View single post by Esprit2
 Posted: 10-03-2019 07:12 pm
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 458
During closed throttle over-run (engine braking, or coasting), a very high manifold vacuum condition is created that leads to a spike in oxides of nitrogen emissions. There are a number of ways to mitigate that, but most involve some manner of catching the throttle before it fully closes, and slowly (several seconds) closing it the rest of the way. A dashpot damper is typical.

The Zenith-Strombergs take a different approach. They let the throttle slam shut, then use the Throttle Bypass Valves to shunt a metered flow of air around the closed butterflies. The net effect is the same. The engine is allowed to breath a little bit, at a deminishing rate, for a few seconds after you let off on the throttle pedal.

The small metal cylinder (tin can) mounted toward the rear of the manifold is a control valve. A hose connects it to a spigot on the intake manifold. It gets full vacuum from the manifold, but only passes a metered amount on to the Bypass Valves, deminishing to zero over a few seconds.

In 'Constant Depression' carbs (Z-S, SU, etc), opening the throttle starts a chain of events that eventually feeds the engine more fuel. It doesn't take very long, but it's a detectable lag in throttle response. Venturi carbs (Dellortos, Webers, most carbs) have a much more immediate response... very crisp compared to the Z-S carbs.

The Throttle Bypass Valves introduce another layer of delay on top of the Z-S carbs' already slow throttle response. Some sportscar folk expect qucker throttle response from an engine, and one of the motives for converting a 907 to Dellortos (besides power potential) is to have a more responsive throttle.

Deleting the Throttle Bypass Valves doesn't improve the basic Z-S carb's throttle response (it is what it is), but doing so does remove that extra layer of delay that the TBVs add to the already slow throttle response. It's not a fix as much as it removes an add-on something that makes the matter worse. The 'fix' would be converting to Dellortos or Webers... with a heavy personal preference for Dellortos.

While I'm on a roll...

The 907s Zenith-Stromberg carbs are tuned emissions lean. They could just as well be tuned sporty-rich, but they're not. They were chosen because they have the resolution that allows them to be tuned that way, and the Dellortos/ Webers don't. They're not bad carbs because they're tuned emissions lean... they're that way because someone (gov't emissions standards) tuned them that way. They can also be tuned 'sporty'... forgetting that throttle response issue.

An engine can run acceptably well (least worst) on an emissions-lean mixture in warmer weather; but during a frigid Winter, a slightly richer mixture is helpful. One option would be to tune the carbs rich enough for Winter operation, and it will still run well during the summer. But the emissions standards didn't allow for that.

So the Temperature Compensators are an alternative. A bi-metallic strip opens & closes a port in response to ambient temperatures. The result is an emissions-friendly mixture for warmer seasons, and an automatically adjusted cold-friendly mixture during the frigid Winter months. Since most J-H don't get driven in the Winter (no Florida... a 'real' Winter), the Temperature Compensator isn't really required, and it's just something else that can fail and cause problems.

An alternative is to remove the Temp Comp's cover, and turn the adjuster screw in tight enough to ensure that the port will be forever closed, regardless of ambient temperature. Then tune the carbs for 'best running' during the warm part of the year. Don't worry about Winter when the car is tucked away, awaiting Spring.

Tim Engel

Last edited on 10-03-2019 07:15 pm by Esprit2