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HIF Carburetor Rebuild
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HIF Carb Rebuild Tech Session
March 25, 2006 - Downers Grove, IL
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. 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 Schroeder for the use of his heated garage and well stocked fridge.
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 was a little prior business to attend to at the other end of the car; jacking up the rear end and changing 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. Too 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.
The first oddity encountered with these carbs was that one had the butterfly plate poppet valve soldered shut, but the other one didn't. The poppet valve is designed 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. As more and more members arrived, it seemed 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 you 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.
The next step was 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 were being somewhat stubborn. Look out! Cowboy ended up wielding his favorite tool again! No problem, the screws were finally removed without damage.
Finally, the carbs were 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 was encountered which required removal and re-installation of the carbs. 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 final proof is in the pudding, as they say. Dave Mullis took his car for a test run and returned with a big grin of approval. The entire job was finished in 5 hours including the minor rear suspension work, plenty of show and tell during class room time, cleanup and test drive. The half of the motley crew remaining at 3 PM was happy to mug for the camera. Less than an hour later after the car was returned home and the Team Mullis has a short personal cruise, Carol and Dave shared 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 had 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 for work where parts would not be intermingled with other carbs. A third set was present for show and tell, questions and advice. 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.
See cover photos.
Also see 40 photos of the HIF carburetor rebuild at www.chicagolandmgclub.com/photos/hif_carb