493 carburetor tuning question

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Mattax said:
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.
View attachment 1715726265

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.
View attachment 1715726279

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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So I pulled the distributor today and I couldn't find anything wrong with the springs or weights. They have no slack, consistent tension between the two and they move smoothly. Just for the sake of it I lubricated them with some white lithium. Put it back together and started the car. After warming up I set up a timing light and without touching anything I could watch the timing move anywehre from 20 degrees to 32 degrees. So I pulled it again and looked more closely and there were some markings on the shaft on the distributor where it goes into the drive gear. (See photos) they are on opposite sides as if it was slapping in the slot of the drive gear. So I pulled the gear and took some measurements. With the gear fully engaged 3/8ths or .375 inches of the distributor shaft engage with the gear. I took measurements and being generous I found the MOST the gear is engaging is actually .226 inches. Which causes some some play in the mechanism and most likely caused the marking in the picture. I think this explains my inconsistent mechanical advance. My question is do I get a different gear or do I need to bite the bullet and get a different distributor. Obviously if I get a gear with a longer shaft it may not engage on the cam as much since it will be sitting .149 inches higher supposedly...

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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 :)
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The best way to adjust your carb is to use a vacuum gauge and adjust til you get the highest vacuum. Your power valve may not be the correct one and causing it to idle rich.You need to find out what the vacuum reading is on your engine then install the correct power valve if needed.
 
Zachary Egnitz said:
So I pulled the distributor today and I couldn't find anything wrong with the springs or weights. They have no slack, consistent tension between the two and they move smoothly. Just for the sake of it I lubricated them with some white lithium. Put it back together and started the car. After warming up I set up a timing light and without touching anything I could watch the timing move anywehr3 from 20 degrees to 32 degrees. So I pulled it again and looked move closely and there were some markings on the shaft on the distributor where it goes into the drive gear. (See photos) they are on opposite sides as if it was slapping in the slot of the drive gear. So I pulled the gear and took some measurements. With the gear fully engaged 3/8ths or .375 inches of the distributor shaft engage with the gear. I took measurements and being generous I found the MOST the gear is engaging is actually .226 inches. Which leads to some play in the and most likely caused the marking in the picture. I think this explains my inconsistent mechanical advance. My question is do I get a different gear or do I need to bite the bullet and get a different distributor. Obviously if I get a gear with a longer shaft it may not engage on the cam as much since it will be sitting .149 inches higher supposedly...

View attachment 1715728080

View attachment 1715728081
Click to expand...


If the timing is moving like that you have the wrong springs for the weights or you have a timing chain issue. Or both.
 
Could be the wires to the amplifier are reversed. Assuming this isn't an RTR model.
1/4" of engagement of the tab in the slot should be enough. Doesn't sound like you've got a B in an RB block. I
'll leave that for the guys who have built B/RB engines.

Up down motion of the distributor drive shaft should be zero.
The intermediate gear moving up down can cause an apparent shift in timing because of the helical gear. 12* seems like a lot for that. 3-4* OK maybe.
Same with the distributor tab into the slot of the gear. If you place the distributor in the engine, can you move the rotor 6 degrees?
 
Mattax said:
Could be the wires to the amplifier are reversed. Assuming this isn't an RTR model.
1/4" of engagement of the tab in the slot should be enough. Doesn't sound like you've got a B in an RB block. I
'll leave that for the guys who have built B/RB engines.

Up down motion of the distributor drive shaft should be zero.
The intermediate gear moving up down can cause an apparent shift in timing because of the helical gear. 12* seems like a lot for that. 3-4* OK maybe.
Same with the distributor tab into the slot of the gear. If you place the distributor in the engine, can you move the rotor 6 degrees?
Click to expand...

I don't have a protector but I took some rough measurements and did some maths and it probably has around 3 degrees of play in the engine.
 
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