I do agree that folks simply shouldn't "dive in" and start modifying the carbs idle system, or anything else for that matter. Even with that said, idle fuel MUST be adequate for the engine or you are not going to get a good result. All q-jets are application specific, and very few were not emission calibrated, so expect to have to modify most of them in some area for a good end result.
Idle fuel MUST be adequate to the mixture screws for all applications. The ideal situation is to have full control with the mixture screws from rich to lean, and be completely on the idle system (no nozzle drip). Idle fuel is also available to the transfer slots so it helps with transition from the idle to main system as well. For emission calibrated carburetors they are often insufficient in fuel delivery at both places, so we will need to address these areas as part of the carburetors calibration when we are setting it up exactly for the application.
Novadude, the ONLY reason your carb is doing fine with the 108LSA camshaft is that you have small lower idle airbleeds. Some later carburetors had small LIAB's and they will get by with smaller idle tubes and DCR's when larger than stock cams are being used that increase overlap and degrade idle quality. Very, and I mean very few q-jets have the small LIAB's without a HUGE upper one, so in this particular case you hit the lottery and your carb doesn't require larger idle tubes and DCR's to make the engine happy. The cam is also pretty small for 355cid at 9.6 to 1 compression, so another big player in the deal, and the 108LSA isn't a deal breaker for it. There are just times with this hobby when you just happen to find a combo that works well, even if the parts from the outside looking in aren't the most ideal.
For the OP, a carburetor number is ALWAYS required for anyone to make tuning recommendations. Rochester made quite a few different models and changes to each one thru the years of production. Quite surprisingly aside from some of the later units being quite limited for idle fuel delivery, the main fuel isn't going to be that far off in many cases.
What folks also do NOT realize, is that things done to the engine to make them more efficient, may not require more fuel anyplace, they may actually need less.
Ignition timing is also a big player when tuning the primary side of the carburetor. We can add more timing at light throttle cruising to help burn leaner mixtures, which can help some as you sneak up on the idle jet/metering rod combination and APT setting.
We also build engines here, and dyno nearly every one of them. We also tune a LOT of engines brought here after significant engine modifications, so our learning curve with these things is pretty extensive.
For the most part, the very WORST cam choices are tight LSA grinds with short seat timing events. We would NEVER under any circumstances put a 108 or 110LSA cam into an engine build that was headed for a street driven vehicle. Even the smaller cams on "tight" LSA's don't make the grade. They close the intake valve too early in the cycle and KILL any potential for the engine to make strong upper mid-range and top end power as they lack duration (too small), and the early intake closing and tight LSA pull power DOWN in the rpm range, and narrow up the power curve (torque) at the same time. A by product of the narrower power curve is INCREASED octane requirements all else being equal. So wider LSA will not only spread out power, it will decrease octane requirements at peak torque (max VE), and it will occur slightly later in the rpm range, when cylinder filling events are happening quicker, so even less chance for detonation.
For the most part cam manufacturers offer tight LSA cams to provide an "aggressive" idle quality, which lots of folks love to hear at car cruises and car shows, etc. I've had more complaints in recent years from folks using the "thumper" cams, Voodoo, and other designs on tight LSA's than any other. Always the same thing, "rough" idle, stinky exhaust, and lacking in power on top end. Several very good customers of mine who bought into the "bling" from those cams have extracted them and replaced them with larger cams on wider LSA's, and/or cams closer to stock specs, and been quickly rewarded with smoother idle quality, smoother off idle, and strong/broad/flat power curves with PLENTY of top end power. At that point they usually say the only good sound the "tight" LSA cams made is when they "tinked" off the dumpster!
Enough on camshafts, I have dyno sheets and DIRECT test results to back up my comments, as well as a few really good stories to tell about that topic as well.
The best way to get good results with your engine is to make good choices for it during the rebuild. To this day I can't believe folks make such piss-poor choices for heads, compression ratio, and cam choice with all the good information right at the keyboard in front of you. There as so many good combinations out there to choose from, that it absolutely makes no sense to come up with a less than desirable end result from your new engine project.
Even so, the phone rings off the hook here, and nearly half of the folks who contact us have simply miss-calculated the compression ratio of their new engine, and simply chosen the WRONG cam for it. This past week was a banner week for those type of complaints, I got WORN SLAM OUT from calls from folks who were having all sorts of running issues with new engine builds, and some of them absolutely HATED them. 9 out of 10 simply made very POOR cam choices, buying into the BS that 108-110LSA cams with short seat timing events (modern lobe profiles) where the hot ticket for their application.
We had one caller who had installed a 204/214 duration cam into a 455 engine build with 10 to 1 compression, and another who built a stroker 454 and at over 10 to 1 compression and put a Comp XE268 cam into it. Keep in mind here that a "stock" replacement cam for a low compression 350 Chevy engine has 274 degrees seat timing. Why would you, or how could you think a cam with LESS seat timing on a tighter LSA is going to be the "hot ticket for nearly 500cid at 10 to 1 compression?
Then there was the guy who did a complete rebuild of his 396 BBC engine and was talked into a Mutha Thumper cam on a really tight LSA and 230/240 something duration at .050" tappet lift. He said it idled like crap, sucked tons of fuel, stunk up the garage, his clothes, and anyone stupid enough to pull up behind him at a stoplight! The wife REFUSED to ride in the car because it was so stinky, so he wanted us to correct the issues with carb tuning, which is NOT going to happen as you simply can't get past the basic laws of physics with these things.
Another BIG player in this deal while were on the engine discussion topic is establishing tight quench. 99 out of 100 folks calling here complaining about how poorly their new engine performs don't have the first clue as to what the quench distance is in it, or the true static compression ratio.
In most cases, their engines actually have LESS than ideal quench. This happens from buying pistons that are pretty low in the holes at TDC, and thick "rebuilder" gaskets were used during the overhaul. Many also team up those shortblocks with heads that have large and poorly shaped combustion chambers, which makes things even worse everyplace.
Very, very few of their engines were decked and squared either, which we consider a MANDATORY part of the engine building process for used blocks, or any block that doesn't make the grade when you mock it up to see how far down in the holes the pistons are.
Tight quench combined with decking and squaring is EXTREMELY important to getting a good end result. I don't like to see over .040" quench on any of these engines for any reason. I also hate to see over .005" deviation bank to bank or front to back. In most cases we have to "split the difference" after mocking them up, but it's almost always possible to establish tight quench and pretty even bank to bank in one milling operation.
There are several ways to get quench distance to ideal specifications. First mock up the engine and measure how far down in the holes the outer 4 pistons are at TDC. Write these findings down and provide them to the machine shop doing the work. You can deck/square for zero, then use a head gasket in the .035-.039" range crushed to come up with a good quench distance.
Another option, especially if the pistons are pretty far down in the holes at TDC, is to deck/square to about .015" in the holes and use a .020-.025" thick head gasket.
Tight quench does several things for us. LESS timing will be required to make optimum power. Octane requirements will be lower, even if the process increases the compression ratio a bit. The engine will make more power (torque), and transfer less heat to the jacket water (runs cooler), all else being equal. A third benefit is that now we can accurately calculate the true static compression ratio and make a good cam choice for the engine. So instead of your "fresh" flat top piston 350 SBC being 9.32 to 1 compression because the pistons were .020" in the holes at TDC and you used .060" thick head gaskets on it with 64 cc Vortec heads, it's now 10.23 to 1 compression instead. BIG difference for sure and a larger cam will work much better in the higher compression engine.
That previous paragraph also explains why some engine combinations flat ROCK, and a nearly identical engine combination is somewhat of a "turd" using basically the same parts. The spread can be even greater if good cylinder heads aren't used, but that is a completely different topic, and I'm getting long winded enough here.
Anyhow, for anyone wanting detailed assistance with carburetor tuning, it is REQUIRED that you provide a part number. It also helps if you have taken the time to measure all the items inside the carburetor related to tuning, such as idle bleeds, main bleeds, idle tubes, DCR's, bypass air, jets, metering rods, etc, etc.
Even though at this point in my learning curve I know what most of these carburetors should have for specs, it helps to know that it hasn't been previously altered or modified in some way, which could effect the end results considerably if we made specific recommendations for it. Hope this helps some?.......Cliff
