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This is a discussion on Mykk's Carb & Ignition tuning guide within the Carbureted forums, part of the Performance category; I've been getting EMails on carb & ignition tuning, so I wanted to make a guide I could refer people ...
I've been getting EMails on carb & ignition tuning, so I wanted to make a guide I could refer people to. I'm strictly talking about performance aftermarket square bore carbs.
Here it is:
Mykk's Carb & Ignition tuning guide
Picking the right size carb for your engine. (I'm using my own SBC for examples)
357cu/2 = 178.5
178.5 x 3.76 =671.16 cfm assuming 100% volumetric efficiency (VE).
Things like Camshaft, Heads, Intake manifold, exhaust, forced induction, even ignition timing all affect VE%. A factory mid-80's smog era 350 would be running around the 71% VE range, performance factory small blocks of the 60's era would be in the 83-85% range. A built & blueprinted engine with well thought out components working together could reach as high as 95% VE, a supercharged engine can put out 105%-115%. For the sake of my build we'll use 87% for our formula
671.16 x 0.87 =584 (rounded) cfm the engine will need to spin 6500rpm, in this case I would choose a advertised 600cfm carb, because most carb manufacturers under estimate the cfm output. (I'm actually using a 650cfm demon now, we'll get into that later)
But, luckily for us lazy dumb people there are several online calcs that will do the math for you!
Most engine builders will tell you that for a street driven application pick a carb that flows slightly lower than what your engine will handle. The idea is this will prevent from having stumbles, flat spots and just generally over carb your engine.
I have a friend in the town that dropped in a stock L31 5.7L 350ci into his 1967-72 short bed 1/2 ton, pretty clean little truck (my favorite era of GM's) He used a Vortec intake for square bore carb, and put on a Holley 750cfm carb. Granted, his truck is pretty quick and he claims his engine is more responsive and quicker with the 750cfm than it ever was with a 600cfm.
My idea as to why is because the 750cfm is adding more air/fuel ratio at a lower throttle than the 600cfm could possibly deliver, however if he ever puts his foot all the way into it I'm sure he get's one hell of a stumble that the engine won't be able to recover unless he pulls his foot out of the throttle, and in racing that will cause a slower ET.
Ok, we've determined what size carb we should use on the engine, what kind of carb should we use? Brand/manufacturer is highly personal tase, I'm just referring to manual secondaries vs vacuum secondaries.
As a general rule of thumb, manual secondaries is for manual trans, lighter vehicles. Vacuum secondaries is for automatic transmission equipped and heavier applications.
If you live at any significant elevation above sea level (like me, in Prescott Az @ 5300ft elevation) You will want to immediately re-jet (or rods too) the carb for the elevation, Every carb manufacturer has a different approach to this, I believe on my Demon carb it's 1 jet size lean for every 1200ft, but this will change in a later step.
I'm going to move on to ignition timing, and than we'll come back to tuning the carb. Most diagnosed fueling problems can be resolved in the ignition system.
Setting up your ignition timing curve can be time consuming and frustrating but a well dialed in distributor can make or break the way your performance build reacts on the street.
Some engine builders believe in setting your max time and it doesn't matter where the idle timing lands, this might be fine for an all out race motor. But anything that see's the street, stop lights & cruising will need to be concerned with the entire timing curve.
To set the maximum ignition timing, the vacuum advance must be unplugged and the port on the carb capped. So your dist is relying on mechanical advance only. Different engines like different max timing, it's mostly in the cylinder head design. My Vortec heads should only use a max of 34 degrees, occasionally I sneak up to 36-38 to see what happens. Older head designs can take 38-42 degrees max. Ideally the max advance should be set at the point right before detonation.
I've discovered my engine likes to have that max timing of 34 degrees all in by 3200-3400rpm, so that means the entire mechanical advance is maxed out in the time it takes to go from idle-3200rpm,
Setting the idle ignition timing, called initial timing. Big camshafts require more initial timing in order to idle, my Comp XR276-10 idles good with 13 degrees initial timing. I like my idle a little low, around 650-750rpm a) because I like hearing the lope of the cam b) because I'm not fighting the engine with the brake pedal while stopped at a light.
Ok, we've declared my engine likes to have 34 degrees max timing and will idle fine with 13 degrees. That means in my distributor I will use the 21 degree bushing that came in my kit, as well as lightweight springs for the weights to have my advance come in fast to reach that 34 degrees by 3200rpm. This can get tricky, you can mix match thick & thin springs to get your desired timing curve. I've found by just using only light springs the engine doesn't want to come back down to idle because timing is advanced at a low rpm due to centrifugal weight.
Now, just leaving it like this would be fine for the weekend warrior/race car. But in any kind of driver, cruiser or anything that you hope to get any kind of fuel mileage out of you need to have a vacuum advance hooked up. The vacuum advance on the dist adds more ignition timing on top of what's already in place by the mechanical advance. The idea is that under light load situations the manifold vacuum is high and the A/F ratio is lean. When the load on the engine is light or moderate, the timing can be advanced to improve fuel economy and throttle response due, in part, to the slower flame travel in the combustion chamber from the lean A/F under these conditions. Under light load and cruise the engine will accept a maximum timing advance of about 52 degrees. Anywhere between 48-52 is good.
You can choose different vacuum advance modules that will tailor to your needs, but first you need to decide if you are going to run the vacuum advance from the "ported" vacuum port on the carb or from the manifold vacuum port.
Manifold vacuum allows actual manifold vacuum to the distributor at all times. Ported vacuum only allows manifold vacuum when throttle is above idle. This was used as an emissions control to retard timing at idle in order to reduce emissions. Using ported vacuum can make a more reliable and predictable idle speed since the initial timing will never change based off of vacuum. Does your camshaft duration require additional initial timing in order to idle properly? Do you have the idle screw on the carb open so far your beginning to pull fuel from the transition circuits or boosters? Radical cams will often require over 24 degrees of timing advance at idle. To much initial timing can and will fight the starter motor while cranking and in turn can break teeth off the flywheel & starter. An appropriately selected vacuum advance can plugged into manifold vacuum can provide the needed extra timing at idle to allow a big cam to idle at a lower rpm while allowing the lower timing while cranking when manifold vacuum isn't present.
If you choose to run straight manifold vacuum to your advance in order to gain the additional timing advance at idle, you must select a vacuum advance module that pulls in all of the advance at a vacuum level at least 2” below the manifold vacuum present at idle. So lets say your moderately built engine pulls 15" of vacuum at idle, you'll need a vac advance that maxes out at 13". A more radical motor might only pull 10" vac at idle, choose a vac advance that's max by 8"
My engine only makes 10-12" vac at idle, it cruises around 17-20". But I'm not using manifold vacuum for the advance and I've decided on a module that begins adding timing between 5-7" vac, it's maxed out @ 11-13" and will add 8 degrees of advance max (camshaft degrees, which equates to 16 degrees at the crank) With my max mechanical timing @ 34 degrees + the 16 degrees from the vacuum advance, my engine will not ever go above 50 degrees of timing while cruising (and that's if I cruise around at 3000rpm with the mechanical advance maxed). As soon as I put my foot into it, manifold vacuum goes away, the vacuum advance retards to the mechanical advance curve.
The same thing happens under a load, manifold vacuum is low, the distrubutor is only using the mechanical advance and will provide for more power with less chance of detonation at the cost of fuel consumption.
Ok, we've got the ignition timing thing down. Back to the carburetor.
With the engine idling, fully warmed up and you're running the initial ignition timing your happy with. Set the desired idle rpm you want to use, your carb should be set up from the factory with a generalized base setting for your idle mixture screws. Turn in the idle adjustment screws until the engine rpm barely begins to fall, than turn that screw back out by 1/8 turn. Do the following to the other idle screws. Re-adjust the idle rpm screw if needed to get desired idle RPM and repeat the procedure.
To judge if you need bigger or smaller jets, Keep your eye on your spark plugs, pull one plug out every other day or once a week.
* Normal deposits: Light brown or tan colored.
* Fuel fouled spark: plug Black fluffy carbon deposits indicate an overly rich fuel mixture or possibly a weak spark. Check for such things as a stuck choke, a heavy or maladjusted carburetor float, a leaky needle valve in the carburetor, leaky injectors, low coil output or high resistance in the plug wires.
* Wet spark plug: A wet spark plug means the plug has not been firing. If not due to engine flooding, the problem may be a bad ignition cable (excessive resistance, shorted or arcing). But wet fouling can also be caused by dirt or moisture on the outside of the plug that provides a conductive path to ground, or by an internal crack in the ceramic insulator that shorts the plug to ground.
* Oil fouled spark plug: Heavy black deposits with an oily appearance. These are the result of oil entering in the combustion chamber, probably past worn valve guides, guide seals or rings. Switching to a hotter plug may help prolong plug life somewhat, but no spark plug will survive long under such conditions. The only permanent cure to this condition is to fix the oil consumption problem.
* Glazed spark plug: Yellowish melted appearing deposits on the insulator tip that result from high temperature operation. The engine may be running too hot (check for cooling problems), the EGR valve may be inoperative and/or the heat range of the plug may be too hot for the application. Switching to a cooler plug may be necessary if no other problems are found.
* Damaged plug: If the electrodes have been smashed flat or broken, somebody put the wrong plug in the engine. A plug that protrudes too far into the combustion chamber may hit the piston or a valve. Always follow the plug manufacturers application recommendations when selecting replacement plugs to prevent this kind of problem.
* Overheating If the spark plug: insulator is blistered, white and free from deposits, something is making the plug run too hot. If the heat range is not too hot for the application, check for cooling problems, incorrect ignition timing or a lean fuel mixture.
* Melted electrode: A symptom of severe preignition. The spark plug has been running too hot for a long time (see overheating above). This can be very damaging and may burn a hole through the top of a piston!
* Detonation: If the insulator is split or chipped, detonation (spark knock) may be occurring in the engine. over advanced ignition timing, excessive compression due to accumulated deposits in the combustion chamber, or engine overheating.
There are more specific ways to read a plug, but this should get you going in the right direction for.
Once you're confident you've got your carb & ignition dialed in. For those select few who decided to go with a vacuum secondaries type carb. Don't forget to pick up a secondaries spring kit for your carb and play around with lighter springs! Usually the spring in the carb out of the box is too conservative and won't ever fully open up at any rev.
Vacuum secondaries do not function off of manifold vacuum, like the misleading name implies. They function off of the air passing by a small port inside the venturis. The faster the airflow through the venturis the more the secondaries open.
Idle RPM adjusting screw:
Idle Fuel screws: Speed Demon
There are four idle fuel screws on the speed demon, one at each corner, two on each side. Some carbs only have two screws located on the primaries fuel metering block
The Edelbrock carb has two idle fuel screws located directly on the front of the carb.
Try to keep all the idle fuel screws turned out equally.
The vacuum ports for Manifold vacuum and Ported vacuum:
The edelbrock's ported vacuum is the higher up port on the left. The other two, one bigger port and smaller port are both manifold vacuum. The bigger port is for the PCV hose and the smaller port can be used for ignition vacuum advance
On the Speed demon, the forward most vacuum port is ported vacuum, the one directly behind it is manifold vacuum (tough to see, but my manifold vacuum port is plugged under the throttle position sensor on my carb)
The vacuum spring pod on the Speed Demon has an easy access plug that allows the changes of springs a snap.
Don't be afraid to change up or play with your carb & ignition, as long as you approach what your changing with an educated direction.
If you have any more ideas, or would like to post up what you have done, please do.
Last edited by 04SilveradoMykk; 03-09-2011 at 11:53 AM.
pm a mod to get this stickied.
O2 GMC RCSB 4.8L V8 AUTO 3.73's
rebuilt/beefed up 4L60E(at 43,000miles)
Matching Ninja 500r
i agree, get this a sticky
97 z71 200k+ miles, 4" Rough Country suspension lift, 35" MTR's, dual pipes, some unknown tune
Incredible write up!!! This will be very useful for me as i have very little dealings with carb setups!!! Thanks!!!
Here's an interesting little twist to my vac secondaries...not that I suggest you do this on your own set up.
I was suspect that my vacuum secondaries weren't opening... I mean, my truck is pretty quick. But it just felt like something was holding it back. So, today I rigged up a small camcorder focused on the secondaries vacuum pod & linkage and went for a short drive...Sure enough, my vacuum secondaries never opened. Despite what throttle position, rev or load the truck was under. I was already using the lightest secondary spring holley makes too. I took the carb off, cleaned the air passages and made sure the port in the carb for the vac secondaries wasn't plugged or blocked.
The problem still persists.
BTW, vacuum secondaries won't open while reving in the driveway. They need to be under a load to function. And even different kinds of air filters can delay the opening of vac secondaries.
So I was looking at my carb, and I saw that the secondaries had a plate & rod that would be forced closed when the primaries close. And this same plate & rod would limit the movement of the secondaries according to position of the primaries.
As an experiment, I removed the vacuum secondaries spring entirely (with the check ball still in place). Just to see what would happen. I was expecting a bog & hesitation from hell. Because research and theory tells me that the lower vacuum signal from prematurely opening secondaries wouldn't be able to draw fuel out of the boosters until the higher revs. And without the second accelerator shot that manual secondary carbs have, the engine should just fall flat on it's face.
I couldn't believe it, my throttle response was improved. More power, more off idle torque. Part throttle response under a load was increased. Higher RPM power increased. Better than it ever has been. I think my saving grace is the fact I left the check ball in place, and that allows the secondaries to open smoothly and fluently and not flop open.
I'm wondering if my secondaries were stuck closed when I ran 14.1 @ 94.6mph in the 1/4 mile in early Jan, motor only with my heavy 22" rims & tires. Now I'm itching to get back and see if I can hit low 13's... :)
Last edited by 04SilveradoMykk; 01-31-2010 at 07:05 PM.
The other end of the spectrum: dialing in a performance carb for (slightly) less fuel consumption.
With drastically increasing gas prices it's come to my attention to devote a little time to setting up a carb for daily driven use that might not make as much power in the top end but will help (slightly) in the fuel consumption department. I'm going to use my Annular booster Mighty Demon 750cfm as the example. Why am I using a double pumper huge carb as my economy test mule. Simply because his carb has Annular boosters, a more efficient booster design that requires smaller jets much less vacuum signal before drawing fuel through them compared to traditional downleg boosters. My personal engine runs much smoother and responds much better to this carb than the previous Speed Demon 650cfm Vac secondaries downleg booster carb.
Re-jetting and Power Valve: A generic performance carb has four equal sized venturis and equal sized boosters. Why do the primaries have 7-8 sizes smaller jets than the secondaries? Because the power valve supplies the extra fuel through the primaries when needed. The power valves that come in Demons & Holleys are rated at 6.5", that means at more than roughly 6.5" of manifold vacuum the power valve is closed and prevents the addition of more fuel. As you put your foot into the Go Pedal (or increased engine load) manifold vacuum gets real low and can even disappear entirely. At below 6.5" of manifold vacuum the power valve opens and supplies more fuel through the primary boosters to compensate for the smaller jets. You can go down your local auto parts stores and get differently rated power valves. In my street 750cfm I've replaced the 6.5 power valve for a 4.5 valve, thus delaying the addition of more fuel until even less manifold vacuum is present. Most carbs made today only have one power valve, some older holleys have 2 valves. One in the primaries and one in the secondaries. If you even felt compelled, you could drill out the fueling passages in the primary metering block for the power valve to allow more fuel flow at WOT and than significantly drop the primary jets sizes for a lean cruise. I wouldn't recommend making such changes without a wideband O2
I've previously discussed how to roughly gauge jetting a carb to your engine by reading the plugs. Going one step leaner (smaller) from ideal can help consume less fuel without harming a motor (as long as your not spark knocking/pinging or overheating) at the sacrifice of some power. Be sure you have your carb jetting right to your motor before going leaner.
Delaying the secondaries:
On my carb, it has manual secondaries, which ultimately means the gas pedal controls when the secondaries open. In my experience by delaying the secondaries 'til the primaries are open further can actually improve throttle response and can get rid of a bog in acceleration if it's a big carb like my Mighty Demon. My theory, the higher RPM of the engine puts a bit of a vacuum signal under the open primaries as opposed to all four barrels opening sooner. Also, in my carb, delaying the opening of the secondaries also prevents the second squirter from dumping raw fuel down the venturis until a higher throttle position. in a vacuum secondary carb you simply install a thicker, or the thickest spring in the secondaries pod to delay the opening rate.
If one wanted, you could disable the secondaries entirely by removing the link bar. Thus turning your performance 4 barrel into a 2 barrel. But when you put your foot into it all the way it's not nearly as gratifying as all four barrels in action. Not to mention you wouldn't be able to successfully tune for adequate cruise and WOT A/F ratio without modifying the power valve fuel passeges. But as long as you never put your foot the floor it could work.
I made a new link bar out of coat hanger wire and bent the secondaries throttle bracket all in the name of having the secondaries open later and in my case not open completely vertical. A No-No in the racing world, but in performance street it'll swing. Especially with a carb too big for the motor.
Utilize the vacuum advance and pay close attention to your ignition timing: The vacuum advance is a fuel economy tool. The scientists and engineers of yester-year discovered that the lean air/fuel ratio during high manifold vacuum situations burn much slower than a rich fuel ratio and can utilize much more ignition timing, we're talking about around 24 degrees at idle and as much as 52 degrees while cruising. So they designed the vacuum advance to be hooked up the manifold vacuum. Than came the beginning of controlling emissions output, and the engineers moved the distributor advance over to a ported vacuum source so it wouldn't add timing at idle. The idea wasn't that high initial timing would make more smog, but rather the delayed ignition timing would make the combustion cycle at idle finish much later and in the exhaust manifolds creating more heat in the exhaust and burning off excess fuel in the pipes. The same idea behind catalytic converters before they were invented.
By moving the vacuum advance over to manifold vacuum, the first thing you'll notice is the idle rpm jump drastically. You will need to lower the butterfly idle adjustment screw and reset your idle fuel screws. More initial timing, less air & less fuel to maintain idle = less fuel consumption. The catch is, you will need to install a vacuum advance in your distributor that is maxed out by roughly 2" less manifold vacuum than your engine produces at idle. And these days, vacuum advances are getting tougher to find. Particularly ones with specific open and max vacuum ratings.
Also, once you've accelerated and your cruising down the road. The engine braking affect of taking your foot off the gas to slow down is greatly reduced. The engine just wants to keep pulling forward, and even stopping abruptly can be a chore. so be careful. Other engine factors play a role on how much initial timing should be present, big cams require more timing.
Gimmick products: Most guys will tell you to avoid gimmicky products like the plague, such as things designed to swirl and spin income air for example. But, I've found that one gimmick in particular can help with fuel consumption (as long as the carb is set up right) but does sacrifice some power in the top end. This expensive piece of anodized aluminum is made by airaid. It's a 1" four hole spacer that has spirals cut into the bore. there are several copy's and no name brands that can be found online for cheaper. In my experience, with my single plane intake, it helps to atomize and blend the air/fuel ratio while low throttle cruising and can help make up for that one or two jets sizes you leaned out earlier.
Remove your PCV valve: This may, or may not be good for your specific engine. Personally, with my choice of demon carbs I've noticed the engine responds well to not having the extra vacuum leak known as the PCV valve. I've even been able to turn in the idle fuel screws because they were no longer compensating for the extra air sucking into the engine. Theoretically you would also be able to drop down a jet size without PCV. But, the engine oil will get dirty faster. Crankcase gases will build up and be forced out of valve cover breathers and can leave an oily mess (use baffled covers to help), if you really wanted to go full out you could use a race style evac-pump to keep the crankcase fume free.
You could also put in a vacuum limiter inline with your PCV valve to make the vacuum leak much slower/smaller. I've had good luck using an old 1/4" drive socket with a small head inside the 3/8"s vacuum line to the PCV valve.
Finally, intake manifold design plays a big role in fuel consumption. Ultimately I will be swapping my RPM air-gap back in place of the Super Vic type single plane I'm currently using. And than I'll be dialing in the carb and playing with various spacers once again.
Edit: I just bolted on my revamped carb & swirly gig spacer. The first thing I needed to do was turn in all four corner idle screws for it to idle. Before with this same carb it needed 1 3/4 turn on each screw to idle at 750-800rpm in gear. Now if I go any more than 1/2 turn each it gets way too fat and even lowers engine RPM. I've got in gear idle around 900rpm to make 10" of vacuum. Any lower idle speed and manifold vacuum gets real low and isn't able to open the vacuum advance, which causes even lower idle rpm drop until there isn't enough manifold vacuum to keep the power valve closed and than it idles really fat and smokes out of the pipes. Park/Neutral idle is around 1300, that sounds high but there is no dieseling or run on when the engines is turned off.
I really want to spend some time with a wideband so I can tune my idle, cruise & wot A/F ratios. Until than I'm going to leave the PVCR and idle & high speed air bleeds alone (despite that I've heard Demons are rich at cruise and lean at WOT)
If you have any more tips or advice, please feel free to post them. Cheers ~Mykk
Last edited by 04SilveradoMykk; 03-09-2011 at 12:02 PM.
I pulled the dual plane intake out of storage and I'll spend the next couple of weeks cleaning up castings, and matching the runners to the ports & gaskets. I decided to use the 1" four hole-tappered spacer with this manifold and I'm matching the squarebore flange to the spacer. I'll also knife edge the crossover between the planes.
put about 15mins in last night just to get started.
Taking off the single plane Super Vic copy intake and replacing it with the worked over copy RPM Air-Gap should really aid in the fuel consumption department.
Installing the dual plane intake:
The distributor received an accel adjustable vacuum advance, dialed in to start pulling advance around 5" and max out 11 degrees advance by 8" manifold vacuum. Roughly 2" less than manifold vacuum present at idle by my engine. The dist was also set up with a 21 degree mechanical advance bushing.
Theoretically this should provide for: 36 degrees max mechanical timing all in by 3200rpm, 15 degrees while cranking, 26 degrees of initial timing at idle, and a cruise timing between 34-47 around highway rpm's.
Finished the intake swap and dialed in the distributor. The results went beyond my expectations. An off idle hesitation that was evident with the previous set up is now completely gone. I thought I was chasing down a carb tuning issue, it apparently was an ignition tuning problem. The engine is reacting very well to the new intake and distributor settings, the vacuum advance ended up adding 15 degrees of timing for an initial of 30 degrees.
The engine has never started up, idled and reved so quickly in it's entire life as it does with this set up. I'll be curious to see what and how it affects fuel consumption.
OUTSTANDING write up..