Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Carb Starting point

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BBvetteDRVR:
Carb started dripping pretty good at idle and loading up when stopped at any lights.
Checked all the usual things (timing, vacuum leaks, etc) without finding anything obvious. Decided to pull the carb apart to see if anything jumped out. Carb was rebuilt 4 or 5 years ago

468 (454 +.030) with somewhere between 9 ~ 9-1/2 : 1
Automatic
comp cams hyd roller XR270HR),     .510 lift, dur 270/276
Short tube headers
69 GM aluminum intake, very low rise, has the hot slot but the intake/head gaskets are blocked off.
3 gasket sandwich on the base

71042216
78/48b CM
Idle tubes .044
Down channel .050
upper air bleed .069
lower air bleed .077
idle bypass air .139
idle mix screw holes .079

everything nice and clean, all passages clear

Idle has never been spectacular but could usually get it down to 650-700 in gear but not now that it's dripping
Points with B26 van, usually on manifold vacuum
15" (bounces) vacuum at idle

Fuel pressure was at about 8-ish at idle (bouncing around from 7 to off the scale of this old snap-on vacuum / fuel pressure gage) and smoothed out to a solid 7 when the engine revs up a bit

I bought an adjustable fuel pressure regulator but haven't installed it yet.

Idle adjustment screws at at 2-1/2 turns and do cause the engine to stumble and slow down if I bring them in to about 1 turn or less.
@-1/2 turns seems about the highest vacuum reading, any more doesn't really change it (still bouncing but around 15")



Going to re-assemble the carb tomorrow and re-install. Looking for any guidance at all to help trouble shooting in a logical sequence
Thanks
B

Cliff Ruggles:
8psi fuel pressure is fine with that carb, assuming it's a 7040216.

Pretty small cam for a 454 build and tight LSA with early closing intake.  I've tuned a few Chevy engines using those cams and all exhibited "quirky" idle quality, especially the Big Blocks.

I see the same symptoms with their "famous" XE camshafts, and attribute some of the problems to slamming the valves down pretty hard and bouncing them on the seats, or at least that's what the engines act like and there was some associated valve train noise with them similar to having Rhoads lifters installed.

Not a fan of any of their camshafts, flat or roller and they leave BUTT LOADS of power on the table if you are using the "shelf" grinds.

I attached a copy of some dyno sheets from a 455 Pontiac engine, nothing exotic, just a simple 455 engine build with factory cast iron heads and 9.3 to 1 compression.  Comp suggested the XR276HR cam, it didn't make the grade anyplace and the customer noted "quirky" idle quality and EVERYONE at the dyno shop blamed the Q-jet for the engine being such a "turd".

When I was contacted about it, I quickly moved away from the Q-jet "bashing" they were throwing at me and asked about cam specs, compression and head flow.  I told them to throw the XR276HR cam in the dumpster behind the shop and gave them specs for a custom ground HR cam instead.

I had them grind it on a 114LSA with the ICL set at 110 degrees. 

Not only did the new cam make a BUTTLOAD more power at every RPM, the engine builder noted that the "quirky" idle characteristics disappeared and the engine actually idled nearly dead smooth with the larger cam on a wider LSA.

Enough cam bashing, but for sure for what you are doing I'd give me a call at some point.  If you aren't getting fantastic idle quality with a TON of initial timing with the timing off the scale and some nozzle drip changes are in order for sure.......Cliff

BBvetteDRVR:
It wasn't the greatest cam choice by any means but I was trying to make everything more for boulevard crawling and I this was my best guess at a hyd roller. I did (and forgot to mention) go for a set of 1.72 ratio rockers for an insignificant increase in the lift.

The timing was set to 36 total (vac adv disconnected) at about 2600
16 at idle (vac adv disconnected) at 850ish

When you say "...If you aren't getting fantastic idle quality with a TON of initial timing with the timing off the scale and some nozzle drip changes are in order for sure"

What kind of timing numbers are you suggesting? I was thinking of maybe 18-20 initial would be lots, I could monkey with the vac-advance and try to stop it at 38-40.

I've finished checking everything on the carb and started putting it back together, anything I should note/check or change before I do?

Thanks again for looking this over
B

77cruiser:
Have you tried lowering the float a bit?

BBvetteDRVR:
It's already down a little. It was about .350 I actually move it up closer to .3+

Carb is back together and installed, now just to decide on where to begin re-timing the engine and start the adjustment process again.
B

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