air bleed questions

[wallaby]

my carb # is: 17056502    (71jet/41k rod)

Your book states that not all carbs have 4 main air bleeds, and not always are upper & lower of the same size.
My question is: on the units without air bleeds in the throttle body, does that mean there is no hole there, or does it mean that there is no brass restriction in the hole?

My carb has a large hole (about .125) as an air bleed in the throttle body, and a small hole (about .028) in the air horn. I guess I'm wondering if this is ok, or am I missing something (like a brass orifice)...or are the top & bottom mismatched maybe?

I tried to adjust my APT with the "tip in" method, but it wants more fuel that I can adjust into it. I backed out the APT until I could feel it losing contact with the spring , and it still wants more fuel at part throttle cruise. It seems to run great under load....even without the secondaries, but I have a lean misfire at cruise.

[wallaby]

Further reading of your great book  took me to the page that deals with this issue.
As luck would have it, I have a set of 77 jets and  some 47k rods. I think I would prefer a 49 rod for more lean adjustment if needed, but I'll try what I have on hand for now and see how it turns out.
Does it sound as if I'm on track?

As for the giant bleeds I have, is there a reason to downsize them? I mean.. other than their need for larger jet & rod sizing, is there some other advantage?

[Cliff Ruggles]

Some Chevy carburetors were set up with huge lower main airbleeds, and restrictions in the uppers.

A 77 main jet is HUGE if used with a "K" metering rod, and it will be pretty rich at heavy/full throttle. 

If you can't tune within an acceptable range with the APT using the 71 main jets, move up to 72's, not clear to 77's.

Also keep in mind that the float level, fuel inlet seat diameter, fuel pressure, etc, effect the A/F ratio some.

In many cases, one can simply raise the float level slightly and add enough fuel to get within the range that APT will eliminate the lean condition.

In any case, make small changes when tuning, just one jet size is usually enough.....Cliff

[wallaby]

77 sounded big to me also, but I didn't choose them by myself....I followed recommendation from your book.
To quote:"If your carburetor has the large main air bleeds, all of the modifications listed for each example still apply, except for the main-jet and metering-rod choices. Increase the main jets to .076 or .077 inch. Use a metering rod about .030 inch smaller as a starting point. The metering rods must have the smaller .026-inch tips..."

I thought I was going to be lucky with these jets/rods. I'm not even sure where I got these jets...I thought maybe they came with the carb originally? I don't know. I have lots of spares 72 or smaller.
After installing the 77/47k setup, I started with 2 turns out on the APT and still had to back it out a couple turns to dial in the tip-in.

Should I just make all my bleeds .070 and use common size jets, or is there hope of tuning this carb with the big bleeds?

[Cliff Ruggles]

The problem with your logic is that your carb doesn't have 2 pairs of the large main airbleeds.  The upper main airbleeds have the most impact on the final A/F curve since they are in direct alignment with the fuel supply passages.

The lower pair are "indirect", and have less effect.  To date I've never seen a Chevrolet carburetor from that year have a .028" upper main airbleed and .125" lower main airbleed.   In any case, the .028 upper main airbleed would call for a pretty small main jet.  .077" main jets are HUGE for that set-up, and the carb will be putting down a LOT of fuel at heavy throttle openings, even if you can get it dialed in at part throttle with a large metering rod.

If I were building the carburetor here, both pairs of main airbleeds would be replaced with .070/.070", and I would install our custom jet/rod package, along with a high flow needle/seat assembly, new float set at 1/4", HP accl pump assembly, and the rest of the items from our rebuild kit. 

Then I could install the carburetor on the test engine and it would come right in for A/F across the speed/load range......Cliff