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MG MGB Technical - SU HIF4s dripping fuel

Recently rebuilt these HIFs (standard kit) and installed on my 79 roadster along with tubular header. Ran great (!) for a month, then last week after a 30 minute trip down the freeway I was off the freeway in traffic when the car suddenly refused to idle. Managed to get it off the road, and when I looked under the hood imagine my horror to see fuel dripping out of the foam air filters.

Popped the filters off and saw the fuel level was up past the bottom of the pistons. Lifting the pistons made the level drop, but letting them back down brought it back up. (Ignition was off, fuel pump not on.) Opened throttle and the fuel gushed out with a bubbling hiss which subsided fairly quickly. After this, thie fuel level dropped out of sight.

Mopped up the remaining fuel, waited for 2 - 3 minutes, then started the car (required a couple of tries, and stumbled a bit but then smoothed out) and drove home. Next day, at the end of a short trip in the neighborhood (no freeway) it started running rough, no idle; managed to coax it home, popped the hood, same story: fuel dripping out of the carbs.

Any ideas?

Thanks!
Chris Foleen

Chris - Your problem sounds like one a friend of mine had with his HIFs. In his case the float pivot pin had come loose and the float was just flopping around in the float chamber so they couldn't shut off the flow of fuel. The other possibility would be a flooded float that is not doing it's job. If you are using a standard SU fuel pump, there is not enough pressure to force fuel past a properly operating float and needle valve. If, however, you have a high pressure replacement pump and a pressure regulator, it is possible that the regulator is not doing it's job.
Good luck - Dave
David DuBois

Thanks for the input, Dave. The fuel pump is stock.

I neglected to mention that I had to put shortened spacers between the carbs and intake manifold in order to clear the brake booster (common mod, as far as I can make out). While I can't see how this could be a problem, I thought I'd mention it.

I checked the floats when I rebuilt the carbs, and they appeared to be sound (no leaks, nothing sloshing around inside, no bubbles when submerged in water). I have heard that the floats in HIFs can be unreliable; how would I check for intermittent sticking (assuming the pivots are okay...).

Thanks again!
-Chris
Chris Foleen

Chris,

Both of your carbs are flooding? Except for coincidence, that pretty much rules out the sinking float, dirt in the needle and seat or problems with the pivot. Either it's something common to both carbs, like the pump is too high a pressure, or something was done wrong to both carbs when you rebuilt or replaced them.

If you have a vacuum/pressure guage, it's pretty simple to connect it to the filter outlet to rule out pump pressure.

Other than that, is it possible that you accidentally connected the fuel line to the vent ports? Is it possible you made a mistake in your method to set the float levels?
Matt Kulka

Sounds like the needle and seat are not closing correctly. Could be dirt. Hate to say it, but your probibally gonna have to pull them and see whats happening.
J Arthurs

I experienced flooding with SU HIF's; I replaced both float needles with some used ones I obtained. These had a rubber-like material around the seating area, unlike the original ones which had a metal seat surface. Also checked the float height to the recommended value; flooding problem gone.
Willie L.

Thanks for all the input, guys. I'm going to "have 'em off" and look at the floats; it is possible that in reassembling the carbs I somehow boogered both floats.
However, I can't help thinking the problem has more to do with the obvious pressure/vaccuum "lock" released when I open the throttle. Could this be the result of an intake or exhaust manifold leak somewhere, or a compromised evaporative fuel recovery system?
Chris Foleen

Dave replied off-line to ask if I'd checked "float drop" and if I was using the standard needle valves instead of the Grosse. In the interest if keeping this thread complete, I'm copying my reply here.

I did check and set the float position as recommended in the manual, by inverting the carb so the needle valve was held shut by the weight of the float, and then bending the brass pad until the float would rest in the correct position (spec. is 1mm +/- 0.5mm below the rim, I set to 1mm). Also checked the free motion of the pivot at this time. I spent some time at this because I had been forewarned that the floats in the HIFs could be a source of problems.

I am going to remove the carbs and re-check everything, including the hose routing. I also suspect that I should have put sealant on the manifold gasket before bolting everything up...
Chris Foleen

This thread was discussed between 01/02/2003 and 02/02/2003

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