10 replies to this topic

[RHDmini74]

So I got my new motor up and running. Set the ignition timing without the vacuum advance tube connected as I should. After I reconnected it the idle went up, like it should, so I adjusted it back down. With a small blip of the throttle then revvs jumped up and just hung there and wouldn’t go back down unless I turned the idle adjustment screw. Then it would sometimes drop too low and want too die out, but I i reved it up a little again it would want to hang and not drop. Without the vacuum advance tube connected the idle is fine. Dashpot has oil. No other leaks anywhere. What else shouldnI check? Maybe too rich? The car has high compression flat top pistons, an AEG530 cam, big valve head, forged rockers.

[Spider]

What Carb(s) are you running?

[RHDmini74]

What Carb(s) are you running?

Single Hif44

[Spider]

Ah, OK.

 

The Port on these Carbs is usually in a less than desirable place for Distributor Vacuum Advance.

 

On all other SUs, it is Ported such that at Idle, you don't get much or any Vacuum, however, in these, they have near full Manifold Vacuum at Idle. This is ~ OK ~ (manageable) with stock Cams, but when anything with as much as a slight lump at Idle is fitted, the Manifold Vacuum is up and down at Idle, so giving more and less Advance, which will change the Idle Speed and likely a by a fair bit.

 

2 Solutions that I can think of;- 1 - (the easy) is to disconnect it and block it at the Carb, or 2 - (and what I usually do), is to block off the stock port and fit a new one so that when the Butterfly is in the Idle position, the port is aligned with the edge of the butterfly, that will give next to no vacuum at idle on that port.

[RHDmini74]

Ah, OK.

 

The Port on these Carbs is usually in a less than desirable place for Distributor Vacuum Advance.

 

On all other SUs, it is Ported such that at Idle, you don't get much or any Vacuum, however, in these, they have near full Manifold Vacuum at Idle. This is ~ OK ~ (manageable) with stock Cams, but when anything with as much as a slight lump at Idle is fitted, the Manifold Vacuum is up and down at Idle, so giving more and less Advance, which will change the Idle Speed and likely a by a fair bit.

 

2 Solutions that I can think of;- 1 - (the easy) is to disconnect it and block it at the Carb, or 2 - (and what I usually do), is to block off the stock port and fit a new one so that when the Butterfly is in the Idle position, the port is aligned with the edge of the butterfly, that will give next to no vacuum at idle on that port.

I see, i was just reading your other thread that was talking about that. Is it okay to run the car with the vacuum advance port blocked off? would I lose anything with it like that? i wouldn't mine modifying it in the future, i just don't want to drive it around with the advance blocked if its bad for the engine

[Ethel]

You only have significant manifold vacuum when the engine's throttled. Running without vac will affect fuel economy, but won't do any damage.

 

Suppose it'd be possible to rig up a valve to close just before you hit maximum depression, but it'd be simpler just to drill a new hole.

[RHDmini74]

Okay sounds good. I’ll play around with it and see if I can get it better or I’ll drill the new hole and try it that way

You only have significant manifold vacuum when the engine's throttled. Running without vac will affect fuel economy, but won't do any damage.

Suppose it'd be possible to rig up a valve to close just before you hit maximum depression, but it'd be simpler just to drill a new hole.

[ACDodd]

I keep rebuilt ported vacuum HIF44's on on the shelf if you needed one quickly.

Ac

Attached Files

•  IMG_20180719_194240.jpg   52.67K   2 downloads

[RHDmini74]

Nice. I’ll pull mine off and make the new fitting this weekend. Until then illdmrive it the way it is. How far away is the new hole from the old one? Or about how much of the butterfly opens when your at idle?

I keep rebuilt ported vacuum HIF44's on on the shelf if you needed one quickly.

Ac

[ACDodd]

The new drilling g is positioned such that at idle it is covered. Opening the throttle any amount uncover s the drilling g such that it is vented to manifold vacuum.

Ac

[Spider]

The new drilling g is positioned such that at idle it is covered. Opening the throttle any amount uncover s the drilling g such that it is vented to manifold vacuum.

Ac

 

Following on with AC's info here, from the top, this is where I position the new port, you can see it next to the original one

 

 

These next few shots are of a HS4, but as far as this Port goes, it's the same on a HIF44

 

Inside, it needs to end up at here - Butterfly Closed (Idle Position and covered - nothing to see)

 

 

Just off the closed position