I don't think there's a spec for that. I'd say if the vacuum advance works properly, the vacuum at the port is good.

At idle there should be no vacuum reading. Vacuum starts when carb is starting to come off idle position and is controlled by RPM's. Walt.

Walt is correct. Ported vacuum is created by air flow by the port, which should be next to o at idle, as it is created by a venturi-like effect. Thus the more flow past the port, the higher the vacuum. If you are getting vacuum there at idle you likely have open throttle at idle, and should have a high idle problem. If your idle is low or normal, and you have significant vacuum at the port, I'm afraid your port is somehow continuous with manifold vacuum via some kind of leak. Or your vacuum gauge is messed up?

Hi guys - thanks. I took a look at the base of a spare carb I have lying around. It looks like the little hole in the throttle bore that is connected to the port for the vacuum advance is right at the spot where the throttle plate closes. I assume that at idle the throttle plates have to be open a little (otherwise how would the air get into the engine), but just a crack. If there's not supposed to be any vacuum there at idle, then it means that the vacuum advance is not engaged, so the base timing is the timing. But, I thought that at idle you needed a lot of advance to ignite the very lean mixture early enough to get an efficient burn. Especially since the manual recommends setting the timing to 0 deg. Why is this?

The throttle plates/valves are nearly closed and air still gets into the motor via the air bleed/bypass channels that provide air, mixed with fuel via idle jets, and then enters intake at the throttle valves. Since the throttle valves are nearly closed at idle, minimal air flow means minimal vacuum to vacuum port. When you are idling at 500 rpm, and disconnect the vac advance to adjust timing, you should see minimal change in RPM/timing. If you do see a big change, then throttle plate position is contributing a lot to your idle air flow, and you may need to clean the bypass/bleed system.

It is advantageous to have higher advance at idle in a cold engine (slower burn), so in the 70s some cars had manifold vac applied when cold, then switched to ported vac when warmed up.

Wow, thanks for the great explanation. I was looking to understand how there could be such a significant difference in both RPM and timing at idle with the vacuum advance hooked up vs. disconnected (with the port blocked off). It seems that from your explanation it could be that one of the air bleed/bypass channels may be blocked.

BTW - do you recommend setting the timing at 0 deg. as indicated in the manual, or do you advance the timing up to 4 or 6 deg before up to the point of almost pinging under high load?

Thanks again.

The manual says the "0" setting is based on 80 octane fuel, and you may advance it according to the fuel being used. But not further than 1/2" from the TDC mark on the flywheel. Beats me why they'd use this "inches" reference rather than degrees, but . . .

And yes, the "light pinging under load" is still a good reference.

Thanks for the advice. Now i just need some dry roads to try this out on. And a nice long hill.

When you jack that timing up a bit, you won't even have to leave the driveway to tell the difference. Quicker throttle response. And on the road, significantly better mileage. Went from 15 to 18 on the road with my Twin-H Hornet.

Park, what do you use to set the timing? A light or vacuum gauge?

For what it's worth, I use a timing light, and i get the engine up to operating temp and adjust the idle to 500 with a tach first. What I've been doing is disconnecting the vacuum line that runs between the throttle body of the carburetor and the distributor vacuum advance, at the carburetor end. I close off the connection at the throttle body by installing a vacuum gauge. Then I set the little distributor pointer on the block to the center line (zero degrees), tighten the bolt, and then adjust the timing with the bolt at the bottom of the distributor to get the light to flash when the long mark is level with the indicator on the bell housing. Once this is all tightened down that sets the base timing to zero when the pointer is right in the center.

Where i have been fooled is that once i take out the vacuum gauge, which typically reads around 10" Hg or so (not correct, from what I read above), and i reconnect the vacuum advance line, the RPMs jump up to ~600 and the timing appears to be about 6 or so degrees BTDC. It sounds like I might have a carburetor idle circuit problem that is causing the throttle plates to be open too far (to achieve the 500 RPM base idle) and that's what's causing there to be so much vacuum at idle. I'll investigate that and report back sometime over the weekend.

Michael, it sounds like you're still setting the basic timing to zero. A bit modest with modern fuel. See above re the 80 octane reference.

Kerry, I use a timing light, but I have to admit it's a bit awkward to see the flywheel marks accurately due to the parallax. I have no issue with the vacuum gauge method, but it seems to me one is optimizing the timing at idle, which may not be optimum overall. Or do you do it at a higher RPM?

Park,do you know of anyone putting timing tape on the dampner corresponding with the TDC on flywheel and diameter of dampner to facilitate timing setting and advance checking?
Probably best to use a micrometer thru spark plug hole; extremely difficult to see timing mark.
Don't think it would be difficult. Would have to install pointer.

Don't know of any, Richard, but doesn't seem that it'd be difficult. Not a bad idea at all! Aside from difficulty of seeing the marks accurately, the large diameter flywheel has the advantage of greater spacing between them.