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Old 01-15-2020, 09:29 PM
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Tuning carb with no wideband/afr gauge?

How was tuning a carb, more importantly for a performance engine, done without a wideband or AFR gauge? How would you know if you were stoich, rich or lean?

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Old 01-15-2020, 09:58 PM
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Originally Posted by bcjjones View Post
How was tuning a carb, more importantly for a performance engine, done without a wideband or AFR gauge? How would you know if you were stoich, rich or lean?
Rich smokes, smell of raw fuel, the engine lays down, the plugs get blackened.

Lean it wants to back fire, runs hot though that may not show on a coolant gauge but the exhaust manifolds or headers will be really hot, plugs will be white, the engine will be at risk of preignition and or detonation.

Stoich without an AFR gauge is a place you might be at or close too in a reasonably wide band. This is where the Chevy inline 6 got its name on Blue Flame Special. Back in the 1940's and 50's following WW2 it comes from the cockpit crew observing the engine stack flame color, blue flames meant you were in the acceptable stoich mixture range. If you have a gas stove you can observe mostly blue flame on the burners which means the same thing of the mixture is approximatly correct.

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Old 01-15-2020, 10:36 PM
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Start with factory specs on timing then adjust for your parts so it idles good both cold and hot.
Install new plugs.
Drive around for a week on those plugs.
Pull the plugs.
Read the plugs then adjust.
Install new plugs. Putting the old ones on a shelf.
Check the plugs next week.
Once you get it correct you can reuse the old plugs. I check and replace mine(if needed) every 3rd or 4th oil change depending on how often the engine is ran. Keeps the engine happy and the plugs not frozen in the heads.
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Old 01-16-2020, 08:32 AM
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Using a AFR gauge will help in tuning but you can't always go for the magic numbers you hear about depending on the build as some engines are meant to run more richer then leaner depending on the build and the size of cam inside it and compression etc. It is just a helper to get the best tune possible but you still have to adjust and get it to where your engine gets what it wants to not be to much or to little adjustment wise.

To many try to lean out big cam engines and you can't do that with bigger cams as with more duration and overlap the more the mixture has to be richer on the idle circuit in order to maintain a proper idle but with bigger cam engines they don't like to sit and idle long or plugs will get black and need to be changed. That is why you have to have the whole setup from timing to carb tune and the proper cam matched to your heads and proper stall if running a automatic transmission and also rear gears to keep the build in the rpm range the cam needs for it to run the bests it can as well for the carb to do it job along with everything else.

I don't use a AFT gauge but use old school techniques and it has never failed me and I get good mileage and get my engine what it wants to make it run good and make my plugs not foul out and look good for what the build is and needs. Its a art that is learned over time just like anything else.
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Old 01-17-2020, 05:33 AM
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Learn to read your plugs......here a link to help with that:


You can only correctly read plugs after a full throttle hit and shut off after the run. Only need to go 1/8 mile to do this. The fuel ring will tell you rich or lean....it will take a few passes to get a read on timing, and that will also tell you more or less advance. Idle time, part throttle operation, and cold start will ruin the readings. You'll need a fresh set of plugs, fully warmed engine, and a safe, LEGAL place to make the hits....Doesn't matter if it's a street car, race car, or something in between, you'll be able to tell what the engine wants. Once it's tuned, if you choose, you can look at the o2's to see what they are doing, and that is good for diagnostic stuff, but unless you have an o2 sensor in each exhaust port, they are poor for tuning an engine.....I tune based on best mph, which is what you should do as well. Richen until car just slows down then back up to last jetting before it slowed. Get fuel right before you do anything with timing....and add timing as little as possible, also until mph stops increasing, and look for signs of detonation as well. You'll want to drop it down a degree or so below that point. The engine will be happiest there and will give you the best performance overall.
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Old 01-17-2020, 07:48 PM
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Idle Mixture & Main Jets

You do have to do a lot of testing. Always a good thing about the internet and a forums like this. Base lines you can glean from other people with similar setups. With Holley's for example they pretty much come stock with safe Main Jet sizes. Then it is a matter of reading plugs as others hear have written. Always start richer as lean can hurt and engine. But work towards the leanest you can without detonation. Idle mixture is pretty easy with Holley's and most carbs. Just get out the Vacuum gauge and go to the highest level. You can pretty much know when you are fat at idle as you barely can handle the smell.
Then it is a matter of setting the timing as well in harmony with it all. Trial and error. Re-Read plugs and do this off and on. Weather can change this especially if you are running on the ragged edge.... I am running right at 10:1 C/R with Iron Vortecs and a tight squishy deck and good AFR I am doing fine on 91 Octane.
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