Cam LSA discussion and possible causes of my smelly exhaust

[jwilson645]

With the help of this forum I rebuilt the 350 in my 77 Silverado last year. The current 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
Decked 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

After a full rebuild on the Qjet with the parts and expertise from Cliff Ruggles, it runs good. My issue now is that it still has a strong exhaust smell and light smoke from the exhaust at idle. It was suggested that XE series cam is the culprit of the smell. [quote]Short seat timing events, advanced intake lobe, tight LSA and super-quick opening/closing ramps just don't make for a good end result. Stinky idle and "rough" at idle speed is directly related to the LSA.[\quote]
Before I go changing camshafts, I want to learn a bit more about why the 110LSA would cause "stinky idle" more than say at 112LSA as is being recommended. This is the replacement cam that has been suggested to me that would be a better fit for my truck because of the 112lsa and higher duration. https://www.summitracing.com/parts/sum-1103

If I understand correctly, the LSA has to do with the amount of time that both valves stay open together and the tight 110lsa is allowing unburnt fuel out the exhaust. Am I thinking about this correctly?

[BossHogg69]

I'd check my timing and idle adjustment (I don't know much about Q-Jets, but if they were used stock by GM, I'd think they would be able to idle cleanly) before I started looking at pulling a cam. What do your spark plugs look like?

A tight LSA allows more overlap between the intake & exhaust cycle than say a 112 or 114, etc. I myself don't consider a 110LSA 'tight'. That being said, a 212*/218* .447/.454 cam is pretty mild on duration & lift, so even with a 110 LSA, I wouldn't think it would pollute your idle very much if any. Proper tuning can solve a LOT of idle/cruising issues, but the process of doing it is lost on a lot of people.

[geezer#99]

That's a mild cam. Just right for your motor.
You need to close up the primary throttle and cover the transition slot. Likely you have the idle cranked up and are idling on the power circuit. You'll need more initial timing to do that. More initial will speed up the idle allowing you to close the throttle back down.
There's a section here on the transfer slot. It's on a holley but they're all the same.
http://www.crankshaftcoalition.com/w...EI_distributor

[jwilson645]

Originally Posted by geezer#99

That's a mild cam. Just right for your motor.
You need to close up the primary throttle and cover the transition slot. Likely you have the idle cranked up and are idling on the power circuit. You'll need more initial timing to do that. More initial will speed up the idle allowing you to close the throttle back down.
There's a section here on the transfer slot. It's on a holley but they're all the same.
http://www.crankshaftcoalition.com/w...EI_distributor

It shouldn't be on the power circuit as I checked all that during this last rebuild but I'll double check. I will also play with the timing some more.

[geezer#99]

What's your apt set at?

[jwilson645]

Originally Posted by geezer#99

What's your apt set at?

IIRC, it's 3 turns out. I'd have to check my notebook at home to be sure.

Timing is something I need to check into. My initial WAS at 14 but I bumped it back a tad due to hard starting and I'm not sure what it is now.

[jwilson645]

Played with the timing some yesterday. Currently set at 12BTDC initial and the smell is better but not completely gone. Still a little gray-ish smoke from the driver side tailpipe. Total is questionable as the mark kept bouncing but it should be 32-34

[cadillac_al]

It sounds like the perfect truck engine. A couple questions that may help everybody, is this the original 77 carb? Is the vacuum advance hooked to manifold vacuum?

[jwilson645]

Originally Posted by cadillac_al

It sounds like the perfect truck engine. A couple questions that may help everybody, is this the original 77 carb? Is the vacuum advance hooked to manifold vacuum?

This carb is off an 84. The original 77 carb was long gone and what was on it was junk. This 84 was untouched original so I built it instead. I've been running the advance off ported and just moved it to manifold this weekend. Seems to have helped it some. It runs really good. I would just like to get the smelly/smokey idle corrected if possible.

[geezer#99]

Have you looked in the carb while it's idling?

[jwilson645]

Originally Posted by geezer#99

Have you looked in the carb while it's idling?

I have. No nozzle drip.

[geezer#99]

Might try setting the float an 1/8 inch lower.

Moving to manifold vac for the pot and getting improvement means you need more initial.
If you're maxed out at 12 degrees you might need to try an interuppter switch in the power wire to the dizzy.