Timed vacuum or manifold vacuum?

[2059]

230I 236E
Haven't mapped the curve.
At idle, none on the ported side.
Didn't check manifold vacuum.

What are you thinking as far as initial to try?

It will run cooler with some more timing at idle. What's the cam duration at .050 lift? Have you mapped the entire ignition curve? How much vacuum is it pulling and how much timing is the vac advance adding?

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I'll go back out and hook up the test equip't and check.

 

[Hemirunner]

That's not a very large cam at all. My street driven 572 cam is .278/.284 @ .050 with 12:1static compression just for a comparison. I wondered how much advance your vac can allowed. If it's around 10 degrees, you could move from ported to manifold for a source and run around 20 degrees of advance and see how it likes it. You'll most likely have to adjust idle screws as more timing = leaner mixture given the same carb settings.

 

[2059]

That's not a very large cam at all. My street driven 572 cam is .278/.284 @ .050 with 12:1static compression just for a comparison. I wondered how much advance your vac can allowed. If it's around 10 degrees, you could move from ported to manifold for a source and run around 20 degrees of advance and see how it likes it. You'll most likely have to adjust idle screws as more timing = leaner mixture given the same carb settings.

good point. here's a quick map:

Manifold vacuum 10-12 HG at idle

Timing:
1000 10*BTDC
1500 12
1750 19
2000 24
2250 28
2500 32
2750 42
3000 48

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i also checked my vac can by hooking to manifold vac. timing went from 10* to 40*. So ~30*. Ya, if I start tinkering with the timing I'll adjust the idle screws. kind of a pain on the crossrams.

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Guess I could hook it to my vacuum pump and take it to 15-20 hG to get the total contribution.

 

[Hemirunner]

I guess with your application, you need to decide if your goal is mileage and driveability or performance. Then tune in that direction. If it's driveability, then all I would do is give it 5 to 10 at idle and reduce the vac by the same amount and see how that drives. If Haines built it, it's probably very well done already.

 

[lewtot184]

the vacuum units have a number stamped on the stem that connects to the plate. all of them i've seen are stamped 7,8,9,or 10. this tells how many distributor degrees in the unit. double that number for crank degrees. with that cross ram i'd probably run a hose that connects to both carbs with a tee in it and connect that to the distributor. i'd try to run 15-20 degrees initial.

 

[2059]

Interesting little experiment yesterday. I bumped the initial to 16* and disconnected the vac can. My total ran about 34* at 3000. Reset the idle and went for a drive. After everything was warmed up, I stood on it and under full load, I was getting backfire through the exhaust and black smoke. tells me I’m not finishing the burn, which means I’m not lighting soon enough, which means I need more total advance than 34* I was surprised that no one commented on the 48* total. It seemed like a ton from what I’ve read however, after yesterdays test drive maybe not.
I’ll “T” the advance off of the two carbs on the ported side, set the initial to 20* and pin the weights to give me the rest of the total. We’ll see how that goes.
The whole point is to get the car to run cooler and allow me to get the car to Idle below 1300.

 

[Hemirunner]

I would look at the possibility of a rich mixture or idle and fuel collecting and igniting late due to not having the advance at low rpm. Do another experiment of locked timing (no mech advance and vac disconnected) at 34-36 degrees and see what that does. 48 total degrees at 3000rpm under a part throttle with no load is pretty typical with the vac advance hooked up. As soon as you go to full throttle and manifold vacuum drops, that's going be much less.

 

[2059]

I would look at the possibility of a rich mixture or idle and fuel collecting and igniting late due to not having the advance at low rpm. Do another experiment of locked timing (no mech advance and vac disconnected) at 34-36 degrees and see what that does. 48 total degrees at 3000rpm under a part throttle with no load is pretty typical with the vac advance hooked up. As soon as you go to full throttle and manifold vacuum drops, that's going be much less.

I'll stick a guage on it and drive it but, If I'm on the ported sie of the system wouldn't the vacuum be high through that port since the vacuum is created by the air flow through the venturi flowing past the port and creating the vacuum?

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I understand the maniforld vacuum being non existant because basically you "open up" the system when under full throttle and there is little resistance/restriction below the butterflys.

 

[62 Dart Convertible]

wouldn't the vacuum be high through that port since the vacuum is created by the air flow through the venturi flowing past the port and creating the vacuum?

Well, yes, no, kind of. Yes, vacuum will be high, but because the vacuum port is just exposed to manifold vacuum by the butterfly opening just off idle...

 

[Hemirunner]

If you have 10* at idle with the vac advance connected to ported vacuum, you're not getting any vacuum advance being applied at idle, then when you romp on it, it goes from no vacuum to no vacuum and has little to no ignition timing. Lock it out at 34 degrees, (no mechanical advance)leave the vac advance disconnected, adjust the carbs and take it for a drive. It'll be a night and day difference. Most advance curves I see in hot rod muscle cars are the exact opposite of what these engines actually want.

 

[67Satty]

leave the vac advance disconnected, adjust the carbs and take it for a drive. It'll be a night and day difference. Most advance curves I see in hot rod muscle cars are the exact opposite of what these engines actually want.

^^^^^^I picked up a tenth in the 1/8 and my motor seemed WAY happier when I tried this^^^^^^

 

[383man]

If you have black smoke its to rich at that throttle position as at 34 total it should not blow black smoke or backfire thru the exh. The timing off a little will not cause black smoke as thats only from to rich which of course is to much fuel or not enough air. You may have something else going on. Ron

 

[Meep-Meep]

If you have 10* at idle with the vac advance connected to ported vacuum, you're not getting any vacuum advance being applied at idle, then when you romp on it, it goes from no vacuum to no vacuum and has little to no ignition timing. Lock it out at 34 degrees, (no mechanical advance)leave the vac advance disconnected, adjust the carbs and take it for a drive. It'll be a night and day difference. Most advance curves I see in hot rod muscle cars are the exact opposite of what these engines actually want.

From my post #27

"With a ported signal you can only go advanced based on the mechanical curve plus any vacuum signal to the diaphragm. If no vacuum signal to the diaphragm then you default to the mech advance in the distributor, which has the basic timing curve necessary to run the engine under load. The latter is very important. "

My point is pretty much the same thing you said here, which says the mech curve must be correct. The ported vac advance should NOT be used as a band aid for a poor mech curve. It's an enhancement for street drivability.