Timing ?'s, Idle Speed and Ported vs Manifold Vacuum - Hot Rod Forum : Hotrodders Bulletin Board
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Old 11-02-2004, 03:35 PM
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Timing ?'s, Idle Speed and Ported vs Manifold Vacuum

Ok so the saga continues..

Heres the history..

ZZ4 Crate engine.. about 7k miles on it, Barry Grant ZZ4 crate demon carb. Stock ignition (hei) etc.

So I had some initial issues setting the timing but got it set to 12 degrees btdc at around 800rpm with the vacuum advanced disconnected and plugged.

Ive got the carb butterflies closed as far as they can, ie idle screw all the way and the secondary butterfly's stop all the way out.

So after setting the initial timing and plugging the vacuum advance back in to the manifold vacuum port the advance jumps up (as its supposed to) but as a result of that the RPM also increased. So the RPM jumps to around 1200-1300 and I cant get it down any further. I can get the RPMs down by screwing in the idle mixture screws but I have a feeling thats not the right way to do it.

As a temporary fix Ive connected the distributor vacuum advance to the ported vacuum port. This removes the advance at idle and as a result a slower speed. Im experiencing some 'tip in' pinging on light part throttle acceleration and have a feeling its a result of the ported vacuum advance. Im running 92 octane guess fyi.

So I know that roughly 80% of the people are saying the full manifold vacuum is the way to go but for what ever reason I just cant get it to work..

So my questions are:

1. ***!!!! why cant I get the RPM down? Ive never not been able to lower the rpm of a carb

2. Is 12 degrees btdc optimal? I can drop it down to help lower the RPM maybe put it at 8? what would that do to the engine and its performance

3. The base advance is set as 12 with the vacuum advance disconnected, with it connected it jumps up to about 25! Is that right? It seems really high to me, like the vacuum can is putting too much advance in.

Thanks guys for looking and helping!

Joe

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Old 11-02-2004, 04:35 PM
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Update #1

Well.. I figure Id post my trials and tribulations here in hopes that it'll help someone else down the road.

I bumped the initial timing to 8 degrees, this allowed the rpm to drop enough to where I could put the distributor back on full manifold vacuum while having the idle speed around 900 rpm.

So I took the car out for a spin and Im still getting that tip-in pinging I was getting earlier. So that means thats theres most likely something gacked with the idle mixture?

I still think theres a problem that I cannot get the timing to the factory specs as well as the idle mixture. I'll pick up a vacuum gauge tomorrow as for whatever reason I cant find mine and adjust the idle mixture screws. Ive got the basicly to factory spec so I dont think there will be a lot of adjusting to tune it optimally.
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Old 11-02-2004, 05:50 PM
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First off, you are setting your initial correctly. And it sounds like you have 13 degrees of vacuum advance. What is your mechanical advance? If you set your initial at 10 degrees, when you raise the rpms to 3000, with the vacuum disconnected, what is your timing? You must get a timing tape/cover to find this out, or do it the old fashioned way and measure what your markings are on the harmonic balancer, then continue the length and mark it youself up to 50 degrees.

Once you have done that, set your timing at 3000 rpm to equal 33 degrees as an initial starting point. How does it idle now? Can you bring it down now or set it how you want?

Do you have an adjustable vacuum can? If so, plug your vacuum in and see what your timing is at 3000 rpm. Most vacuum can can be set with an allen wrench where you plug in the hose. Adjust your time to 48 degrees total as a base line.

You stated "So I know that roughly 80% of the people are saying the full manifold vacuum is to he way to go but for what ever reason I just cant get it to work" Not so, it is whatever your engine likes. It sounds like your engine likes ported, which only works when the throttle blades are opened. All of mine have always like ported, and I have tried to see what it liked.

Drive your car after you have set the timing with the vacuum unplugged, if it pings, back off a bit until it stops. If no pinging, then advance until it does ping, then back off until it stops. Then after that has cleared, plug your vacuum hose back to the can, then then get to cruise rpm, then give throttle, if no ping, adjust can for more until it pings, then back off. If on the initial throttle romp, it pings, then back off until it stops. You should have your timing set for your engine then. Take it home and record what the initial, total, and vacuum total is for future reference. Hope this helps.
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Old 11-02-2004, 11:11 PM
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So is it better to set for total timing at 3000-3500 or should I set for initial timing? When I checked last I had 36 total in at 3600 rpm.


To me the issue is that under ported vacuum idle timing is 12 degrees but under manifold vacuum is at around 25 degrees or so. That just seems like a huge difference to me and Im trying figure out how thats affecting the idle speed and the tip-in pinging Im getting.
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Old 11-02-2004, 11:23 PM
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When you have mainfold vacuum at idle, it is pulling all your vacuum advance in. That's not so hard to understand.

I have never believed this thing about using manifold vacuum for your advance. You had might as well not use the vacuum advance at all and just set your initial at 25 degrees. Of course, then it would be harder to start.

Vacuum advance was invented with using ported advance, no advance at idle and it builds up from there. Just makes sense.

If you aren't happy about how you advance works, best to get the distributer recurved. Years ago, they used to do that all the time. Seems to be a lost art. Or maybe, people think by just using vacuum advance, this can be avoided.
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Old 11-03-2004, 12:34 AM
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Re: Timing ?'s, Idle Speed and Ported vs Manifold Vacuum

Quote:
Originally posted by JoeS
Ok so the saga continues..

Heres the history..

ZZ4 Crate engine.. about 7k miles on it, Barry Grant ZZ4 crate demon carb. Stock ignition (hei) etc.

So I had some initial issues setting the timing but got it set to 12 degrees btdc at around 800rpm with the vacuum advanced disconnected and plugged.

Ive got the carb butterflies closed as far as they can, ie idle screw all the way and the secondary butterfly's stop all the way out.

So after setting the initial timing and plugging the vacuum advance back in to the manifold vacuum port the advance jumps up (as its supposed to) but as a result of that the RPM also increased. So the RPM jumps to around 1200-1300 and I cant get it down any further. I can get the RPMs down by screwing in the idle mixture screws but I have a feeling thats not the right way to do it.

As a temporary fix Ive connected the distributor vacuum advance to the ported vacuum port. This removes the advance at idle and as a result a slower speed. Im experiencing some 'tip in' pinging on light part throttle acceleration and have a feeling its a result of the ported vacuum advance. Im running 92 octane guess fyi.

So I know that roughly 80% of the people are saying the full manifold vacuum is the way to go but for what ever reason I just cant get it to work..

So my questions are:

1. ***!!!! why cant I get the RPM down? Ive never not been able to lower the rpm of a carb

2. Is 12 degrees btdc optimal? I can drop it down to help lower the RPM maybe put it at 8? what would that do to the engine and its performance

3. The base advance is set as 12 with the vacuum advance disconnected, with it connected it jumps up to about 25! Is that right? It seems really high to me, like the vacuum can is putting too much advance in.

Thanks guys for looking and helping!

Joe
If you're idling too high then you'll be pulling vacuum even with a ported source.

If you're idling high you're getting air from someplace, it just won't happen otherwise. Make certain that your secondaries aren't hung up, that your choke is totally and completely off. Check for vacuum leaks, a rich condition with a vacuum leak will raise idle.

K
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