Stuggling with bad idle and off-idle bogging

[jwilson645]

I have a 17084292 carb on the 350 in my 77 Silverado. It performs pretty well after about 2000 rpm.   I put one of Cliff’s rebuild kits in it about 4 years ago when this engine was 100% stock.  Since I did the rebuild (see below for specs), I am working on getting it dialed in. It’s getting better but the idle up to about 1800-2000 rpm is rough. It will sputter, pop and hesitate.   I put a FP regulator on it because the stock replacement fuel pump from NAPA was pegging my gauge at 10psi. That helped a good bit. It also has 73 size jets in it per an earlier recommendation from Cliff.  Also, I’m not getting much response from the mixture screws.  Could it be that the APT needs to be adjusted? I haven’t touched it yet.   I also know Cliff recommended that I get away from the XE cam which I haven’t done yet.

Engine specs are:

350 Block (4 bolt main) decked .010
.040 over bored
.010/.010 crank journals
1999 Vortec 350 heads (freshly rebuilt)
SpeedPro DDP pisons- http://www.summitracing.com/parts/slp-h423dcp40
FelPro Head .016 gasket- http://www.summitracing.com/parts/fel-7733sh1
Edelbrock 2116 performer vortec dual plane- http://www.summitracing.com/parts/edl-2116
Deck the block .010

Piston: http://www.summitracing.com/parts/sl...cp40/overview/

Head gasket: http://www.summitracing.com/parts/fe...make/chevrolet

.043 squish.

9.56:1 static compression ratio


Comp Cams- http://www.summitracing.com/parts/cc...34-2/overview/


Cam Style:Hydraulic flat tappet

Basic Operating RPM Range:1,000-5,200

Intake Duration at 050 inch Lift:212

Exhaust Duration at 050 inch Lift:218

Duration at 050 inch Lift:212 int./218 exh.

Advertised Intake Duration:256

Advertised Exhaust Duration:268

Advertised Duration:256 int./268 exh.

Intake Valve Lift with Factory Rocker Arm Ratio:0.447 in.

Exhaust Valve Lift with Factory Rocker Arm Ratio:0.454 in.

Valve Lift with Factory Rocker Arm Ratio:0.447 int./0.454 exh.

Lobe Separation (degrees):110

[Cliff Ruggles]

I have NOTHING go to say about that line of camshafts.  Short seat timing events, advanced intake lobe, tight LSA and super-quick opening/closing ramps just don't make for a good end result.

A truck engines needs smooth idle, strong power off idle, and broad/flat power curve, so that cam works against you just about everyplace.

From what I've seen here when tuning engines that used them is that they require more fuel at idle than what the cam specifications would typically dictate.

They have a "quirky" idle quality, and more times than not considerable valve train noise at idle and low rpm's.

At a minimum I would take a look at the idle system and open things up a bit to put more fuel to the mixture screws and transfer slots.  There is really no reason it doesn't idle well below 1800rpms or so, even being an XE cam, but I'm betting it will respond well to increased idle fuel delivery......Cliff

[jwilson645]

Thanks for the input. I'm going to pull it apart again and clean out all the passages and make sure there isn't something blocking one. I guess the idle isn't really "bad" but it smells strongly and has a little bit of white/gray smoke when sitting there at 800rpm.  Not pleasant to be standing outside of it for sure.

[Cliff Ruggles]

"It’s getting better but the idle up to about 1800-2000 rpm is rough"

Stinky idle and "rough" at idle speed is directly related to the LSA. 

Even so with that little tiny cam I wouldn't expect it to be rough past about 1000-1200rpms, so you may have other issues that you haven't figured out yet......Cliff