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SU Carb Rebuild and Identification

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goloch

goloch

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Having finished my work on the ignition system, my attention is now focused on the carbs as I try very hard to get my 71B through emissions inspection. The next obvious step to me is tuning the SU carbs. Is there anything else to check (already did valve clearances twice) before going to the carbs? And while I'm working on the carbs is it worth it go ahead and rebuild them? What are the chances of screwing something up irrevocably (aka needing to pay someone else to fix it) if I start mucking around with needles, jets, etc?

Also, my car seemingly has the wrong model of carbs for its year. From everything I've seen, SU HS-4 carbs should be on a 71. I have no separate float chamber which implies HIF-4. Is there any chance the HIF-4's are original to a 71? The engine is missing its ID plate so I have no idea if the engine is even original. I'm guessing the answer lies in the model number of the carbs (AUDxxx); where do I find that?

As an aside the (cracked) distributor is from a 65 based on the Lucas part number stamped on it. I'm not even sure if it's actually compatible with the (now-reinstalled) manifold advance. What a mess...
 
HIF-4 Carbs are noted for the 1972 model year for which the build start was AUG 1971. Is it possible that to the end of the 1971 model year someone ran short and put on a pair, sure it an MG after all. What is the build date on your car?
 
[ QUOTE ]
What are the chances of screwing something up irrevocably (aka needing to pay someone else to fix it) if I start mucking around with needles, jets, etc?

[/ QUOTE ]

- I'm sure you know this, but not using any power or air tools to remove carb parts would be a good start to not screwing something up.


[ QUOTE ]
From everything I've seen, SU HS-4 carbs should be on a 71.

[/ QUOTE ]

-I believe you are correct. Mine was built 12/70 and has the
HS-4's (which has a little piece of stamped tin under one of the float bowl screws that denotes the AUDxxx.. I think on the HIF's it should be under one of the screws that hold the dashpot on)

It sounds like the PO was, um.. Creative under the hood.
 
I have a '70 with HS4's. I rebuilt them last year myself. If you can get the SU video featured in Moss. Moss sales it for a ridculas price so see if you can't find it on ebay or if someone in your British Car club has it for you to borrow. Its REALLY good and shows you how to rebuild the carb in live time as well as tune them. The rebuild really helped me understand the simple and elegant design.

I don't know where you are in NJ, but one of the foremost SU experts on the planet is in NYC. Joe Curto. Here is his site, I bought my rebuild kits from him as well as the Haynes manual and tuning kit. Good luck!

https://www.joecurtoinc.com/
 
Based on what I read in this tread your carbs should be HS4 and you are running HIF4.

The HIF4 was the last generation of SU carbs on the MG and as better characterisitcs in regards of antipolution but they must have all the correct hardware.

I thing the biggest problem you are facing is that the HS4 and HIF4 were extremely different in one regards that is the advance vacuum for the distributor on the HS4 is on the side of the carbs and the HIf4 is on the intake close to the engine. Although simple for difference it is in fact a major difference because it will change all the advance characteristics on your distributor.

If your DPO was wise enough he or she would have changed the distributor at the same time.

ALL MGB of those years ran the 25D4 distributor but there is a small plate on it giving specific specs. Please look at the attached link to identify yours.

https://www.mgbmga.com/tech/mgb21.htm

Can you not have your car pre-tested before the official smog test?
 
Eric, In goloch's original post he stated : "As an aside the (cracked) distributor is from a 65 based on the Lucas part number stamped on it." So he might have the 25D4 dizzy, but might not have the proper vaccuum unit. and looking at the chart you so nicely provided that makes a huge difference in the timing.

goloch
 
If he wants to replace his distributor I have a brand new not used 45D4-41427 ported vacuum a direct replacement for the pre 73 MG's.
I have no use for it since I have those famous HIF4 that would require a 45D4-41599.
 
[ QUOTE ]
the HS4 and HIF4 were extremely different in one regards that is the advance vacuum for the distributor on the HS4 is on the side of the carbs and the HIf4 is on the intake close to the engine.

[/ QUOTE ]

It is?
ponder.gif
I only say that because my hs4's are hooked up to the intake manifold as well.
 
If you are running HS4 with a distributor 25D4 ported (see the list I attached earlier) you must have poor performance under acceleration.

If a 25D4 ported is attached to the manifold maximum advance is applied at idles leaving nothing else under trottle opening.

Suggestion. If you run HS4 with a ported 25D4 ten un-plug the vacuum from the intake, plug the hole and hook the vaccum to the ported side of the carb (HS4). The re-adjust timing with strobe light to proper settings.
 
Forgot to add that the distributor must have the proper vacuum unit and it must be in working order. Again refer to list in previous post.
 
Thanks for all the responses. I'll have to go over all the details later on as I'm in school and have to prepare a test I'm giving later today, but here's some pics of the engine to see if it helps figure out what on earth is going on under the hood.

https://goloch.net/mgb/view_photo.php?set_albumName=turkeyday04&id=IMG_1265
https://goloch.net/mgb/view_photo.php?set_albumName=turkeyday04&id=IMG_1284
https://goloch.net/mgb/view_photo.php?set_albumName=turkeyday04&id=IMG_1285
https://goloch.net/mgb/view_photo.php?set_albumName=turkeyday04&id=IMG_1286

The first one is especially interesting. That is how the DPO left what I assume to be the manifold side of the vacuum advance. The dizzy had no vacuum hose attached at all. I have since replaced this hose, but have not had the car on the road (no brakes at the moment) to check on the effects. All of the other pictures are after the work I've done on the ignition system and some vacuum hoses (including the poorly run, but reconnected, vacuum advance hose and the formerly missing hose from the valve cover to canister).
 
Well it appears you have the correct manifold, but I can't tell anything aout the Vaccuum advance unit. Anyone else?
 
Hello Goloch,

the 1972 MGB U.S.A. model used HIF4 models AUD434F and 434R for what it's worth.
Incidentally, when accelerating hard, the vacuum advance doe snot come into play, only the mechanical advance as the engine speed increases. Disconnect the vacuum advance when strobe timing the engine.

Alec
 
I do not completely agree with the last post made here.

Mechanical advance is in direct relationship with rotation speed of the shaft (thus the engine) it has no bearing on trottle opening.

This being said on HIF4 equipped cars the vacuum must be un-plugged for initial strobe timing.

Vacuum advance was introduced in cars to advance timing suddenly for a short time period to compensate for the following physics item.

Trottle opening change in order to induce acceleration will change the mixture ratio of gas and air. A higher ratio of fuel to air burns more slowly and thus must be ignited in advance until ratio is equalized.

Vacuum advance is only useful at low rpm's which is one of the reason that racing distributors don't have any. Also if you run a radical camshaft the vacuum characteristics at low rpm's are so poor that vaccum advance becomes ineffective.

As a reference there a book written by Peter Burgess (How to power-tune an MGB) which has a whole chapter on ignition.

Finally please be aware that UK and North American version of the HIf4 carb are different whereby the north american model has no provision for vacuum (thus on the manifold) and the UK version has a ported vacuum set-up similar to the HS4.
 
[ QUOTE ]
as I'm in school and have to prepare a test I'm giving later today

[/ QUOTE ]
I see you are working just as hard for your test today, as I am studying for mine tonight /ubbthreads/images/graemlins/tongue.gif
 
OK, my brain hurts from pouring over all of the leads this discussion has given me.

It seems like we all agree that the carbs are indeed HIF-4, not HS-4, and that the manifold matches. I checked a photo of the distributor (the car lives at my parents' house > 1 hour away /ubbthreads/images/graemlins/frown.gif) and it's 40897 - a 25D4 from 1965 with ported advance according to the table Eric provided. It's probably a safe bet that the DPO just swapped the distributors and didn't even pay attention to the advance unit.

Now that I've confirmed the identity of the carbs, the next puzzle is how to ensure that the vacuum advance is working correctly. The plate with the engine number is missing so I'm not even sure this is the original engine; the carbs and manifold now indicate it might not be. Any tell-tale signs that can help identify the engine and give some clues as to which advance unit should be there?
 
just a thought.. if you are wanting hs4's at the moment there is a decent auction going on eBay (item 7966209468)
There is about a day left on it.

figure maybe $75 (including shipping) for all of it, then approx $135 for the rebuild kit... beats the $535 Moss wants for a new set.

it won't help solve the unknown engine type though. but it's a thought.
 
Got the carbs off and they are AUD550R; I'm assuming there's no difference between AUD550 and AUD550R. Still don't know if the engine is correct.
 
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