Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Jetting
blazer74:
Hi, Just thought I’d throw this out there.
17056228 on 74 K5 Blazer. 355 sbc Vortec heads Performer vortec intake, HE268 old school Comp cam
218/454/110. Compression approx 8.7:1, Headers/Duals, 4:10’s/35”
35 Idle tube, 52 DCR, 70 air bleeds mains, uppers and lower idle bleeds. 71 jets/Cliffs 44’s / 98 bypass air. APT is adjusted up 1.5 turns to eliminate a slight off idle dead spot until fully up to temp then goes away. No cruise issues whatsoever even with the APT adjusted all the way down. As a note the DCR was at 47 with 89 bypass, jet 72 after opening both my Hot starts are much cleaner, doesn’t load up. Then installed the 71 jets.
Would 70 jets be a step to small in order to be able to tune the APT? 72 jets were way to rich with black tailpipes. 71 jets tailpipes cleaner but still slightly dark. Slight exhaust smell but no eye burning like I had with the 72 jets. Quite a change with just one jet size smaller, runs better also. Running non ethanol fuel.
Building second carb 17057504 set up the same except with 55 DCR (already there)and want to try 70 jets. The 17056228 does have the larger primary bores.
Thanks for your opinions, Craig.
blazer74:
Mix holes 93, 2.5 turns out, will drop idle less than 2 turns and kill engine all the way in.
Cliff Ruggles:
With only 8.7 to 1 compression and 110LSA cam with 218@.050" you're going to get a little stinky exhaust and sooty pipes simply due to the overlap. Not a deal breaker but the jet size is NOT a player at idle in any fashion. If you change one jet size and it fattens up the idle or leans it out a simply turn with the APT screw would have duplicated that exactly scenario.
Tune for jet size FIRST. This means making pulls on the primary side up steep grades without the secondaries. See what it likes and wants, then leave the jets alone.
Next tune the APT system (this assumes the engine is happy with your idle system mods) for smooth right off idle and best power, performance and fuel economy in the "normal" driving range or light engine load.
No mention of timing, vacuum advance, etc, but it is a BIG player in what you are doing. Adding timing allows the engine to effectively burn a leaner mixture, so don't forget to do some tuning there as well.
I don't remember if you called the shop or not for parts/metering rods or how to set the carb up, etc? I do so much of this sort of thing it all runs together these days. I might suggest a different approach with the 17057504 carb that may help clean things up some and work better for what you are doing......Cliff
blazer74:
Somewhere in the process of dropping jet sizes and opening the DCR from 47 to 52 and bypass air 89 to 98 cleaned up the idle quite a bit and the eng likes the smaller jet.
Didn’t mention 2000 stall also.
All the parts are from you. Didn’t speak directly to you just followed recipes in your book.
Compression could be a bit higher than stated.
Vortec heads flat tops with 2 valve reliefs .040 in the hole. .028 compressed GM head gaskets.
Timing is 10 advanced, mechanical comes in 1100. Mallory replacement HEI w/ vac advance limited to 7 IIRC. 15” vac @ 700, rpm drops 50-75 in gear.
Couldn’t run ported due to nozzle drip with the 35 ICR /47 DCR /89 bypass.
Haven’t tried ported with the 35/52/98.
Need to lean up the secondaries also. Using your DR rods with K Hanger. POE is 52 with slight notches in the flaps.
17057504 is 35/55/ 70 main and upper idle air bleeds, lowers 76 52 POE.
I will call this or next week.
Thanks Craig.
Cliff Ruggles:
Pistons are .040" in the holes at TDC?
.068" is a lot of quench for an SBC build. The chambers in the Vortec heads will help offset that a bit.
When you open up DCR's you put MORE fuel to the transfer slots so it often requires taking some fuel away thru transition. I typically compensate for that here.
Most customers have bought something from Comp Cams and it's ALWAYS on a tight LSA. I learned early in this game to put some fuel to the idle/off idle system to avoid problems and complaints.
I'm in the shop 7am-4pm. Phone call volume is HEAVY so if I don't answer leave a number and I'll call you back......
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