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Vacuum

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memphis

memphis

Jedi Hopeful
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Hi All,

My car is setup with a Mallory dual point w/o a vacuum connection. The small copper vacuum connection at the carb is pinched off. The engine seems to run ok, but I am wondering if anyone else has this sort of setup?

Why would the PO pinch off the vacuum advance connection and not use a distributor with vacuum advance? Any advantage to this setup?

Thanks,
Bob
 

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My car is setup with a Mallory dual point w/o a vacuum connection. The small copper vacuum connection at the carb is pinched off.

Why would the PO pinch off the vacuum advance connection

It is pinched off because the Distributor he chose to use does not utilise vacuum advance and he did not
want a vacuum leak at the intake manifold to upset the carburetor mixture adjustments.---Fwiw--Keoke
 
Hi Bob,

The technical reason [inserts nerd smiley] for vac advance is because the flame speed in the combustion chamber gets slower as vac increases due to having less fuel molecules in the same space = they are further apart and so it takes longer for the flame to move through them all. Without vac advance on a standard engine, the peak combustion pressure will occur slightly later than ideal at part-throttle which means some of the useful energy is lost into the exhaust pipe. Net result is slight reduction in fuel economy during light throttle cruise. The standard solution to this is to advance the spark a few degrees under high vac/cruise conditions to compensate.

For an engine with a 'lumpy' cam or multiple carburettors there is often very little vacuum because some of the mixture is spat back into the inlet before the intake valve closes. This means there is not much need for vac advance- it is usually considered one more thing to go wrong on a race engine which is not concerned with cruising fuel economy.

If your engine has a standard cam and pulls reasonable vac then you will get a small improvement in fuel economy by reinstating the vac advance and probably a minor decrease in exhaust temp too.

IMO dual points are old school hotrod tech, a Pertronix ignitor and standard dissy will give better performance on an otherwise standard engine.

Andy.
 
Hi Andy,

So it sounds like I should be on the look out for an original distributor, then buy a Pertronix Ign.? Correct?

I assume that with the original dizzy, I would need to reconnect the vac advance to the carb?

Thanks again,
Bob
 
memphis said:
Hi Andy,

So it sounds like I should be on the look out for an original distributor, then buy a Pertronix Ign.? Correct?

I assume that with the original dizzy, I would need to reconnect the vac advance to the carb?

Thanks again,
Bob
Click to expand...

That'd be what I'd do but then again I'm a perpetual fiddler :-)

If you do find a used dissy, suck on the vac advance pipe and be sure it holds vacuum. The diaphragms often rupture.

Andy.
 
So it sounds like I should be on the look out for an original distributor, then buy a Pertronix Ign.? Correct?

Well Bob, if you just like spending money you might go do that---Keoke-- :laugh:
 
Hello Legal Bill,

In general, just trying to understand how these cars are supposed to be setup. The engine runs, overall, pretty good. The idle seems a bit irradic on certain days and gas mileage is low at around 16mpg.
Thanks,
Bob
 
A couple SWAGs:

The idle speed will vary a little (50rpm +/- or so) depending on engine temp and ambient temperature and humidity. If your car has a 'lopey' idle you may have a cam with more overlap and/or more lift than stock (wouldn't be surprising given the distributor mod). My more-or-less stock BJ8 seems to idle best around 750rpm (I've noticed most modern cars are set to idle around 750, too). With HD carbs--which yours appear to be--sometimes the choke cables and levers don't return to their stops and the engine will idle fast (and erratic if one does and one doesn't).

I get 23-24mpg at highway cruising speeds (up to 70mph), 19 over 70 and 15-16 in stop-and-go city driving. If you're getting 16mpg with mostly highway driving I think you're wasting gas. Does your car have headers? If so, it may be fitted with richer-running carb needles, or the carbs may just be set very rich (headers or not). There's a method for setting mixture using the lifting pins under the carb 'dashpots,' or you can use Colortune plugs or other methods. You can check the balance by ear with a hose or tube, or there are several types of flow synchronizers available.

You'll want to check timing. If your harmonic balancer is stock, it won't have degree markings. Best to get an 'advance' timing light since you only have the pointer on the timing cover and a single mark on the balancer marking TDC (the balancer rings have been know to slip as well).
 
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