vacuum advance, advancing to quickly?

[Soul]

331
Vacuum Advance HEI Dist
600 cfm Holly

Well I'm having a small issue, when I mash it to the floor it surges until you let off of it. If a ease into it, doesn't do it. I'm using the default spring for the power valve, and its a 6.5 power valve. I'm thinking it might be the vacuum advance opening to fast, it was a cheapy dist and thinking springs are weak? Open to suggestions, I won't see it again till this weekend, having to put in axle bearings

[MalcolmV8]

I'd put money on a fuel problem with carb over timing advancing to quickly. My 2 cents

[whisperer]

Easy enough to check weather the vacuum advance is causing the problem. Just pull the tube off of it and plug the tube. No, the timing won't be perfect but no worries, if the surge is gone then you are on the right track.

I would change the power valve for something higher. At least an 8.5. The rule of thumb I start with is 6in below normal cruise vac, sometimes tighter. You do have to put a gauge on it and drive it to find that number.

For the other people out there that don't know what the number means on a Holley Power valve: The number is the point that the Power valve opens and enriches the mixture. The valve is normally closed ABOVE the number, or with a higher vacuum. When you step on the throttle or load the engine more the vacuum drops from say 14 in down to 3 (or whatever). The PV will come in and enrichen the mixture below whatever the PV number is.

If you have a Vac secondary 600 then you might be looking at the secondary diaphragm spring also as the problem.

Of course a regular ol' vacuum leak will do the surge also......

[Soul]

No vac leaks, only happens from about half throttle to above, thats why I'm confused by what it is..

[Chris]

I agree with Malcolm and whisperer, sounds like a fuel issue more than timing.

[plowboy34]

fuel vote here, specially if it only happens past half throttle.

[whisperer]

Give a little more info on which 600 Holley you have. There have been about 20 different list numbers over the years. Also, what general shape is it in? Was it just rebuilt by somebody that knows Holleys, or brand new, or off of the shelf after sitting for a while etc.. I'm assuming it's a 4160 vacuum secondary carb?

If it was in my shop I'd be pulling the secondary bowl off of it for a look-see. The secondaries are closed until 1/2 throttle when the primary linkage un-blocks the secondary linkage, enabling the vacuum diaphragm to pull the secondaries open according to the position of the linkage. If there is a fuel feed issue to the secondary side then effectively there is a huge vacuum leak created as the secondaries open if the fuel feed is not sufficient. It could be as simple as a dirty secondary jet or metering plate, or commonly a cheap metering plate gasket covering a fuel passage.

[jzwartster]

You know this is the same problem as my earlier posts on timing. It ran fine but when you step into she fell flat on her face. Do you have the electric chip under your cap and not points? If so your motor will seem to run fine where you have your timing set at Around 14? With this chip you want your timeing around 32.5 thats where mine is and I tell you what you best be an experienced light weight truck driver! Have fun!

[Soul]

Found the issue, the arm to engage the powervalve was bent, this came from ebay so I think some one dropped it. Runs much better.

Carb was a 600cfm 1830 series (think that is right number)

[jzwartster]

Great!

[whisperer]

I think you are confusing the Power Valve with the Accelerator Pump? Awesome!, glad you got it fixed.

[Soul]

Ya, totally mixed the two up. So far running decent, gonna take to track this weekend and see what numbers I can lay out.