493 carburetor tuning question

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Zachary Egnitz

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Hello all, I am trying to tune the carburetor on my 493 stroker but I have never worked with anything this big and neither has my dad nor my co-workers so I'm hoping I can get some insight from those of you with experience. Firstly here are the specs of the motor.
Engine:
493.41 cu. in.
10.89 compression
Bore 4.35
Stroke 4.15
JE SRP pistons
Eagle crank and rods
Trick flow 240 cylinder head
Edelbrock torker 2, single pane intake manifold
Holly 850 double pumper
1.6 rockers (Harland sharp)
Crane cam duration 234/242 at .050, lift .575/.595* (689551)
Howard lifters
Eagle crank and rods
Doug 2in headers
MSD distributor with mechanical and vacuum advance.
Here's the current situation. The car runs really well, starts well even when cold. Timing is ~32° at 1000 RPM (with the distributor vacuum line disconnected) and ~53° at 3000 rpm with vacuum advance. I'm pretty sure I'm running rich because my tail pipes are black and your eyes burn if you run it in the garage for more than a minute (even with the doors open). My fuel bowls are where they should be (right beneath the center line of the sight window). Here's where I get confused. When I tried setting my idle mixture screws I turned them in all the way and then slowly brought them out one half turn at a time until I got the highest rpm. However, the highest rpm occurs when they are screwed in all the way. When I unscrew them the rpm goes down. This led me to believe jets were too big however with the screws all the way in the engine bogs when i hit the throttle quickly. I took the screws out half a turn and the bog is less significant. I took them out another half turn and the bog is basically non exsitant and the car pulls well. Right now I have them 1.25 turns out and the secondaries are 1.5 turns out I believe and the car runs great, good throttle response, pulls well and doesn't bog.
To reiterate, I don't have much experience with engines this big but I find it odd that my rpms go down when I bring the screws out and my tail pipes are black but if I bring the screws in it bogs and looses power. Is this just normal for this size engine with the cam I have? Should I just keep it how it is? Should I put smaller jets in? Any input would be appreciated :)
 
I don't know alot but i'll through my 2 cents in. The main jets should not have any real bearing on what your describing. It sounds like you need to balance your transfer slots on the primary and secondary to help get some more air in. You may need to get some holes into the plates but thats after the other things have been tried cause thats kinda permanent. Air bleed adjustment,idle feed restrictor sizing may also play in here. I would look where your primary plates are in relation to the transfer slot 1st though. Hope this helps.
 
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scampy said:
I don't know alot but i'll through my 2 cents in. The main jets should not have any real bearing on what your describing. It sounds like you need to balance your transfer slots on the primary and secondary to help get some more air in. You may need to get some holes into the plates but thats after the other things have been tried cause thats kinda permanent. Air bleed adjustment,idle feed restrictor sizing may also play in here. I would look where your primary plates are in relation to the transfer slot 1st though. Hope this helps.
Click to expand...

This is good information for me to look into for a bit thanks!
 
Scampy is correct. Transfer slot relationship at idle is very important. A portion of the fuel at idle comes through the transfer slot. That portion is not controlled by the idle mix screw. It is only controlled by the amount of slot exposed to atmostpheric pressure vs the amount of slot at low pressure.

Go here and scroll down to 1966 for an overview of carb fundementals. Theres some slight differences between Holley and Carter but the main one to be aware of when reading or watching that Master Tech session is Holley uses a 'power valve' for the wide open throttle enrichment. They also skip over the role of transfer slot at idle when they are first introducing how the idle system works. Later they touch on it.
Master Technician Service Conference - Chrysler's Training for Mechanics

Second. Timing and mixtures are closely related. Even with highcompression, that cam is probably resulting in a fairly inefficient idle.
Timing at 1000 rpm probably should be around 22 - 30 deg.
I wouldn't try to use the vacuum advance at this phase of tuning.
Later, much later, you can work on using it to bring the timing while cruising (at 2800+ rpm) to something around 50 to 56 BTC.

here's a guide to ballpark the initial timing.
Distributor starting point for a curve

Depending on high rpm efficency, the maximum timing around torque peak shouldn't be more than 36 or 38 degrees, give or take. Too much at wide open throttle full load can cause damage.

Post here summarizing the concept of timing the spark.
Large RPM and Vacuum drop when shifting from park to gear
 
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All that said, you can focus on initial timing and tuning the idle first.
Then come back to the driving and wide open throttle stuff later.
Just have the discipline to keep your foot out of the pedal on any test drives. Thats the one way the engine can be seriously damaged.
 
Mattax said:
All that said, you can focus on initial timing and tuning the idle first.
Then come back to the driving and wide open throttle stuff later.
Just have the discipline to keep your foot out of the pedal on any test drives. Thats the one way the engine can be seriously damaged.
Click to expand...
Wow! Great documents, thanks a lot this helps a bunch!
 
Agree with the other comments. Pri blades probably open too far, exposing too much t/slot. Engine is idling off increased exposure of t/slot & not the mixture screws, hence idle is best with mixture screws screwed in.
 
It's been said already but transfer slot/throttle blade position is very important. The fact that it doesn't die when turning the screws all the way in tells you it's getting extra fuel--from somewhere--so while you're in there you might put a new power valve in to rule out a blown/leaky one (and double check that it's seated against the gasket properly.)
 
beanhead said:
.........The fact that it doesn't die when turning the screws all the way in tells you it's getting extra fuel--from somewhere--.............
Click to expand...

This.

To the OP, first, the size of the motor has nothing to do with being able to tune your carb. Either you know how a carb works, or you don't. Just say it. Its fine, We're here to help.

What folks have said about the throttle blades and T-slots is generally correct and could be part or most of the problem, but does not answer why the throttle need to be open so far.

Probably the first thing you should check is the carb fuel levels, and that fuel is not coming out any of the boosters at idle.
 
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Blown/leaky power valve does not make the idle rich. Assuming the t/slot position is correct.
 
Bewy said:
Blown/leaky power valve does not make the idle rich. Assuming the t/slot position is correct.
Click to expand...
Incorrect
An 'open' PV won't enrich the idle, but if it's blown or leaking dumps fuel straight into the engine through the vacuum port....at idle or any other time...
 
So this weekend I did some reading thanks to Mattax and I had too much of the transfer slot exposed which is why it was running so rich. Ive leaned it out and now my idle mixture screws actually do what they're supposed to. I went for a strole around the neighborhood, nothing crazy and it drove fine except I still have a bog with any quick throttle input. And I don't mean WOT I mean if I just tap the throttle quickly (maybe 30 to 40% travle) it will bog. I checked the play in my accelerator pump arm and there is none. First I loosened the adjustment nut a half a turn and there was still no play in the spring so I thought maybe it was dumping too much fuel in but the bog seemed unchanged if not worse. So I went back to the starting position. I didn't want to tighten the spring at all without doing some more research as I know if it's too tight you can break the pump at WOT.

Edit: I'm realizing now that if the spring is pre-loaded( which it seems it is) I may be loosing some displacement of the accelerator pump making it less effective.
 
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Leave the pump shot lever spring alone.
See what accelerator pump cam is installed. They are color coded so you won't have to remove it to read the number.

However that said, its best to make sure the off idle is responding well with slower opening of the throttle. Begin with as slow as possible, then try it with more nromal but not jabbing.
It should respond smoothly with no hesitations or hiccups.

Make small changes in initial throttle position, timing, and idle mix. Keep notes of each change you make and whether it improves the idle and off idle power or not.

First, I would make note of how many turns of the idle speed screw it takes to open the throttles so .020" of transfer slot are exposed. Then how many turns to open them so .040" are exposed underneath. Write those down and you'll always know how much the transfer slots have been exposed. Outside of that range it is likely to have a flat spot or a delay.

I'd work with timing next. Measure the rpm and the timing. You can use timing as well as the throttle position to increase vacuum and rpm. So now is a good time to find out where the timing is, and also maybe check timing at 200 rpm intervals so you have some sense of the advance curve in the lower rpm ranges.

If you ever wondered what tuning was, this is it!
Its a lot different than setting it to factory specs type tuning!
Idle and off idle can be the more challenging than some people bargained for, but its really a matter of willingness to iteratively make changes and check results.
 
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Once you get the off idle OK, if there are still issues slowly adding power, say for an uphill at 35 mph, then it may be time to test smaller idle air bleeds. You can do that by sticking some wire in those bleeds. We can address that as needed.
 
Zachary Egnitz said:
So this weekend I did some reading thanks to Mattax and I had too much of the transfer slot exposed which is why it was running so rich. Ive leaned it out and now my idle mixture screws actually do what they're supposed to. I went for a strole around the neighborhood, nothing crazy and it drove fine except I still have a bog with any quick throttle input. And I don't mean WOT I mean if I just tap the throttle quickly (maybe 30 to 40% travle) it will bog. I checked the play in my accelerator pump arm and there is none. First I loosened the adjustment nut a half a turn and there was still no play in the spring so I thought maybe it was dumping too much fuel in but the bog seemed unchanged if not worse. So I went back to the starting position. I didn't want to tighten the spring at all without doing some more research as I know if it's too tight you can break the pump at WOT.

Edit: I'm realizing now that if the spring is pre-loaded( which it seems it is) I may be loosing some displacement of the accelerator pump making it less effective.
Click to expand...
Mattax helped me sort my bog out as well. My issue was both total timing (I had not advanced it enough) and carb adjustment. First thing I had to do was to get the timing resolved. After I did that, I worked on the carb like Mattax said. After adjusting the carb, the car ran great. Many time folks try to make carb mods when basic adjustments are all that are needed. Hope you get yours sorted out so you can enjoy it!
 
Zachary Egnitz said:
Hello all, I am trying to tune the carburetor on my 493 stroker but I have never worked with anything this big and neither has my dad nor my co-workers so I'm hoping I can get some insight from those of you with experience. Firstly here are the specs of the motor.
Engine:
493.41 cu. in.
10.89 compression
Bore 4.35
Stroke 4.15
JE SRP pistons
Eagle crank and rods
Trick flow 240 cylinder head
Edelbrock torker 2, single pane intake manifold
Holly 850 double pumper
1.6 rockers (Harland sharp)
Crane cam duration 234/242 at .050, lift .575/.595* (689551)
Howard lifters
Eagle crank and rods
Doug 2in headers
MSD distributor with mechanical and vacuum advance.
Here's the current situation. The car runs really well, starts well even when cold. Timing is ~32° at 1000 RPM (with the distributor vacuum line disconnected) and ~53° at 3000 rpm with vacuum advance. I'm pretty sure I'm running rich because my tail pipes are black and your eyes burn if you run it in the garage for more than a minute (even with the doors open). My fuel bowls are where they should be (right beneath the center line of the sight window). Here's where I get confused. When I tried setting my idle mixture screws I turned them in all the way and then slowly brought them out one half turn at a time until I got the highest rpm. However, the highest rpm occurs when they are screwed in all the way. When I unscrew them the rpm goes down. This led me to believe jets were too big however with the screws all the way in the engine bogs when i hit the throttle quickly. I took the screws out half a turn and the bog is less significant. I took them out another half turn and the bog is basically non exsitant and the car pulls well. Right now I have them 1.25 turns out and the secondaries are 1.5 turns out I believe and the car runs great, good throttle response, pulls well and doesn't bog.
To reiterate, I don't have much experience with engines this big but I find it odd that my rpms go down when I bring the screws out and my tail pipes are black but if I bring the screws in it bogs and looses power. Is this just normal for this size engine with the cam I have? Should I just keep it how it is? Should I put smaller jets in? Any input would be appreciated :)
Click to expand...
Mattax has you going in the right direction. When tuning any carb for the first time always get brand new plugs it in first, then get your timing at idle correct and you would want something in the low 20*s without vac advance with that cam imo. Then get the total set at about 35* without vac advance. Then get the tune on the mix screws and they with end up some where between 1-2 full turns out and all 4 should be exactly the same turns out. The idle blade screw will be about 1.5 turns out to get the t-slot exposed, you will have to take the the carb off and check. Adjust both pri and secondary t-slot with .020 exposure. If your mix screws need to be less than 1 turn out you will need to change the IFR to a smaller one. If the mix screws are not out enough you will have a stumble because these screws initialize the fuel transfer when you crack the throttle. Check your plugs for how its burning. should be fairly clean on the base circle on the shell of the plug and the ground strap should show the burn to the bend of the strap, if its burned to the weld its too much timing and if its on the tip not enough, on the bend is just right. Others things can influence that as well like different fuel as the ground strap is a heat indicator, but is does help to get timing close. Yes, also make sure the float level is in the middle of the glass with the new holleys and at the bottom if you have the brass plugs before you start. Hope this helps
 
bluefish said:
Mattax has you going in the right direction. When tuning any carb for the first time always get brand new plugs it in first, then get your timing at idle correct and you would want something in the low 20*s without vac advance with that cam imo. Then get the total set at about 35* without vac advance. Then get the tune on the mix screws and they with end up some where between 1-2 full turns out and all 4 should be exactly the same turns out. The idle blade screw will be about 1.5 turns out to get the t-slot exposed, you will have to take the the carb off and check. Adjust both pri and secondary t-slot with .020 exposure. If your mix screws need to be less than 1 turn out you will need to change the IFR to a smaller one. If the mix screws are not out enough you will have a stumble because these screws initialize the fuel transfer when you crack the throttle. Check your plugs for how its burning. should be fairly clean on the base circle on the shell of the plug and the ground strap should show the burn to the bend of the strap, if its burned to the weld its too much timing and if its on the tip not enough, on the bend is just right. Others things can influence that as well like different fuel as the ground strap is a heat indicator, but is does help to get timing close. Yes, also make sure the float level is in the middle of the glass with the new holleys and at the bottom if you have the brass plugs before you start. Hope this helps
Click to expand...

Thank you, I'm hoping this random April snow will go away so I can get back to it. Only have time on the weekends to work with it.
 
Took the time today to measure the Tslot and how many turns from closed I should be at. Also looked at my accelerator pump cam but I need some help determining which one it is cause I'm color blind... see attached. Here are my notes from today. Let me know if I'm on the right track or not. One thing that confused me was at one point my advance dropped significantly for no reason. I set it to
20 degrees and without changing anything ot moved to 9 degrees and yes my distributor was tight. I had to loosen it to reset it and as soon as I moved it the slightest bit timing jumped up to 30 degrees and I had to bring it back to 20. Not sure what the deal with that was but anyway here are my notes for the day...

.020 Tslot is 1 turn
.040 Tslot is 1.5 turns
Idle screws should be between 1 and 2 turns
____________________________________________
Timing: 20° at 700, 23° at 950, total Timing 39°
Current throttle position: 1.5 turns from closed
Idle 950
Idle screws are 1 turn out
Bogs
______________________
Timing: 13° at 850 20° at 970
Current throttle position 1.25 turns from closed
Idle 850
Idle screws 1 turn out
Bogs but better
__________________________
Timing : 20 at 720, 19 at 950
Current throttle position 1 turn from closed
Idle 720
Idle screws 1 turn out
Bog less so far
____________________________
Add 2 degrees Timing to last iteration (22)
Bog is worse
________________
Go to 18 degrees
Idle 680
Bog about same as at 20 degrees so going back
_____________
Timing : 20 at 720, 19 at 920
Current throttle position 1 turn from closed
Idle 500
Idle screws 1.25 turn out
Bog is almost gone but Idle is very low
___________
Turn throttle to 1.25 turns
Idle 700
Least bog so far but Idle dropped to 600
Timing moved to 9 at 600????
________________
Timing : 20 at 720, 19 at 920
Current throttle position 1 turn from closed
Idle 500
Idle screws 1.25 turn out
Bog present but not bad

20210417_155352.jpg
 
Zachary Egnitz said:
Took the time today to measure the Tslot and how many turns from closed I should be at. Also looked at my accelerator pump cam but I need some help determining which one it is cause I'm color blind... see attached. Here are my notes from today. Let me know if I'm on the right track or not. One thing that confused me was at one point my advance dropped significantly for no reason. I set it to
20 degrees and without changing anything ot moved to 9 degrees and yes my distributor was tight. I had to loosen it to reset it and as soon as I moved it the slightest bit timing jumped up to 30 degrees and I had to bring it back to 20. Not sure what the deal with that was but anyway here are my notes for the day...

.020 Tslot is 1 turn
.040 Tslot is 1.5 turns
Idle screws should be between 1 and 2 turns
____________________________________________
Timing: 20° at 700, 23° at 950, total Timing 39°
Current throttle position: 1.5 turns from closed
Idle 950
Idle screws are 1 turn out
Bogs
______________________
Timing: 13° at 850 20° at 970
Current throttle position 1.25 turns from closed
Idle 850
Idle screws 1 turn out
Bogs but better
__________________________
Timing : 20 at 720, 19 at 950
Current throttle position 1 turn from closed
Idle 720
Idle screws 1 turn out
Bog less so far
____________________________
Add 2 degrees Timing to last iteration (22)
Bog is worse
________________
Go to 18 degrees
Idle 680
Bog about same as at 20 degrees so going back
_____________
Timing : 20 at 720, 19 at 920
Current throttle position 1 turn from closed
Idle 500
Idle screws 1.25 turn out
Bog is almost gone but Idle is very low
___________
Turn throttle to 1.25 turns
Idle 700
Least bog so far but Idle dropped to 600
Timing moved to 9 at 600????
________________
Timing : 20 at 720, 19 at 920
Current throttle position 1 turn from closed
Idle 500
Idle screws 1.25 turn out
Bog present but not bad

View attachment 1715725405
Click to expand...
Is your distributor new? sounds like the mechanical advance is hanging up. Could be the weights are sticking, springs, etc. If the distributor does not provide consistent mechanical advance you will be chasing your tail. The cam in your picture looks like mine, which is the stock pink one.
 
Ricks70Duster340 said:
Is your distributor new? sounds like the mechanical advance is hanging up. Could be the weights are sticking, springs, etc. If the distributor does not provide consistent mechanical advance you will be chasing your tail. The cam in your picture looks like mine, which is the stock pink one.
Click to expand...
Yea it's a brand new distributor which is a little annoying...
 
Zachary Egnitz said:
Took the time today to measure the Tslot and how many turns from closed I should be at. Also looked at my accelerator pump cam but I need some help determining which one it is cause I'm color blind... see attached. Here are my notes from today. Let me know if I'm on the right track or not.
Click to expand...

looks like the stock pink cam, i wouldn't mess with that just yet.
make sure you are resetting idle mix each time you make changes and make sure the arm that pushes your accelerator pump is tight against the pump arm. sometimes idle screw adjustment will create some unwanted clearance at the accel pump.
set idle mix with the engine at operating temp, gradually lean all idle scerws out until you get the highest idle or most vacuum and then go back maybe 1/8th turn rich...
 
I'll log in some time tommorrow to spend more time on what your notes.
Zachary Egnitz said:
One thing that confused me was at one point my advance dropped significantly for no reason. I set it to
20 degrees and without changing anything ot moved to 9 degrees and yes my distributor was tight. I had to loosen it to reset it and as soon as I moved it the slightest bit timing jumped up to 30 degrees and I had to bring it back to 20. Not sure what the deal with that was but anyway here are my notes for the day...
Click to expand...

This sure sounds like something isn't right in the mechanical advance. I think it would be worthwhile looking at the advance mechanism. It sounds like there is slack in the advance springs.

Pink cam provides the one of the least amounts of initial pump shot. If nothing it else it will be useful for testing the gradual opening of the throttles from idle.

I do not worry much about how many turns in or out the idle mix screws are. Most important is they provide a control.
 
Zachary Egnitz said:
.020 Tslot is 1 turn
.040 Tslot is 1.5 turns
Click to expand...
Interesting. That's pretty sensitive (1/2 turn of the screw). I'll check a 4779 double pumper next time I'm at the garage.

I was pretty tired last night. Once I started looking at your notes I completely agree there is an issue with the distributor.
upload_2021-4-20_19-41-56.png


On an MSD, use springs in pairs so the tension on both weights is the same.
Timing will advance in a fairly straight line.
There should be enough initial tension so the initial timing stays pretty much the same up to the actual idle rpm.
If the engine is idling at 950 rpm, then when its put into gear (assuming automatic transmission) the goal is the least drop in rpms. But if it does drop a little, and it probably will, its important the timing stays the same. Otherwise manifold vacuum will go down with the reduction in timing. With less vacuum, less fuel pulled through the idle systemswhen if anything it needs a little richer when placed in gear.

Something like the solid green line would be reasonable.
And then when experimenting with a little less initial, the whole advance shifts down as illustrated by the dashed line.
upload_2021-4-20_20-4-30.png


Leave the vacuum advance disconnected for now, but one of the reasons for using a slow rate of advance is planning for the vacuum advance to be used.
 
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