Can vacuum and mechanical advance curve tuning help my lopey cam bucking issue? - Page 2 - Hot Rod Forum : Hotrodders Bulletin Board
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  #16 (permalink)  
Old 10-25-2012, 02:45 PM
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Nope i was wrong holley 750 dp uses drilled plates not notched ones. Drilled should be fine for the way the idle circuit works.

You are correct with the drilled plates. Sorry had to research the carb to find out. Didnt want to talk out my arse.

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Old 10-25-2012, 03:18 PM
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Okay, I checked the vacuum can plugged into full manifold. It's adding 8* at idle and ~14* all in.

18* initial
36* total
WITH VACUUM
26* initial
~50* total

I thought the vacuum can should be giving all of it's advance at idle. Anyone else think it's weird that both distributors I bought for this engine came preset for 18* mechanical advance? And if it did, why isn't the vacuum can also preset for low vacuum situations?

Am I doing something completely wrong? Here is the dizzy I have now.
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Old 10-25-2012, 03:24 PM
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Quote:
Originally Posted by Zerocyde View Post
Okay, I checked the vacuum can plugged into full manifold. It's adding 8* at idle and ~14* all in.

18* initial
36* total
WITH VACUUM
26* initial
~50* total

I thought the vacuum can should be giving all of it's advance at idle. Anyone else think it's weird that both distributors I bought for this engine came preset for 18* mechanical advance? And if it did, why isn't the vacuum can also preset for low vacuum situations?

Am I doing something completely wrong? Here is the dizzy I have now.
Thats way to much vacum advance. Does it have two holes where the little rod can go or a piece that pops out and can be tunred so you get less total. Also little flat index plate that can be moved to stop advance motion it should be full in or one back from full.
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Old 10-25-2012, 03:27 PM
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Quote:
Originally Posted by Custom10 View Post
I agree the gears are not right,,,

For some timing help,,,First off the thumpers like manifold vacuum adv, 30+ deg at idle so if you have the vacuum adv at the desired 10-12 limit then you need a initial at 20-22. May need to limit the mech advance if you bump up the initial so you dont go past 36. However initial becomes much less significant if you run manifold vacuum adv but the vacuum advance cant be more than 14 deg of the equation so do the math. Also too high an initial and it will buck the starter when hot.

Check how much the vacuum adv is adding at idle with manifold vac, this will tell you where you are at. Let us know

Shot for no more than 48-50 full advance initial + mech + vacuum adv. "bucking" or surging can be caused by too much advance,

Also try and jet up, maybe the AF ratio is lean? get that AF meter yes good plan.
This is the correct setup. No need to retype. More int. Less vacum and mech. But have the mech come in sooner lightest springs in the kit usally.
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Old 10-25-2012, 03:30 PM
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Quote:
Originally Posted by ap72 View Post
I think he's saying he readjusts the carb when its on manifold vacuum and when he tips in it dies-
What I'm saying is, hooked up to full manifold, when the idle dips down real low it has a hard time bringing itself back up to regular idle.

Like when coming to a stop or something that can lower the idle, if the idle falls down to ~500, it struggles to bring itself back up to ~1000 (where I have it set to idle). It seems like, with full manifold on the vacuum can, the car is depending on that vacuum advance to stay alive, and when something (like brake booster) drains the limited vacuum that turns the vacuum advance to zero, causing the idle to struggle to come back up and turn the vacuum advance back on and then idle correctly.

Also, let me re-state to be clear:

WITHOUT VACUUM: 18* initial, 36* total.
WITH VACUUM: 26* initial, ~50* total.

That means the vacuum is adding ~8* at idle, and ~14* over 3k rpm. Isn't a vacuum advance can supposed to be giving it's full advance at idle?
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Old 10-25-2012, 03:41 PM
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Quote:
Originally Posted by Zerocyde View Post
What I'm saying is, hooked up to full manifold, when the idle dips down real low it has a hard time bringing itself back up to regular idle.

Like when coming to a stop or something that can lower the idle, if the idle falls down to ~500, it struggles to bring itself back up to ~1000 (where I have it set to idle). It seems like, with full manifold on the vacuum can, the car is depending on that vacuum advance to stay alive, and when something (like brake booster) drains the limited vacuum that turns the vacuum advance to zero, causing the idle to struggle to come back up and turn the vacuum advance back on and then idle correctly.

Also, let me re-state to be clear:

WITHOUT VACUUM: 18* initial, 36* total.
WITH VACUUM: 26* initial, ~50* total.

That means the vacuum is adding ~8* at idle, and ~14* over 3k rpm. Isn't a vacuum advance can supposed to be giving it's full advance at idle?
What about the mechanical advance. It should be giving some above 3k.
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Old 10-25-2012, 04:12 PM
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Quote:
Originally Posted by Zerocyde View Post
What I'm saying is, hooked up to full manifold, when the idle dips down real low it has a hard time bringing itself back up to regular idle.

Like when coming to a stop or something that can lower the idle, if the idle falls down to ~500, it struggles to bring itself back up to ~1000 (where I have it set to idle). It seems like, with full manifold on the vacuum can, the car is depending on that vacuum advance to stay alive, and when something (like brake booster) drains the limited vacuum that turns the vacuum advance to zero, causing the idle to struggle to come back up and turn the vacuum advance back on and then idle correctly.

Also, let me re-state to be clear:

WITHOUT VACUUM: 18* initial, 36* total.
WITH VACUUM: 26* initial, ~50* total.

That means the vacuum is adding ~8* at idle, and ~14* over 3k rpm. Isn't a vacuum advance can supposed to be giving it's full advance at idle?
If your idle is dropping that much at a stop light you obviously need a higher stall in your transmission. Getting it tuned right will help some but you will always have that problem until you get a decent converter. If you do some research you may be able to use a higher stall from a different car that you can pick up from a junk yard for cheap.

you never did answer to what stall you have meaning its likely stock and likely (but not necessarily) really low. NOT ALL FACTORY TC's ARE THE SAME!!!
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Old 10-25-2012, 04:39 PM
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Quote:
Originally Posted by ap72 View Post
If your idle is dropping that much at a stop light you obviously need a higher stall in your transmission. Getting it tuned right will help some but you will always have that problem until you get a decent converter. If you do some research you may be able to use a higher stall from a different car that you can pick up from a junk yard for cheap.

you never did answer to what stall you have meaning its likely stock and likely (but not necessarily) really low. NOT ALL FACTORY TC's ARE THE SAME!!!
Sorry, I have a manual. Any advice about my vacuum can findings?
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Old 10-25-2012, 04:59 PM
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Quote:
Originally Posted by Zerocyde View Post
What I'm saying is, hooked up to full manifold, when the idle dips down real low it has a hard time bringing itself back up to regular idle.

Like when coming to a stop or something that can lower the idle, if the idle falls down to ~500, it struggles to bring itself back up to ~1000 (where I have it set to idle). It seems like, with full manifold on the vacuum can, the car is depending on that vacuum advance to stay alive, and when something (like brake booster) drains the limited vacuum that turns the vacuum advance to zero, causing the idle to struggle to come back up and turn the vacuum advance back on and then idle correctly.

Also, let me re-state to be clear:

WITHOUT VACUUM: 18* initial, 36* total.
WITH VACUUM: 26* initial, ~50* total.

That means the vacuum is adding ~8* at idle, and ~14* over 3k rpm. Isn't a vacuum advance can supposed to be giving it's full advance at idle?
You need a canister that will be fully pulled in by 7-8 "hg your idle vacuum is 9 "hg correct?

What is the duration of the cam?, what is the vacuum level at crusie speed? get a gage for the inside of the car, extend the hose or what ever, what is the starting point of movement of rod travel in "hg vacuum of the cansiter? what is the end point of rod travel in "hg?

Set it to start to move at 3-4 "hg, get it to stop moving at 7-8 "hg, limit the added advance to 12-14 at the crank. If it is not adjustable get one that is, crane or accel maybe MSD.
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Old 10-25-2012, 05:02 PM
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Got any pix of the dizzy with the cap off. Showing the vacum adavnce and wieghts. I might be able to help. Also any parts you have left over it says it comes with the recurve kit. Should be a bunch springs and parts.
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Old 10-25-2012, 05:11 PM
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Zerocyde,
Just my two cents. Try disconnecting the vacuum adv. and plugging the carb port, and take it for a short test drive. Keep it short because it will be retarded somewhat. Make sure its allready hot before the test.
If the bucking/surging stops you know its probably too much advance from vacuum can. Then get a vacuum limiter plate that Crane sells and install it, and try limiting your vac. adv. as someone else suggested. Its a pretty cheap
test that you can get rid of if you don't like it. You'll also be able to use it later after you change your gears to optimize your mileage. You can also use it with a adj. vac. can.
Usually a good sign of too much vac. adv. is surging when deccelarating. Forgive the spelling. What happened to the spell checker?
I'd leave it connected to ported vac in you current tune IMO.
FWIW
ssmonty
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Old 10-25-2012, 05:19 PM
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Quote:
Originally Posted by Zerocyde View Post
Sorry, I have a manual. Any advice about my vacuum can findings?
so in neutral it just drops down to 500rpm when you slow down? you have carb problems.
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Old 10-25-2012, 08:09 PM
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Quote:
Originally Posted by ap72 View Post
so in neutral it just drops down to 500rpm when you slow down? you have carb problems.
No, only when doing something that uses up the limited vacuum like braking after engine braking or something like that.

Quote:
Originally Posted by hcompton View Post
Got any pix of the dizzy with the cap off. Showing the vacum adavnce and wieghts. I might be able to help. Also any parts you have left over it says it comes with the recurve kit. Should be a bunch springs and parts.
I know it has a vac lockout piece, but the canister itself isn't adjustable I don't think.

Quote:
Originally Posted by Custom10 View Post
You need a canister that will be fully pulled in by 7-8 "hg your idle vacuum is 9 "hg correct?

What is the duration of the cam?, what is the vacuum level at crusie speed? get a gage for the inside of the car, extend the hose or what ever, what is the starting point of movement of rod travel in "hg vacuum of the cansiter? what is the end point of rod travel in "hg?

Set it to start to move at 3-4 "hg, get it to stop moving at 7-8 "hg, limit the added advance to 12-14 at the crank. If it is not adjustable get one that is, crane or accel maybe MSD.
So, if mine is adding 8* at idle, and 14* at 3k, then does that mean it's starting to advance under 9hg and finishing sometime after? Okay, so how do I adjust where it starts to advance and where it is fully advanced? Is that the alan wrench down the tube adjustment?
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Old 10-25-2012, 08:23 PM
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There is lockout for Mechanical advance and vacum advance. Both need to be set very restrictive and go for more initial.

You can close off the vacum line and setup with out vacum advance and see if it will help smooth things out better. Just to get the hole vac can system out of the equation and see if it needs to be tuned. It may turn out to not effect the lope at all.
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Old 10-25-2012, 08:38 PM
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Quote:
Originally Posted by Zerocyde View Post
No, only when doing something that uses up the limited vacuum like braking after engine braking or something like that.



I know it has a vac lockout piece, but the canister itself isn't adjustable I don't think.



So, if mine is adding 8* at idle, and 14* at 3k, then does that mean it's starting to advance under 9hg and finishing sometime after? Okay, so how do I adjust where it starts to advance and where it is fully advanced? Is that the alan wrench down the tube adjustment?
yes possibly it needs more than 9 to finish advancing, that is why I suggested to first see what the motor pulls under different conditons and rpm then see what the range and starting/stopping points are on the can. You can then decide what you need to do with it.

yes the range is set with an allen key via a set screw through the hose nipple opening, some have it some dont, I think the size is 3/32 or there abouts.
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