Ported vacuum for vac advance (1 Viewer)

[landcruiserrich]

I have a 1974 fj40 which I am rebuilding the carb on. I have checked the tech section and found alot of good info on this carb. I want to modify it for ported vacuum so I can run a vac advance dizzy. First of all do I need to, and secondly should I attempt it myself? I read the post by Ed Cook : www.ih8mud.com/tech/carbinfo.php

I am wondering if anyone else has done this?

 

[Degnol]

You should have a port on the driver's side which is used for the pull-off or whatever it is called on the carb. I just got a "T" fitting and ran an extra hose to my dizz. I run a GM HEI, but it should work the same for you. A lot of carbs have been ruined by trying to add an extra port. Try the "T", it will cost you less than $2 to find out if it works for you.

GL
Ed

 

[landcruiserrich]

Thanks, I'll give it a try

 

[65swb45]

I have a jig set up by my drill press to do porting during the rebuild process.

 

[landcruiserrich]

If I can just tee off the fitting on the driver's side, why would I have to drill a port? Is the fitting that Degnol mentioned not the proper place to take vacuum from? Why can't I take vacuum off the manifold for vacuum advance?

 

[Degnol]

I used to run with no pulloff(PO deal) and the truck ran just fine, I used the one vacuum port for my distributor only and it worked fine. Found and rebuilt a carb with the pulloff and needed a second vacuum port. Somebody on MUD suggested the "T" and it has worked just fine.
So...I have no science to explain why it works well, and I do know there are several places to get vacuum.
Mark can probably explain in something other than anecdotal form as he is a carb shaman.
I think I have a carb base with two ports on it if you want to trade.

GL
ED

Don't run with scissors!!!!

 

[landcruiserrich]

Thanks for the idea. I'm going to go with it but it just sounded too easy to be true! I'm still curious about why anyone would go through the trouble drilling holes in their carb.

 

[FJ40Jim]

Thanks for the idea. I'm going to go with it but it just sounded too easy to be true! I'm still curious about why anyone would go through the trouble drilling holes in their carb.

If the carb is a non-US model, then it should already have a ported vacuum fitting.
All 69-74 Fed-spec carbs have no ported vac fitting, hence the need to add the fitting.
Connecting any distributor to manifold vacuum is counterproductive.

 

[65swb45]

Connecting any distributor to manifold vacuum is counterproductive.

WORD

 

[rgentry]

I clipped the following from: http://www.bob2000.com/carb.htm

This agrees with my understanding of engine vacuum and why you would
not want to mix up manifold vacuum and carb/ported vacuum. Seems to
me that if you are using the U.S. vacuum RETARD distributor, then it makes sense
to hook this to MANIFOLD vacuum. If you are switching to any sort of
vacuum ADVANCE distributor, then you need to provide PORTED vacuum.

All of this seems to be a never-ending source of confusion.

QUOTE:

Vacuum ports and what they do.

Not all Holleys have the same ports, but you can tell by location and size what each one does. Any port at the very bottom of the carb (in the aluminum baseplate) will pull manifold vacuum, which is strongest at high vacuum situations such as idle and slowing down, and weakest at low vacuum situations such as wide-open-throttle (WOT). Common manifold vacuum connections at the bottom of the carb are PCV (usually a 3/8" diameter tube) and EGR/distributor retard/smog device hookups (usually smaller tubes under the primary fuel bowl). The other type of vacuum connection on a Holley is ported vacuum, which is weak at idle, strong at cruise, and weak at WOT. This one is always (if supplied at all-some competition carbs don't have this port) on the passenger (US) side of the carb, above the idle mixture screw, and is generally used for distributor vacuum advance.

 

[landcruiserrich]

So the fitting on the base of the carb that goes to the emissions control box isn't ported vacuum?

 

[FJ40Jim]

So the fitting on the base of the carb that goes to the emissions control box isn't ported vacuum?

Correct, The fitting on the base that faces away from the engine on a 1970-1974 F carb is full time, manifold vacuum. It is used for the choke breaker.

FWIW, there are some late 74 Kalifornika spec carbs that have ported vac fitting in the carb base on the valve cover side. I've only seen a few.

 

[landcruiserrich]

Ok so now all of that is cleared up, what are my options? I can have a port installed on my carb ,but how much would that cost? Can I use a base from a later carb? Should I just save my money and get a Webber or Holley? The plan was to get a non-USA vac advance distributor, but if this carb ends up costing me too much I won't be able to do that for a while.

 

[rgentry]

Sounds like Jim and Mark know where to drill the hole for ported vacuum.
You might find a junk '74 carb on the shelf at a place like cruiseroutfitters
or landcruiserparts.net and try and install your own vacuum advance port.

I see the old vacuum advance distributors for sale all the time. If you are
willing to try this yourself, I bet you can rebuild the carb and install the
vacuum port and get a used dissy all for $100. A lot of detailed work involved,
so you might get Jim to do the carb for $300 and buy a new vac advance
dissy for $250 and know that it has been done right.

 

[landcruiserrich]

Not only am I cheap, but I'm also a sucker for punishment. I'm considering trying to install the vacuum port myself. I have an extra carb from my parts cruiser, the only problem is it has god knows how many miles on it and the primary shaft moves up and down in the housing, so on top of the port, I'll have to have a bushing installed. I won't try drilling holes in my good carb which I'll probably end up sending to Jim or Mark when I throw the other carb against the wall in frustration

So I was wandering what the hardest part of installing this vacuum port is and if anyone has a picture of the finished product.

 

[Degnol]

Hey, I owe everyone an apology, I guess. The PO had the dizz hooked up to the vacuum port on the driver's side, where the choke pulloff is normally ported. When I installed the HEI, I just "T'd" off of that and everything seems to run not only fine, but great. I do have a throttle body part of a parts carb that has the ported vacuum on the pass side, so I'll swap that out and maybe I'll be more pleased with the performance. Currently I'm only running 9 second quarters.
I'm sorry I was passing along BS. I hate that. Just seemed to work for me.

Apologies to all!
Ed

 

[Pin_Head]

I'm considering trying to install the vacuum port myself.
So I was wandering what the hardest part of installing this vacuum port is and if anyone has a picture of the finished product.

The vacuum port for the vacuum advance ties in above the throttle plate in the narrow part of the bore. Vacuum there is near zero at idle and increases with increasing air velocity. I dont think the location is particularly critical, but you don't want to drill into any fuel circuit. Make it a nice press fit and JB weld it and you should be good to go.

 

[landcruiserrich]

Thanks guys, I'm going to go drill a hole in my carb now. I'll post results when I finish the rebuild.

 

[Pin_Head]

If you go to the spector web site and look at the carbs, you can see a port next to the idle mix screw that looks like it might be appropriate.

Non-USA version with the vacuum port hidden behind the wire near the idle mix screw:

 

[Degnol]

FWIW, there are some late 74 Kalifornika spec carbs that have ported vac fitting in the carb base on the valve cover side. I've only seen a few.

So... Jim C...If I have one of those, which I do, I can use the engine-side port for my vacuum advance HEI?
Apologies again, they all look the same to me. I had no understanding of ported vacuum vs manifold vacuum. But it makes sense.

Thanks
Ed