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MG TD TF 1500 - SU carbs running rich

I know that there has been an epidemic of such problems, so just thought I would add my contribution. SU carbs are not new to me - like many Brits of my age, I've been fiddling with them all my life. So when it came to rebuilding and adjusting the carbs on my TF I had no fears about it. The only real issue I was pondering was whether I might need richer needles than GJ after making various mods to the engine. Well it turned out that I was way off beam. I bought the kits from Burlen and rebuilt the carbs including new spindles, bushes, throttle discs, gaskets, jets, etc. All went well or so I thought!. Making all the standard adjustments she ran rich. So dismantled the carbs, readjusted and tried again, She still ran rich. Used the Colortune and yellow all through. Next step was remove the carb suction chmbers and pistons and check the height of the jets relative to the bridge. Big difference and as far as I was concerned they were too low, I ran out of adjustment - even with the jet adjusting nuts up tight. No good. So looked at what Dave Braun was recommending and sure enough the jets were too low. So dismantled again and checked the dimensions of everything against the original parts with a micrometer. I found that the new jets were 1mm shorter than the originals, however I don't think that was the cause. The bores on the old jets were worn so there was no way they were going back anyway and I continued to use the new ones. The significant differences I am sure were in the thickness of the bottom bearing washer - new ones 50% too thin, and the top bearing washer - new ones 10% too thick. Together these conspire to lower the jet. In addition, to my surprise I found washers jammed so tightly into the jet adjusting nuts that I did not know they were there. So removed them and put all back together with the old washers and new jets now sitting bang on Dave Braun's recommended position. Next step use his method to set the fuel level in the float bowls. I had carefully used the std method with an 11mm round bar, but although one was OK, the other was way off - much too low. Anyway all that done, reassembled the carbs and test ran it. All OK, Colortune now blue when its supposed to be.
Dave H
Dave Hill

Dave< Thank you for your thread. I too, am having great difficulty with my carbs on a newly built and modified XPAG. Carbs are fat at idle but respond very well off idle. I can't seem to get the rpms to settle down and idle without it loading up. I'm running new GJ and 090s With the needles flush with the bottom of the pistons and the jets at a start of 6 flats. I have run them all the way to 1/2 flat with little change. Floats set at 7/16. I have developed a plethora of fuel leaks that I cant overcome so I am awaiting fresh gasket sets and will start again. I will look at D Braun's site. I have the Two popular SU manuals but neither seems to be helping me at this point. I'm a newbie to these carbs and fighting with them as I have is not helping my learning curve an iota

Cheers

Bill Chasser jr
TD4834
W. A. Chasser Jr

Dave Hill, could you state the document on DB's site you are referencing. I couldn't find it

Cheers

Bill Chasser Jr
TD4834
W. A. Chasser Jr

Bill - were your carbs completely rebuilt? Carb bodies re-bushed and new throttle shafts and butterflies? If you can't get the idle down, it usually means an air leak. Plenty in the archives.

Tom Lange
MGT Repair
t lange

If those washers I mentioned are the wrong thickness its going to be very difficult to get the jet in the right position at anything like 9 flats open on the jet adjusting nut. The jet top should be around 1.8mm below the bridge for starters.
Setting the floats with a bar is hopelessly inaccurate, as I found out. The problem I think is that the forks get misshapen and don't settle on a round bar evenly, besides you can bend them all you like but what matters is the flat area that rests on the needle. This tends to develop a hump with frequent bending and it all goes to pot. The only accurate method that ensures both carbs are darn near equivalent is to make sure the fuel in the jet is the same level for each carb and in the range of 7 - 8mm below the bridge. You lower the jet until the meniscus is level with the top, then measure the depth with the butt end of a micrometer (a tyre tread depth gauge should also work, though less accurate). Then bend the fork until you get the depth in the right range, It took me three attempts. They don't need to be spot on, just close. When doing this stuff on the car I disconnected the fuel line and used a small auxiliary fuel tank with tap, suspended above.
Its on the website somewhere, forget exactly.
Dave H
Dave Hill

Dave,

Thanks for your kind comments about my paper on setting up SU Carburetors. There is an epidemic of sorts regarding the washers you spoke of. Here in the USA Joe Curto and Jim Taylor years ago conspired to supply the properly dimensioned washers. Also, when trying to raise the jets high enough, against the incorrectly stacked height washers, the jet adjusting nut sometimes fractures.

Bill, on the front page of my website http://www.dbraun99.com/ is a link to a document titled "•A PDF of the detailed instructions I send out to clients after I rebuild their carburetors." I hope the instructions work for you.

As far as needles go, the fuels today require leaner needles, if you are running GJ, you must have the H4 carburetors, and they require a spring to add 'weight' to the piston because the piston is no longer weighted or brass. In addition, a lack of oil in the dampener chamber will make for a richer idle. However, how the MG company selected the GJ needles in the first place always seemed odd to me for the minor power increase of the 1250 Mk II and TF XPAG and the XPEG engines. The only thing I could surmise is that the greater airflow capable with the H4 is never really put in practice, so the richer portion of the needle was never in play. After all, the same throat diameter is used on the MGB with an 1800 engine. If the GJ is too rich at idle and fine at speed AND you have the proper height of fuel in the bridge, you could try the GN needles with are leaner at idle.

Warmly,
dave
Dave Braun

Dave, if you have the original carbuettors on a TD, what would you recommend as a needle for the modern fuels of today? As you mention, mine always run rich too.
kmclemore

Just because this thread is about SU carbs, I thought I'd put in a little info I ran into. After rebuilding my carbs they worked perfect, but I used the original float valves. I was told I should get a new set of Gross jet valves and install them, so I did. Now the rear carb floods out due to the Gross jet not sealing properly! Tapping on the bowl will seat it temporarily. I can't do anything with it so both Gross jets are coming out, going in the trash and the old style float valves are going in their place. I'll never buy another Gross jet. Just thought I'd throw that bit of info out. PJ
Paul S Jennings

Dave Braun, Thank you. I found it. I clicked on it this morning and the site said it was no longer available. I tried again later and it came up. My carbs are H4 TD MKIIs I have ES needles out of my H2's and possibly a set of LS1 I can play with. Rather than rehash what I've done here , you can go the "TD4834 Engine Installation" thread and read what I have done thus far

Cheers

Bill Chasser jr
TD4834
W. A. Chasser Jr

Dave. Yes, H4 carbs with GJ needles, blue springs and SAE 20 oil. I'll try the leaner needles as you suggest. I agree with your analysis entirely - the carbs were surely never more than partially open for max power on such a small engine.
Paul. Good comments, but have used Grose jets in the past and decided to avoid them this time, so I'm using the std needle and seat arrangement.
Has anybody contacted Burlen Fuel Systems over the thickness of the washers supplied?
Dave H
Dave Hill

kmclemore,

I would first try lowering the fuel height in the jets to something around 0.20 below the bridge. Then I would try and see if the ES needles respond properly with the jets at about .060 below the bridge. And I would make sure a jet isn't hanging up on 'enrichment' they should return smartly to the jet adjusting nut when the cable is pushed in.

If all that fails, I would try leaner needles. AP was the standard leaner needle for the XPAG with the twin H2 carbs, but that was developed with the old distributor cam... and frankly it isn't a whole lot different from the ES.

Warmly,
dave
Dave Braun

Bill you mentioned of FB that you were going to sort out the overflow pipes? did you do this ok? the float chamber needs to have a link to external air/atmosphere
mog

Thanks, Dave - I'll try that. I have some Teflon o-rings coming in the mail soon, so I'll install them and then try the settings you suggest.

One other question - what are the PROPER thicknesses of the two washers for the jet? (i.e. top and bottom washers) I'll make them if I have to, but I really want to get this right this time around.
kmclemore

Mog Yes I made new tubes and sweated them onto the fittings and blew air through them before the solder set. Lines are clear.

D Braun. I would like to know the washer answer also as I read that they have been an issue.

kmclemore let me know how yours turn out and what you ended up doing. you and I seem to be on the same page...

Cheers

Bill Chasser jr
TD4834
W. A. Chasser Jr

Will do, Bill!
kmclemore

back to the top
L E D LaVerne

Who sells a variety of needles? Moss only sells the GL for the 1.5 carbs. If the GN gives a leaner idle, does the taper of the needle allow for the same flow at speed as a GL? I would like a leaner idle but not change the mixture at speed. PJ
Paul161

Burlen Fuel Systems are the main SU people.
Dave H
Dave Hill

Joe Curto has the best needle selection in the US.

Tom Lange
MGT Repair
t lange

Thanks Tom, I check with him. PJ
Paul161

HI !
What is the goals of these washers ?
for the top one, I thought it was to prevent leakage from the outer jet cavity to the inlet. A sort of gasket between the top bearing and the inner top body of the carb.
I also thought that my running rich problem of one carb was due to a small leakage of the top washer. Allowing succion of gaz from the outside of the jet.

I can't understand why the mixture could be dependant on the thickness of these washers.

So Dave H, what is the thikness you measured on the old and the new washers ?


Laurent.

LC Laurent31

Sorry but it was a few months ago now, so I have forgotten. Also I am at my French house and the car is in England, so can't measure anything.
Dave H
Dave Hill

This thread was discussed between 10/05/2015 and 21/09/2016

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