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MG MGB Technical - HIF4s still overflowing

I did a partial rebuild on my HIF4 carbs a few weeks ago. When I took them apart I found that a rebuild had already been done within the past 10 years so I concentrated on the float bowl area to try to solve my intermittant overflow problem.

I replaced the floats, needles, seats, and bowl gasket among a few other unrelated items on the carbs.

At first it looked like this had solved the intermittant overflow problem (car off, ignition on). However, it has returned.

If I'm working on the car and turn on the key to test something sometimes I'll get pump clicking and overflow but most of the time I won't.

I've got a pressure regulator on the fuel system set at 2psi and a filter just before that. I've got new fuel lines from the filter to and through the carbs.

If I drain the bowls by running the car when an overflow occurs and then let the fuel rush in that will clear the problem.

The only thing I didn't do that I can think of is replace the float mounting/pivot screws. Perhaps there are some minute burrs on those causing the float to hang.

Any other thoughts?

Thanks,
BH
BH Davis

Sometimes its grit in the fuel that stops the valve seating. I found the neoprene tipped valves worked well, maybe they just squidge over the grit. It does sound as though you have contaminated fuel, a blast clears it, then when it comes in more slowly some grit settles in the valve.
Stan Best

I had to swap in a set of old neoprene tipped valves in my HS4's because the new SU needles were not shutting off and constantly flooding the engine(they also had noticeable ridges in the tips after a very short time). I also couldn't get the float height correct. But the old needles have cured the problem. I'd suggest finding a set through Joe Curto or a few of the other vendors.
Ron

After years of misery with HIF 4s, no matter how many times I stripped and replaced parts. I could not stop intermittant overflows.
According to an article in Enjoying MG, modern fuels do not suit the SU. So if you are using it occasionally gum residue attracts any particles.
Additives may help, or starting it up reguarly, but I fitted HS4s they will still overflow but it's minutes to cure.
c cummins

Despite my respect for Roger Parker there were several things in that article that certainly did not concur with my experience with both a 4-cylinder (HS) and a V8 (HIF). And if the V8 isn't going to suffer from fuel vapourisation under test at temperatures of over 40C then nothing is. Thousands of these cars ran in desert states in America with no vapourisation problems, and many hundreds still do. For fuel to vapourise and cause starvation it has to do so at a rate approaching 2 pints per minute for the HIF. However it might be possible for the HS to suffer problems if the heat-shield is damaged as vapouristion in the jet pipe could occur.

As far as problems with modern fuels go then there are certainly reports from America of problems with rubber melting and gumming, but then they have some pretty exotic mixtures which we don't have (yet). I've never had float valve sticking open or shut on the roadster, and that is despite having the same tank of fuel for around six months over the winter with only very occasional running. I did have problems with the V8 overflowing some years back, but that proved to be a leaking float, since replacing that it has been fine. That also does little winter mileage unless the roads are drty and salt-free, with the same tankful lasting up to 3 months.
Paul Hunt 2

Check that the two overflow pipes are not blocked, they also act as a breather to the HIF carbs. Remove them and clear, using thin wire or an airline, replace and see if the problem has gone!
Paul
P Bass

replace the needle and seat with grose jets which are far better than the originals
these are a ball-bearing built into the seat which they say will not leak as long as the pressure isnt over 5psi
moss part number is 386-340
Ste Brown

BH
Make sure you have an inline fuel filter fitted. I agree with Ste regarding fitting grose valves.
Rob
R J Collier

Thanks guys.

The polution control system was removed from my 73B before I acquired it, so the overflow tubes on the carb vent to hoses that go below the car (away from the exhaust). This is where I was getting the overflow. If I left the key on for any reason sometimes I'd get a puddle below the car, and sometimes I wouldn't.

I have an inline pressure regulator just before the carbs set at 2psi and and inline filter just before that.

The carbs had Grose jets in them when I took them apart. My first replacement were the original style with steel tips, and the problem remained.

I have now followed advice above and installed the neoprene tipped needles from Joe Curto. So far all looks good.

I finally received clear instructions on the float level setting with the Joe Curto needles. I had them a tad too high. One would think that would close the needles earlier though, reducing the amount of fuel in the bowl. Perhaps that was making the carbs overlow for some weird reason though.

BH
BH Davis

Float height has very little effect on valve closure. Once the float has left its bottom stop it will apply the same amount of pressure to the valve regardless of where the fuel level is. If it is leaking, then the fuel level continues to rise but the float doesn't, which applies even more pressure to close the valve. If you are getting leaking from the vent tubes then the valves aren't closing, either because they are faulty, there is dirt in the fuel, or the floats are partially sunk and not applying enough pressure.

Opinions on Grose jets (jets?) vary. It seems they *used* to be OK, but now aren't as good as the original type, and certainly not as good as the neoprene-tipped type.
Paul Hunt 2

In thirty years of fixing MG's I had more success with the original brass needle & seat than with the Neoprene ones but they do need replacing about every 100,000 miles. I have had two sets in my car at 280,000. There are many pirate ones available with varying degrees of quality. When examined with a magnifying glass I have seen them with chatter marks on the needle & seat, from the machining. I use only SU packaged ones. (WZX1101A NOT Z) On the HIF carb it is important that the needle is the correct height as otherwise the closing tab on the float may be at an angle to the needle end causing it to catch. Some of the pirate ones come with a washer. It is also important that the float is at the correct height -1mm below the bottom face when hanging against the needle. There used to be two different seat hole sizes available - can't remember the numbers now but someone else may be able to help with this.
Garth
Garth Bagnall

I too had an overflow problem with newly rebuilt carbs a while back. My first guess was trash in the needle and seat. After removing and checking I could find nothing wrong, well a few days later it happen again. Pulled the carbs with the same results, couldn't find any trash or sticking needle valve. Only when It happen for the third time while parked on an uphill drive I discovered the cause. A bit of wear on the float pivot rod and in the float where the rod passes was allowing the float to twist just enough to hang up on the side of the float chamber. A new set of floats and a small washer behind the screw head of the float rod to give some new surface for the float to pivot on solved my problem. I suspect this problem will start showing up more as our cars get older and more wear starts showing up in places we haven't seen before.
Bill K

Is thisa a problem that occursin hot weather, heavy traffic or such like, when underbonnet temps go up?
Expanding fuel in a hot fuel line can overcome float/valve shut off pressure.Originally the fuel line csossed the rear of the engine bay in front of the heater box, but behind a small steel shield. If you have added filters and regulators the pipework might be too exposed to radiated heat from the block, etc.
When this happens it is usually the first carb in line which floods. Try some temporary shielding with aluminium foil. Sometimes helps to remove part of the engine bay/bonnet rubber seal at the back of the compartment, to make a better through flow of air.
Allan Reeling

This thread was discussed between 21/07/2007 and 09/08/2007

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