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MG TD TF 1500 - Carburetor - Lean Mixture? (video)

Another carb question ... the opposite of Geoffrey M Baker's "too rich" thread.

Are the SU carbs on my '55 TF-1500 set too lean? see video link at end of this post.

What problem in the engine could cause the carbs to need a lean setting?

I adjusted them to maximize engine RPM at idle using Laurie Alexander's video. (enrichen the rear carb until RPM slows; lean the mixture until RPM slows; find happy medium. Do the same from front carb. Fine tune both carbs again.)

The engine runs best with the jet adjusting nut on the rear carb only two flats from the top (fully closed). The nut on the front carb is about four flats from the top.

The engine idles nicely at 800 RPM and performs well at speed.

But lifting the suction piston ~1/32 inch causes the engine to stumble.

Engine related repairs since I bought the car in Dec 2013:
> Replaced the 5 PSI Facet fuel pump with a 2PSI Facet pump.
> Rrebuilt the SU AUC 6020 carbs using the SU Master rebuilt kit Moss #375-518 a couple of months ago. I watched and followed Lawrie Alexander's "SU Carburettors" video at each step of the process. Everything is back in the right place and I believe that I did a good job.
> New throttle shafts and bushings
> Tried to set the floats and jets using Dave Braun's method of measuring the fuel level below the bridge of the throttle bodies. I wasn't confident with my adjustments, so I reverted to the 3/8" float drop stated in the WSM.
> Balanced the carbs with Moss's version of the Unisync. The Unisync seems to bog the engine down equally when used on either carb. Is that a problem?
> Adjusted the mixture as Lawrie instructed.
> Carb and manifold bolts appear tight. I believe that compression is OK and uniform (need to check again)
> New points, rotor, plugs, distrib cap, ign wires
> Timing approx 10 deg BTDC
> Adjusted valves
> All adustments and gaps at WSM specs

Photobucket video:

Thanks for any advice or suggestions that you can give to me.

LM Cook

It actually sounds pretty good, idles smoother than my TD. I didn't hear any spitting or hesitation. I did notice a lot more and sudden RPM drop from the rear carb, so make sure your throttle sync is correct. It seems the rear carb has more effect than the front, etc . Quick check: at idle, put your palm over the carb throat briefly. The rpm drop should be about the same for both carbs. If a lot different, the throttle sync is definitely off. Throw the unysync away. Get a piece of fuel line, put it up to your ear, the other in the carb throat and listen, hiss should be the same at idle. Or you can use a cheap stethoscope- remove the round end part and stick the tube in the carb throat. George
George Butz

And here's mine:!9417&authkey=!AIgp9bw45ilbxUs&ithint=video%2c.mp4
Geoffrey M Baker

Sorry about that, the previous link is to an older video. See this one:!9626&authkey=!ABcQlKeicq1F29A&ithint=video%2c.mp4

For some reason, it's flipped sideways, hope I don't give you all a neck crick... :)
Geoffrey M Baker

Update -

RPM fell from about 800 to about 200, then died when I put the air cleaners on. Original Volks air cleaners with original oil wetted clean mesh elements.

George - each carb lost RPM when I used the Unisync. The loss was about equal for each. I guess that's similar to putting my hand over the intake. I believe that the carbs are balanced.

The Unisync didn't seem to affect engine RPM in Lawrie Alexander's video.

Why would my carbs be so sensitive to flow restriction? Is that related to the apparant lean mixture setting?

I increased RPM to 800-900 after I installed the air filters. Engine runs good again.

Could I damage the engine with this lean mixture setting? (2 flats from top on rear carb / 4-5 flats from top on front carb)


LM Cook

If it "runs good again" drive and enjoy. If you just cleaned and oiled the filter elements, there was probably just too much oil on them at first causing restriction. Maybe drive for a hundred miles and then pull the plugs to check if you are worried. It will not run well and will hesitate/bog if too lean. BTW, the number of flats has no bearing on anything, due to differences in many things. George
George Butz

Thanks, George,

I can see how the number of flats may not necessarily be relevant.

I am still concerned about the severe drop in RPM when the air flow is restricted.

Maybe I'm just looking for a problem that doesn't exhist. I will be installing an AEF118 TF-1500 head soon to replace the 22952 TC/TD head that is currently on the engine. I am trying to tweek all systems to max performance before I install it.

Good idea about checking the plugs in a hundred miles or so.

The air filter elements were cleaned and wetted about a hundred miles ago.

BTW - I noticed in my video that the shaft connecting the carbs wasn't connected. It ran the same after I tightened the connecting clamp.

LM Cook

Contrary to George I find the unisyn quite useful as I've gotten older and the hose method I used for years doesn't work as well with my rock n roll abused ears.

First of all you shouldn't need the screw driver to lift the damper on your carbs. They have a lifting pin designed to just that. If the unisyn is killing the car, then I suggest you open up the air flow on it as you likely have it turned down so far it is restricting the air flow.

I like to start with the carb adjusting nut turned down about six flats and work from there.

The rear carb is severly lean based on the video . I'd turn down the idle a little as it sounds higher than the indicated reading on the tach and attempt to tune the carbs again.

I verify the sync of the carbs before hand with the unisyn or if you trust you hearing you can use the hose method.

MG LaVerne

Lonnie, I'm having exactly the same issue (except for black smoke). When I have the filter off, the car runs pretty well, but as soon as I put the filter on, I have to increase the airflow so much that it starts to run very unevenly...

And I'm running with the carb mix nut almost shut off on both carbs, so I have no idea why it should run rich...

I've ordered some pancake filters which should be something of an improvement, but I'm perplexed as to why there is so much of a difference between filter/no filter.
Geoffrey M Baker

> LaVerne
I'll lower the idle speed, remove the air cleaners, and adjust the mixture again. The RPM on the video is a good smooth idle when driving. I'll also increase the opening on the Unisyn and rebalance.

I get the same results when I push up the lifting pins.

I tried the hose-method once, but couldn't hear much difference. I'll try again and see if the results are the same as the Unisyn.

> George
I may have mislabled my post (lean vs rich). Sounds like you and I have the same problem. My car spit out raw black stuff at startup and idle, too. It covered anything downstream of the exaust in my garage. I rebuilt the carbs, corrected some ignition problems (points, plugs, cap, rotor, wires, wobbly-distributor head, and timing.) Then I drove it on some 30-min 40MPH runs. It hardly blows black stuff at all now.

Here is a thread that has a video of my black exhaust that was taken after most of the above was completed, but before tweaking the float level and mixture nuts. And most importantly - before the longer/faster driving runs. The thread has good comments and suggestions from forum members. The video link is in my opening post.

(Chris and webmaster - thanks for modifying the BBS software to allow us to link to posts in the archives.)

Keep in touch. I'm sure that we will get this worked out together.

LM Cook

This thread was discussed between 24/04/2014 and 25/04/2014

MG TD TF 1500 index

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