Quadrajet Power - APT system description and adjustment

[@robertwest3093]

And the choke flap trick for apt adjustment is BRILLIANT!

[@georgekalafatis7286]

And that fully tapered rod is the way to go

[@georgekalafatis7286]

Thank you for being here I used to do quads all the time I just it's been so long you remind me of everything that I forgot and you're real good thank you

[@waynep343]

APT adjustment on the car/truck that can be driven. with the Tool to adjust the APT.. i also use a rochester carb dipstick. thru the round fuel bowl vent. its 39MM from the end of the stick to the beginning of the 1/32" ruler scale. but anything that will reach down the round vent tube and you can feel the primary rod holder and push it down against the spring.. you can adjust the APT screw up till it just touches the metering rod holder pin.. and give it 3/4 more turn.. then with the engine warmed up.. left foot on the brake.. right foot ready on the gas pedal.. car/truck in drive.. give it a short blast of part throttle.. like you would do after the drag racing burn out to check the traction of the tires on the way to the staging beams on the starting line.. if the carb hesitates... adjust the APT up 1/4 turn.. get back in.. back up and do a dry hop again.. if the carb hesitates... give it another 1/4 turn upward.. you will keep doing this until you find the sweet spot.. trust me you will know when adjustments no longer improve the part throttle acceleration. you won't believe the difference.. but it will also improve your fuel economy. your cruise power .. and driveability at part throttle. APT.. at idle the fuel is limited by the tapered tips of the idle mixture screws.. when you tip into the throttle you start to expose the vertical idle transition slots. but the small primaries of the quadrajet you are thru the idle transition circuit and beginning the primary main circuit.. if the primary needles are too far down in the main jets.. do to too low an APT setting.. the carb will bog. if its really low the carb will backfire. bringing it up the APT adjustment 1/4 turn at a time.. allows you to find that exact sweet spot where the manifold vacuum pulling down the primary rod holder and held up by the APT screw is just right for your application. you can take it from a quadra bog to a rip roaring carb. earlier quadrajets also have a device that limits the downward/lean limit of the primary metering rods.. its that little wire thing sticking up from the base plate between the primary throttle blades. the screw is locked in to adjust that.. but if its too lean.. you can remove the carb.. take the two long screws out of the back.. take the 2 or 3 base plate screws out.. and kinda unloop the accelerator pump rod.. so you can bend the little wire that sticks up from the top of the base plate up just a fraction of an inch. the threads on the APT screw are like 28 threads per inch if i recall. since the APT screws you need to get it within about 1/2 a turn.. each turn of the apt screw raises the adjustment just under 0.04" so you might want to measure where you started out at and raise in 0.04" and reassemble and try it again.. you don't have to touch the top of the carb at all other than the 2 long rear screws..

[@francoismachine]

Thanks for taking the time to write all of this down, really appreciate the info :)

[@peadarmacewen4710]

Excellent video - no clowning or music - clear images and brief, comprehensive, information.

[@pixL01]

nah its a commercial, americans will eat that up tho

[@bobdixon4998]

I've been helping my neighbor with his 71 Chevy k20. It has a 1980 quadrajet on it and your videos have been great in instructing me on the rebuild. I'm continuing to watch your videos. Thank You!

[@QuadrajetPowerLLC]

Thank you!

[@mrlastname3065]

Great, I just watched this video & see now where the 'extta spring' I found after a quad rebuild goes. Of course, it goes underneath the ATP screw. Oh well, maybe I'll get the initiative to slow down this time & try to understand how to set the float using the gauge the kit came with while I'm there. Quadrajet rebuilds are alot like life, there are no shortcuts~

[@dirtyburd71]

Your videos are extremely informative and easy to follow. Thank you!

[@QuadrajetPowerLLC]

Glad you like them!

[@adolphmadrigal6186]

Awesome video I've seen other videos and people seam to never miss with that screw I'm glad I found your channel and I got me a rebuild kit for my quadrajet

[@QuadrajetPowerLLC]

Thank you.

[@ourkid2000]

Great video! Love it, always wondered about this APT thing

[@georgekalafatis7286]

I've always enjoyed the way the quadrajet works it starts good cold it runs smooth and I miss it I'm going to take that Holley off and get working on it but the weather's been real cold and I don't want to start my car to get you the information just as soon as the weather warms up

[@panafax21]

Awesome Videos Guys ,very informative have learnt a lot cheers. New Zealand Garry.

[@fulgenciomarcos9350]

Thank you for the valuable information.

[@hoytswinderman6295]

Thanks for the great info, appreciate it very much.

[@robertwest3093]

The Qjet is so infinitely adjustable and so precise I won't use anything else. There really isn't a need for any other carburetor considering what the Qjet is capable of.

[@quadjunkies7381]

Thank you for your help I just rebuilt my carburetor and it would not idle at all I didn’t think that screw did anything so I turned it in all the way

[@octaviofuentes7781]

How did it do after adjusting it ?

[@logantucker364]

Great video! Thanks

[@QuadrajetPowerLLC]

You're welcome!

[@DanVanDanVan]

great video thx

[@QuadrajetPowerLLC]

Glad you enjoyed it

[@robinschiele4874]

Does the APT system also affect the idle mixture screws? Could it be that if my idle is too rich (AFR of 12) that I would also need to turn down the APT screw in order to get the Idle Mixture screws effective?

[@QuadrajetPowerLLC]

The APT can have some effect on idle mixture. Lowering would lean your idle mixture some.

[@stanthrasher4815]

Great info tks!

[@jamesdeligio2761]

I have an 88 Monte Carlo ss. Original. Had the carb rebuilt last June. Damn think keeps hesitating and bogs. Had to put the rpm’s up a bit so it doesn’t keep dying on me. That top adjustment screw I heard would fix that problem ?

[@Spiritof_76]

Isn't that an electronic QJet? It's different internally and needs different procedures.

[@jamesspalding5559]

Excellent

[@buddymartin7923]

Can you adjust the apt screw while running or do I shut down, adjust, crank, then test again?

[@QuadrajetPowerLLC]

You can adjust with it running.

[@buddymartin7923]

@@QuadrajetPowerLLC thank you very much. The jets and custom made rods you talked about; what are the benefits? My Qjet is replacing a 2 barrel Holley on a stock Ford 351M with 2V heads. Weiand intake. This is a 3/4 ton truck for use, not racing. Will I see any improvements or are the stock rods/jets fine as long as they’re in good repair?

[@QuadrajetPowerLLC]

@@buddymartin7923 our custom rods are full taper, which give you much better tuning ability for part throttle over the factory rods.

[@mikeuva2273]

@@QuadrajetPowerLLC how do you adjust running? go to 2000 rpm and tune to best idle?

[@HateCrewReaper]

I set the idle at 2k. I tipped the choke blade a bit and the rpm raised. I turned the screw counterclockwise a 1/4 of a turn to start. I repeated the procedure until I was now at 3 and a half turns out and the screw was literally loose. The rpm was still rising when I tipped the choke blade. What could be causing this?

[@aztecahernandez6539]

Thankx for sharing your knowledge. By the way my quad is leaking from the acceleretor pump shaft and when the engine is off I can see gas dripping on the drivers port from the jet. Any recomendation Thankx in advance.

[@QuadrajetPowerLLC]

Check you float height. Sounds like it is too high.

[@rockerpat1085]

It can also be the fuel is boiling in the bowl due to the ethanol!!! If the fuel is too hot it will boil and in the bowl and also in your fuel line pushing the fuel into the float bowl past the needle and seat!!! Try cracking the line at the filter when it's happening and see if it stops pushing it out!!! Happened to my Vette just the other day!!! Uncle Tony has a video about how to solve the problem!!! Good luck to you fella!!!

[@aztecahernandez6539]

Is a 1975 Cadillac quad with additional air horn air holes worth the trouble to rebuild or a 1979 Chevy quad a better choice. I have both. Thank you.

[@QuadrajetPowerLLC]

79 Chevy carb is much better to work with.

[@edwardpurks3883]

Would the pr44k metering rod work on a 78 gmc motorhome? Currently it has 53m's and it's too lean in the power tip region before secondaries open. I notice the pr44k has a smaller power tip which may help

[@QuadrajetPowerLLC]

If you are lean at heavy throttle you need to change your secondary rods. For part throttle, you could use our full taper pr50m rods and set up adjustable apt for proper mixture.

[@edwardpurks3883]

@@QuadrajetPowerLLC thanks for the recommendation. It's lean at about 50% throttle. This is before secondaries begin to open so I believe it's still only on primaries. Full throttle afr is actually okay, around 12.2 afr. 50% throttle when it's lean is at 16.5 afr. Light throttle is 14.6 afr. This is with size 76 main jets and a adjusted apt to help improve the lean area. It was worse with the stock 74 jets. The 76 jets helped bring the lean spot down to 17.5 afr (from 18.5 afr) and the apt adjustment brought it to 16.5 afr. It's not missing or hesitating so I may be over worrying here. Next step is maybe rebush the throttle shafts? There not terrible (55k miles) I did order your full taper metering rods a just to see if it would richer it up in that that area due to having a smaller power tip. I was kind of worried because it's a heavy Rv and lean afr while accelerating at 50% throttle up hills may not be good for long-term engine life. We hardly ever use the secondaries

[@inell6]

The mysterious APT screw. I've heard after the screw is bottomed out, it should be 2, 3, and 4 turns out, from three different opinions. Is there a basic adjustment I can try on a 1979 Corvette 350 5.7 Carb #17059507. Thanks.

[@QuadrajetPowerLLC]

It should never be bottomed out. Start about 2.5 turns up from seated and tune from there.

[@bryanw2400]

do you do calibrations on them for pontiac 455

[@franciscotrujillo4961]

One more question friend, I'll tell you, I have a Rochester Quadrajet carburetor and I want to change the primary nozzles along with the primary needles, and since there are different numbers of nozzles and primary needles, I have to deduce that depending on the number of nozzles, I must set the needle number equal to the nozzle number? I mean, if I put number 0.72 primary nozzles, should I put number 0.72 primary needles? I understand that in this way, the idling engine has the primary nozzles blocked with the primary needles, since they are of the same diameter, and that when accelerating and losing vacuum, the primary needles begin to rise, allowing fuel to pass to the primary jets, is that correct? all the best

[@QuadrajetPowerLLC]

What is your quadrajet code number? There should be about a .030 difference between Jet size and primary rod upper size.

[@franciscotrujillo4961]

@@QuadrajetPowerLLC kzbin.info/www/bejne/nIOodpSIlpqlqK8 They took it to be cleaned and they painted it and I don't know if I'll be able to see the number, as soon as I know something I'll tell you, yes I know it looks like the same one from this video of yours, without a screw to regulate the primary needles, greetings and thank

[@Thirdgen83]

Does the APT affect idle?

[@theungoliant9410]

ok.... what if i don't have an apt at that location?

[@tammycouts6239]

Did you find out anything? Mine doesn’t have one so I’m trying to find out why

[@lennycoluccio9129]

Can they be calibrated for a camshaft

[@QuadrajetPowerLLC]

Yes

[@geoffernie]

Can I use a 69 jet with your custom apt rods. Thanks

[@QuadrajetPowerLLC]

You could, but would probably not be the best option for you. What is your carb code and engine specs?

[@oswelysguillen6221]

Hola quiero el video pero en español soy fe venezuela por favor

[@oswelysguillen6221]

Tengo uno carburador parecido necesito arreglarlo pero esta en ingles

[@jasonbeason4780]

Mine starts smoking when I close the choke.

[@QuadrajetPowerLLC]

Yes, creates a rich condition and can create black smoke.

[@jasonbeason4780]

@@QuadrajetPowerLLC strats up super snappy and runs clean with no hesitation. When I step on the gas all four barrels do extremely well no bog doesn't feel like it starving for gas. Works flawlessly. Thanks to you and your modification kit I installed in it. My adjustment on apt is 3 turns up from bottom and the two front air fuel mixtures are bothe set at 3.5 turns out from bottom. I also decided not to reinstall the black cup. I couldn't ask for a better quadrajet. Thanks for the kit and videos.

[@mikeuva2273]

do you know what that black cup does? what difference is there if i use it or dont use it?@@jasonbeason4780

[@jasonbeason4780]

@@mikeuva2273 I left mine out

[@stanthrasher4815]

Awesome

[@69sexymonk69]

Link for the Chevy truck ones?