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Runs great UNTIL vacuum advance line is connected

Old 07-09-2018, 12:28 AM
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big block ken
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Default Runs great UNTIL vacuum advance line is connected

I tuned up the '66 427/390, which is factory stock and original with 90k miles. It ran perfect, with the timing at factory spec 4* at 600 rpm and dwell at 28-30. But as soon as I unplug the vacuum advance line and connect it to the diaphragm, it sputters and runs terrible. Manifold vacuum is connected to the factory Holley 3370 carb.

Rather than advance it, as I had done before following Lars timing procedure, I'd like to get this to idle as it should per factory specs. I'm not looking for optimal performance, since I basically show the car. I was also getting some pinging under hard load. Is it possible that the original vacuum diaphragm is bad, after 52 years?
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Old 07-09-2018, 12:34 AM
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65GGvert
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Do you have a vacuum gauge to check your advance to see if it has a leak in the diaphragm?
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Old 07-09-2018, 12:45 AM
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Originally Posted by 65GGvert View Post
Do you have a vacuum gauge to check your advance to see if it has a leak in the diaphragm?
I use a vacuum gauge on the manifold vacuum line from the carb to adjust idle mixture. But no, I don't have a hand held vacuum pump tool. I see your point. I think I'll stop at Harbor Freight tomorrow.

Is the diaphragm supposed to pull any vacuum at idle? I just don't understand how it can suddenly run like sh*t as soon as I connect that vacuum line to the dizzy.
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Old 07-09-2018, 01:07 AM
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It is if you do have it hooked to manifold vacuum instead of ported.
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Old 07-09-2018, 01:08 AM
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I assume it also runs badly if you just unplug the vacuum line and leave it disconnected? That would be the same thing as having a hole in the diaphragm.
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Old 07-09-2018, 06:47 AM
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Start by connecting a piece of vacuum line to the vacuum advance control and suck on it to see if it holds a vacuum. The diaphragm may be tore and it's causing a big vacuum leak when you connect the hose making the engine mixture lean.
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Old 07-09-2018, 07:13 AM
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Originally Posted by tbarb View Post
Start by connecting a piece of vacuum line to the vacuum advance control and suck on it to see if it holds a vacuum. The diaphragm may be tbutore and it's causing a big vacuum leak when you connect the hose making the engine mixture lean.
Sounds silly but this is how I check it too. Been doing it for years...you can suck enough vacuum on it and let the hose end stick on your tongue from the vacuum and it should hold there for at least 10-15 seconds; if not something is leaking...

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Old 07-09-2018, 07:30 AM
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Chuck Gongloff
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Originally Posted by big block ken View Post
I tuned up the '66 427/390, which is factory stock and original with 90k miles. It ran perfect, with the timing at factory spec 4* at 600 rpm and dwell at 28-30. But as soon as I unplug the vacuum advance line and connect it to the diaphragm, it sputters and runs terrible. Manifold vacuum is connected to the factory Holley 3370 carb.

Rather than advance it, as I had done before following Lars timing procedure, I'd like to get this to idle as it should per factory specs. I'm not looking for optimal performance, since I basically show the car. I was also getting some pinging under hard load. Is it possible that the original vacuum diaphragm is bad, after 52 years?
FWIW... recently, my 56 Chevy hot rod started running poorly. I hadn't put a timing light on it for a long time, as I'm of the "if it ain't broke, don't fix it" school of thought...

When I pulled the vacuum advance hose to the distributor.... it made no difference in the idle/running of the car. Plugged the hose, and the car idled much better.

"Sucked" on the distributor vacuum advance can and found that the diaphragm had ruptured. I had a 100% vacuum leak through the diaphragm.

So.............. it does happen..
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Old 07-09-2018, 08:04 AM
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Originally Posted by Frankie the Fink View Post
Sounds silly but this is how I check it too. Been doing it for years...you can suck enough vacuum on it and let the hose end stick on your tongue from the vacuum and it should hold there for at least 10-15 seconds; if not something is leaking...
Well it holds a vacuum. That sucks. I pulled the entire vacuum line that's a short rubber line from the carb manifold port, to a metal tube, to a longer (the white striped) rubber line to the diaphragm. That way I could tell there were no leaks in the line also. But it holds strong. I had two dwell meters that gave me two different readings, so I had bought a new one also to make sure that was set right. And I bought a new advance timing light as well.

The factory spec of 4* BTC at 600 rpm just seems so low, especially since using Lars theory of setting it at peak street rpm of 2800-3000 (I assume big blocks would be the same as small blocks, but he doesn't say), I had timing set at @ 34* total then. And then it just fell wherever it was at idle. When I initially checked it before adjustment, it seemed to be at 10-12* advanced at idle. I read a recent CF post where someone with a ' 67 427 was setting his at 16* at idle! So I may have no choice but to play with the timing to get it where it idles smoothly.
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Old 07-09-2018, 08:12 AM
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Originally Posted by big block ken View Post
Well it holds a vacuum. That sucks. I pulled the entire vacuum line that's a short rubber line from the carb manifold port, to a metal tube, to a longer (the white striped) rubber line to the diaphragm. That way I could tell there were no leaks in the line also. But it holds strong. I had two dwell meters that gave me two different readings, so I had bought a new one also to make sure that was set right. And I bought a new advance timing light as well.

The factory spec of 4* BTC at 600 rpm just seems so low, especially since using Lars theory of setting it at peak street rpm of 2800-3000 (I assume big blocks would be the same as small blocks, but he doesn't say), I had timing set at @ 34* total then. And then it just fell wherever it was at idle. When I initially checked it before adjustment, it seemed to be at 10-12* advanced at idle. I read a recent CF post where someone with a ' 67 427 was setting his at 16* at idle! So I may have no choice but to play with the timing to get it where it idles smoothly.
go back to 10 - 12 and see what happens
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Old 07-09-2018, 08:21 AM
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Originally Posted by big block ken View Post
Well it holds a vacuum. That sucks. I pulled the entire vacuum line that's a short rubber line from the carb manifold port, to a metal tube, to a longer (the white striped) rubber line to the diaphragm. That way I could tell there were no leaks in the line also. But it holds strong. I had two dwell meters that gave me two different readings, so I had bought a new one also to make sure that was set right. And I bought a new advance timing light as well.

The factory spec of 4* BTC at 600 rpm just seems so low, especially since using Lars theory of setting it at peak street rpm of 2800-3000 (I assume big blocks would be the same as small blocks, but he doesn't say), I had timing set at @ 34* total then. And then it just fell wherever it was at idle. When I initially checked it before adjustment, it seemed to be at 10-12* advanced at idle. I read a recent CF post where someone with a ' 67 427 was setting his at 16* at idle! So I may have no choice but to play with the timing to get it where it idles smoothly.
NONE of my classic cars are ever set at factory specs for timing.....those were lazy, conservative setups for one-size-fits-all driving by everyone from Hot Rod Roy to Grocery Getting Granny.... using a variety of '60s gas from Sunoco 260 to the Mom & Pop corner RunFunny single pump general stores...

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Old 07-09-2018, 08:22 AM
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FYI, there is nowhere on the carb to get a ported vacuum. Only the factory manifold vacuum port at the base, as shown below.



Holley 3370 carb for 1966 427/390 4 speed
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Old 07-09-2018, 08:25 AM
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There are untold acres of debate on this but generally "most" agree that full time vacuum results in a smoother idle and cooler running....

As stated, I wouldn't be obsessed with running the car precisely at factory specs...

Last edited by Frankie the Fink; 07-09-2018 at 08:27 AM.
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Old 07-09-2018, 08:28 AM
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Originally Posted by Frankie the Fink View Post
NONE of my classic cars are ever set at factory specs for timing.....those were lazy, conservative setups for one-size-fits-all driving by everyone from Hot Rod Roy to Grocery Getting Granny.... using a variety of '60s gas from Sunoco 260 to the Mom & Pop corner RunFunny single pump general stores...
That's true. When my late brother Joe bought this new, I was 12 and I raised the octane on my bike by putting baseball cards in the spokes. In the '80's I owned a '66 427/425 and was able to get Sunoco 103 octane then, and I could feel the difference. So it makes sense to make adjustments for today's dishwater fuel. I'll just knock timing up until it smooths out, then drive and hope there's no ping (which is what I was getting at times before this tune).
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Old 07-09-2018, 08:36 AM
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If detonation is a concern then bump it up until the idle smooths back out at least; then you can decide how far you want to push it...
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Old 07-09-2018, 10:37 AM
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If you set initial timing at 4* BTDC and can't get the engine to run with the vacuum advance connected something is wrong. Could the TDC mark on the damper have slipped on the rubber or something amiss inside the distributor.

That carburetor # does not sound right to me, I thought the Holley # was 3811.
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Old 07-09-2018, 10:45 AM
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Vacuum advance check with timing light. Remove vacuum advance hose and check timing. Hook up vacuum advance hose and check timing. Pinch vacuum advance hose with needle nose pliers, check timing.

This issue is either as described in post #16
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Old 07-09-2018, 10:46 AM
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Originally Posted by big block ken View Post
FYI, there is nowhere on the carb to get a ported vacuum. Only the factory manifold vacuum port at the base, as shown below.



Holley 3370 carb for 1966 427/390 4 speed
Ported vacuum on a Holley is taken from the metering block via a port just above the idle mixture screw on one side. To your point, if you look at the 3370 metering block, you will see an unmilled boss where the ported vacuum nipple would be installed. It is not included on this carb as Holley considers this a performance carb, not an economy or emission carb.
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Old 07-09-2018, 10:50 AM
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Originally Posted by Frankie the Fink View Post
There are untold acres of debate on this but generally "most" agree that full time vacuum results in a smoother idle and cooler running....

As stated, I wouldn't be obsessed with running the car precisely at factory specs...
^^
Yes, the theory is, the sooner the spark occurs, the more time all the fuel has to be burnt up before it hits the exhaust system. Thus, less is going on in the exhaust, less radiant heat from the exhaust, lower EGT, more efficient engine.

I find that most original engines run well with a very wide range of idle timing settings. For example I have a 5.3 that idles almost the same whether it has 10* or 30* of timing at idle.
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Old 07-09-2018, 10:50 AM
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[QUOTE=big block ken;1597560427
But as soon as I unplug the vacuum advance line and connect it to the diaphragm, it sputters and runs terrible.[/QUOTE]

I can't make any sense out of your statement. The VAC signal line must be removed from the VAC and plugged with a golf tee to set initial timing. The vacuum and centrifugal advance specs are in your CSM. I think the max centrifugal is 30 @ 5000, which is very lazy, and maximum vacuum advance is 16 @ 12" and the VAC is stamped 360 12. So you need to check both. VACs eventually fail due to seizing up or a ruptured diaphragm,which will cause a significant vacuum leak.

Total WOT advance should be in the range of 36-40, as high as you can go in that range without detonation, and with lighter springs you can set it a few hundred revs above the point of maximum advance. With 30 centrifugal, initial should be in the range of 6-10, and total idle advance with the VAC connected should be in the range of 22-26 and stable.

Duke



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