Set up tweaks for extremely mild small block chevy

[dphipps]

I had been trying to figure out a off idle stumble off and on for some time. It was never much concern but I decided to dig into a bit the other day. I found that with the accelerator pump disabled the problem went away, so I figured it's running rich.

So off the carb came and I noticed it was wet on the bottom so I re checked the well plugs and sure enough the primary ones leak so I have repaired that and will be re assembling the carb this week. I did check this when I originally rebuilt it but the bubbles were so small that I probably just didn't observe close enough in the beginning. I feel this leak was causing a lot of the problem? I imagine once it gets warm and has vacuum on it the leak is bigger than It seems?

So I figure while it's off I'd see if there's any tweaks I can make while its apart. I have piles of scrap carbs and parts so I do have a pretty good selection of jets/rods/hangers, but I am a complete novice as far as tuning goes.

I hadn't used the carb before rebuilding it so I'm not sure how it functioned prior to rebuilt. The carb was rebuilt around 2 years ago with a kit from Cliff's, this stumble problem was there from the start. No modifications were made to the carb and I put bushings in the baseplate.

Specs:

-1979 halfton, 350SBC with stock heads and an edelbrock intake

- performer cam (Intake duration: 204 Degrees • Exhaust duration: 214 Degrees • Intake lift: .420” • Exhaust lift: .442” • Lobe separation: 112 Degree • Intake Center Line: 107 Degree )

- 3.08 gears, TH350 and stock converter with a shift kit

- 14* inital timing and somewhere around 36* total (haven't checked again after some more fiddling)

- headers with 2.5" dual exhaust


Carb:

- 17084285

- 43K primary rod

- 73 jets

- CH secondary rod

- K hanger

As you can see it's not a hot rod but it might as well run as good as it can with what I have.

[Cliff Ruggles]

Part of the stumble off idle issue could also be the engine build. 

The aluminum intake isn't helping nor is the larger than stock camshaft.   If the heads were ever off or engine rebuilt it will almost always have a LOT more squish distance than the original engine did as aftermarket head gaskets are a LOT thicker than the factory .020" steel shim gaskets they used back then.

I see this sort of thing ALL THE TIME. 

Well meaning folks make changes to the engine thinking that there are improving power and performance and more times than not they are hurting it.

I'm not familiar with that carburetor part number, might be a SR carb?  Is it a single main airbleed model?........Cliff

[dphipps]

I have never had the heads off and the truck is pretty low mileage so I don't think they've ever been off.

Is the cam really a problem? It was pretty much the smallest there was so I didn't think it would be much issue. The aluminum intake is just a dual plane and I was told it's mostly a copy of the stock one.

The stumble issue went away with the accelerator pump disabled. Even spun the tires!

I'm not sure how to identify the air bleed question.. I just dug out the book and I'm going to say it is not a single main air bleed.

I attached a picture of the airhorn.

[dphipps]

Also, I was advised to try going up a jet size when I put headers on so that's all that has been changed from the factory calibration. I guess I'm mainly wondering how the current calibration looks.

[Cliff Ruggles]

Things done to the engine to improve efficiency may have it wanting LESS fuel, not more fuel.

I've never been overly fond of the RV camshaft they sell for SBC engines.  It's a solid 10 degrees bigger than a stock cam everyplace and unless the compression ratio is raised at least one full point everything suffers....idle quality, throttle response, efficiency at low RPM's, etc. 

The best cam to use in a stock compression ratio 350 CID build with smog era 76cc iron heads on it is the Speed Pro CS-274.....IMHO......Cliff

[dphipps]

Good things to know. Oh well, can't learn without doing, even if it means a couple mistakes along the way.

Thanks

[Cliff Ruggles]

Dating clear back to the 1970's when I first got into this hobby EVERYONE would tell you to put a "high volume/high pressure" oil pump in your SBC, and a "double roller" timing set, and some sort of RV or aftermarket "high performance" camshaft. 

The "RV" cam was and has been a frequent recommendation all these years and at least half of the folks who call here will have one version or another of it in their fresh SBC build.

So, the oil pump does NOTHING except rob power, wear out the distributor gear, cam drive gear, timing set faster, and it can shear the pump drive shaft off as well.

The "double roller" timing set is a step backwards for the cam drive.  A Morse chain is a LOT stronger, constant tooth contact, and provides a cleaner spark curve than a roller chain.  Not to mention most roller chains aren't that great.  Even the high end seamless true roller variety running on billet sprockets aren't as tough as a Morse chain drive.  If you don't think so pull apart a 4 x 4 transfer case some time and see what type of chain they use in it?

The RV cam isn't all that bad, but you will LOOSE power (torque) right off idle and at low RPM's unless you raise the compression ratio one full point to compensate. 

95 out of 100 folks that "rebuild" their late model SBC 350 do just the opposite.  They install pistons that are way below the deck at TDC, then a big thick rebuilder head gasket which lowers the compression half a point or so and increases quench distance killing off combustion efficiency.

To add insult to injury many use 882 smog heads on them, killing of a bunch more power.  Top all that off with some Thumper or Thump-Ya-Mutha cam on a 107 LSA and you just built a complete "turd" for a new engine, and are now on the phone with the machine shop, and a bunch of other folks trying to figure out how to get it to idle in and out of gear and make acceptable power.

I see this sort of thing play out ALL THE TIME here, as there isn't a day goes by some poor soul calls the shop for tuning or carb modification assistance.....and I have to be the bearer of bad news and tell them that there isn't a whole lot I can do for them, or anyone else for that matter......Cliff