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MG TD TF 1500 - Running rich - do I need leaner needles?

I've had a good trawl through the archives but I thought it might be worth raising this again to see if there was a fresh insight to be had.

I've a reconditioned set of H2 carbs on the TD. It runs rich unless I have them fully adjusted up (as opposed to the recommended 6-flats down), but even then it's on the rich side, and the plugs still tend to be sooty (although they clean up after a decent run - so I'm assuming it's not that far out). The car always starts v easily on full choke, but is happy to have the choke pushed in v quickly afterwards, even in cold weather.

Tickover is lumpy - a sort of bub, bub, BUB, bub, BUB, bub, bub, bub. At speed, it runs and pulls OK but there's an underlying raw, coarseness when it revs - not as smooth as I expected, but I may be expecting too much.

There's no smoke out of the exhaust, but plenty of soot in the tailpipe. There's a very slight weep of petrol on the base of the jet to LH carb and I was thinking of rebuilding that shortly (even though it's fairly new).

What I've done so far:

1. Set the timing and points (used a dwell angle meter on the latter), balanced the carbs.

2. Checked all around the carbs and manifold with a propane torch for leaks, esp. close to the petrol weep at the base of the LH carb - no change in revs at all.

3. Checked and adjusted the carb float levels - again, no problems.

4. Checked the carb pistons for centering etc. - they're OK. Brass pistons, no springs, good healthy clunk when let go. Oil OK in dashpots

5. Checked air cleaner - seems OK, no restrictions on air flow or collapsed feet etc.

I've got ES (std) needles fitted. I'm thinking of trying APs (lean).

I also read (way back in these archives) that some of the replacement needles produced for these SUs were short so, yesterday, I set the needles on mine about 0.125 ins longer (they had been set with the collar exactly level with the piston). The piston still seated OK, BTW. It *seemed* to run smoother - but I didn't get a chance to give it a test run and look at the plugs etc. so I can't be sure what difference (if any) there would be out on the road.

I'll let you know the results of that as soon as I can (it's chucking it down with rain here ATM and is likely to go on all day, turning to wet snow this evening). In the meantime, are there any other ideas anyone might have, please?

- Tom.
Tom Bennett - 53TD 24232

I have the same problem on my TF. Tried everything you listed and eventually traced the lumpy running to a bad condenser. Replaced it with another only to find that it happened again within 30 mins. Talked to Moss about this and apparently there have been problems with condensers. Got it sorted now with another condenser.

Anyway mine still runs rich at tickover and gives sooty plugs. I haven't been able to run mine far yet as it's still off the road following a major off body re-build. I thought about setting the needles lower as you have but haven't got around to it yet as I discovered a few other problems when I drove it around the corner for the first time. I have GJ needles in mine which is what the SU website recommended for my car.

AJ
A R Jones

I believe that running at "tickover" (ideling) will always give sooty plngs and the false impression of a rich setting,,,
STEVE WINCZE

The best way to check actual float levels is to remove the cover and piston. with the key on and the pump stops clicking see if the actual fuel level is 1/16" below the bridge.
Some times the gaskets fitted between the float bowls and carb body are too thin and this will raise the operating float level.
darnoc31

Had the same problem...bought AP needles, and made absolutely no difference....
I then found out that the leaner needles only have an effect at higher rpms, not at idle....Don't waste your money.
Keep looking for other causes. (such as a vacuum leak in the intake manifold).
Edward
E.B. Wesson

Fuel level should be 1/6" or .166" below the bridge.
.140" to .190"
Len
Len Fanelli

Thanks Len. A slip of my senior finger. glad you caught it.
darnoc31

"but there's an underlying raw, coarseness when it revs - not as smooth as I expected," does this mean acceleration or cruising steady at higher speeds?

You can be plagued by rich overall, but lean on acceleration. When giving it throttle, it will drop way lean, in spite of the oil dampener; juggling different viscosity oils may tweak it a touch. I installed an air/fuel ratio meter to keep an eye on rich/lean situations running a supercharger.
If it is sluggish on acceleration, you might pull out the enrichener and see if it is better or worse.
I'm checking out some Harley Davidson carbs with accelerator pumps as replacement to the SUs we've run.

Good Luck!
JIM
Jim Northrup

Assuming you adjusted everything properly, are you absolutely sure that the jets are the correct .90 size and not worn (already)?

The jet and needle can wear quite fast if the jet is not centred correctly. If they are worn they will cause rich idle.

Another cause can be the cork sealing rings, not sealing properly.

Please be careful with concentrating too much on the idle mixture. You don't want to be too lean when driving, as that will cause more damage by overheating than a lumpy idle ever will.
Willem vd Veer

If you want to see how lean you are at speed, run along at cruising speed for a few miles. Cut the engine and coast to a stop and pull the plugs. The color should be tan, not black, not white.

There are a bunch of washers which fit under the jet bearing head that are too thick. They lower the entire jet down and rob you of the last couple of turns of the jet. But my experience is that you should start with the fuel level set at .160 below the bridge, and have the jet about .060 below the bridge. I always start at .070 and send the carburetors that I refurbish out that way.

warmly,
dave


Dave Braun

Tom (Bennet) mentions in his item 2 that he checked for petrol leaks with a propane torch. I'm just wondering if you are still on the planet?
R A WILSON

What I meant to say was that I wonder how you do that and remain on the planet !!
R A WILSON

Surely he meant an /unlit/ torch! ;-)
Rob Edwards

I presumed Tom was looking for air leaks, but reading back he held the propane torch close to the petrol leak.....
Willem vd Veer

Obviously, Tom meant unlit....
The propane will seek intake manifold leaks, and cause the idle to increase, when the gas hits the cylinders.
A very common and much-used method.
Edward
E.B. Wesson

Just noticed RA's point (thanks for the other comments, BTW. I'll reply to those when I'm back home, later).

Sloppy posting on my part. To reassure everyone that I wasn't trying to incinerate everything, yes I meant to say: "Checked for air leaks with an unlit propane blowtorch, with the car out of doors, running the nozzle over and under the areas where leaks were likely - mainly the intake manifolds". I included the underside of the carbs where, on one of them, the slightest trace of a petrol weep is apparent although, thinking about it, this probably was futile.

There wasn't any change at all in revs, which suggested to me that I don't have an air leak.

- Tom.
Tom Bennett - 53TD 24232

One interesting, but maybe, not relevant, suggestion, is to check the union at the gas tank....
Because of the way it is mounted, it won't necessarily leak gas, but can cause the pump to draw air....
This happened to a friend's '32 Ford (which has a similar fuel union), and he went through three carbs, before he corrected it....He had converted to an electric fuel pump, as the first of the "fixes", then discovered the loose union, after returning the 3 carbs, thinking they were all defective.
Edward
E.B. Wesson

That's an interesting point, Edward. I have a glass fuel filter which I fitted, in line, just before the fuel pump, at the same time I changed the carbs. I later found out (via this BBS) that it should be *after* the pump (to protect the pump) but it's on one of my lists of things to do "ASAP after the winter".

It's never shown any air bubbles coming into the filter from the direction of the fuel tank but (and I think I mentioned it on here last year), the filter shows petrol draining back towards the tank, when the pump isn't operating, with the filter then filling with air from the top (pump) end. When the ignition is switched on again, the fuel soon gets pulled back into the filter, and on to the pump; switch off, and the petrol slowly goes back towards the fuel tank, leaving the filter full of air. The pump is brand-new from Burlen, BTW.

I couldn't work out, then or now, how the air is entering the system as the pump switches off. There's no trace of a petrol leak on the fuel delivery lines, filter or pump, which suggests the air ingress isn't on the +ve pressure side of the pump. If it has to be on the suction side, doesn't that also mean there's a potential for air to be drawn by the pump into the fuel, and then on to the carbs?

I'd always assumed that any air in the fuel would be vented via the overflow pipes on the float chambers but in view of what you're saying here, could this be implicated in the rich running?

Cheers.

- Tom.
Tom Bennett - 53TD 24232

Thank you to everyone who has replied:

AJ: I'll try a new condenser. I know the little blighters can cause all sorts of problems and I've been meaning to order a couple of spares. I think I also need to sort out the weep on the base of the LH carb so I'll check the level of the jets at the same time and also re-centre them both.

Steve: I'd tried giving the car a good run but with any setting other than the jet screwed right up (as opposed to the recommended 6 flats down) the power seems down when road-testing and the plugs end up sooted. If I have the flats wound fully up, the plugs are clean - tan, tending towards white. I can't seem to get the usual test for mixture, of just lifting the piston 1/16th and noting what the revs do, to work for me - it doesn't seem to make any difference.

Darnoc31 & Len. I'd adjusted the float level last year when I noticed the problem, but I'll check it again.

Jim: It accelerates OK (with the jets adjusted fully upwards to give the whitish plug colour) but that's when it feels raw, with a very rapid, very slight vibration from the engine - and a throatier roar from the exhaust, which is not unpleasant at all. I haven't driven another TD so maybe that's how it should feel. But I know that the rawness disappears when I enrichen the mixture or retard the timing.

Willem: I'll look at the jets when I take the carbs apart. They've only done 500 or so miles from the major refurb (done by a reputable specialist here in the UK). But the seals are suspect, bearing in mind the weep on the LH one and I do have the service kit which I'm intending to use. I am concerned about running too lean, so I'm being cautious although my runs, so far, haven't been very long or particualrly hard.

Edward: I take your point on the lean needles. I'll hold off ordering those for the time being.

Dave: A photo is worth a thousand words. I have a service kit for the H2s from Burlen. I'll have a look when I take the carbs apart, to see if anything differs. It certainly *seems* that the jets are too low and are not giving me the proper scope for adjustment (leastways, if that is the cause it would explain a lot).

Thanks again. All suggestions and advice are, as always, gratefully received.


- Tom.



Tom Bennett - 53TD 24232

As Willem vd Veer mentioned, a common problem is with the jet seals. The oem type is oil impregnated cork. Many were replaced with short pieces of rubber hose...in fact that is what used to come in the moss kit years ago. If the seals are a poor fit or dried out, raw fuel will get sucked past the jets and the engine will run rich no matter what you do! The best seal is still cork, soaked overnight in oil. The only one that works is genuine SU. It can be gotten from Joe Curto (joecurto.com).
Steven Tobias

The only thing the guys have not already covered I can think of: It is easy for an old cork seal to be stuck and hidden in the top or bottom jet bearing fitting, the new one placed on top of it, and it will leak. Have had same problem with the too thick copper washer Dave mentions (the tiny, super- thin one between upper jet gland housing and the carb body I think) as well as leaky new corks. I have used the teflon o-rings from the fellow in Maine in place of the corks with great sucess. How did the car run before the carbs were rebuilt? Need to verify ignition/timing, valve adjustment, compression, etc if nothing else found to be wrong. George
George Butz

Thanks for the advice on the propane torch - I didn't know you could do that !
R A WILSON

RA: As long as you acknowledge that it comes with the usual disclaimers. You know, the ones about no reponsibility etc. for burning your car, house and garage down, melting your contact lenses or nylon shirt, singeing your whiskers, and so on .......
Tom Bennett - 53TD 24232

This might be a long shot but is your choke cable possibly hanging up somewhat?
Mike
Mike TD/TA

Thanks, Mike. No it's not that, I'm afraid. The cable adjustment is fine and the jets go fully up into the body of the carbs when the choke is off.

- Tom.
Tom Bennett - 53TD 24232

You need to do the drop test of the piston/chamber fit. Many old carbs have been mutilated by people sanding the ID of the chamber or OD of the piston. .0001 makes a difference. If this clearance is too great, the piston will ride lower, increasing the fuel feed and making it go rich. Problem can be expected to be worst near idle, ie, the lower 1/2" of the chamber, which can be hard to spot.

FRM
FR Millmore

Interesting comment in the archives under "Carb Adjustment", with regards to today's fuel and the needles.
Mike
Mike TD/TA

Thanks FRM. That's something I hadn't thought of and I'd not seen mentioned before. I haven't sanded the pistons/chamber sides, but that's not to say a PO hasn't.

I suppose if there is too much clearance, there's not a lot to be done, except to replace both of them as don't they usually come in matched pairs? I don't suppose greasing the piston sides will help give more of a seal (straw-clutching here, BTW)?

- Tom.
Tom Bennett - 53TD 24232

Gentlemen,

One more cause of rich running. If the cotter pin retaining the pin where the choke linkage meets the bottom of the jet is left too long vibration can cause the pin to rotate until the long leg of the cotter pin points up. When the choke returns to the off position the cotter pin can hit the adjusting nut holding the jet down by a small amount. Took a while to find.

Cordially,

Jim Haskins

1953 TD
J. M. Haskins

http://www.britishcarforum.com/bcforum/ubbthreads.php/topics/687269/SU_Pistons_TR3

Great John Twist video showing drop testing
George Butz

Tom -
Grease very evil! It attracts dirt and scores the bejeezus out of the parts, and it sticks.
The diametral clearance is .0001-.0003", so half that per side. This is picky stuff (and why you must keep the piston/chamber really clean). If the piston is undersized, it could be plated and finished back to size. Trouble is, the more likely problem is that the chamber is worn, corroded, or otherwise damaged where the piston spends most of its time.
This would be at the very bottom, since that's idle and slow speed driving; or, in the case of cars that have spent a lot of time at constant speed. someplace farther up that corresponds to that speed/load. You need to watch very carefully to pick this up on a drop test. We spent a year figuring this out on a customer MGB once. He drove long distance while religiously observing the speed limit; the chambers wore at exactly that point, but nowhere else. When he got to that speed, the piston would lose its vacuum and drop, which had exactly the same result as putting the choke full on. It would foul the plugs and kill the engine. Once the car came to a stop, the plugs would dry up and it would start and run perfectly. Worse yet, since none of us drove like he did, we could not replicate the problem. All made worse because we were in hilly city (Pittsburgh Pa), but he drove mostly in flatlands like Ohio.

FRM
FR Millmore

Thanks, George. I'd forgotten that video but I've had a look at it and I'll give it a go when I get chance, and then let everyone know - I'm away this weekend.

I'll also look at the same time at the piston chamber for wear, FR, and see if I can come up with anything there. The grease gun will stay well away from the carbs, BTW.

All the best.

- Tom.
Tom Bennett - 53TD 24232

FRM & George: I got home early enough today to take the carb piston housings off and test the clearance between those and the pistons.

The test showed that they're both a snug fit and fairly well matched, with a very slow drop on both. There's no sign of uneven wear to the any part of the brass pistons, or the inside of the housings.

I'm not sure if that slowness is significant, but they certainly drop a lot more slowly than in John Twist's video, although I balanced them last year and found that they rose and fell, promptly and in tandem, as I blipped the accelerator. There were no signs of hesitation or sticking, then.

I'll find time in the next couple of weeks to explore the other suggestions and come back to you all. I'm still suspicious of the petrol weep to the base of the LH jet. I'll have to screw up some courage to take the unit apart, as it's not something I've done before.

- Tom.
Tom Bennett - 53TD 24232

Reactivating old thread from March 2012 ...
============================

> TOM BENNETT ...
Did you fix your rich carb mixture problem?
How?

> BRUCE C ...
You posted a similar question in August 2008
http://www2.mg-cars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=8&subjectar=8&thread=2008081717024514404

I've had the same problem ever since I bought my TF-1500 a year ago. I rebuilt the carbs plus replaced glands and seal washer twice.

Thanks,

Lonnie
TF7211
LM Cook

I've never been able to get my TD to be other than rich even with the jets screwed all the way up with standard needles. Drop test is normal and even. Choke is not hanging up and the ignition is a Moss supplied Crane electronic system. The car starts fine, choke can be pushed home in a few blocks, and the car develops good power and an even idle but plugs are always black and tail pipe sooty. Recently did a test with the Colortune visual spark plug system and confirmed it is running rich. Color in the window shows bright orange and should be light blue. The only way it goes blue is on the overrun coming back down to idle. I debate leaving well enough alone or sending the carb set to Joe Curto for expert attention.
John Quilter (TD8986)

Hi,

I cannot remember who told me, but they said always change the needle and jet at the same time, you might only see wear on one of them but they will always wear together if the needle has never been centred properly. The wear always occurs at the base of the needle, so it will be rich all through the range due to wear in the jet, but worse at idle because that is were both jet and needle are worn.

Regards Bernie
B W Wood

This thread was discussed between 04/03/2012 and 01/12/2014

MG TD TF 1500 index

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