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MG MGB Technical - 74 MGBGT - Gift Horse

Warning: This is a long post.

I have been lurking on this board for almost five years but seldom posted anything. Most of my experience has been with US scrapiron. I really don't know enough about these cars to respond to the problems of others and my 71 roadster has been like the energizer bunny. Plenty of premium gasoline, change the oil and a little grease and it just keeps running and running and running. If it ain't broke, don't fix it. Anyway, I have just been lurking around and trying to learn something.

My Daddy always said: "Don't ever look a gift horse in the mouth." A few weeks ago, I met a man who had a '74 GT that he said had been sitting in his driveway for about 18 months. I tried to buy the car and he would never make me a price. Finally, last week, he said; "I will give you the car if you will move it out of my driveway" The price was right so it is now sitting in my driveway.

The car looks to be in reasonably good shape It is a white, wire-wheel car with tan interior. It appears to be virtually original and not boogered up by a DPO. It has a small dent in the left front fender and several spots of bubbled paint. Metal under the bubbles is rust pitted but there is no sign of rust-through. Inside, it needs carpet, door panels and seat covers. Everything else looks to be in good shape.

I replaced the battery; drained the gasoline; and replaced it with 93 octane. The old gasoline had a funky reddish brown color but no sign of trash or rust particles. With a new battery and new gasoline, it turned over really well but would not start. I could smell gasoline and it wasn't getting fire. I replaced the points, rotor, condenser, cap and wires. It started right up. The engine sounds solid and the gage said 75 pounds of oil pressure; but it will not idle, black smoke is rolling out the exhaust and there is a really strong gasoline smell. I had a little trouble figuring out where the gasoline was coming from. The car has all of the original emissions controls and gas was coming out of the anti run-on valve and draining down onto a gravel drive. It wasn't visible fron the top. With the ignition switch on and with or without the engine running,the fuel pump pumps gasoline straight through the anti run-on valve and out onto the ground. Any ideas as to why? Because of a recent move, my manuals are buried deep in one of many unopened boxes in my basement.

I fear that the problem is in the carburetors. They appear to be original(HIF-4's I believe)and I don't think they have ever been touched. I have never worked on an SU carb but I have watched the ones on my 71 torn down to the little bits. The HIF-4's look to be more complicated and the linkage is really intimidating. HELP!!!

A couple more questions and I quit.

(1)When I pulled the dizzy (25D4)there was a knob or nut and spring on the outside of the case. The Moss catalog says it is an adjuster. What does it adjust?

(2)There is a lot of discussion on this board about distributor advance curves that are way over my head. The inside of the distributor is clean but it looks like the heater control valve has leaked and the vacuum unit on this car is covered with rust. I doubt that it is working. My question is do I really need it? My 71 has a Petronics ignitor and the vaccum unit is disconnected and the intake manifold is plugged. I don't know how it would run with it, but it runs extremely well without it. Prices of a vacuum unit in the Moss catalog range up to $95 US. I don't want to spend that kind of money if it isn't going to help.

I cannot close without Kudos for the people on this board. The advise is fantastic and sometimes sooo simple. The gas tank on the GT doesn't have a drain plug and I had no idea how to drain it. I visited this BBS and , wonder of wonders, someone had the same problem and the siolution was unbelievablely simple. Remove a gas line, drop it into a suitable container and turn on the ignition switch. Brilliant!!! I emptied almost 10 gallons of gas in a matter of minutes.

A Sneed

Sounds like the floats in the carb are not doing their job. You can try giving the carbs a good whack while the fuel pump is running and see if that cure the problem. Most likely the gas had turned to varnish in the bowls and gummed up the works. Fresh gas will disolve some of that varnish so you might be able to get away with just seating the needles. Otherwise you will have to dismantle and clean.

The screw on the distributor fine tunes the timing. Most people rarely use it for anything. Just because the vacum canister on the dizzy is ugly, do not assume that it does not work. Even if it does not, the car will run reasonably well, however the gas mileage will suffer without vacum advance (or retard, depending on how you look at it). If all else fails, load it on a trailer and ship it to me, be glad to free up the space in your drive ;)


Nice gift! The HIFs are simple - the linkage looks a bit Heath Robinson but you can't go far wrong. If in doubt take a couple of pics of it in situ. Access to the float bowls is from below (4 screws) but if you are really lucky a smart tap to the side of them will free the stuck float.
Steve Postins

Thanks Pete and Steve. I tried the hammer bit; it didn't work. So I guess it is pull the carbs and clean them. I was afraid that might be the solution. I guess I had better start opening some boxes and try to find my manuals. I think I also have a video on SU carbs somewhere. I just got an e-mail from the president of the local British car club asking if I would host this month's meeting. We have a couple of shadetree mechanics in the club. Wonder if I could get one of them to do a tech-session on HIF's :)?

Thanks for the offer Pete but I think I'll keep it. If I can't get it to run, I can always cut the top off of it and make a nice flower pot.
A Sneed

The tech session is a good idea. You will not be the only person that learns something. A meeting with a tech session is always more interesting than sitting around and talking.

Clifton Gordon

Get some fuel injector cleaner, put it in the tank, drive it around a bit to get it mixed up and into the carbs. It may well free up the float valves without taking things apart. I am not a fan of "magic remedies", but this stuff has saved me a lot of trouble on a number of occasions.
FR Millmore

I had never reworked a carb before, either. I have HIFs, and felt pretty intimidated by the prospect of going into the "innards", too. But, I did, and they're pretty simple. I had a slightly different problem, but replaced the needles and seats, all by myself...well, after helpful advice from this site.

They don't scare me, anymore.
Fred Doyen

Well, I'll toss my 2 cents in here. I have had a few dozen brit bits and I would say replace the carbs with a Weber. I know, I know, the purists are shaking their heads and moaning. Gee, keep the old carbs if you want... but use a Weber to drive the car. I wrestled with the carbs on my 74 roadster over and over. They won. Of all the SUs I have played with on Austins, Triumphs and MGs, they are the most difficult I have encountered. Now, with a lovely Weber, I have a very reliable enjoyable (albeit non-original) car. Give it due consideration... After playing this game for 30 years, I go for ease and enjoyment... and forego the purity at times! BILL
William Andrew


Take the carbs off and disassemble them Clean the old gummy varnish off and make sure the lines from the bottom of the bowls to the jet are clear. Blow through them. I think that your problem is, that the floats are not stopping the gas from entering the bowls when the proper fuel level has been reached. Also check to see if you have a SU fuel pump. If the previous owner had a problem with the fuel pump and replaced it with a regular electric fuel pump,the pressure may be too much and is overpowering the float needle,causing the bowl to flood. SU fuel pump has a low pressure (I think about 3 lbs) Get a SU manual for $10.00 and follow the directions. Loosen the inter connect shaft that joins the carbs, set the jets,set the idle, set the floats, all easy stuff. Finally reconnect the interconnect shaft without moving throttle plates on either of the carbs. Buy a Uni syn carb tool, a necessary item for balancing airflow through the carbs. As I said easy stuff and a chance to educate yourself about the tuning of SU carbs. Alyn

I found it really useful to read a book that explains how the carbs actually work. Once you get your head around how it works (in some detail), the idea of taking it apart becomes far less intimidating!! I read "Fundamentals of Motor Vehicle Technology" by Hillier (ISBN 0-7487-0531-7) which has a very good chapter on carbs. It also explains in some detail all other aspect of the mechanics of cars of around the era of the MGB. Lots of diagrams etc.

I D Cameron

If you disassemble the carbs, do them one at a time. While SU carbs are pretty simple, it is nice to have one intact example when trying to re-assemble a pile of pieces that you are not familiar with. Just remember that the two carbs are mirror images of each other.

If you have a digital camera (or even a film camera), take some pictures of the linkage and hoses before you remove them. It will help when you re-install them.

Paul Noble
Paul Noble

I agree that no doubt the problem is with the carb floats - assuming that your fuel pump is delivering the right pressure. And it's likely that the problem is as everybody has suggested: varnish and/or crud from bad gas. However, don't overlook the probability that the floats themselves are leaking and full of gas, therefore sinking in the float chambers. Regardess of what you find, save yourself having to do the job over: REPLACE the floats. This has only happened to me once and it was caused by a leaking float. It happened in front of a hotel 800 miles away. Moral: always carry spare floats on long trips. While the old HS4s used the same float front and back, on the HIFs, they're different, and accordingly, more expensive, of course!

I'll have to respectfully differ with Bill on Weber carbs. Over the years, I've known of about six or eight friends who switched to the downdraft Weber. In all cases, disappointments with performance arose. I suggested rebuilding the SUs as necessary and going back to them. They all did and they ended up with improved performance. If your SUs are hard to tune, either they're worn out, the engine is worn out, there's a vaccuum leak, or there's a problem with valves or ignition. If all these are reasonably OK, (always do ignition before messing with carbs) SUs are a breeze to set up, and I typically find that I can leave them alone for 25-40,000 miles. There's nothing simpler than an SU carb. I'm not strongly committed to originality, but when something works so well, I'm not inclined to replace it.

And now, excuse me while I go play with my new MSD ignition...

Allen Bachelder

I appreciate all the comments; and as I suspected in the beginning, I am going to have to rebuild the carbs. I have made some improvement, however. I bought a spray can of carb cleaner and I soaked the carbs inside and out. I sprayed as much as I could into the float bowls and let it sit for about two days. It still doesn't idle but it starts easily and I don't have gas pouring out of the anti run-on valve.


When I bought my "B" several years ago, people told me that SU carbs were high maintenance. I bought all the necessary tools, including a Uni-Syn. I also bought repair manuals and a video on SU carbs. They are presently hidden away in one of many unopended moving boxes in my basement but It looks like I may finally get to use them.


As far as SU's and Webbers are concerned. I have a "71 mgb roadster and a "70' E-Type Jag. The Jag has three SU's and they haven't been touched since I bought the car. I have had the Jag for about four years. While building a new house and some other personal distractions, the Jag sat in a friend's basement for two years without being started. I recently retrieved the car. I charged the battery and I don't think it turned over more than a dozen times until it was running like a Swiss watch.

When I bought the B, it had carb problems. A local shop sent them to an SU specialist; I may be wrong but I think his name was John Taylor. The carbs were totally rebuilt, including oversized throttle shafts and removing the little spring thingies from the throttle plates. The carbs were re-installed and adjusted and they haven't been touched since. And, that was at least five years ago. The car starts and runs beautifully. It has never failed to start and I don't ever need to use the choke. From my limited experience with SU's, I have to agree with Allen. SU's that are properly setup are very reliable, and I certainly don't have any trouble staying up with some of my friends who run Webber conversions.

Again, thanks for the input.


A Sneed

I was always told that SU's reguired a lot of "fiddling with". After two healy's and three MG's I have learned. They only require fiddling with if you fiddle with them. Get them rebuilt, adjust them using the correct tools (syncronizer and a color tune are invaluable) and leave them alone. The one's in my TD have 15 years without touching the adjustments.

The one's in my B are out for rebuild as part of the restoration.
Bruce Cunha

A Sneed, is it possible that your carbs were sent to John Twist? He's a wizard with them!

If you want your distributor gone through, you can email me and I'll give you the info to ship it to me. I can make sure the curve is (close to) accurate and clean and lube it for you. At the same time, I can test the vacuum unit and throw on a used one cheap if you need it. I highly recommend it for performance reasons. You'll understand why it's there when you have it working!!! All this for the cost of postage (and a week without your car running.)
Jeff Schlemmer

This thread was discussed between 02/08/2005 and 12/08/2005

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