Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
idle help
73ss:
Specs are @ .050 : Intake closes at 46.3 ABDC opens at 6.3 BTDC.
Exhaust closes at 5.3 ATDC opens at 61.3 BBDC.
Your thoughts? (Tried to post pic of card but I can't shrink them small enough to post)
77cruiser:
My 421 sbc idle about 12" cam is 227/239/107/103. 18 deg initial 10 vac.
Carb has LIAB .063 move the UIAB to the cover .043 tube is .031 I think DCR is .05 I think.
This is the best one I've done.
Cliff Ruggles:
My thoughts.
Anytime I've found myself having to run more than about 10-14 degrees initial timing to get good vacuum at idle the cam has been just a tad big for the CID and static compression ratio.
I ran into this about 15 years ago after assembling a 455 Pontiac engine. I did NOT choose any of the parts, just hired to assemble and dyno it.
It used the same cam I've put in other 455 builds and the same CNC ported aftermarket aluminum heads. The only difference was this engine had lower compression, around 9.8 to 1 instead of 10.5 or so.
One wouldn't think that would be a big deal but that engine didn't like 10-12 degrees timing at idle, and it needed more idle fuel and bypass air to be happy. I'd also add this it wasn't quite as "peppy" right off idle, at low RPM's, and it made less power on the dyno as well.
Not a deal breaker for sure, but the experience taught me that it would have been a little happier with about 5-8 degrees less duration, or a half point or more compression......
73ss:
I'm slowly making some progress. I shrunk the LIAB's down to .063 and it did not like it. Lost control of mixture screws and had a noticeably stinkier exhaust, lost a tad bit of vacuum. Opened them back up to .070, seems better. I'm able to get a decent idle, in and out of gear, 11~12"vacuum with 18 Degree initial and another 10 from the vacuum advance for a total of 28. vacuum wanders from 11~12".
We tried a friends QFT 750 holley. With that carb, I'm able to get a steady 12" vacuum without the added timing from the vacuum advance, for a total of 18. We cracked open the secondary adjustment screw to stay of the front transfer slots to get a nice idle. Exhaust had a better note to it as well. The QFT has the 4 corner idle and only needed 7/8 turn on each screw to be happy. Better throttle response when slowly cracking the throttle open also. I probably could have taken a few more degrees of timing out at idle and still had a good idle. Nice and steady on the vacuum gauge.
This tells me that the engine is capable of producing at least 12" without the need for added vacuum advance at idle that the Q-jet wants. Not sure what to try next. It didn't like the LIAB's taken down to .063. A little better with them opened up to .070. No difference when pulling off a small vacuum line. It picks up about 30 rpm and a tad more vacuum if I partially close of the choke.
Since it responded to opening up the idle down to .058 I'm thinking opening them up to .060. Take the idle tubes up to .039~.040?
Any thoughts or tips would be appreciated. Thanks
Cliff Ruggles:
The results make no sense to me.
IF you close down the LIAB this puts more fuel to the mixture screws.
IF you tip in the choke flap and get an improvement in vacuum richening it up, then it wants more idle fuel at the mixture screws.
In some cases when making the LIAB smaller I've also had to use smaller idle tubes and/or smaller DCR's at the same time to get control of things.
One thing for sure is that on engines with low vacuum at idle speed smaller LIAB's produce more sensitivity at the mixture screws and clean up the idle some.
I've also found that with some engines smaller IAB's just work better. This simply happens when the vacuum is low and signal to the idle system is a little "weak".
Holley carbs shine in that area because they were NOT produced to have to pass emissions so very generous in the idle and off idle system, always have been. This is good for idle with big cams but heavy fuel consumption is typically the result.
The Q-jet actually has an EXCELLENT idle system and the bypass air being a controlled vacuum leak is superior to drilling holes in plates or opening up the plates to add air. If we raise the throttle plates in a Q-jet to add idle air they very quickly pull from the boosters (nozzle drip) as the system is considerably more efficient than a Holley style carburetor.
In any case you are still baffling me with the engine wanting all that timing and "low" vacuum even when you throw timing and fuel at it. I've build hundreds of engines and the only time I've ran into that sort of thing was full-race stuff with super tight LSA camshafts, locked out timing, and "flooding" them with fuel to keep things happy. In a "race" engine nobody cares anyhow.......Cliff
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