383 Stroker Carb Issues

Hey all, new to this forum. I have a trans am with a 383 stroker with a Barry Grant 750 Carb. The issue I have is whenever I crank the car the AFR starts out about 14:7 which is perfect. But within less than a minute it will go to around 12:1 AFR, then about another minute or two after it will go straight to dead lean. I don't understand why it is doing that. The car idled just fined before I let it sit for a couple months now all of sudden it doesn't run right. Any help?? I've messed with adjusting the screws and I've tried changing the primary jets in the metering block but it just makes it run even more rich the ultimately goes dead lean within the minute. I am STUCK!!

Welcome to the forum, Zaneyboi. Just to be sure we're talking the same on ratios, an AFR of 14.7 to 1 air to fuel is the stoichiometric ratio for maximum efficiency - a nice target number for highway cruise, but likely never seen at idle or acceleration. As we go "rich" the amount of air to fuel drops - the 14.7 goes lower. As we go "lean" we're pushing more air for each volume of fuel, and the 14.7 goes higher. When you say that in less than a minute your AFR goes to around 12:1 you're getting richer than you were on initial start which doesn't make sense. Even in Alabama on a cold start your choke is set, so I'd expect an AFR of more like 10.5 or 11 to 1, then after a minute or less the choke plate is opening and the AFR would bump to maybe 12 to 1.

On your idle stop you need to adjust your idle air screws to maximum vacuum, right? The old school approach was to close one until RPM dropped, open until RPM dropped, and set halfway between the two spots but vacuum is better. From you saying it goes "...straight dead lean" I'm wondering if maybe you went the wrong way on tweaking the primary jets?

I've never messed with a Street Demon carb, so I apologize if I'm leading you down a rabbit hole. Maybe one of the guys will jump in and help out a bit, but glad to have you here!

Mr Spears, you're explaining it right. first, toss out that number right after cranking. it's not reliable. the readings have to be at operating temperature and after a run. sitting in your driveway at an idle and "evaluating" isn't the right way to look at this. You need to take a test drive of varied rpms and then return to where you started and then note what you observed during the different rpm ranges. You say it was good until it sat for months.. have you checked for vacuum leaks? Or perhaps the pvc valve isn't "right" somehow? Is the air cleaner clear /clean and in the right place? Could a critter maybe have "moved in"??

I'm curious yo know how you are measuring your AFR.

Welcome to the site.

Your Idle Mixture passages are clogged from sitting. Remove the mixture screws and squirt cleaner through the holes and follow with compressed air or other source of high volume air (leaf blower), do this a couple of times, put the screws back in and adjust.
Cover stuff cause this is kinda messy

Quote:

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Originally Posted by chasracer

We have a tendency at the track to shoot some carb cleaner into the air bleeds when the engine is running a bit rough - most of the time we get away with it but there's been a time or two where we made it worse and had to pull things apart to fix it.

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Actually, it didn't make it worse, it just sped up the inevitable. Sometimes the magic doesn't work.

Thanks for the reply! I'm more used to fuel injected applications and am fairly new to carbs but still know my way around one in terms of basics. But yeah what you said makes no sense about it going richer than initial start then dead lean in less than a minute after. But I have tried changing the primary jets from the 78s that were in it all the way to 82s and it will change how it first idles but still ultimately goes dead lean. I do not understand in the slightest what is going on.

The only vacuum I have goes to the vacuum advance and brake booster. I have a carb spacer with a vacuum fitting that I used to T off for both vacuum advance on the distributor and the brake booster. And I do not have a PVC valve I just vent to atmosphere. And I will take the carb off and try cleaning it perhaps...

I am using an AFR gauge.

Thanks I will try that after work.

You need to think in terms of circuits. Sitting at idle in the driveway, you'll be using ( you guessed it) the idle circuit! The 2 needle valves can effect your readings at the AFR... that's it, they will do little else for this conversation. The jets you speak of will only work off idle, so changing them to any number and then doing readings at idle doesn't work.

That's why I was saying to drive the car around and at different rpm ranges, then take readings at a given rpm/speed! There is also a power valve for wide open throttle,these are known to leak on some carbs. So you will have an opportunity to get the opening value when you have it apart. With that number on hand... get a vacuum gauge and take a reading, usually you use a valve that's 1/2 of what ever your highest reading is on the gauge. But if you want to "tune" the valve you can use a higher or lower setting to open the power valve sooner or later as you desire. These are all just basics.. I'm hoping you can report back that you found some debris that fouled the carb and get rid of those huge jets you went to! LOL..