atikovi

Not on a Corvette but an '88 454 in a motorhome and I'm rebuilding the carb. I can move it a bit side to side but can't pull it out gently. Don't see it listed on any exploded view in the kit instructions. Is it supposed to come out?

johnt365

That is the APT (adjustable part throttle) screw. Do a search for quadrajet apt and you will see how it can help fine tune your A/F ratio.

If you remove it, count the number of turns so you can reinstall it at the same depth.

jnb5101

rather than counting the number of turns the piston is screwed into the body, use lars' papers to adjust the rods to the proper height. email lars at v8fastcars@msn.com for copies of his excellent write-ups.

atikovi

Thanks for clearing that up. No mention of that in the kit.

What rods? It's just like an idle mixture screw with a spring under it. I was surprised I actually had the correct tool to turn it with.

jnb5101

that's why you need lars' papers. a tang from the primary rod hanger contacts the top of the adjustable piston and limits the rods movement. it's a critical adjustment that effects idle and off-idle response.

Shark Racer

Lars only recommends this if you know or suspect APT has been adjusted from its factory height. His recommendation is a baseline.

Idle and off-idle response are determined moreso by the idle down channel and fuel tube circuit; the APT mostly controls cruise AFR. Essentially, it determines how low the rods can rest in the jet in high vacuum situations. When the throttle angle is low enough, you run off of the idle circuit only and the main circuit is not a significant contributor to fueling.

jnb5101

sorry, shark
that's not how i read lars' paper 'how to adjust a APP'. he specifically mentions how this feature helps control cruise, idle and wot. but beyond that, my intent was to inform the poster about the informative papers that lars took the time to write, and that will allow him to successfully rebuild the carb.

Shark Racer

APT has no effect on WOT.

Its effect on the idle system is minimal at best; it is used to tune cruise. Think about how APT and the power piston works. The APT screw is a lean stop - the rods will drop no further into the jets than the APT screw will allow. If you go WOT, vacuum on the power piston goes to zero and the needle's power tips remain in the jets, which presumably are the same size. (You should use the M rods with single MAB carburetors like the one pictured by atikovi, basically late 70s-mid 80s truck carbs, the main air bleed outcropping looks like a trapezoid vs a double hump like a passenger car dual air bleed carburetor).

At very minimal throttle angles, APT has less effect because the MABs are minimally exposed.

I agree that it's best to tune APT first before you fine-tune IMS, but it's not going to make or break your idle.

Playing with idle air bleeds, down channel restrictions and idle fuel tube restrictions are not for the faint of heart, but these are the strongest actors in the idle and off idle characteristics of a QJet.

atikovi

Got it done today. Don't know why that gold finish seems so worn off though.











Will install it tomorrow and see how it runs.

atikovi

And what is that round part in the center for?

atikovi

Got the carb back on and it runs good except the electric choke doesn't seem to be working. Let it run 10 minutes and it still wasn't fully open.

Shark Racer

Is it opening at all?

Make sure you don't have a gasket between the choke housing and choke element.

If it is opening, you could have a weak spring or could have it set too tight.

I like to test my e-chokes on the bench with a DC power supply.

atikovi

Yes it opens a bit when the engine starts from the vacuum break. And there is no setting, it's fixed in position by a grove in the housing matching one on the choke. Probably should have tested it first.

Shark Racer

Is that the same setting you had it at before? If not, you can loosen the screws and adjust it. IIRC, towards the rear of the carb (towards the secondaries) should tighten the choke spring (increase choke duration) and towards the front of the carb (fuel inlet) should relax the spring (decrease choke duration). Make a reference mark and experiment to find something that works.

You also have to make sure that you got the lever arm on the choke spring caught correctly in the groove on the arm inside the choke housing.

atikovi

Maybe I wasn't clear before but with this design the choke is NOT adjustable. It fits into the housing at a fixed location and then is secured by rivets.

Shark Racer

It is adjustable - remove the rivets and replace with self tapping screws.

atikovi

When I take it off I'll shoot a close-up of the housing and choke body to show that a bump on the choke fits into a grove on the housing so that it's can't be rotated. BTW is that a painted finish?

Shark Racer

Perhaps a anti-tamper "feature" they added in the 80s. You could grind the bump off to make it work, or pick up an aftermarket choke cap.

It's a Caswell yellow cad plating.