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Emission-Carb Rebuild

Making sense of all those extra smog parts

Some 1980s emissions carburetors are horribly complicated, and require a bit of head scratching to sort out all the widgets. Fortunately, most of the rebuild job is a tabletop affair; although, it's a good idea to ask permission before spreading bits of carb all over the dining room table. Some wives and girlfriends think this is worse than the toilet-seat felony and, if they start throwing plates, you could lose some of the smaller carb pieces. (Of course, if the mechanic in the family is from the distaff side, there's no problem, right?)

Complex Carbs

Post-1990 fuel injection is almost universal now. Further back in time, people using old carbureted Jag or MG engines or whatever would only have a couple of simple SU carbs to rebuild, which is usually quite easy. In the middle period, you have complex carb technology to deal with.

While the temptation is to remove all the emissions stuff off an '80s engine and just keep it simple and well tuned, in some states you need the emissions gear visibly left in place to get through testing, so don't bin it. And in Canada, theoretically you need to keep all the stuff on your engine for its existence to be checked, as well as getting acceptable readings.

The carb shown here actually came off a Jeep Grand Wagoneer, which runs a 360 cubic-inch (5.9-litre) small-block AMC V8. It was a surprise to find a Ford Motorcraft carb on it, hidden under all the macaroni of vacuum piping for the emissions gear. The carb itself was vaguely familiar, akin to one seen on a Ford 2.8 V6. It's only a two-barrel and, even though you'd get more power with a four-barrel, there's more or less enough air and fuel getting in for the torque needed to heave around the Jeep's enormous bulk (provided you're not in a rush).

The Jeep won't be chasing musclecars even with the carb rebuilt, but hopefully it will yield better fuel consumption as well as getting through the emissions test. Before starting, there are a few general points worth mentioning.

Starting Points

First, disconnect the battery if the engine is in a car. The risk of fire is small, but not worth taking.

Check every inch of piping and wiring to the carb for cracks and splits. A small split in a vacuum pipe could be the cause of all sorts of trouble.

The PCV or Positive Crankcase Ventilation System is usually a tin can filled with carbon. Check its connections as well.

Be methodical as you strip the carb, and put the discarded bits somewhere off to one side so you know where you are in the process, and don't get mixed up.

Engineers: Don't read this next bit. The hollow nuts securing the fuel pipes to the carb are old, soft and corroded in place. It may be better just to use a monkey wrench on them straight away rather than rounding them off with a spanner first.

The carb is made of soft alloy held together with hard steel screws and bolts. Be careful and gentle or you'll strip threads. When reassembling and screwing the top back on, it's worth spreading the load by tightening screws in stages and in a sequence, rather like tightening down a cylinder head (to be sure the top doesn't warp).

Automatic chokes waste fuel and are generically unreliable. The approach we're taking on this carb is that if the autochoke works perfectly, it can stay. As soon as an autochoke fails, trash it and get a manual choke conversion with which you can turn off the choke after a few hundred yards.

It's also worth checking the (usually two) main fuel filters, as well as the little mesh filter screens inside the carb, and to check the spark timing as well. The Exhaust Gas Recycling valve in this case was also suspect, so it's been replaced. These EGR valves get clogged up like a chimney with carbon, and need regular cleaning or replacement. Their function is to reintroduce inert exhaust gas into the combustion chamber to lower the combustion temperature and cut nitrogen dioxide smog emission. When dodgy, they can give you similar symptoms to a carb in need of a rebuild, and the nitrogen count fails emission tests.

To get the right rebuild kit, check out the numbers stamped on the carbs. Many of them are casting and manufacturing references, but one of them will be the number you need to identify the carb. If the ID plate hasn't been lost, good news. If you still have the vehicle's VIN number and the carb is original, that will help too.

There may be substitutions in the box of rebuild kit bits. The power valve in this case looked completely different to the original, but a piece of paper in the box told us it was a new version and still compatible. Presumably they've found a way of making it more cheaply.

The only slightly tricky bit of the process is adjusting the float height, but it's usually just a matter of checking it. Tempting though it may be to do it first and read the instructions later, this is one instance where you should read the comic first.

Okay, my gorgeous wife is out of the house, so it's time to strip the carb on the coffee table.