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Carburetors Carb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.

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Old 03-11-2005, 04:38 PM   #1
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heavy vibration under load in 6th gear

Holley VS single pump 600cfm. While cruising in sixth gear, if you give it a little more gas I start to feel a low frequency vibration, like a repeated stumble I guess. The more gas I give it the more it does it, if I give it a moderate amount of gas (like less than 8" vac) it does it very hard and shakes the car pretty bad. I figured since it's worse at low vacuum then my secondaries are too rich so I dropped from 70s to 66s and it still does it. Any thoughts? I have 62 primaries in now. Any other gear and it's fine, probably because there's less load being put on the engine.
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Old 03-11-2005, 05:18 PM   #2
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Well here's a clue, it looks like my secondaries aren't opening completely. I put a paperclip around the shaft that comes from the diaphragm so that when the secondaries open they'll push the paperclip down the shaft. So I made a couple 0" vac runs and the paperclip was pushed down some but when I manually opened the secondaries all the way it pushed the paperclip a lot more, it looked like they only opened about half way.

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Old 03-16-2005, 11:29 PM   #3
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Holley tech guy said to try a heavier secondary spring because they might be kicking in too early. I'm kinda doubtful that's the problem, though, because the shaking is continuous, not just when the secondaries first open.
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Old 03-17-2005, 04:58 PM   #4
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What RPM is the engine at when you do this? Opening it up alot, at low RPM, under a heavy load like 6th gear, makes the engine work REALLY hard... could even cause spark knock.
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Old 03-17-2005, 08:43 PM   #5
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It happens under 1750, the lower the rpm's the worse it gets. I didn't have this problem with my old 750 on there, I could cruise in sixth at 1400 with no problem. Now, once I get the speed above 1750 the problem goes away.
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Old 03-17-2005, 08:53 PM   #6
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hehe, you were probably making more power with the 750, hence the lack of vibration. why are you running a vacuum secondary carb with a stick?
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Old 03-18-2005, 08:32 AM   #7
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I dunno, it definitely feels faster with the 600, though I haven't had it to the track this year yet. I switched to the 600 because I was having low end bogging problems with the 750 which I attributed to it being too big and not enough velocity to atomize the fuel well as a result. As for the vac secondaries, I guess I never really put much thought into having them vs. mechanical.
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Old 03-18-2005, 09:08 AM   #8
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Quote:
Originally posted by Ukraine Train
I dunno, it definitely feels faster with the 600, though I haven't had it to the track this year yet. I switched to the 600 because I was having low end bogging problems with the 750 which I attributed to it being too big and not enough velocity to atomize the fuel well as a result. As for the vac secondaries, I guess I never really put much thought into having them vs. mechanical.
Remember I was telling you about low vacuum at low RPM?
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Old 03-18-2005, 09:08 AM
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