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MG MGB Technical - HIF Carb adjustment

I have a 78 MGB. Would I be right in saying that this has HIF carbs as opposed to HS carbs? They look vastly different to whats detailed in the haynes manual. Now, whats happening with my engine is its racing on idle, backfiring occasionally, is lacking in power and absolutely drinking petrol. The smell of fumes is overwhelming so that would say to me that the engine is running far too rich. The problem I have is I cant find an insctruction sheet anywhere to sort out these carbs. All instructions Ive come across detail the HS carbs, and thats no use to me. Im of the opinion that my carbs need a rebuild, so thats what Im going to do. I need instructions with exploded diagrams and details on adjustments necessary before I can start though. Can anyone point me to an online manual or link which details the carbs for me so I can get my car running right. I cant afford to run it with the amount of fuel its using and Im afraid of the amount of fumes in the car as well. This is the single biggest problem I have with my car now and I do need to get it sorted out, so any help that can be provided would be very much appreciated. Thanks.
Ross Kelly

Sounds like a problem with the fuel inlet valves. See the earlier thread of "HS4 vs HIF" for info on this.
David Lieb

I learned much from this: http://www.niksula.cs.hut.fi/~mdobruck/siililand/mini/diy/3/su-tune/su-tune.html
Pete

Originally a UK spec 78 would have had twin HIFs, but it could have anything now. Haynes should show both, unless it is an early edition, and you will need a later addition for other features and components on your car. The main difference between HS and HIF is that the HS has the float chamber bolted to the side of the carbs - in front of the front carb and behind the rear carb, and the main jet is exposed below with the jet pipe going into the bottom of the float chamber. With HIFs the main body is extended downwards and the main jet and the float are contained within it.

Setting up the two is very smiliar, the main difference being on an HS you use a spanner to turn a nut on the main jet underneath the carb, and with the HIF you turn a screw on the side, facing forwards on the front carb and rearwards on the rear.

Gross richness and massive fuel consumption could be caused by leaking float valves, but in this case fuel would be pouring out of the overflow/vent pipes and onto the ground and be very obvious. On the other hand if a vent pipe is blocked it prevents the float from rising to control the float valve and fuel floods into the intake, making things even richer.

You might like to have a look at http://www.mgb-stuff.org.uk/wn_fuelframe.htm and click on 'SU Carbs' for info on problems and setting-up.
Paul Hunt 2

I have completely rebuilt one carb now, and with dirty hands and before I get fully going on the next one, Ive already found a problem. The needles in each carb are different. One is longer than the other. Theyre not a matching pair. The whole piston in fact is slightly different, as is the jet from what Ive seen so far. Will this create a problem? Do I need at least one new carb or a new needle and jet? The rest of the body of the carb looks identical. Its just that. Where do I go from here? Ill finish rebuilding them anyway but is this going to be a problem?
Ross Kelly

If the bodies are in fact identical but the pistons, needles and jets are different, that seems wrong to me. Someone may have swapped parts out of a different carb. When I work on my carbs, I don't even let the front piston get put back into the rear carb vice versa, as the pistons and carb bodies are very closely matched. This very well could be the reason for your rough running, gas smell, and fuel consumption.
If one piston were to get stuck up in the chamber, maybe that would cause the rich running especially at idle.
See if you can find an identifying mark on the needle base, usually starting with the letter A, and usually having 3 letters. My guess (but couldn't find a definitive answer) is yours should be ACD. If one of your pistons has an ACD needle, I would bet that is the correct one. I think you need to find the correct parts for your carbs, and make sure the carbs are correct also (in Canada, 1978 Bs had single Zeniths, but in the UK they may have had 2 HIFs still).
Erick Vesterback

My car should definitely have the HIFs. It was only US spec cars that had the Zeniths if I understand correctly. Anyway my carbs are back together now and ready to go into the car aside from one other job. That job is to set the float level. People keep telling me to set it so the top of the float is in line with the aluminium casting of the carb, but this is clearly bolloxology cause the float doesnt sit that low down regardless of what you do. Wtat reference point on the float should I be using to line up with the top (bottom really) of the casting when the carb is upside down?
Ross Kelly

OK I think I have the floats sorted out. We'll see tomorrow anyway. With the needles, further research has led me to the conclusion that in one carb I have a spring biased needle, and in the other a fixed needle. Hmmm. I think Ill be picking up a new set of needles. The weird thing is though, nowhere does it say that the HIF carb ever came with fixed needles. Does the piston from a HS carb fit a HIF carb? Maybe thats whats after happening.
Ross Kelly

Whilst export markets got the Zenith UK continued with SUs to the end.

The jets *do* differ between th carbs as they are handed. The carb bodies and floats are also handed, but everything else is the same for both carbs including jets, pistons and covers. If there are visible differences between things that should be the same you will probably never get the car to run right as the carbs will have different characteristics. My advice would be to cut your losses and swap them for a reconditioned matched pair.

The change from fixed to swinging needles probably was with the change from HS to HIF. The part numbers for the pistons and covers are different, but that could just be down to the different needle arrangements and other than that an HS piston my fit (to some extent) an HIF body and vice-versa, I've never tried personally. Pistons and covers were supplied as matched sets for both types and must not be mixed up, the internal clearances are precision.

The highest part of the HIF float sits above the rim of the float chamber. A straight-edge is laid across the chamber over the middle part of the 'U' formed by the float, which sits below the rim. Or to put it another way the straight-edge is laid across at right-angles to the float pivot pin and about mid-way long its length, or the middle of the chamber. There should then be a gap of .04" +-.02" between the bottom of the straight-edge and the nearest part of the float directly below it.
Paul Hunt 2

Im thinking I will pick up a set of reconditioned carbs actually. Ill have nothing but trouble with the ones I have. I just couldnt get them working properly on the car today. One thing I will have to do as well is solder the poppet valves shut in the butterfly valves cause in the front carb its leaking, making tuning the engine a bastard. ill do that during the week. Im not rushing out to buy carbs just yet. I dont have the money, and I want to weigh up my options and find places to get them.
Ross Kelly

Ross,

This may help explain some of the features of your set-up.

As a supplement only to the excellent advice above, from Clausager (P80):

"With the introduction of the first 18V engines in 1971...Home market cars continued with the HS4 carburettors, now specification AUD 492 fitted with spring-loaded AAU needles...found on engines of types 18V-581, 582 or 583 with a letter F after these code numbers in the prefix. {export cars fitted with SU HIF4}.

HIF4 carburettors were introduced on home market cars in November 1973 with the engine types 18V-779 or 780. These were also fitted with the AAU needles and carburettor specification was AUD 616. These carburettors were carried over on the 'rubber-bumper' models in late 1974 but needles were changed to ACD and the carburettor specification became FZX 1001. The final change to the carburettor specification on home market cars was with the introduction of tamper-proof carburettors of specification FZX 1229. These can be recognised by having much shorter necks to the bellhousings of the suction chambers. It is not certain when these were introduced, but they possibly came at the start of the 1977 model year (car number 410001 in June 1976)."

For someone who is about to have his first serious go at re-setting his HS4s in conjunction with engine tuning I appreciate all the contributions above. I understand so much more than I might have otherwise.

Regards
Roger
Roger T

I managed to find someone here selling a set of carbs that came off a car cause the owner was replacing the carbs with a weber twin venturi jobbie. Anyway €100 plus the rebuild kit I ordered, and Ill strip and rebuild them and Ill be happy!
Ross Kelly

Grrrrrrrrrr I really need those new carbs! My car died today in traffic and wouldnt restart, and its running far too hot. Im going to borrow a company car tomorrow and get the distributor and carbs pulled out in anticipation of the new parts. I spent 20 minutes pulled into a loading bay today after work getting my hands oily trying to find out why the car died. I sprayed WD40 into the distributor to clear any water that might have lodged in there from when I washed the engine earlier with degreasant and a pressure washer (looks much better now!), and I checked all the cables, looked for a spark, which I had so deduced that it had to be the carbs, so I checked to see if they were getting fuel, which they were, so I tried restarting. Still nothing. Then I had the bright idea of putting the choke on to start it, and she fired up and after a few revs, idled OK and I was on my way. But I cant live with it being like that. Ill order the new carbs tomorrow. I was waiting to get paid but I need this sorted now.
Ross Kelly

This thread was discussed between 28/03/2008 and 02/04/2008

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