Anyone familiar with Q-jets?

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My427stang
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Anyone familiar with Q-jets?

Post by My427stang » Sun Jan 14, 2018 5:29 pm

I am working to dial in the GTO a little better, and although it runs pretty well, I can't get it out of the transition circuit at idle. Actually, a better way to describe it is that I cannot close the primaries enough to shut down ported vacuum. I assume I am into transition, but the real reason is I want the vac advance to be dead at idle

The carb is a 17057256 Quadrajet, which looks to be from a 75-79 BOP, so not the original carb for a 67 400 HO, but the car is a very nice driver, and he isn't ready to buy a numbers matching carb and moreover, we may go Holley and RPM intake later (Q-jet guys, don't be offended, we'll keep it if it does well).

I know Holleys well and with a Holley I would give it a little secondary idle so I could close the primaries and get out of the transition circuit. I likely don't need much

I understand the Q-jets have a fixed idle orifice/air bleed that is generally larger on engines with bigger cams, smaller on later versions. Seeing as this is a later model M4MC version, I would assume it's got the smaller bleed. If I am understanding this correctly, this air feed will do what I want and I just need to sneak up on the size I need to get idle air from somewhere else and close the throttles.

1 - Can anyone point out a picture on which bleed I am looking at? I haven't found anything online that shows me the fixed idle feed.
2 - Does anyone have the original sizes for that fixed idle bleed for my part number, or direct me to where I can find stock configuration? If it's very small <.028 then it immediately says I am likely on the right track.
3 - I heard some Q-jet versions are considered 650 cfm versions simply by limiting secondary throttle blade opening, can anyone confirm that and tell me if they expect that with mine? (this is just for giggles and later tuning, don't need it for what I am doing now)

Not looking to do any fuel curve tuning yet, but I need to get this motor a little more air at idle so I can close the primaries for basic setup, any help or direction is appreciated
70 Mustang, 489 FE, TKO-600, Massflo SEFI, 4.11s
71 F100 SB 4x4, 445 FE, hot rod 4x4

grandsport51
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Re: Anyone familiar with Q-jets?

Post by grandsport51 » Sun Jan 14, 2018 5:48 pm

I have used this info in the past worked well!!

http://www.73-87.com/7387garage/drivetrain/qjetidle.htm
LIGHT 'EM UP

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Re: Anyone familiar with Q-jets?

Post by pamotorman » Sun Jan 14, 2018 5:50 pm

change the location of the distributor vacuum hole in the throttle body to above the butterflies and epoxy close the lower one. i have done this to holleys
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Re: Anyone familiar with Q-jets?

Post by tjs44 » Sun Jan 14, 2018 6:04 pm

migth contact this guy,pontiac racer and QJ specialist.
https://cliffshighperformance.com/

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Re: Anyone familiar with Q-jets?

Post by pamotorman » Sun Jan 14, 2018 6:40 pm

My427stang wrote:
Sun Jan 14, 2018 5:29 pm
I am working to dial in the GTO a little better, and although it runs pretty well, I can't get it out of the transition circuit at idle. Actually, a better way to describe it is that I cannot close the primaries enough to shut down ported vacuum. I assume I am into transition, but the real reason is I want the vac advance to be dead at idle

The carb is a 17057256 Quadrajet, which looks to be from a 75-79 BOP, so not the original carb for a 67 400 HO, but the car is a very nice driver, and he isn't ready to buy a numbers matching carb and moreover, we may go Holley and RPM intake later (Q-jet guys, don't be offended, we'll keep it if it does well).

I know Holleys well and with a Holley I would give it a little secondary idle so I could close the primaries and get out of the transition circuit. I likely don't need much

I understand the Q-jets have a fixed idle orifice/air bleed that is generally larger on engines with bigger cams, smaller on later versions. Seeing as this is a later model M4MC version, I would assume it's got the smaller bleed. If I am understanding this correctly, this air feed will do what I want and I just need to sneak up on the size I need to get idle air from somewhere else and close the throttles.

1 - Can anyone point out a picture on which bleed I am looking at? I haven't found anything online that shows me the fixed idle feed.
2 - Does anyone have the original sizes for that fixed idle bleed for my part number, or direct me to where I can find stock configuration? If it's very small <.028 then it immediately says I am likely on the right track.
3 - I heard some Q-jet versions are considered 650 cfm versions simply by limiting secondary throttle blade opening, can anyone confirm that and tell me if they expect that with mine? (this is just for giggles and later tuning, don't need it for what I am doing now)

Not looking to do any fuel curve tuning yet, but I need to get this motor a little more air at idle so I can close the primaries for basic setup, any help or direction is appreciated
yes they limit the air flow by restricting the air flap opening on some carbs. you can just adjust this by cutting the stop to allow full opening. you can also drill the primary butterflies to get more air at idle. some Q jets did have this
Last edited by pamotorman on Sun Jan 14, 2018 6:43 pm, edited 1 time in total.

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Re: Anyone familiar with Q-jets?

Post by F-BIRD'88 » Sun Jan 14, 2018 6:43 pm

The flow rate of the pcv valve also effects the idle flow rate and carb throttle opening at idle speed.
Change the pcv. If the motor is cammed you probabily also need to increase the base initial spark advance at idle.
( and re- curve the distributor curve too)


Not all pcv valves flow the same at idle... Big effect on net idle speed and throttle opening at idle.

Non stock camshaft? What cam?

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Re: Anyone familiar with Q-jets?

Post by F-BIRD'88 » Sun Jan 14, 2018 6:49 pm

Some oem qjets have the seconday throttles open to a angle other than 180 degrees wide open at WoT..

This is to DIRECT air and fuel flow in the manifold
Flow directional bias.

Some OTHEr oem qjets limit secondary air door open max angle to limit max flow rate.

2 different things.

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Re: Anyone familiar with Q-jets?

Post by 77cruiser » Sun Jan 14, 2018 7:04 pm

Here's a pic of the Idle air bypass. If the carb has the holes they are probably around .050. Depending how far you need to close the throttle you could to about .075-.085 & try it.
Bypass2.jpg
you might have to give it some fuel too via. IFR & idle down channel restriction.
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Jim

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Re: Anyone familiar with Q-jets?

Post by pdq67 » Sun Jan 14, 2018 10:05 pm

Fwiw?

There is a guy handled, "Lars", on the car boards that works on them. There is another guy too, but I can't remember his handle right now?? Also, would Doug Roe be of any help?

The early Firebird 400" Pontiac engines had their rear barrels shorted so that they wouldn't open up as much as the 400" GTO engines did to limit Hp production. 325hp for the 'Bird and something like 350 to 360 for the GTO 's here if I remember right?

I think the 'Bird intakes were filled in so the BIG rear hole's were, "D"-shaped, instead of round.

My, "big-headed", B-I-L had a '67 'Bird 400"/4-speed and he wouldn't leave the Q-Jet alone after I reset the rear butterfly's a couple of times. He got drunk one Sat. morning and jumped up on top of it and STOMPED the hell out if it!! Then traded it in on a cheap 1/2 ton Ford P/U... Sucker was, "Army Green", with a white interior in it! The Dealer was over at Paris, MO back then, early to mid '70's...

pdq67

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Re: Anyone familiar with Q-jets?

Post by Geoff2 » Mon Jan 15, 2018 4:26 am

What are the cam specs & comp ratio? Idle vacuum? More info needed for a fix for the problem.

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Re: Anyone familiar with Q-jets?

Post by panic » Mon Jan 15, 2018 9:48 am

increase the base initial spark advance at idle

This^^^

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Re: Anyone familiar with Q-jets?

Post by Firedome8 » Mon Jan 15, 2018 9:50 am

77cruiser is on to what I think u are asking about,,,study your particular carb and open up the idle air bypass and re test , this is a common quad modification and a reason quads are adaptable. I would not touch ifr or the down channel till you get the idle ware u want it . you may need to open the idle discharge depending on your combo a good pre emissions is ,090" thair and or about. When idle is good you can disable acc pump and slowly turn up idle, it should transition smoothly if not you may need to look at IFR and down channel restrictions. I like to dyno with sec. locked out and go from thair...find doug Rohe quad book ,,it says it all.

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Re: Anyone familiar with Q-jets?

Post by My427stang » Mon Jan 15, 2018 10:23 am

Thanks everyone for the quick response, I will look close at you said. For some reason I though the fixed idle bleed I need to drill was in the air horn, but I will look close from the bottom as shown

As far as the car and other options, I am currently at 12 degrees initial, of course I could go more, but cam is not big and the carb is known to come off a later stocker, so I was initially going that way. Cam is 278/80 adv 226/230 110 LSA on 107, nothing radical

As far as timing, I moved initial around to 16 or so, when troubleshooting and it didn't get me there and I really don't like too much initial for a pure street car. Let me do some measuring and see what I come up with.

Thanks again!
70 Mustang, 489 FE, TKO-600, Massflo SEFI, 4.11s
71 F100 SB 4x4, 445 FE, hot rod 4x4

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Re: Anyone familiar with Q-jets?

Post by F-BIRD'88 » Mon Jan 15, 2018 11:16 am

That motor with that cam can use up to 24 deg initial base timing..

Put the drills away and set the friggin timing right.

Not the first time I have had to tell someone this..

Re curve the distributor to allow the needed base timing
....(shorten the mech advance curve to about 10-14 deg based on best found initial timing..

The engne don't care what you want for timing..


the egr effect of the cams overlap slows combustion
speed down, requiring more timing advance at base idle..
It is just that simple..

20 to 24 degrees.

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Re: Anyone familiar with Q-jets?

Post by F-BIRD'88 » Mon Jan 15, 2018 11:19 am

All the extra idle air needed comes thru the pcv at idle.
And they are not all the same..

Fix the spark timing.. Leave the friggin carb alone.

Automatic trans or manual transmission ?

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