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That's exactly my question. I played around with my car this weekend and I'm wondering if having a double pumper with mechanical secondaries is not the best thing. One problem is my car is missing the timing pointer,so I'm kinda guessing. I need to get one ASAP. I advanced my timing from 25 - 35 degrees. Kept it conservative since I have no pointer. Definitely improved the performance.
My carb is the only issue I have. It does'nt bog at all, but I am still leary. What do you guys think? Look at my mods and feel free to make suggestions or express opinions.
Thanks in advance for all your help.
 

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Old rule of thumb I've always heard is vac. sec carbs with an auto and mech. secondaries with a stick...
 

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Hmm, interesting that you have the TFS 205 heads with a relatively small cam and displacement. Might be a dog down low (low velocities with large heads) and a DP carb will only make matters worse, unless i'm misreading something on your stats.
 

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I don't see any mods listed. However a relatively factory setup on a 289 or 302 should not exceed 480 CFM.
 

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650 DP, 3.55 gear, 2800 stall, you're just fine....
 

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First get your timing sorted out, and then I'd put it on a dyno to check the a/f ratio and across the power band. We were having some acceleration issues with our '66 and ended up having to jet the primaries down 2 sizes and the power pump down one size to hit the sweet spot. That was on a .040 over 289 with a Holley Street Avenger 570 cfm carb with vacuum secondaries..
 

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Our 69 SportsRoof with an H code 351W had a 650 cfm dp on it when we bought it. There was no provision for a kickdown linkage for the FMX (which is why you don't use a double pumper on an automatic equipped car), so we pulled it and replaced it with a 600 cfm vacuum secondary Holley with kickdown provision. It's amazing how the ability to downshift improved the car's performance. The 650 cfm dp is a pretty desirable carb, so you can probably sell it for a decent price on e-bay.

If you don't have a pointer you can always use the old auditory approach. Advance your timing until the car starts to ping under hard acceleration and then back it down slightly and try again. When you reach the point just below "pinging" you're pretty much dialed in. Are you saying that you moved your initial timing from 25* to 35*? That would basically be all in from the get go. What are the specs on your cam? I've got an aggressive cam in my 302 F code convertible but I'm running 14* initial with 32* all in at 2500 (IIRC). Could there be something amiss with your distributor? The first cam I had in my car would allow us to move the timing up to 35* initial before it would start to ping, but it turned out my distributor was seriously screwed up. When I rebuilt the engine I had a professional custom curve a 68 J code distributor and that's how I got to 14/32. It works out great.
 

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There was no provision for a kickdown linkage for the FMX (which is why you don't use a double pumper on an automatic equipped car)
Ken, his specs show a '66 ... the kickdown is all part of the accelerator linkage, and the carb has nothing to do with it. He also shows a TCI 2800 stall converter, which will work great with mechanical secondaries.
 

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Like the others said, with your combo mech seconds are OK. Linkage concerns aside, stick or auto really has nothing to do with it. The basic deciding factors are car weight and gearing. Light car with high gears means the engine rpms will accelerate quickly so the engine won't bog from the quick acting mechanical secondaries. With a heavy car or too little gear the engine can't rev quick enough to use the extra air/fuel that mech secs dump into the engine so it bogs and stumbles when you floor the pedal. When using an auto the converter has to be part of the equation also since that affects how quick the engine will rev as well. Vacuum seconds are generally preferred for street cars because they can offer better mpg and smoother secondary kick in without loosing power but if you can control your right foot a DP can work just fine on the street too.
 

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gotstang said:
Old rule of thumb I've always heard is vac. sec carbs with an auto and mech. secondaries with a stick...
I'm not sure where you heard this or why it would be true. I am running vacuum sec. w/ a T-5 with absolutely no problems thank-you-very-much. ;)

I think it is a matter of preference, but I chose vacuum because it is easier (for me, that is, because I am experienced with it).
 

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IMO you can't even talk about driveability problems if you don't know what your timing is. Both the initial timing and the timing curve are critical to performance.
 
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