Enlarging a transition slot

[ArizonaGuy]

What carburetor are you working on ?

It’s a 4160 Holley, 750 vacuum secondary

[rgalajda]

Do you have the List no. ?

[bill jones]

---I use a 32 tooth hack saw blade
---you can only get about 3 teeth into the slot on about a 45 degree angle
---the sides of the teeth need to be lightly and precisely ground flat to the same width as the blade
--- blunt end needs to slanted back 45 degrees or so---so the blade can poke into the slot 1/4"
---the blade needs to be narrowed up to where it's only about 1/4" wide with that slanted nose
---I make a handle and bolt the blade to the handle and make the overall length of the blade about 3" long
---you are actually only going to be using like 3 or 4 teeth to be pointed to PULL the metal out of the T-slots
---this can be done with the throttle blades held wide open
---the actual cutting end of that blade is only like about 1/4" long---and a couple of inches to fit onto a handle

[mag2555]

I would use a cut off disc in a one of the small battery powered Dremel tools.

You will first have to ware the disc down to a smaller radius on a bench grinder stone or on concrete.

You have to use the low speed setting on the throttle plate.

[rgalajda]

"I’m seeing some threads mentioning beveling the edge of throttle plates, or filing small notches in the plates (mirroring the slots themselves)… can anyone elaborate on either of these?"

Some of the old AFB carbs had notched throttle blades. Simple to do with a file . Much easier than lengthening the slot if it gives enough exposure.
Plus you can go back by replacing blades.

The reason I asked for the Holley List number was that I can not remember ever seeing a holley carb that did not have t slot exposure. It is usually a matter of too much t slot exposure with long duration/race camshafts. I have about 30 holley carbs here that are fine.

The total length of the t slot is irrelevant in your case. It is a matter of where it starts below the throttle blade and the thickness of the throttle blade.
Thick brass or thin aluminum/steel blades. Big difference.

Holley 3310-1 780 cfm
transfer slot .023 x .220

Holley 3310-2 750 cfm
transfer slot .026 x .210

Holley 80557 750 cfm
transfer slot .022 x .240

Holley 4803 780 cfm
transfer slot .024 x .210

Quick Fuel 770 cfm
transfer slot .028 x .225



Holley 850
transfer slot .024 x .200 --- short

Holley 750
transfer slot .027 x .230

Holley 600
transfer slot .024 x .260

Holley 1850-3 600 cfm
transfer slot .024 x .270 --- using thick brass throttle plates.

These are carbs that I measured the transfer slots on . I used pin gauges to measure the width of the slots. Notice how they vary so much. And look at how long the slots are on the 600 cfm carbs with thick brass blades.

[ArizonaGuy]

Do you have the List no. ?

80508-8. Sorry I didn’t respond sooner. Lately I’m a one-legged-man in an ass kicking contest

[ArizonaGuy]

"I’m seeing some threads mentioning beveling the edge of throttle plates, or filing small notches in the plates (mirroring the slots themselves)… can anyone elaborate on either of these?"

Some of the old AFB carbs had notched throttle blades. Simple to do with a file . Much easier than lengthening the slot if it gives enough exposure.
Plus you can go back by replacing blades.

The reason I asked for the Holley List number was that I can not remember ever seeing a holley carb that did not have t slot exposure. It is usually a matter of too much t slot exposure with long duration/race camshafts. I have about 30 holley carbs here that are fine.

The total length of the t slot is irrelevant in your case. It is a matter of where it starts below the throttle blade and the thickness of the throttle blade.
Thick brass or thin aluminum/steel blades. Big difference.

Holley 3310-1 780 cfm
transfer slot .023 x .220

Holley 3310-2 750 cfm
transfer slot .026 x .210

Holley 80557 750 cfm
transfer slot .022 x .240

Holley 4803 780 cfm
transfer slot .024 x .210

Quick Fuel 770 cfm
transfer slot .028 x .225



Holley 850
transfer slot .024 x .200 --- short

Holley 750
transfer slot .027 x .230

Holley 600
transfer slot .024 x .260

Holley 1850-3 600 cfm
transfer slot .024 x .270 --- using thick brass throttle plates.

These are carbs that I measured the transfer slots on . I used pin gauges to measure the width of the slots. Notice how they vary so much. And look at how long the slots are on the 600 cfm carbs with thick brass blades.

Thanks much for doing all that work and posting the data See my post directly above… I may have to de-prioritize this 750 for the time being. But when I can get back after it I’ll update this thread

[Geoff2]

The AFB carbs that had notched throttle plates were MUCH thicker than typical H blades. They were alum & about 0.100" thick.

[rgalajda]

The AFB carbs that had notched throttle plates were MUCH thicker than typical H blades. They were alum & about 0.100" thick.

Geoff, your right, I just measured some. The only holley blade that thick are the old brass ones.

His carb should have throttle blades marked "172" which are about .040" thick/steel . Still I can't see why there wouldn't be enough T slot exposure on that carb, although I don't have one here to look at.

[ArizonaGuy]

The AFB carbs that had notched throttle plates were MUCH thicker than typical H blades. They were alum & about 0.100" thick.

Geoff, your right, I just measured some. The only holley blade that thick are the old brass ones.

His carb should have throttle blades marked "172" which are about .040" thick/steel . Still I can't see why there wouldn't be enough T slot exposure on that carb, although I don't have one here to look at.

Indeed, it has 172 plates in it. Gimme an hour or two, I’ll post some pics

[ArizonaGuy]

Suspect throttle body

0033C024-F735-45FB-BF1C-462C38744B12.jpeg

[ArizonaGuy]

One that I know for a fact is ‘right’.

A33B575B-DC4B-42C8-B0E8-0E67436AA603.jpeg

I just put that^ TB on the suspect carburetor, we’ll see what happens. I measured and compared a bunch of stuff… I have a suspicion but I’ll wait to post it until the road test (with alternate TB). But I’m pretty sure I’ve found the problem

[ArizonaGuy]

Problem solved well, positively identified at least. The alternate TB (.025 x .250 slots) worked like a champ. So, indeed the suspect TB has slots that aren’t exposed.

The reason: or at least part of it, the shaft bores are .015 off center. Too low. That moves the throttle plates away from the slots, and the slots don’t see any vacuum until the plates are open quite a bit. That’s why it was lean off idle and early throttle opening.

I got this suspect carburetor for $30. Now I know why

[ArizonaGuy]

I have a scrap throttle body that I’ve stolen parts off of. I’m going to practice lengthening the slots on that, see if I like it. If not, I’ll notch throttle plates. Stay tuned

[rgalajda]

I have had the idle screw backed out far enough on these that there was no transition slot showing and there were no adverse effects/lean off idle.