Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
idle help
73ss:
I need some help getting the idle dialed in.
Recipe: 17084228, single main air bleed.
Main air: .088
Upper idle air bleed: .070, Lower: .077
idle tube: .038, idle down: .058
Idle port: .090, Bypass: .125
75 jet, 50m rod, 1/4 float.
Engine: 489 BBC, Brodix race rite oval port heads. 10:1 squeeze. Domed pistons.
pistons 10 in the hole, block has been decked. .040 cometic head gaskets.
cam: Howards roller, intake: 232, ex: 246 @ .050. .601, .612 lift. 114 lobe sep, 110 intake center line.
Timing: 20 degrees lead, stable in and out of gear. 850 rpm.
I can only get 10in of vacuum with 20 degrees lead. Engine idles fairly well, doesn't stall. Slight hesitation right of of idle. If I disable the accelerator pump, The off idle hesitation is all but gone. I assume this is because too much transfer slot is exposed.
If I close off the choke at idle, I get 11 inch of vacuum and about 50rpm increase. This tells me I'm lean on idle. Mixture screws are unresponsive past 4.5 turns out. I did open the idle down from .054 to .058. It did make a difference but it acts like it wants more. I currently have 2/3 of transfer slot exposed. Boosters are not dripping at idle. Engine will stall with mixture screws seated. Plugs are white.
Carb feels good otherwise, No stumble going to WOT from stop, May need to tweak ATP or primary rod size yet.
Any suggestions on what to do next, Increase idle tube, idle down channel? Decrease lower idle air bleed?
Thanks for any suggestions or help.
Kenth:
I have measured mid-70´s Chevy BB and 350 HD Truck Q-jets with up to .044" idle tubes from factory.
I would say your race-engine needs larger than .038" idle tubes.
Also, most if not all BB Chevy in the high performance era uses .138" idle air bypass.
And i would open the idle needle ports to .095".
HTH
Cliff Ruggles:
You would be better off reducing the LIAB than putting in larger idle tubes.
Before doing anything remove a small manifold vacuum hose and see if it wants or needs any additional idle bypass air?
A few days ago I set up one of those later truck carbs for a 690hp Big Block Chevy engine and used .038" idle tubes. I reduced the LIAB's to .063", .070" uppers, and .055" DCR's with .136" bypass air. It was well on the idle system and excellent response with the mixtures screws with only .089" holes under them.
The larger LIAB's vent enough to require much larger DCR's and idle tubes with heavy cammed engines. Even when you open them up to compensate the response at the mixture screws isn't nearly as good vs closing down the LIAB's.
I'd also add here that you aren't making nearly the vacuum I'd expect from that cam in a 489CID build at 10 to 1 compression with 20 degrees initial timing in it. Not quite sure what's up with that deal but it shouldn't need much past 10 or 12 degrees timing to make 12-14" vacuum at 750rpm's with that cam.
What are the actual seat timing numbers (advertised) duration and where are they rating it at, .006" etc?.........Cliff
73ss:
Advertised duration is : 284.6 in 298.6 ex. It's not listed as to what the rating is.
I am puzzled as well with the low vacuum and 20 degrees of lead. I did verify TDC and the timing pointer when the heads were off, spot on. I would like to think I got the quench good. The shop said pistons are 10 in the hole, I measured around .008, It was hard for me to get a precise measurement due to the domes and several thousands worth of piston rock at TDC. I went with the cometic gaskets , they have a smaller crevice volume than the fel-pro equivalents. I was expecting at least 12"s of vacuum with around 14~16 degrees of lead.
Pulling a small vacuum hose has no effect, I did block off one of the idle bypass slots in the air horn with a piece of tape. RPM's dropped but idle did smooth out somewhat, so I think I am close on that.
The throttle is open far enough that the ported vacuum advance port is just on the verge of working, But not far enough to cause nozzle drip.
I verified the ignition system is working correctly by swapping out the MSD setup with "old reliable", a known good working points distributor that I have for troubleshooting. No difference, Pops right off at the flick of the key.
The original '73 7043202 ? BB carb does in fact have the .138" bypass ports. Idle discharge ports measured out at .082.
Thank you for the replies!!!
Cliff Ruggles:
That is NOT a big cam in that build. I suspect late intake closing or the compression ratio is less than calculated, or a little of both. The 114LSA is the right way to go with the big CID engines, they thrive on it. I like the ICL around 109-110 as well. I typically use cams around 244/254 @ .050" in them if they are around 10 to 10.5 to 1 and pump gas street builds. They will act no bigger than using a 234/244 @ .050"ish cam in a 454-468 build.
My smaller 455 CID engine at 11.2 to 1 compression idles with 13.5" vacuum with only 10 degrees initial timing. My cam is close in duration at 289/308 @ .006", 236/245 @ .050", 112LSA with the ICL at 109.5 degrees.
I've built 496-505cid engines with bigger cams than you are using around 10 to 1 compression and they idled with at least 12" with only 10 degrees initial timing at 750-800rpm's and nothing really special required from the carb in terms of bypass air, idle tubes or DCR's......FWIW......Cliff
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