newbie carb question???

[phegan]

why is a mecanical secondary better than a vacuum secondary carb. i'm hunting around for a cheap used carb
to rebuild but it seems the vac's are a dime a dozen.
i know the answer will seem obvious once explaned :)

[Eric455]

the secondaries are more of a headache. they have tendancies to get stuck and once that happens fuel all over the place motor runs bad, they work good when new, and when they are installed in cars. but for a boat applicatin just go with mechanical's ask hotcrusader for more info.
hope any of this info helps
eric

[Eric455]

vacume secondaries that is......

[OMEGA_BUBBLE_JET]

I'll lay it out real simple. vacuum secondaries are only designed to be open for short periods of time. Like in a car for instance. With the pedal on the floor they are only open at 3500 rpm and up. Or as the vacuum builds to open them. Then you shift and the rpm's go back down. etc. etc. In a boat however you may sustain 5000 rpm or more for 10-15 seconds or however long. Mechanical secondaries are designed for high rpm engines. Also the throttle response is much crisper as you don't have to wait for the secondaries to open. As soon as you mash the pedal to the floor they are open. Vacuum secondaries really aren't designed for marine which is why the folks that do run usually end up putting a spring kit in them. Hotcrusader76 knows about this and so does Duane at HTP. both of these guys are real pros and can answer your question more in depth if you like. Bottom line mechanical secondaries work better. Don't question it just do it. Oh and Duane sells new ones cheap!!!! No worries on rebuild.

[jim@pj]

Mechanical secondary carbs have numerous advantages. Some have already been mentioned. In my opinion the big difference is in the secondary circuit design. First off, the mechanical secondary carb has a secondary power valve which dumps more fuel to an engine in the transition phase( form idle to W.O.T) The vac secondary carb does not have this and ends up with a lean condition before the jets pick up the flow. Second, a vac secondary carb does not have secondary jets, just a merering plate. You can buy a kit that converts this plate to a metering block with jets but no power valve. Third the vac secondary carb has no secondary squiter. All this adds up to less fuel in the seconary , which can cause trouble if you are not sure of how to set your motor up. Jim www.performancejet.com (http://www.performancejet.com)

[78Eliminator]

I remember my first car had a 750 with vacuum secondaries and when they would open, the car would actually buck like you hit the nitrous button. It was pretty cool. But, like it was mentioned above, it takes a bit for them to kick on. One thing that was not mentioned about mech secondaries is the fact that they are great for fuel conservation......
There are also a few more moving parts and as far as adjustment goes they are a little more complicated, you will need to talk to the carb guys in here as I have never messed with them (vacuum secondaries) before. I like to keep things simple so I just went with mech secondaries.
[ April 21, 2003, 04:02 PM: Message edited by: 78Eliminator ]

[phegan]

thanks guys, i think i got it.
vac sec's use the high initial engine vacuum at low rpm to open the butterfly valves where as the mec's use the cable or linkage position to open the secondarys.

[Oldsquirt]

NO, the vac secondaries use the high venturi vacuum in the primaries, not manifold vacuum, to open the secondaries. This vacuum increases with airflow. By changing springs they can be set to open at different vacuum levels. They should be considered "load sensitive". No load, they don't open. Load increases, airflow in primaries increases and that raises the venturi vacuum to the point where the secondaries are pulled open.
Mechanical secondaries are just what they sound like. The secondaries are opened by a mechanical linkage to the primaries.
[ April 21, 2003, 08:21 PM: Message edited by: OLDSQUIRT ]

[OMEGA_BUBBLE_JET]

let's make this easy. If you want a carb for a car, buy a vacuum secondary. If you want a carb for a jet boat, buy a mechanical secondary. It's really that simple.........

[Blown 472]

phegan:
why is a mecanical secondary better than a vacuum secondary carb. i'm hunting around for a cheap used carb
to rebuild but it seems the vac's are a dime a dozen.
i know the answer will seem obvious once explaned :) Vac secondary carbs are also set up a little leaner than mech secondaries, if you are running this on a jet go with the mech sec. as you will need to run a little fatter on the top end any who.

[Hotcrusader76]

Well not to be the devil's advocate but the Vacuum secondary does work great on big block applications. Without all of the technical mumbo jumbo, the 750 #3310 carburetor in stock form and jetted around 76 works great under normal operating conditions.
If you're after balls to the wall performance with max tuneability then by all means look into a double pumper. They are crisper in throttle response and will offer you an improved top-end result over the vacuum version.
Can a #3310 Vacuum Secondary carburetor perform on larger HP motors? Yes. It will take some appropriate modifications and the addition of some performance components to do so, but nonetheless it's suitable.
Is it worth the cost to modify a vacuum secondary? IMHO, not for Jetboaters. But for a 'class' specific regulation stating the use of only a #3310 carburetor, then by all means yes. Now if you just can't separate from the vacuum secondary due to fear of money loss in fuel consumption, then get it tricked out! wink
If you want any more details to what the differences might be in retrospect to this topic feel free to call me up.
~Ty

[Hotcrusader76]

Blown 472:
phegan:
why is a mecanical secondary better than a vacuum secondary carb. i'm hunting around for a cheap used carb
to rebuild but it seems the vac's are a dime a dozen.
i know the answer will seem obvious once explaned :) Vac secondary carbs are also set up a little leaner than mech secondaries, if you are running this on a jet go with the mech sec. as you will need to run a little fatter on the top end any who. Actually, the #3310 carburetor has a richer Primary metering block, showing many similarities to the 850 double pumper's block. This is why the carburetor still works on big blocks as does the double pumper. The airbleeds are also very similar in size, but the idle circuit still shows a 0.003" difference in stock form.
Now when the secondaries kick in, it does have a slight lack of fuel delivery in comparison, but you have to remember this is engine vacuum dependant and the amount of fuel dumped is primarliy because of booster size. Some secondaries never open full tilt on mild mannered motors anyhow.
Just my 0.02 wink

[phegan]

Is this a great forum or what? idea
great info guys. I want to try and rebuild a holly carb. does any one have an old one for sale cheap?
It will be going on a shop built ford small block.
(302 or 351w). the motor will have flat-top pistons and a rv cam.
I'm thinking a 650 cfm would be ok.I like the idea of running it a little rich to keep things cool. will a 650 be enought?