changes back to pure gasoline and this year no problems.


Jim Haskins
J. M. Haskins

Jim Haskins has touched on a point that should be the first step in adjusting the float level. The float chamber should be in a perfectly vertical position. This is achieved by positioning the float chamber at a right angle to the carb body. If the "right angle" position does not give you a vertical float bowl, you have the wrong float chamber. Unless you start with the float chamber in this position, all the measuring and adjustments will not give you the proper fuel level.


George Raham [TD4224]

Thanks for the remarks and encouragement. I went on today and used a camera to record what is happening at the jets top when it is moving down. The fuellevel is still at the WSM position, so quite down. It is on youtube.
Half way the video you can see the reflection of the lamp in the fuel within the jet that gets bigger, demonstrating the upcoming level.
Then it starts flooding over the edge of the jettop and then it completely floods.
I find it still hard to unambiguously define what the level is. Which point do you pick?

I had put in new Grosse jets but since one kept leaking, I replaced them again by the original ones.

Trying to raise the level made me make the lever almost flat.
The rear carb now reads 3.9 mm (measured after the fuellevel within the jet reached the top, just before flooding) front carb still reads 5.1 mm so I need to adjust that again.

Still not an easy operation for me but yes, a challenging one.
greetings, Huib

Huib Bruijstens

This thread was discussed between 29/03/2012 and 31/03/2012

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