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When the vacuum advance is connected to full manifold vacuum it can cause the type of surging you are experiencing if the vacuum threshold for operation of the vacuum diaphragm is exceeded. When this happens the timing fluctuates based on the varying vacuum level.

 

Without really knowing what you have for timing you are shooting in the dark. Before you remove the carb disconnect the vacuum advance and set the initial timing with a timing light and then test in gear. You will probably need to turn the curb idle screw in to get it to idle. If that solves the surging problem you should probably run the advance off a ported vacuum source. You can then proceed with your carb tuning. If you can get the car running decently I would suggest mapping the advance curve and based on that optimize the timing and then run through the carb again.

73 conv. 460, D0VE large valve heads, Performer RPM manifold, Voodoo 227/233 cam, Holley 950 HP carb, C6 trans, 3.25 trak-loc.

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Hey guys, went out of town for a little while but finally got back to the car and figured out the surging issue. It was a bad PCV valve. Should have figured it out, but it seemed to go in and out.

 

got it timed precisely too. 12 degrees initial, 35 degrees total. My vacuum gauge method had it set somewhere like 30degrees initial :/.

 

Ever since I've don't this work however, a new problem arose. from 1000-1500 RPM it runs like SHIT then its like a turbo kicks in and everything seems fine again. I'm thinking due to the new cam its running rich and I have to go down an inch or 2 in power valve size. My quick-fuel 600vs comes with a 6.5. Which isn't too far off but I'm thinking i need more like a 5. My vacuum gauge is acting up so I still haven't got a solid read but I thought my idle RPM is around 12-13 which should have been fine with that. Going to have to double check and also I'm going to drive around and get the read through those RPMS . There is also a chance i blew my diaphragm in it because I had a few carb back fires while dialing everything in. The problem just realllly seems like a power valve.

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If you have 13-14 in/hg vacuum at idle you do not need a power valve that opens at 5.0 in/hg. Chuck

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Can you be a little more specific as to the exact symptom? Runs like shit covers a lot of ground. If it is stumbling when you floor it the accelerator pump probably needs attention and/or the secondaries are opening too soon.

73 conv. 460, D0VE large valve heads, Performer RPM manifold, Voodoo 227/233 cam, Holley 950 HP carb, C6 trans, 3.25 trak-loc.

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So if I'm accelerating from a stop, or cruising real slowly, from 1000-1500 RPM it does what really seems to me is like a rich blurble and runs rough. (I'm going to pull some plugs to check color) Then at 1500 or shortly after it smooths out in one immediate moment with a boost of power. I'm not flooring it or anything, this is driving around the city. I'm getting awful gas milage, something like 7 mpg. It has also died while accelerating in this range when its cold, but only once. I will check the accelerator pump, and then the secondaries, never have adjusted secondaries. I'm going to replace my power valve regardless because I have had at least 3 to 4 carb backfires since installing, I know this can rupture the diaphragm and if I'm going to take it apart to check it i might as well put a new one in for $8. I will get one rated at half whatever my idle vacuum is.

 

Thanks!

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The backfires indicate a lean condition. I would start with accelerator pump, although I would probably go through the entire carburetor, check all passages, jet sizes, etc. New does not mean correctly assembled/inspected. There might be debris in it from assembly or installation. The RPM range you're having problems with could indicate a problem in the transfer slots.

 

 

“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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The backfires were before I tuned the mixture screws and timed it, so I wouldn’t count that on the list of symptoms. And yeaaaa I've been thinking I oshould tear it down and put it back together to make sure, just been procrastinating since its my daily driver. Thanks

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The backfires were before I tuned the mixture screws and timed it, so I wouldn’t count that on the list of symptoms. And yeaaaa I've been thinking I oshould tear it down and put it back together to make sure, just been procrastinating since its my daily driver.  Thanks

 

That carb and basically all Holley style carbs made since the 90's have a backfire protection check valve so it is extremely unlikely the power valve diaphragm is ruptured.

73 conv. 460, D0VE large valve heads, Performer RPM manifold, Voodoo 227/233 cam, Holley 950 HP carb, C6 trans, 3.25 trak-loc.

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Most aftermarket carbs are shipped rich, it is safer than lean. Try dropping the primary jet size 2-4 numbers, readjust mixture screws and see if things improve. Chuck

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