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.

 

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.

 

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.

 

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.

 

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.

 

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?

 

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?