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MG TD TF 1500 - Setting up SU H4 Carbs for XPEG Engine

I tried to find a particular thread about SU H4 carbs, but couldn't, so I am after more information about rebuilding and setting up my H4 carbs. I recall Willy Trevitt offering great advice on the choice of needles and this essentially what I am after. My carbs are 6031 and 6033, (off an MGA I understand) but I am rebuilding them to the TF specs, with all new components.

So Willy, (and others) if you happen to see this thread, can you gift me with some more of of your valuable knowledge?

Cheers, Maurie Prior
R M Prior

Aren’t MGA carb bodies a different length than TF carbs. If your stuffing this in a TF you might have a problem with air cleaners? I’m new to TFs so I may be blowing smoke. Someone else will have something to add.

As far as rebuilding H4s in general I’ve built MGA carbs to TD specs and also MKII TD carbs. What do you need help with?
W A Chasser

Fitting an air fuel meter is essential to really knowing what kind of mixture you are running. I found the LS1 needle to be rich at idle and lean at speed with my car (MK2 xpag bored to +80, about 9:1). The LS1 has a weird bump in the curve. The GS needle for MGA has a very even curve and gives me an air fuel ratio of about 12.5 to 13 at speed. This is the range for max power. It is still a little rich at idle, so I've taken to lowering the needle about 0.05". I use an AEM brand gauge fitted inside the glove compartment. A muffler shop can fit the grommet for the sensor. I have it on the pipe just past the manifold (header in my case). The needle bounces around a lot, but is pretty steady on the road.

I run mostly at sea level, and that can make a difference in the mixture.

Jim
J Barry

As in the earlier thread Maurie GS needle and a red damper spring
William Revit

Maurie, I have the same carbs on my TF 1500. The pancake filters fit fine.
M Hyde

And , run your needle flush with the piston as it's supposed to be, it'll be fine with your 1500+ engine.
Also, your carbs are the same length as the TF carbs, it's the TD2 carbs that have the longer bodies.
willy
William Revit

Willy,

Many thanks, but I cannot remember that previous thread title. Do you remember what it was?

Cheers,

Maurie
R M Prior

Yep, but you've left your run a bit late for getting SU bodies off me.
I'll reactivate the thread-



William Revit

That's interesting, the post is there but the content has gone to computer Heaven

"TF Carb Body Numbers 18 June 2022"
William Revit

William Revit. Thank you for setting me straight.
W A Chasser

Mistake: I was originally using the GJ needle, not LS1. There was a noticeable power increase when I switched to GS.

Jim
J Barry

Jim, THANK YOU!
For many years the GJ needle was ok.
Then after minor modifications it was much too lean.
The LS 1, with red springs, was better but TOO rich from about # station #1 to # 4 or #5 station's.Next I dropped the LS 1 needle .038" and it was a lot better. Today I looked in my pats bin and found a pair of GS needles!, Since the GJ was too lean at all station's, and the LS 1 too rich especially at 1500 to 2500 RPM, the GS needle performed wonderfully at this RPM range and improved acceleration.
Yes still too rich at Idle.
You stated that to correct this you dropped the needle .050" correct? When I dropped the LS1 needle .062" the car was not drivable!
Len
Len Fanelli

Hi Len,

I found the GJ ran in the 14:1 to 15:1 range at speed - good for fuel economy but not great for power. I believe I first dropped the GS needle about 1/4 stage, or about 0.03" and then to around 0.05" when it was still a tad rich at idle, but my memory is a bit foggy. I have to admit I've never been confident of my carb idle settings as there is so much that can go wrong between the float bowl and the jet. One of these days I'll get them properly rebuilt!

Jim
J Barry

Maurie - this was a similar thread on H4 carbs and the various part numbers on TF and MGA applications

https://www.mgexp.com/forum/t-series-and-prewar-forum.46/tf-carb-identification.4518061/
M Hyde

I dropped the GS needles .015” now as close to perfect as can be!
Len Fanelli

Happily, I have been through a steep learning curve - researching and gaining some knowledge of the SU H4 carburettors that were fitted to many British cars in the past.

Since my TF1500 had been incorrectly fitted with H2's by a previous owner, I have now sourced a set of the correct H4's. I understand, that the replacement carbs I now have were from an MGA (6031 and 6033 and the correct units for my car, should be 6021 and 6023)

There are some minor but expensive linkages alterations required and with patient research, I now have all the appropriate parts required to rebuild and restore a set of H4's to suit my TF.

I am currently in the process of a total rebuild of these carbs and interestingly, I have also discovered the significant discrepancies in the prices for the rebuild parts from various suppliers in the UK, the US and even in Australia. Given also, the high costs of shipping halfway around the world, it is a balancing act between buying new reproductions or unearthing originals and refurbishing them. Interesting times for me at least!
R M Prior

Here are two images of my SU H4 carbs.

The first is of how they were when I purchased them, and the second is of them after my rebuild.

Cheers,

Maurie Prior




R M Prior

Hi Maurie
Looking good-
What needles did you end up going for----

willy
William Revit

‘Fuel should be visible in the jet tube (remove the carb dashpot and piston and turn on pumps to see where it settles. We actually set the jet tube height relative to the top of the bridge and NEVER subsequently touch the jet height (i.e. don't mess with raising/lowering the jet with the screw at bottom of the carb). We've found that top of the jet should be between /.060 and .065 below the top of the jet bridge. Best atomization happens with this "jet drop." With the jet higher than that, the fuel actually "mists" too much, and with the jet lower than that, it actually "puddles" and doesn't atomize well enough. Once the jets are set relative to bridge height, the next step is to set the fuel level in float bowls. You should see the fuel about 1/8" to 3/16" below the top of the jet tube. Any higher and it's likely spilling over when the motor fires/vibrates. Blow down in jet tube (with pumps running) to see where the fuel level settles"
1/8 to 3/16 + .060 -.065 = about .220" below the bridge. I found to achieve .220" at the forks use 15/32"instead of 7/16" Also it is easier to lower the float level 1st, then raise it up to /220"
And YES .060" to .065" does make a difference!!
Len
https://abingdonperformance.com/
Len Fanelli


Hi Willy,

Exactly as you suggested - GS needles and red springs, and I am glad you asked because I was wondering if altitude will affect this selection?

Where I live, is on the New England Highlands of NSW and is at 3500 feet above sea level. Most of my touring will mostly between this altitude level and down to 2100 ft?

Cheers,

Maurie
R M Prior

SU's are quite good at coping for elevation changes up past where others need an adjustment--It all works off barometric pressure so with a carb with a set jet like say a Holly, maybe a little adjustment at your upper level,--- but SU's have varying jet size for most of their running so not really an issue
With a carb like a Holley, the rule of thumb is to leave it alone up to probably 2000ft then drop one jet size for every 1000 after that--
When you go down to the seaside you 'might' have to richen your idle mixture if you've gone for lean/economy at home but I doubt it will be an issue--it would be one flat 1/6 of a turn at the most
The main reason for the GS needles is that the std needles were boderline lean at the best of times and with the crappy fuel with unknown methanol content, GS is a perfect choice to get a decent mixture going

Don't overthink it, otherwise you'll be out there changing needles every time your barometer moves
It'll be fine-

willy
William Revit

This thread was discussed between 08/08/2022 and 25/08/2022

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