Tech Tips: Spring 2006

Stuck Shaft

Q: I have a Weber DCV carburetor on my car. When it is cold the throttle tends to stick. Is this a cable problem?

A: The throttle shaft to body clearance on the DGV is very tight. During cold weather if the manifold is not heated the throttle shafts can seize in the cold carburetor body. This tight clearance can give you problems even in summertime if the shafts get contaminated. Spray some lubricant on the throttle shafts, and work them a bit to see if they free up. Usually this will clear out any contaminants and free things up.

Part Numbering Systems

Q: I’ve noticed that Moss Europe uses factory part numbers on their website. Why doesn’t Moss in the U.S. do the same?

A: The question is, of course, what is the best numbering system to use. There is no universal numbering system that is going to cover all the vehicles during the entire production time period. Numbering systems are constantly changing, and the only time the changing stops is if the originator of the system is no longer in business. As an example, until the last year of operation there were new Rover numbers being created for classic MG and Triumph applications. Most parts are not sourced from the vehicle manufacturer, so alternate numbering systems such as Lucas and Girling/Lockheed numbers have to be used to source and compare parts. There have also been many cases over the years where Triumph, Jaguar and MG applications used the same part, but under different factory numbers. In cases where factory numbers are used, often a company has to make up new numbers for items that the factory did not supply. These can become confusing, as they look like a factory number but really aren’t.

To clarify this situation, in the early ‘70s Moss Motors created a numbering system that was easy to enter on a 10 keypad and would have the flexibility to be used with many suppliers. An added benefit was that the standardized 3-digit system eased the layout of catalogs and price lists. Originally the numbers ended in a 5 or 0, leaving room for future additions. This foresight has proven very valuable, as it has allowed us to offer alternate brands and distinguish between original concours quality and replacement alternatives.

Lever Arm Shock Testing

Q: I ordered new lever arm shocks for my car, but when I hand-test them they have spots with virtually no resistance. Why aren’t they working properly?

A: It’s important to understand how the lever arm shock works. Damping resistance is created by a piston moving in an oil-filled cylinder, forcing pressurized fluid through a spring-loaded valve. Common applications use two pistons: one for compression (bump), the other for rebound. The two pistons are connected to a rotating lever arm by a connecting rod, crank and spindle. During the cycle, oil passes from one cylinder to the other.

The two pistons are connected to a rotating lever arm via a con-rod, crank and spindle. The two cylinders are arranged such that oil passes from one cylinder to the other as the lever is cycled. The damping is set by the stiffness of the valve springs and the initial pre-load. This design allows both bump and rebound damping to be set for the particular vehicle.

Under smooth road conditions, very little damping effect is necessary, but slow movement must be allowed or the ride will feel harsh. To achieve this, there are channels ground into the valve cone surface that allow the passage of oil under slow movement below the threshold necessary to lift the valve from its seat.

During shipping, a lot of air can get entrained in the damping oil. This will give the feeling of dead spots if the shock arms are moved by hand. This kind of testing will not load the shock absorber enough to open the damping valves, so the only movement felt is allowed by the oil leaking through the passages in the cone valve cone surface. This does not give an indication of the operation of the shock under load. This type of testing can only point out a failed shock, as there will very little resistance to movement if the oil has leaked out or if the valves have failed to open.