I picked up an old cheap 80457 carburetor for my dad's 360 pick-up yesterday. While going through it, I noticed there isn't a port for ported vacuum, but the provision in the main body is there (circled red). Can I just drill the small hole in the metering block (circled yellow) and drill and tap for a small hose barb fitting to add a ported vacuum port? Are there any down sides to doing this?

Is there a corresponding hole in the baseplate?

Is so, then yes...... It should work fine.

I looked up that part number, and in my old holley book it says it should have come with a timed vacuum port, so I don't know if there was a change to the build specs or if the metering block got changed at one point.

Additionally, "usually" when the metering block has the "large" headed mixture screws, it's of the "backwards" idle circuit variety.
Meaning, they are an adjustable air bleed for the idle cicuit, and the fuel flow is fixed.
That type of screw should have a shorter, more blunt point on it compared to a normal mixture screw.

The backwards idle circuit carbs are "emissions" calibrated and usually don't work very well on anything that's a bit "cammy".

It's been quite a while since I worked on one of those, but if it is one of the backwards idle circuit carbs, as I recall there would be a small hole about 1/2 way up the venturi, leading towards the metering block, and there would be no brass inserts where the idle air bleeds are(it would just be a hole about .100" in the body).

There is a corresponding hole in the base plate near the transition slot. I tried spraying carb cleaner through the hole in main body (red circle) and it came out of the corresponding hole in the base plate, so I should be good to go. Thanks!

The carb is actually a 80547-5, if the (-5) means anything. I've noticed some versions have a timed port and some don't. What does your book say about a (-5) version? I've noticed some also come with different type of needle assemblies. The one I have doesn't have the typical adjustable float levels/needle seat assembly that I'm used to.

That's interesting about the idle circuit. I hadn't pulled out the mixture screws yet, but I did notice the IABs are present and on the small side and the IFRs look pretty large. I hadn't noticed any small holes near the venturis either, so I don't believe it's the reverse idle variety, but I'll have to double check. It's going on a mostly stock LA 360 (XE256H cam), so I'm hoping it'll work out okay.

I noticed in the pic of the metering block, and maybe it's just a shadow, but it looked the there was some "soot" in the idle well, which is also typical of the backwards idle circuit carbs.

The reason I refer to them as "backwards" is because of how the mixture screw works.
On that type of carb, it's an air restrictor, so when you back it out, which would be richening a normal carb, you're actually leaning it out by allowing more air into the mix.
What the screw does is "backwards" compared to a normal carb.

My old holley book only goes to the -2 of that carb.
It's an old book..... 2005.

Thanks for the info. I had no idea about the reverse idle circuits. I'll definitely have a closer look when I get home. I'll also add the timed vacuum port.

Most reverse idle metering blocks had a shiny sticker around the metering screw with an arrow pointing to Lean. If even part of that sticker is there, it is reverse idle.

The metering blocks look different, if you lay them side-to-side.

My first new Holley was a 6619, emissions carb from the 1970s. It was one of those reverse idle carbs, same as a 6909.

I stuck it on out of the box and it ran super and increased fuel mileage over stock 2-barrel by 3mpg. After listening to some friends, I took it to a local tuner and he supposedly rejetted. I never looked inside again, but just a few years ago I pulled it off the shelf to look inside and I saw it still had the close tolerance stock jets.

There is a mistaken idea about what the idle metering screw does. It took me a lot of thinking to get my head around the concept. For a metering block that is "turn in to lean out", turning the screw only changes the volume of idle air/fuel going into the carb. The actual a/f ratio of the idle component is set by the Idle Feed Restriction and Idle Air Bleed.

It worked out that it is okay to think of the screw changing the mixture, because 99% of situations work that way. But in difficult-to-tune cases, the IFR and IAB may need tweaking.

R.

^ every Dog has its day & today is that day (good info!).

Thanks, dude!

I am pretty sure that the 80457 started production around 1980 and by that time Holley had stopped the reverse idle idea.

The 80457 is a newer and IMHO better 600 carb than the old 1850. I have never seen an 80457 without a ported vacuum stub.

R.

Also as far as I know all of the carbs that use the reverse idle system where the mixture screw controls just the air bleed in the idle circuit all have left hand threads. That way it still works the same to anyone adjusting the carb that dont know any better. Turning the screw in will actually back it out and lean it out because of the left hand threads and vise versa. Ron

Thanks for the info guys. Checked this 80457-5 yesterday and it is not the reverse idle variety.

Just curious........ Did you compare the mixture screw tips between the one with the big head in the pic vs a std one?

No, the reverse idle metering blocks did not have left hand threads. That is someone's attempt to rationalize what they didn't understand.

But I'm going to have to dig out a metering block to prove it.

Might take months!

R.

I don't know why someone inserted the reverse idle system into this conversation. 80457s were never produced with that kind of metering block.

R.

I did, because I saw the big headed mixture screws, which I have only seen used on the reverse idle circuits, and what looked like soot in the idle well, which is common on a carb with the reverse idle circuit.
Also, since the 80457 is supposed to have the timed vacuum port, and his carb doesn't, it thought there might be the possibility it has the wrong metering block on it.

So..... I was "asking" if it had it or not.

I do notice in my old catalog however, that Holley did sell the "large head style" mixture screws (26-101), with no mention of them being for reverse idle....... So it appears they exist.

Pardon the detour, and carry on with your regularly scheduled thread.....

Yes, I pulled the screws out. They were the same as all the others I've seen, just happen to have large heads.