View single post by Mark Rosenbaum
 Posted: 11-02-2005 06:23 pm
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Mark Rosenbaum

Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
Welcome, or welcome back, as you prefer.  The phrase 'old and dumb' hardly seems applicable to someone who's sufficiently 'experienced and wise' to buy a JH.   :^}

Vacuum Switch.
Used only on cars with Strombergs.  The vacuum switch is just a valve intended to open and close at specific vacuum levels.  It should open ('pass vacuum') at 17.5" Hg to 20.0" Hg, and should just close at 15.5" Hg to 16.5" Hg.  The test procedure, such as it is, is contained in section RM55 of the shop manual.

Replacement switches are listed in the Delta Motorsports catalog, but I don't know if they remain available.  However, I've seen quite a few switches attached to carburetors being sold on eBay.  Such switches have a good chance of being defective, of course, but may be repairable.  Some repair information is present in the multii-part carburetor article that appeared earlier this year in the JHPS magazine. 

The vacuum switch and throttle bypass valve system prevents engine stumble during the latter part of a rapid open-throttle / closed-throttle / full-throttle transition, and provides a decreased likelyhood of engine stalling or stumbling during sudden transitions from open to closed throttle.  The arrangement also supposedly ameliorates a number of emissions issues related to these driving activities.  The vented spigot that screws to the engine side of the rear carb's inlet flange, that I call a 'calibrated leak', determines how long the throttle bypass valves remain open once the vacuum switch closes in response to decreasing manifold vacuum.

Since the normal manifold vacuum of a 907 engine is about 14" Hg to 15" Hg, the vacuum switch in conjunction with the calibrated leak allows extending the hold-open period of the throttle bypass valves to a duration suitable to the engine's needs.  The arrangement would also prevent 'throttle flutter' which could otherwise occur in response to manifold vacuum variations that resulted from the opening or closing of the throttle bypass valves.  (This is a 'positive feedback' situation which at best is very annoying.  If carried to an extreme that's unlikely to be possible with Strombergs, it could result in a complete loss of the driver's control of the throttle, with potentially lethal results.)

As the system does provide a net benefit, my recommendation is to ensure that it remains functional.  If this is not practical for some reason, the manifold vacuum line can be run solely to the distributor.  However, I don't favor using the vacuum retard function at all, so you might be better off just capping the manifold vacuum port.

Carbon Canister.
Used only on cars with Strombergs.  The fuel tank is normally sealed.  A small diameter pipe runs from the top of the fuel tank to the engine bay and is connected to one of the spigots on the carbon canister.  A second canister spigot is plumbed to the canister ports of the carburetors.  The canister's third spigot is capped off.

Expanding vapors from the fuel tank pass through the pipe and are absorbed by the carbon granules in the canister.  Air to replace consumed fuel passes back through the pipe and into the fuel tank.  When the engine is idling with a nearly closed throttle, a vacuum occurs at the carbs' canister ports.  This causes air to flow through the canister and into the carburetors.  Absorbed fuel vapors mix with this air and are consumed.

The canister's rather elevated position is related to the height and location of the fuel tank, and is one of those unfortunate design tradeoffs that must be made from time to time.  If liquid fuel should somehow reach the canister, it will drain out and if ignited should provide fiery thrills.  A lower canister location would have decreased the likelyhood of liquid fuel sloshing around the engine bay, but would have increased the likelyhood of liquid fuel leaking out, particularly if the car should be parked nose-downhill and with a full fuel tank. 

Rather surprisingly for an early emissions system, the carbon canister actually works more or less as intended, and does not adversely affect performance or idle if the carbs are in good shape, so it may be prudent to keep things in working order.  However, there's nothing at all peculiar about the canister used in the JH, and if an exact replacement is too expensive, you may wish to check the local wrecking yard for something that can be made to fit (starting perhaps with mid-1970s to mid-1980s GM cars).  Alternatively, you might replace the existing pipe with one that vents under the car, as was commonly done prior to emissions controls.

I advise ensuring that the fuel filler cap seal remains airtight, as it is highly undesirable to have fuel gushing out so near to the occupant(s) in the event that the car becomes inverted due to mishap or misadventure.