Anyone familiar with Q-jets?

[F-BIRD'88]

Be sure that the REQUIRED GM oem starter motor support brace bracket is on the starter motor.. If it is missing get one and install it...

It is a REQUIRED critical part.

[shiftbyear]

can you post a picture of the carb and which port you are using for the distributor vacuum, might clear up some issues. Also later model carbs required tighter emission requirements and were leaner from the factory, 10% ethanol fuel also requires more. Your idle circuit may have to be enriched for proper throttle blade positioning. As the previous post suggested email or call Cliff Ruggles. Good luck.

[kimosabi]

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!

I have a 10.7:1 400 sbc with a Qjet and it does not want lower initial timing than 18-20degrees. Cam is 232 .540 lift 108 at 106 ICL hyd roller. Not a big one. I shortened the timing advance curve down to about 12 degrees travel to a 32 total and quite fast advance all in at 3300. Could be faster but I played it safe.

Rule is always the same. Timing first. Takes a while and some testing but it's crucial that you nail the timing and curve before you dig into the carb, otherwise you'll be chasing your tail.

[My427stang]

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.

I respect you and appreciate the advice, I came looking and you answered, and will consider, but why would a wedge with a mild cam need so much initial? Granted I am not a Pontiac guy, but I have hundreds of Ford and Chevy builds and none of them want that much initial. I am not afraid to recurve, and open to discussion, but this combo from the factory had the same overlap, later intake valve closing and stock specs were 6 BTDC. We are talking sub 60 degrees overlap, stock cam was 61.

Don't mistake me asking for questions that I am inexperienced, unfortunately I have not yet found the documentation of a Q-jet like I have for Holleys, but what I have found is that the fixed idle air bleed in these carbs accounts for just this effect, and moreover, it's a benefit in a Q-jet that most other carbs don't have. If I had the factory carb, as I understand the RA3 and 400HO carbs have this very change for this very reason

That being said, I'd be a fool to not try just giving it some initial, it's a no cost, no time experiment, so I will

[My427stang]

So, as another follow up, I'll pull the carb this week and get some pics. However, I realized my own error and it looks like 77Cruiser and Firedome knew exactly what I was talking about.

I dug around a little on the Wallace Racing site and here is the description of what I am trying to do.

http://wallaceracing.com/qjetidle.php

First, and most likely, the butterflies are set at too high an angle in an attempt to get the idle speed up. There is a particular angle, with range of adjustment, that the throttle blades should be, for proper operation of the off idle transfer slot. The factory has included something you won't find on Holleys...an air bypass. This air bypass takes air from next to the venturi, through two slots(rt & left side) in the air horn. The air travels down through the main body, and exits through a calibrated, factory drilled hole on each side of the base plate, just below the butterflies. These holes vary in size dramatically from the factory, depending on the size of the camshaft. To run a large cam (245 or more at .050", mine is 254intake) you need to drill these orifices to .100-.125". Once done the throttle blades can be moved back to there proper position, and still retain a good idle speed. But now, the air goes around the venturis.

I will of course verify my throttle blades in relation to the transition slot, and of course a bunch of initial could allow me to close the throttle too, I will look.

What I did originally was misread the info above as it was in the air horn, but it clearly says that the orifice is in the base plate and later shows a picture in the bottom right of the collage.



This picture was way down the page and apparently I was easily confused LOL but it clearly shows the air bypass in the baseplate. Whether I fix this one with timing or the carb, I'd really like to get to know these carburetors a bit better. More to come!

[77cruiser]

With that cam you're going to need a bit of both. FWIW that's how I pull the idle tubes out, heat it up a bit & they come out surprisingly easy.

[pdq67]

Be sure that the REQUIRED GM oem starter motor support brace bracket is on the starter motor.. If it is missing get one and install it...

It is a REQUIRED critical part.

I have ran SBC V-8 starter's without the starter brace or the knurled starter bolts fine!

pdq67

[My427stang]

Be sure that the REQUIRED GM oem starter motor support brace bracket is on the starter motor.. If it is missing get one and install it...

It is a REQUIRED critical part.

I have ran SBC V-8 starter's without the starter brace or the knurled starter bolts fine!

pdq67

This one was missing it's bracket too, but now it's got a Powermaster mini on it, not enough room with the Doug's long tube headers

[blykins]

Make sure that the REQUIRED air pressure is in all 4 tires as well.

If the REQUIRED air pressure isn’t there, the tires will either wear in the center or on the outsides.

[F-BIRD'88]

Your cam has a lot more effective valve overlap than the factory pontiac cams did.

Thus much more EGR effect at idle, wanting more initial base timing.
Give it all it wants 20-24deg.

[gnicholson]

Another thing to verify is that your throttle shaft isnt loose from the shaft bore being wore in the throttle body. They will bypass alot of air and cause all kinds of idle issues. Ive put a lot of bushings in those things

[F-BIRD'88]

The 30deg valve seat angle of a pontiac V8 amplifies the low lift flow at TDC
Both good and bad (reversion, exhaust dilution) at idle making the running effect of the increased valve overlap at TDC (your new cam) even more of a change.
More exhaust dilution wants more spark timing advance at idle.

[randy331]

Make sure that the REQUIRED air pressure is in all 4 tires as well.

If the REQUIRED air pressure isn’t there, the tires will either wear in the center or on the outsides.

TRUE !

It's AMASING how fast a tire will wear when FLAT or OVER INFLATED !!!

Randy

[Geoff2]

Stang,
Think about this. Pontiac engines used wedge chambers also. 10.75 CR, 389-400 cu in idled at 600 rpm in gear....from the show room floor. They also idled with 26* of timing, & had mild cams with wide LSAs.
Yes, 26*, not a typo. Used until 1968 when we lost manifold vac adv to the USELESS ported vac adv. The Pontiac engines idled with 6* initial & had 20* added at idle by the VA connected to man vac [ MVA ].

Advance your dist to around 30* [ amount not critical ] at idle. This will simulate what MVA will do. While at 30*, try re-adjusting the carb idle & see if it improves/cures your problem.

[novadude]

If running ported vacuum be careful which port you are using on the carburetor. Some of these 75-79 carbs had a "ported" source for the EGR with a bleed higher up in the bore, and these will not behave the same as a true ported source. I ran into a '79 carb once that had no true ported nipple, just this EGR port.