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MG TD TF 1500 - Am I running Too Lean?

With my newly rebuilt carbs, the engine idles like a dream, all the way down to 850 rpms, even though I set it at 950 and has a smooth progression up to about 3500 rpms. Once hitting 3500 rpms it seems like it's asking for more fuel as I can feel a very slight flat spot. Someone else might not get this impression, but I've never encountered this before, this engine has always kept on climbing with no hesitation. Nothing has been changed on the timing only the carbs were changed. So, am I running too lean OR?? Give it your best shot, what do you think? PJ

What damper oil did you use?
G Evans

Try pulling the choke out at 3500 - will richen it at the moment its asking for more. have you checked the level in the float bowls?

I used 20W oil in the dampers, do you think I should go with a lighter oil? When at idle and taking off in gear it revs right up without hesitation, it's just around 3500 the flat spot kicks in, it's not much just enough for me to feel it. PJ

The go to oil for your carby dampers is ATF, be surprised if that cures your problem if your using 20W. Maybe confirm you dont have a manifold leak.
G Evans

Drive for a couple of miles at3-4000rpm or so, do a clean cut by turning off the ignition, and pull off the road. Remove all 4 plugs and examine their colour.
(It's a good idea to take a spare plug since the chances of breaking one rise exponentially when doing this). If they're too light-coloured (ie white), you're too lean. Pulling out the choke a little will also give you a clue, as suggested.
And do you have the right needles? It would be interesting to know if the "rich" needle made a difference to the flat spot due to it's different profile. Does anyone have any experience with the effect of std-lean rich needles?
T W Moore


I second the use of ATF Automatic Transmission Fluid for the dash pots. Has worked for my the last 13 years, Probably not the cause of the flat spot. I know you had a pro rebuild but it would not hurt to check that the needles are set flush with the bottom of the piston in each dash pot. Anyone can make a mistake. Even infallible me.


Jim Haskins 1953 TD
J. M. Haskins

Thanks fellas, Now I have some work to do, I will get back with the results later. PJ

The oil in the dashpots will have no effect on the problem. It is only there to slow the rise of the pistons when you increase throttle. After a very brief time, the pistons are as far as they will go and lighter oil will not make them go any higher.

If the car runs well at lower RPMs but not higher, then the problem could be timing advance, incorrect needles, or air starvation.

If you suspect leaning out at higher RPMS, just richen the mixture a bit and go for a drive. No need for anything fancy, your butt will tell you if the car is performing better.
Steve Simmons

Concerning TW Moore's comment about correct needles. If you are running rich, should you use a lean needle, to lean it down, or a rich needle because you are rich. I need to lean down a bit, but no doubt have the thicker washer when I rebuilt the carbs about five years ago. I have the adjuster nut all the way up. The car runs strong on 91 octane non eth. My plugs are black soot.
G B McGovern

Ok, I checked the plugs after a run and they are a light tan color, all are very close to being the same. I'm going to enrichen them one flat at a time and see how it goes. Bad weather here, lots of rain, so it will have to wait until it drys up a bit. Indeed, I appreciate all the help, thanks a bunch! PJ

MMO, marvel mystery oil works well for dashpots.
TRM Maine

Paul 161: sounds like you're right onto the problem. Please let us know what you find out when the rain lets up.
GB McGovern: my TF is the same. I'm interested in the effect of a lean (fatter) needle on the situation. In the meantime, the car runs fine (on hi-test to avoid the ethanol).
T W Moore

TW, I use nothing but non ethanol gas, 89 octane,($1.98 per) I'm fortunate to have 2 Conoco stations close to me and a Phillips 66 that has 4 grades of non eth and 1 grade of ethanol 87 type gas. I use it in my lawn mowers also. Ethanol ate the diaphram out of one of my mowers carbs so I avoid it anymore. PJ

Before adjusting, check the basic stuff first- remove the dampers and make sure the pistons rise and drop freely, check the actual fuel level in the float chambers (get up to speed and into the flat spot, kill engine, coast to safe spot and check level then), and check for air leaks carb to manifold. If your idle and off idle acceleration is fine, I would just pull the choke out a little (as mog suggested) and see if that changes the flat spot. Adjusting the jet height may mess everything else up. George
George Butz

We can't get non-ethanol very easily here in the Phila area, but I can tell you that it definitely screws up the tuning on our LBCs. It runs far too lean at speed and I suspect that's what you're seeing.

Steve is correct when he states that the oil does not affect steady running at a given speed. The oil is there to provide an accelerator pump effect when you press the throttle. Depressing the throttle sends vacuum to the bell chamber as well as to the back of the air valve piston, and that bell chamber vacuum in turn attempts to raise the air valve piston (the thing with the needle in it)... but the oil and the dashpot (damper) piston (attached to the knurled cap knob) inhibit the rise of the air valve. This in turn temporarily increases the vacuum over the needle and jet, sucking in more fuel than normal, enriching the mixture and giving the engine a quick shot of extra fuel to spur more power on acceleration.

As you can see, the heavier the oil in the dashpot, the more accelerator pump effect you get. Using a very light oil like ATF will reduce this enrichment significantly, whereas using engine oil (most LBC manufacturers' recommendation) will give you more enrichment.

I use what the mfg suggests... engine oil or straight 30W. It has served me well for over 45 years now.
Kevin McLemore

George, the carbs were rebuilt by an expert with many new componets and they work like new. Only thing wrong with the carbs is they might be set a little lean, other than that they are in like new condition. There are also no vacumme leaks in the system. The distributor has no issues. The coil didn't break down before, so I see no reason it would go bad just by changing the carbs, but I might try a new coil with a higher output, problem there is I havent found a modern coil with the proper screw in connections. I'll eventually solve the issue as I have pursued many of the suggestions by all and using the method of elimination. Thanks to all. PJ

Same as Kevin, I've been using engine oil since first start up many years ago,,,

Steve Wincze

If you set the mixture correctly at idle, but must adjust for a richer mixture at speed, then you may find that the idle mixture has become too rich. As TW Moore suggested, perhaps the choice of needles needs tweaking. I had a similar conversation with Joe Curto last week.

Do you know what needles were in the carbs before you had them rebuilt? Perhaps they weren't standard GJ needles, but the rebuilder installed new jets and GJ needles. Discuss with your rebuilder. He may suggest different needles.

> Testing for correct needle on page 11 of "Tuning S.U. Carburetters"

> Needle charts start on page 38 in the book. Too many choices for my feable brain. I need someone with experience to choose the needles for me.

FWIW - A ColorTune shows that my carb mixture is correct, but my plugs indicate a lean mixture. I'm going with what the plugs tell me.

LM Cook

Paul, I would still check actual fuel level- could have a blockage, lots of problems with needle/seats recently, etc. I'm sure the rebuild is great, however I have meticulously worked on/rebuilt many things over the years that ended up not working right for some reason or other. Does not sound like ignition to me. George
George Butz

Thanks Lonnie that link is a great read. As an aside what percentage of ethanol is added in the US, I avoid it like the plague in Aus although it is only 10%.
G Evans

Pending what other mods you have incorporated making your engine non standard, this might provide some insight;
G Evans

If you want to confirm your suspicians that you are going lean, I'd suggest you find a tuning shop near by that has both a rolling road dyno and an exhaust sniffer. A few minutes on the rollers with the sniffer and you can see whats going on with the mix at various rpms. It would cost me a 100 bucks to do that here. I wouldn't recommend a smog inspection shop that you might find near by as you wouldn't get a true picture unless the car was under load.
While you could richen the mixture at idle in an attempt to cure what you believe is a lean condition further up the rpm range, I wouldn't suggest that. You will most likely end up fouling plugs and finding the performance suffers in all ranges.

Changing the oil wgt in the dampers will affect the mix during wot... heavier oils will richen the mix and lighter oils will lean it. The damper springs can also be changed to give the same effect.

The old adage.... 90 percent of carb problems are electrical.... runs pretty true. While I know the only thing you changed was the carbs, I would do a good inspection of the ignition system. The condensers today are crap. Once that you are certain that it's not an issue and that fuel delivery isn't an issue, I think I would give Joe Curto a call and discuss a possible change in the jet needles.

Just my thoughts.
MG LaVerne

What about running the car in third gear upto 3500 rpm. If the flat spot occurs there also, than it could be related to ignition.
I suppose if there is a fuel shortage in fourth gear at 3500, it would not yet appear in third gear at 3500.
If the flat spot does not occur at 3500 in third, I would increase rpm's till flat spot comes in again (if rpms are still not tooo crazy).
Wouldn t such a test discriminate between ignition (rpm related) and fuel flow?
Just my thoughts, Huib
Huib Bruijstens

Springs can affect mixture, but oil only affects the mixture until the piston is as high as it wants to go. At that point you can run anything from molasses to nothing at all, doesn't matter. So at a sustained throttle, it should make no difference.
Steve Simmons

Excellent clarification!

Steve Wincze

Looking for lean needles for my TF1500, I called Moss and Abingdon. Not available.
Joe Curto was suggested by Abingdon but he also didn't have anything on hand. He did suggest that a "GR" needle looked close in profile to the shop manual lean needle, which is a "GL".
I haven't contacted Burlen but that's probably the next step unless someone has other sources.
T W Moore

Search on Burlen Fuel Systems, SUs then "Advanced Search", GL Needle. No issues with availability.
G Evans

Or you can try SUmidel:
Willem van der Veer

This thread was discussed between 14/11/2015 and 21/11/2015

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