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MG TD TF 1500 - Jet Return Spring

I was out for a liesurely drive in the TD today, and noticed that when I was at idle, the idle was suddenly lumpy. When I got home, I cleaned the soot from the plugs (I run a bit rich, as my carbs probably need rebuild). I checked all the ignition connections and wires, but there was no change. I went to the carb side to look for anything obvious, and I noted what I believe is the jet return spring on the front carb just dangling from the carb. It had come out of the notch in the (jet lever?).

Now I know pretty close to beans at this point about SU carbs (I'm studying though :-) so can someone explain the effect this was having on the engine. I don't think it has been this way long, as the car always idles and runs just fine.

Thanks in advance

Larry
Larry Karpman

Larry, If the jet has dropped down you would get the effect of choke during all throttle positions and I would expect that you would be fouling at least the front two plugs if not all four. Running slightly rich all the time isn't much better than running lean. You will get cylinder wash on the rings which will accelerate bore wear and dilute your sump oil with petrol risking damage to all bearing surfaces.

LaVerne
LaVerne

Thanks LaVerne. As soon as I reconnected the spring all returned to normal. As I said I'm running at the end of carb adustent (mixture/jet nuts all the way up), so I will be rebuilding very soon.

One question I have though is this. I set the front and rear idle adjustment screws on the carbs as per the shop manual. Both equal. However, when I turn the interconnect throttle screw (the one just outboard of the front carb idle screw) it does nothing to affect the idle, no matter how much I crank it up or down. I thought this was to adjust the idle uniformly once both carbs idle screws were set the same. I cannot get the idle below 875 RPM (on my advance timing light reading). Where am I going wrong?

Thanks in advance

Larry
Larry Karpman

Larry, First thing I would do is check the float bowl level. If the floats are set too high you will most likely run rich no matter where you set the jets. As for the idle. First step is to loosen the connection joint between the two carb shafts so that both carbs will operate independently. Next check that both carbs are pulling the same amount of air. I used to be able to do this very well with a short piece of hose and listen to the hiss coming from each carb. My hearing isn't what it used to was, so now I depend on the old Unisyn. If nothing you were doing on the front carb has any affect on the engine speed it indicates to me that the rear carb is supplying all the fuel mixture for combustion or your butterfly shafts have such serious air leakage that the idle adjustment has no effect. The idle screw is just opening or closing the butterfly valve to adjust the air flow. If you can get the two carbs pulling the same amount of air then you can attempt to set a reasonable idle speed and go about adjusting the jets. Good luck.

LaVerne
LED DOWNEY

Ok, my fault. The screw that did nothing when adjusting was actually the choke adustment I guess. Perhaps the terminology "mixture throttle control" screw threw me. If I'm off base please someone correct me.

So now I'm a tad unsure as to how to set the idle, as I folowed the shop manual. According to it, it's the first step before checking air flow. I do now have a unisyn, so that's should be easy to do.

So, can I assume that:

1. Loosen the connection joint between the two carb shafts (the manual doesn't mention this, or I missed it).
2. Set each idle screw so that there is only a thin paper's width between each screw and its stop, and tighten each 1 turn.
3. Start the car and adjust the idle by turning each screw an equal amount.
4. When desired RPM is attained, tighten the connection joint, and go to the Unisyn.

Please someone tell me if I have it right or wrong.

Thanks in advance

Larry
Larry Karpman

Larry - As LaVern stated, when you adjust the individual idle screws on the carburetors (with the interconnecting linkage) you need to use something (Listen tothe his of air through each carb with a piece of tubing to the ear and the other end just at the carburetor opening, or a better method use a unisyn air flow meter) to determine when the two carburetors are pulling the same amont of air through them. When bothe carburetor is pulling the wame amount of air AND the idle is where you want it, tighten the interconnecting linkage. By the way, the other screw you were turning on th efront carburetor is the fast idle screw that increases the idle when the choke is pulled on. Cheers - Dave
David DuBois

Thanks Dave, and LaVerne.

Yes, Dave I figured the fast idle part out in short order - DUH! As for the idle, OK, the manual calls for setting the base idle first, then the air flow. So, you are telling me to set the airflow AND the idle with the Unisyn simultaneously.

I also see that the manual DOES mention undoing the connecting rod between the carbs. I must really be getting senile to miss that!!!!

Thanks all, I belive I'm good to go, and sorry for such basic questions.

Larry
Larry Karpman

Larry,

If your float level is off (too high in the bridge) your carburetors will run rich no matter how high you screw in the jet adjusting nuts. The proper setting of the float results in a fuel level that is .125 to .166 below the bridge. Since fuel seeks its own level, the height in the bowl must be adjusted so a 7/16 drill rod will slide under the adjusting arm when the float needle is closed.

The float level is important because these are constant pressure (vacuum) carburetors. The air is metered by the throttle disc, and the piston movement is determined by the air flow under the piston. The tappered needle which fits to the piston rides up and down in the jet providing a larger or smaller anulus as needed to meter the fuel.

Since the piston is damped by the dashpot (and perhaps a spring as well) the initial rush of air at a larger throttle opening causes the pressure to drop under the piston, sucking more fuel into the airstream. The piston, working against the dashpot eventually rises, seeking a steady state, and that raises the needle to a new position in the jet. With the pressure change beneath the piston the amount of fuel is no longer enriched for acceleration, but is only enriched for the smaller needle diameter at this point in the piston travel.

If the fuel level is to high in the brige, you will always be sucking too much fuel into the airstream, and no amount of jet adjustment can change that.

Once you have the float level and height of fuel at the bridge correct, you can adjust the initial jet setting. The initial setting is six flats down. But I like to check with a dial caliper by adjusting the jet surface to be .070 below the bridge. You can see that the fuel level and the jet surface both reference off the bridge in the carb. From there you will only be adjusting the jet vs. bridge height about .010 total to get the correct mixture setting.

warmly,
dave

p.s. Another explanation of SU fuel bowl operation follows (mostly for MGBs but applicable to our cars as well):

The Mystery of SU Carbuerettors Finally Explained!

Posted 29 August 2004 at 15:32:51 UK time
Rick Ingram, central Illinois, USA, mowog1@aol.com


The basic secret of SU carb function is that inside each SU carb are thousands of tiny gnomes; each with a small bucket. As you open the throttle, more of these gnomes are allowed out of their house and into the float bowl, where they fill the buckets and climb up the carb's passages to the intake, where they empty their buckets into the air stream.

But, if you don't drive the car for a while, bad things can happen. Tiny bats take up residence in the chambers of the carb, and before long the passages are plugged up with guano. This creates a gnome traffic jam, and so not enough bucketfuls of fuel can get to the engine. If it gets bad enough, the gnomes simply give up and go take a nap. The engine won't run at all at this point. Sometimes you'll have a single dedicated gnome still on the job, which is why the car will occasionally fire as the gnome tosses his lone bucket load down the intake.

There has been some research into using tiny dwarves in SU carburettors. The advantage is that unlike gnomes, dwarves are miners and can often reopen a clogged passage. Unfortunately, dwarves have a
natural fear of earthquakes, as any miner should. In recent tests, the engine vibrations caused the dwarves to evacuate the MGB vehicle and make a beeline for the nearest BMW dealership. Sadly, BMW's are fuel injected and so the poor dwarves met an unfortunate end in the rollers of a Bosch fuel pump.

Other carb problems can also occur. If the level of fuel in the float bowl rises too high, it will wipe out the Section 8 gnome housing in the lower parts of the carb. The more affluent gnomes build their homes in the diaphragm chamber, and so are unaffected.
This is why the car is said to be "running rich."

If the fuel bowl level drops, then the gnomes have to walk farther to get a bucketful of fuel. This means less fuel gets to the engine. Because the gnomes get quite a workout from this additional distance, this condition is known as "running lean."

The use of the device known only as the 'choke' has finally been banned by PETG (People for the Ethical Treatment of Gnomes) and replaced by a new carb circuit that simply allows more gnomes to carry fuel at once when the engine needs to start or warm up. In the interests of decorum, I prefer not to explain how the 'choke' operated. You would rather not know anyway.

So, that's how an SU carburetor works. You may wish to join us here next week for electricity 101, or "How your car creates cold fusion inside the starter, and why the government doesn't want you to know about it." This class can be found under the heading "Never Let the Smoke Out!"

-Author Unknown
Dave Braun

Thanks Dave. I'm going to have to get a lot smarter on SUs before I check and adjust the floats without adult supervision. Too bad about the gnomes :-))

Larry
Larry Karpman

Dave, how did you come up with .125"-.166"? Len
Len Fanelli

"As I said I'm running at the end of carb adustent (mixture/jet nuts all the way up)"

I had that problem, as others did, and I changed the needles from whatever it is to EW, available at Burlen. This allowed easy regulation of the mixture, at about the middle of the jet nut.

Denis
1950 TD, 1 1/4" SU
Denis L Baggi

Len,

Personal experience. When I was younger, I always used an eighth inch down. Then, I bought an SU book and it suggested 1/6th of an inch? Who knows, except it is very close to 4mm. I wrote that down in my notes (1/6) and everytime I want to make a decimal point of it, I end up with .166

If you screw the jet all the way up, it should be flush with the bridge. If you then let the jet down until you are flush with the fuel level, you can measure the amount the jet dropped from the outside of the carburetor.

I've tried these settings, and find that if I'm somewhere in that range, I can adjust my mixtures. If I'm higher, I can't get them lean enough, even with the jet adjustment bottomed out. I'm reluctant to go lower, because I don't want to run too lean!

I hope this is more understandable. If any one has different ideas, I would love to hear them.

dave
Dave Braun

Denis: Both Moss and Burlen list the EM needle as the rich needle. No EW. I assume that's theone you put in?

Cheers

Larry
Larry Karpman

Larry,

There is no rhyme or reason to the needle lettering scheme.

The ES is the standard needle, I the AP the lean needle, and the EM is indeed the rich needle. The EW which Denis is using would be even leaner than the AP. Since I'm a flatlander (as you are), I can only surmise that Denis is running his at a higher altitude. Also, the fact that he couldn't get the standard needle to work with the jet all the way up indicates that he needed to go leaner, if float level wasn't a problem on his carbs.

Fletcher Millmore once said here (and I saved it because I found it so useful): For 1 1/4 carbs, only about the first 8 steps are used, 10 for 1 1/2". .100 & .125 needles are longer as they are used in 1 3/4 & 2" carbs. Idle mixture is controlled by the first 2 steps, which are around .089 (1) and .088 (2) for all needles. Normal running by steps 3,4,5. Above this is only for acceleration and high speed. Any improvement in "normal" fuel consumption will be obtained by using a fatter needle at 3-5. Be extremely careful about using a fatter (leaner) needle from 6 up, as this is how you cook your engine.
But: it is a little crazy to experiment with needle selection if the carbs are not already perfect, since any change in the carb will screw up the results of your experiment. A common source of trouble is that the vac chamber springs have been stretched by dunces, which causes rich mixtures above idle and thereby poor consumption. FRM 11/11/2005

Notice from six up, Denis' EW needles start to approach the conventional 'lean' AP needle.


See Graph attached, in order EM ES AP EW

dave


Dave Braun

Dave: I was just curious, as neither Moss nor Burlen listed the EW needle for 1 1/4 SU. Perhaps I didn't check Burlen's site further.
Yes I'm a flatlander, so I assumed the standard needle was the way to go.

Cheers

Larry
Larry Karpman

"Denis: Both Moss and Burlen list the EM needle as the rich needle. No EW. I assume that's the one you put in?"

No, I put in EW as I wrote. I also have some EU which I have not tried. EW was suggested to me by Burlen. On this same board some years ago someone had the same problem, I suggested EW, he tried and thanked me. So I assume it's a common problem, perhaps due to moder fuel, and there is a whole bunch you can try if you look at the Burlen catalog (not Moss).

If you need more info I can look again. But I am sure it's EW.

Denis
Denis L Baggi

EU would be slightly richer between station three and seven (with a slightly smoother curve), than the EW. It matches the EW above and below that point.

dave
Dave Braun

Thanks Dave. I am sure you are right, however I fail to understand why EW allows me - and others - to get a leaner mixture than with the standard needle - forgot what that was. Again, I have the problem that I could not get a mixture lean enough with the jet nut all the way up, and that guy at Burlen suggested that. Incidentally I remember I also put in new jets after dismantling the carburetters.

Maybe I could have reached the same by having the needle slightly below flush?

Thanks for whoever cares to comment. Important thing is that the car runs, not too rich, not too lean.

Denis
Denis L Baggi

Denis,

What altitude are you running?

dave
Dave Braun

This thread was discussed between 26/10/2007 and 01/11/2007

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