Ford Muscle Cars Tech Forum banner

1 - 6 of 6 Posts

·
Registered
Joined
·
2,617 Posts
Discussion Starter · #1 ·
I bought a pair of Holley 450 vacuum secondary carbs (4548 list number) off eBay for the tunnel ram setup I'm putting together. These are listed as Ford replacement carbs for a variety of engines, but I've heard the 450 VS is the best carb to run on a street/strip Tunnel Ram. Both carbs have a 1/4" hole in the middle of the top of the primary fuel bowl, with a lever arm with a rubber plug on the end that is actuated by the throttle... at idle the plug is slightly lifted to allow venting, but as soon as you start opening the throttle the plug covers the hole.

The vent seems redundant to me, as the bowl can also vent through the usual vent inside the air cleaner. The only difference is, this one vents straight to the atmosphere, and will likely be a constant source of fuel seepage. I can't see this being a smog or emissions thing as it goes against most of the smog rules (venting fumes to the air), and it doesn't seem like it would help at all during any kind of driving conditions.

I'm cleaning up the carbs and doing some minor mods, as I don't really see a purpose for the vents I'm planning to leave them off, going to tap the hole and use a spare sight hole screw to plug it. Anyone see any problems with this, or can explain to me why these have the extra vent hole?
 

·
Registered
Joined
·
1,604 Posts
Several decades ago I had an old 585(?) cfm Holley that was original equipment on T-bird that had that vent. Never had a problem with fuel seeping. I don't think mine had vent tubes in the main body so it needed the external vent on the bowl.
 

·
Registered
Joined
·
2,617 Posts
Discussion Starter · #3 ·
This one definitely has both the vent tube in the main body. I think I've convinced myself I don't need it, I'll tap and plug it with a spare sight hole screw, and if it turns out I'm wrong, I can always take the screw out and put the plug back on I guess. Just don't understand what situation it would actually benefit... especially since it's only open when the throttle is closed (idle or engine shut off). Weird.
 

·
Registered
Joined
·
2,617 Posts
Discussion Starter · #5 ·
All the numbers seem to match up properly with other 4548 carbs I've seen on eBay, all of them seem to have this dual vent configuration, as you can see the main body vents in their usual location plus the little rubber lever-actuated plug. And yes, there is an air passage in the metering block to go to the main body vent :).

Here's a pic:

 

Attachments

1 - 6 of 6 Posts
Top