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  Chicagoland MG Club:Club Info

Just a Simple Wrench

by Jake &Ann Snyder


The Œ71 roadster had been running more roughly with each week, and there was not really time nor space to fix it. A much higher priority was replacing the rusted portions on the 72 GT with enough steel to hold a coat of primer over the winter, for which it must be ready.

The big problem with the roadster was that it was missing terribly on acceleration and at engine speeds less than 2000 rpm. This limited the problem to the fuel system or the ignition system, which does not do much to isolate anything. Leaving from a stop light consisted of doing an old fashioned Road&Track start: rev it to 2200 and dump the clutch. Definitely not good for as long as the 72GT would be first on the list to be repaired, as starting like this did not help the roadsteršs drive train.

So, one evening before it got dark, there being no space in the garage, as it were, we put on the disposable vinyl gloves and started with the spark plug wires in the driveway. The vinyl gloves do pass a substantial voltage, so we pulled the plug wires with great delicacy. Got shocked, anyway. The ignition system was fine. Two plugs were changed, after noting that the engine was still fitted with the N11YC winter plugs, instead of the N9YC that are proper for virtually all use. We had left the N11YC plugs in to help pass the EPA inspection in spring, and forgot to change them for summer. Now winter is almost here. (Too hot a plug is very dangerous: They can cause predetonation and body-piercing in a piston.) Then the distributor was advanced a little, with no help. The engine still rocked enough to threaten the motor mounts between 1200 and 1800.

Next, off with the air cleaners. The Uni-syn carburetter synchonizer said that the carbs were as close as they needed to be for air flow, and the mixture on both were nearly perfect: lifting a piston about a 32nd of an inch with the lifting pin gave a slight hesitation, then back to normal revs. The carbs were set perfectly at idle. But something just did not work right on acceleration. With every thing okay, why did it run so poorly at low revs? So more plug wires got pulled with more shocks as the result - the ignition was still fine. At last, flipping the throttle linkage to race the motor showed that the piston on the rear carburetter rose about a quarter-inch before the front piston even started. This is something you can only see with the air cleaners off. Yes, indeed, this was nothing more than a loose "lever" on the carb-to-carb thottle linkage. A nutdriver is hard to find for these, as they are British Association (BA) sized. A 5/16 will work if it and the nuts are both worn enough. Remember to leave a little gap (the book says 12 thousands) on the throttle stop lever when adjusting the lever and pin assemblies with respect to the lost motion levers (Bentley handbook, Plate D.11.). If done correctly, the throttle butterflies will both open simultaneously and identically. That took care of the lope in the low rpm range, and reasonable take-off from stop lights were again possible.

This was a good lesson in what tool would have been nice to own: A few years ago, Ray Glorioso held a demonstration at the spring technical session at Al Piemonte Ford. One of the things he showed us was his little pouch of carburetter tools. A pair of wires were included which fit in the top of the carburetter pistons after the damper rods are removed. The parts that stuck out of the pistons had an "L" bend, and Ray adjusted the ends so they pointed exactly at each other. When the engine was revved up, the pistons moved up, and with good carburetter adjustment and no binding anyplace with the metering needles or pistons, the ends of the wires moved in unison. This gives a dynamic check on the pistons (and therefore, the position of metering needles), which is not as possible to do to do with one Uni-syn tool. Ray also said we could consider bending our own wires if we did not have the kit.

Well, the worst part of the roadsteršs problems were over, but a residual stutter at about 1200 rpm promptly became a major irritation, just as the loping had been. Bending two nine-inch pieces of 1/16 inch brass filler rod to the correct shape was simple, while taking care that the space between them when fit into the carb pistons was about a quarter inch. (The damper rods must be unscrewed and removed, and this is a good time to top up the oil in the pistons with whatever is used in the engine and gearbox.) As the engine was revved, the back piston moved up before the front piston by about 1/10 inch, then the front came up even, to be passed again for a short rpm range by the rear piston. This clearly showed that the air flow through the carbs was not identical, but the reason was not at all clear: It could have been in the carburetters or in in the passages inside the head. At least the reason for the hesitation at 1200 had a reasonable cause, and a very tiny adjustment in the throttle actuating lever for the front carb moved the problem up closer to 2000 rpm where it would not be as irritating. Piston heights, and therefore, air flow and fuel, were the same for both the front and rear pistons above 2000 rpm.

We will not consider a carburetter synchronization and mixture adjustment finished until we check the piston movement with the wires. Thanks, Ray, for helping us get make this pair of carbs almost perfect.

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