Cliff's Quadrajet Parts and Rebuild Kits

Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: 72Campcust on January 26, 2013, 09:47:06 AM

Title: Missing Vacuum Port? what do i do?
Post by: 72Campcust on January 26, 2013, 09:47:06 AM: So, i have a QJet '77 Cali emissions Carb..this is what the front of mine looks like.

(http://i569.photobucket.com/albums/ss134/vartyc/2013-01-26_09-00-55_332_zps28112e42.jpg)

Cliff told me that i need to hook my Dist Vac Advance to the port to the right of the fuel filter, just above the idle screw...well...mine doesn't have that?? ??? I have seen pics of older/other carbs that do have a port on the front right side of the carb so i understand what im looking for..

I currently have my vac advance hooked up to the port just above the fuel line filter housing (how it came when i bought the truck), but it is a "Ported Vacuum" spot.. so my vac advance is getting vacuum at idle.

there are three Manifold ports in the bases plate, the center one is hooked to my PCV, there is one just to the right of it (capped off, seen in the pic) and one to the left of the left side idle screw, also capped off.

So two questions: why is this carb different (no port on the front right)

where/how do i hook up my vacuum advance?

If where i have it is right, ive read in some schematics that there might have been a inline delay valve or check valve of some sort?? Obviously when ALL the vac lines where hooked up it would have been way different..

any help is appreciated..
Title: Re: Missing Vacuum Port? what do i do?
Post by: Ethan1 on January 26, 2013, 04:09:16 PM: Looks like it is in the correct place to me.
Title: Re: Missing Vacuum Port? what do i do?
Post by: Cliff Ruggles on January 27, 2013, 05:59:48 AM: Currently you have it hooked to FULL manifold vacuum.

The lower left fitting in the baseplate is ported vacuum, but may have a "bleed" off port and not work for the distributor, if ported vacuum is needed for the vacuum advance. The bleed off slot can be filled in with epoxy, if it is present, providing strong ported vacuum to that fitting.....Cliff
Title: Re: Missing Vacuum Port? what do i do?
Post by: 72Campcust on February 04, 2013, 10:05:43 AM: Quote from: Cliff Ruggles on January 27, 2013, 05:59:48 AM
Currently you have it hooked to FULL manifold vacuum.

The lower left fitting in the baseplate is ported vacuum, but may have a "bleed" off port and not work for the distributor, if ported vacuum is needed for the vacuum advance. The bleed off slot can be filled in with epoxy, if it is present, providing strong ported vacuum to that fitting.....Cliff

Cliff-- thanks, ive been scratching my head on what to do.. How do I know/identify if it has a bleed off port? How do i know/confirm if i need "Ported Vaccum" for my carb/distributor... i was under the assumption that i did??

ohh.. and you where spot on.. I plugged the hole in the choke housing and cleared up the idle issue.. silly me ;)
Title: Re: Missing Vacuum Port? what do i do?
Post by: Cliff Ruggles on February 05, 2013, 05:02:35 AM: A vacuum gauge hooked to the port with the engine running is th easiest way to determine which port does what.

You can also remove the carb and look at the lower fitting to see if it has a bleed off port to reduce vacuum as the throttle plates open.....Cliff
Title: Re: Missing Vacuum Port? what do i do?
Post by: 72Campcust on February 05, 2013, 03:51:51 PM: Quote from: Cliff Ruggles on February 05, 2013, 05:02:35 AM
A vacuum gauge hooked to the port with the engine running is th easiest way to determine which port does what.

You can also remove the carb and look at the lower fitting to see if it has a bleed off port to reduce vacuum as the throttle plates open.....Cliff

Cliff-- I hooked a vacuum gauge up to the port you suggested, when i throttle the engine up the vacuum climbs in correlation with RPMs. Do i still need to take it apart to see if it has the "Bleed Off" you mentioned? I'm hoping to avoid taking it all apart, unless the chances are pretty good that this does have the bleed off.. (carb #17057512). Am i testing it right by revving the engine up and watching the vacuum gauge..
Title: Re: Missing Vacuum Port? what do i do?
Post by: Cliff Ruggles on February 06, 2013, 05:54:07 AM: The vacuum reading should go to full vacuum right off idle, and mimic what is seen on a manifold vacuum source. The bleed off port reduces signal, so the readings are lower....Cliff
Title: Re: Missing Vacuum Port? what do i do?
Post by: 72Campcust on February 06, 2013, 11:23:20 AM: Thanks Cliff, If i remember right the vacuum pressure climbed with throttle input.. so, guess i need to plug the bleed port..

hopefully last questions..previously you mentioned that the bleed off slot can be filled with epoxy.. can i plug the bleed slot without taking the carb apart, IE just pull the carb off the manifold and flip it over and fill in the bleed slot..or do have to disassemble to get to it.

is it obvious what I'm supposed to plug with epoxy?

thanks
Title: Re: Missing Vacuum Port? what do i do?
Post by: 72Campcust on February 08, 2013, 09:22:57 AM: Quote from: Cliff Ruggles on January 27, 2013, 05:59:48 AM
Currently you have it hooked to FULL manifold vacuum.

The lower left fitting in the baseplate is ported vacuum, but may have a "bleed" off port and not work for the distributor, if ported vacuum is needed for the vacuum advance. The bleed off slot can be filled in with epoxy, if it is present, providing strong ported vacuum to that fitting.....Cliff

Is it obvious what i need to plug if i take the carb off of the manifold.. or do i need to disassemble the carb to do this..?? :)
Title: Re: Missing Vacuum Port? what do i do?
Post by: Cliff Ruggles on February 09, 2013, 03:38:49 AM: Just remove the two long screws in the back and the screws holding the baseplate to the main casting, it will come right off for repairs/modifications....Cliff
Title: Re: Missing Vacuum Port? what do i do?
Post by: 72Campcust on February 10, 2013, 12:54:22 PM: Quote from: Cliff Ruggles on February 09, 2013, 03:38:49 AM
Just remove the two long screws in the back and the screws holding the baseplate to the main casting, it will come right off for repairs/modifications....Cliff

Thanks for the help..hopefully the last question..

Pulled apart..now..which hole do i plug with epoxy??
Top right is the vacuum port i was using, this is view from top
(http://i569.photobucket.com/albums/ss134/vartyc/2013-02-10_09-51-09_613_zps2a5e5432.jpg)

this is bottom side of it..
(http://i569.photobucket.com/albums/ss134/vartyc/2013-02-10_09-51-52_29_zps58943e2a.jpg)
Title: Re: Missing Vacuum Port? what do i do?
Post by: 72Campcust on February 13, 2013, 06:58:15 AM: TTT...
Title: Re: Missing Vacuum Port? what do i do?
Post by: Cliff Ruggles on February 17, 2013, 05:42:53 AM: The round hole just aft of the flat slot is the bleed off hole....Cliff
Title: Re: Missing Vacuum Port? what do i do?
Post by: 72Campcust on February 17, 2013, 08:11:35 AM: Quote from: Cliff Ruggles on February 17, 2013, 05:42:53 AM
The round hole just aft of the flat slot is the bleed off hole....Cliff

Thanks... Im guessing you mean this hole..(where the pencil is)
(http://i569.photobucket.com/albums/ss134/vartyc/2013-02-17_07-35-09_319_zps1e2168c1.jpg)
Title: Re: Missing Vacuum Port? what do i do?
Post by: Cliff Ruggles on February 19, 2013, 03:54:46 AM: The round hole inside the bore just aft of the slot that provides ported vacuum.....Cliff
Title: Re: Missing Vacuum Port? what do i do?
Post by: 72Campcust on February 19, 2013, 03:25:14 PM: Quote from: Cliff Ruggles on February 19, 2013, 03:54:46 AM
The round hole inside the bore just aft of the slot that provides ported vacuum.....Cliff

Cliff-- the hole that im pointing at now (down in the bore) is the only hole in the bore..is that the right hole to plug? (there are no other holes down in the bore that ive got the pencil down inside)

Or am i totally off the mark?? Sorry for so many questions.. i just dont want to plug the wrong hole..

(http://i569.photobucket.com/albums/ss134/vartyc/2013-02-19_12-50-02_483_zpsb4984350.jpg)
Title: Re: Missing Vacuum Port? what do i do?
Post by: Cliff Ruggles on February 20, 2013, 05:19:05 AM: You're getting closer.....INSIDE the bore, just aft of the slot that provides vacuum. As the throttle angle increases, the round hole is uncovered by the throttle plate, and kills the vacuum signal by bleeding off the source back to the main bore.....hope that helps some?.....Cliff
Title: Re: Missing Vacuum Port? what do i do?
Post by: 72Campcust on February 20, 2013, 07:20:30 AM: Quote from: Cliff Ruggles on February 20, 2013, 05:19:05 AM
You're getting closer.....INSIDE the bore, just aft of the slot that provides vacuum. As the throttle angle increases, the round hole is uncovered by the throttle plate, and kills the vacuum signal by bleeding off the source back to the main bore.....hope that helps some?.....Cliff

guess my reference points are off..

Pic #1: Inside the bore there is this hole, which is below the throttle plate..
(http://i569.photobucket.com/albums/ss134/vartyc/2013-02-20_07-06-57_381_zps26001e2a.jpg)

Pic #2: and these two holes are above the throttle plate..Pencil is pointing at the sloted hole, screwdriver is pointing at the round hole..
(http://i569.photobucket.com/albums/ss134/vartyc/2013-02-20_07-09-16_137_zpscb9e9f7f.jpg)
Title: Re: Missing Vacuum Port? what do i do?
Post by: 72Campcust on February 22, 2013, 10:23:03 AM: Bump... :)
Title: Re: Missing Vacuum Port? what do i do?
Post by: omaha on February 22, 2013, 02:44:44 PM: I think it would be the "hole" that the screwdriver is pointing at. That would make sense as it is common with the slot, just at a different position in the throttle bore. I guess as the throttle opens, it provides vacuum to the slot but because the throttle blade is not past the hole yet, it is not a strong vacuum to the slot because of the hole being on top of the blade is bleeding off the vacuum(to the top off the throttle blade) once the blade passes the hole, then, vacuum signal would be returned, BUT, by the time your throttle get that far, the vacuum signal is not as strong any way. (unless your just at light cruise or slowing down from heavy throttle.) That is wild how just one carefully placed hole can have such an effect. like that. Good luck!!
Title: Re: Missing Vacuum Port? what do i do?
Post by: 72Campcust on February 22, 2013, 03:43:32 PM: Thanks, appreciate the feedback.. the wife is getting anxiuos to get the truck out of the garage.. :)
Title: Re: Missing Vacuum Port? what do i do?
Post by: Cliff Ruggles on February 24, 2013, 05:22:48 AM: Correct, the bleed-off hole is behind the slot, and uncovered as the throttle plates open. The first photo is idle bypass air, under the throttle plates....Cliff
Title: Re: Missing Vacuum Port? what do i do?
Post by: 72Campcust on March 06, 2013, 08:02:52 AM: So-- wanted to provide a wrap up for this; plugged the hole as suggested by Cliff.. truck runs great now! I have had to redo all my Idle Screws/APT settings now that the idle has changed but definitely have noticed a difference in performance..power/acceleration at all speeds...now just need to dial in the settings and i think ill be good!

On another note, while i had the carb apart i changed the heater Core (it was leaking BAD), painted the valve covers and repaired some cab rust (since the carpet was out...)

Thanks for all the help Cliff/Everyone-- and patience with me finding the right hole to plug!
Title: Re: Missing Vacuum Port? what do i do?
Post by: Cliff Ruggles on March 08, 2013, 04:07:31 AM: Good news! Strong ported vacuum works great to apply the advance so you get maximum efficiency at cruise, but no additional timing coasting and at idle speed.

Some engines just don't need all that timing at idle, and it's nearly impossible to get them thru emission testing as well, if you are using MVA to the distributor.....Cliff