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Old 01-18-2013, 06:35 PM
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Originally Posted by cobalt327 View Post
If you're going to use a Q-jet, the amount of vacuum the engine makes has a direct effect on what spring to use for the power piston. Too much spring/too little vacuum will have it running pig rich (will be always on the power circuit) anywhere except WOT. Same concept as the power valve on a Holley or the step up springs on an Edelbrock/Carter.

Depending on what the Q-jet carb was originally from, it may need other changes (APT adjustment, jets/rods, bypass air orifice size, more idle fuel, to name a few). I don't say this to dissuade you from using it, just want you to be aware of the possible pitfalls.

Again, for the purpose of getting the engine up and running: Use the Eddy and a dual plane. Use a properly set up distributor.

Once you know what the idle vacuum is and how the cam is, then you can go about tuning it for real.

Also, the cam is the big unknown. It might be any sort of cam. Depending on what class it was built for, it could have a stock lift cam (often they'll have a ton of duration), it could be a cam ground for a vacuum rule, it could be set up to intentionally loft the valves off the nose of the cam lobe to circumvent the rules, could have hydraulic lifters on a solid cam (or vice-versa), it may be excessively advanced/retarded, et cetera.

If you cannot get a straight answer on cam specs/grind number from the guy who owned the engine, you really should take the time to measure it or see if there's some ID on the cam- whatever it takes. Because the way it stands now, you're (and we're) totally in the dark. This makes giving meaningful recommendations a total crap shoot.

3500 stall on the street is not as bad as you may think. But again- w/o knowing what the cam specs are, picking a stall speed is a total guess.
Cam specs are in my original post but it is a summit cam, k1108. This is setup as a race motor and has very little vacuum at idle but the previous owner was able to drive it on the street. This motor was in an all steel 41 willy's 4 door and clocked 10 flat on the strip with a 150 shot of NOS, several times. I know vacuum secondaries are generally better for vehicles less than 3000lbs or so with automatic transmissions and mechanical secondaries are generally better suited for heavier ace vehicles. I want to take this truck to the track but be able to drive it to a cruise-in or car show too, hopefully without changing anything except MAYBE the distributor and the stall converter for now and add my posi later. Which brings me here asking which carb to run. It looks like a 750cfm is the consensus and I'm guessing mech secondaries since I have very little vacuum. Unless a vacuum secondary carb can be tuned to operate correctly with little vacuum, So because I'm also reading that vac sec carbs are more street friendly and pretty good at the track when a person is not so into racing that they're trying to shave tenths off their et. So I'll put the 600 eddy on there so I can get a vacuum number because the previous owner doesn't remember how much the motor had because he didn't care since he ran with fuel injection. From there maybe I'll have a better idea which carb to use. I do the fab and body work part of building custom cars and truck. This is my personal ride and I want it to be fricken fast and track worthy. I'll upload some pics and maybe that will help some of you understand my perspective a little better, if you don't already. I've never had more than 300hp and never been the driver at the strip but I drive my truck like I stole it.
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