Chicagoland MG Club:Photos
MGB SU HIF4 Carburetor Rebuild
March 25, 2006 - Downers Grove, IL

HIF4 carburetor inverted with main fuel jet and adjuster parts removed.

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The patient for the day was the "Team Mullis" 1972 MGB owned by Dave Mullis. It has had sick carburetors for some time with the rear carb flooding badly. We suspect a sunken float, or a stuck float valve, no way to know until it comes apart. With the HIF carburetors the float chamber is in the bottom of the carb, so the things have to be removed from the car for service. Thanks to John Schroder for the use of his heated garage and well stocked fridge, this is going to be a cozy work party.

No shortage of help today. With more than 20 club members in attendance we did our best to spread the work load around, but some of the willing hands had to watch anyway. While the carbs were being removed there a little prior business to attend to at the other end of the car, jacking up the rear end and to change the rebound straps. This is not a big job, as long as you don't break the threaded studs which are a welded part of the rear axle housing. Sometimes penetrating oil will not free a rusted nut, but a nut splitter always will.

The job was getting off to a quick start with piping hot cinnamon apple muffins. To bad there weren't enough to go around, but late comers beware, you snooze you lose. Very shortly the carbs were out of the car and ready for dissection. Notice the square bottoms on the HIF carbs which house the Integral Float chambers. With carbs out there was time for some quick clean up work in preparation for their return. Also time to finish off the muffins while the carbs were being cleaned up a it before public presentation.

Gotta figure this is one of the most pampered cars in the club in recent years. This is the third tech session in a row intended for getting this car roadworthy. Prior sessions attended to all fluid changes, brake flush and adjustment, new hand brake cable, lots of minor electrical fixes, repair of a roll up window, bonnet latch, boot latch, and a general tune up, excepting for this one stubborn carburetor. While we were a little short on help during the heart of the February sub-zero cold snap, there are plenty of hands today. But as these things go, there should be more students than instructors.

The first oddity encountered with these carbs is that one has the butterfly plate poppet valve soldered shut, but the other one doesn't. The poppet valve is intended to eliminate overly rich running and backfiring when high vacuum exists during overrun (coasting down from high speed). For this to work the spring on the poppet has to be precisely calibrated to keep the poppet closed in most running conditions, and to open briefly only when the vacuum is highest with throttle fully closed at higher engine speeds. Unfortunately the thin springs often go soft after many years of heat cycling. When that happens the poppet may remain open and the engine will not idle. The "proper" fix is to replace the throttle plate. The more common expedient solution is to solder the poppet shut. Then you get back to that more "sporting" sound of minor backfiring during high speed coasting. Seems like we could use some more space for observers, so we put the car outside while we proceed with the carb rebuild.

The first check and demonstration requires removal of the dashpot cover and air piston which carries the fuel metering needle. Looking into the throat of the carb we can see the top end of the main fuel jet snuggled in just below the flat surface of the bridge. Turning the mixture adjusting screw clockwise lowers the jet, making the mixture richer. Counter-clockwise raises the jet, making the mixture leaner. We will see more of this later, but for now it is assuring to know that the jet is not frozen in place. Next step is to remove the throttle shaft to check for wear on the shaft and to replace the shaft seals. Here you need to remove a pair of small split tip screws, which are being somewhat stubborn. Look out. Cowboy is wielding his favorite tool again. At least give him credit for using a smaller hammer on a carburetor. No problem, the screws were finally removed without damage.

The first picture here shows the HIF carburetor inverted with the bottom cover removed. The fuel jet is also removed along with the mixture adjusting parts. The "L" shape bracket is held in place with a shoulder screw and spring to be articulated. The mixture adjusting screw bears against the longer leg of the "L" giving it a push to change the rocking angle of the bracket. The darker color straight plate on the short leg of the "L" bracket serves to move the fuel jet up or down as the bracket rocks. Additionally that flat plate is also a bi-metallic strip which will bend according to temperature changes. This allows for temperature compensation for the fuel mixture. When the temperature in the engine bay increases, the fuel mixture tends to go lean. With increasing temperature this bi-metallic strip will bend downward to lower the fuel jet slightly to increase fuel flow, maintaining the proper fuel mixture over a wide range of operating temperature. This feature was used for emissions control during 1972 to 1974 model year production. A nice side effect is that this also makes the HIF carburetor less likely to suffer from vapor lock, which is sometimes a problem with the predecessor H and HS type carbs during hot weather and very slow driving.

The second picture above shows the fuel jet and float removed, and a nut driver about to remove the float valve. That's about all there is in the bottom of the HIF carburetor, so it is not particularly complex or mysterious, once you get to know it up close. The third picture above has Cowboy checking the float level using a digital caliper. The calibration height is not that critical, but the caliper happened to be close at hand. The last picture above shows the bottom end assembly complete just before installation of the bottom plate and O-ring seal.

Here we are using the Dremmel tool to polish out the bottom plate for the rear carburetor. As it turned out the float was okay but the float valve was stuck open, as that carburetor had a nasty accumulation of assorted gunk in the float chamber. Next we have installation of the rubber seals on the throttle shaft, followed my a minor miss-step as Cowboy installs the mixture adjustment bracket before installing the float. Oops. Do over.

Here we have the dual carbs fully reassembled and ready for installation in the car. Fortunately there are only four bolts required to mount these carbs. After initial installation a small problem is encountered which will require removal and re-installation of the carbs. Who can spot the assembly error, visible in the first and third pictures? The throttle shaft had been installed wrong end around, which has the center lever arm pointing in the wrong direction. Oops. Do over again. The last picture above has the carbs off, looking at the outlet ends with the position of the throttle shaft corrected.

The final proof is in the pudding, as they say. Dave Mullis takes his car for a test run and returns with a big grin of approval. The entire job was finished in 5 hours including the minor rear suspension work, plenty of show an tell during class room time, cleanup and test drive. The half of the motley crew remaining at 3 PM is happy to mug for the camera. Less than an hour later after the car is returned home and the Team Mullis has a short personal cruise, Carol and Dave are sharing another grin of approval.

There are often other things going on in the wings at our club tech sessions. Seth Jones brought out for demonstration the club's vintage valve grinding machine which he has recently restored to operational condition. There were also at least two additional sets of HIF carburetors on site. One set was disassembled very early in the day, then moved elsewhere (presumably to the basement) for work where parts would not be intermingled with other carbs. A third set was present for show and tell, questions and advice, not sure those were opened up the same day or not. All in all nearly two dozen people were present for part or all of the session, and many of those people are now qualified to rebuild SU HIF carburetors.

Greetings from Chicagoland MG Club. Visitors are always welcome.

Also see report in April 2006 Driveline.

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