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

[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.