follow up to my electric choke vs manual choke ?

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clutchdust

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i was told that the manual and electric chokes used by holley are interchangeable. well, i was comparing the 750vs w/manual choke currently on my c3 to the 390vs w/electric choke on my little 240z. it looks like it should just swap out *BUT* the thing that makes me curious is the electric choke unit has a vacuum port and tube that goes to the top of the carb body. there's no port on my 750 where i could hook that up.
so what gives? can that electric choke be swapped onto the 750?
 
NOT familiar with Holly carbs, but in general, electric chokes need nothing in the way of any tubes going in/out of the choke mechanism....just bolt to carb/base and hook up the levers, and run the wire to switched +12 and set it to slam shut when cold, and adjust the pull off from the dash pot for open maybe 1/4 untill warm, then straight vertical.....

YMMV, but that's the general idea....never any need for heat/air tubes in a electric choke....

:thumbs:
 
Yes, there are tubes and vacuum connections on electric choke carbs...

On an automatic choke (such as an electric choke), you have to have a choke pulloff (aka "vacuum break") in order to crack the choke open when the engine starts. Holley used 2 styles of choke pulloffs:

1. First style is the external choke pulloff, mounted very similar to a Q-Jet. This uses an external vacuum diaphragm with a vacuum hose running to a manifold vacuum source on the carb. When the engine starts, engine vacuum pulls on the choke pulloff and cracks the choke open about 1/4".

2. Later style, and common on most of the "generic" Holley carbs is an internal vacuum break piston mounted inside the electric choke housing. The piston receives vacuum from an internally drilled hole that goes from the piston cavity into the carb body. Many carbs, if not originally equipped with an automatic choke, do not have this passage drilled in the carb. If mounting an electric choke to such a carb body, you have to do the machine work to create the vacuum connection, otherwise you will get nothing but massive flooding of the engine on cold starrt.

Some other carbs, in addition to the vacuum connection for the choke pulloff, will also have a vent tube (not a vacuum tube) running from the choke housing to the airhorn area of the carb. This tube is only to provide ventilation of the choke housing to prevent it from overheating from the hot electric coil. It draws in a small amount of clean air from the airhorn area of the carb and pulls it through the vacuum break piston leak path. Since this is not a vacuum tube (only a clean air vent tube) you can replace the tube with a filter "sock" over the choke housing inlet. Most later model Holleys with electric chokes simply use a filter screen over this inlet, and do not use the tube running to the airhorn area. But it is imperative that you establish the vacuum bleed source inside the housing so that air will be pulled through the choke housing. Otherwise, the housing can get hot enough to damage the choke coil assembly.

Lars
 
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I installed a lawnmower engine in my Vette and it works fantastic.:smash:
 
turtlevette said:
for what its worth, i switched over to a lawnmower style manual choke on the QJ and it works fantastic.
Click to expand...

I tried that once, decades ago, I forget which carb...but it was a dismal failure...

Lars, I was thinking of those heat tubes that went down into the exhaust crossover and then into the carb vac source....not so much of pulloffs and that piston thing....

:clobbered:
 
so, lars, can this vacuum source be any manifold vacuum? why must i modify the carb? ultimately, i plan to go FI so i'm not really interested in doing much in the way of machinework.
 
clutchdust said:
so, lars, can this vacuum source be any manifold vacuum? why must i modify the carb? ultimately, i plan to go FI so i'm not really interested in doing much in the way of machinework.
Click to expand...

If you have a choke system which uses the integral piston-type pulloff, you have to drill the carb body and do the machine work in order to get vacuum to the piston. Otherwise, the pulloff will not work, and the choke will remain fully closed after engine start, resulting in complete engine flooding. You cannot use a choke with the integral pulloff piston without machining the vacuum oriface. Why must you modify the carb? Because if the carb is not machined for the feature you are installing, the feature will not work. You can't expect the choke system to operate if you're only installing a partial system. It's like asking, "why do I have to install pistons in my engine to get it to work? I don't want to do the machine work to install them..." Holley carbs originally equipped with manual chokes or no choke will not have the vacuum oriface drilled, since it is not used with a manual or no-choke application - the drilled oriface would cause a vacuum leak lacking the choke feature.

If you have a choke system using the external choke pulloff diaphragm, you can use any manifold vacuum source to the choke pulloff and it will operate correctly. However, you still need the choke housing ventilation feature descibed above to assure that it will not overheat.


mrvette said:
turtlevette said:
Lars, I was thinking of those heat tubes that went down into the exhaust crossover and then into the carb vac source....not so much of pulloffs and that piston thing....
:clobbered:
Click to expand...
The tubes you are referring to are tubes to operate the hot air choke system - not an electric choke system. The hot air choke used the same system as the electric choke, with the exception that it did not have an electric coil to heat the choke coil. Rather, hot air heated by the exhaust manifold was pulled through the choke housing using the tubes you referenced. The system worked very well when all of the components were hooked up and intact. Corrosion from heat and moisture killed most of the systems within 5-10 years of operation.

Lars
Click to expand...
 
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o.k. maybe i'm just not understanding what you're saying or i'm not explaining things well enough.
the 390 with the electric choke has a vacuum tube on the choke body that has a tube coming off and connects to a port just below the air cleaner base. so i think this is the kind with the external vacuum source described above. correct? if i understand correctly, i should be able to put this choke on my 750 and simply connect the vacuum to any manifold source. correct or no?
 
There is no vacuum tube coming off the choke housing on any Holley carb. There is no Holley carb with a vacuum connection just below the air cleaner base. The only connection coming off the choke housing on a Holley carb is one of the two types of connections outlined in my earlier posts: It is either a fresh air inlet port to keep cool air circulating through the housing, or it is the inlet for the hot air choke system. Holley carbs with electric chokes use a drilled internal vacuum passage as described above, or an external choke pulloff diaphragm. There are no vacuum tubes coming off the housing going to the top of the carb, and you cannot make the choke work by hooking up the choke housing to vacuum. The choke works as oulined in my previous posts above - if your choke does not have an externally-mounted choke pulloff diaphragm, you have to have an internally-drilled vacuum passage for the internally-mounted pulloff piston.
 
Clutch -
You have me a little befuddled here... If you don't have the resources or capability to spend 50 minutes to drill a few vacuum passages in a simple carb body, how in the world are you going to install and tune an FI system...? If you want a choke, yank that carb off, pull it apart, and do the drilling. The choke will work fine with the vacuum passages in place. I don't think the installation of a bad FI system is going to solve your problems...

Lars
 
maybe i misread your post but i thought you said if the carb didn't come with an electric choke it didn't have the boss for a vacuum port (or something like that). i dunno, reading comprehension is not my forte lately. i'll go back and re-read your post. still, it's not a matter of what i can do, it's more about how much time and effort i want to put into a carb for a car i hardly drive when i know FI is on the 'to do' list anyway.
"hmmm, spend time on converting choke or fix the exhaust, bodywork, paint, interior, electrical, big brake package, t-tops, blah, blah, blah..."
 
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