Vacuum Question

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RPMagoo

Just An Old Motorhead
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I have a early (66) 383 Mopar, pretty much stock, and just installed a Edelbrock 1406 carburetor. Timing set at 15 and 36 degrees. The motor had a flutter under slight acceleration between 1500 and 2200 RPM, with the vacuum hose connected to the "Timed Vacuum" port on the passenger side of the carb. -- I moved it to the "Manifold Vacuum" port on the driver side of the carb, and readjusted the idle, downward. I also moved the accelerator pump bracket from the middle hole to the hole nearest to the carb body (increased the squirt), and the flutter went away.. -- I moved the pump bracket 1st - slight improvement -- then switched the vacuum hose - presto, problem gone. -- Timing, vacuum, and carburetors are something that I am not very adept at, so please don't shoot Me. There are many different opinions regarding where to hook the vacuum to the carb. -- Have I done something wrong, and just got lucky -- or -- ???
 
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I might know a thing or two on carbs. Just saying. Lol

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If its pretty much stock, then use the factory as baseline. They had figured out a thing or two about the timing curves.
I don't have a '66 book but do have a '67. I can tell you the mechanical advance for the 2 bbl looks noticibly different than the 4 bbl.
15* initial at 600 rpm would be about as much advance as would be wanted for a stock cam'd 383. So no, don't add manifold source for th evac advance on top of that.
 
If its pretty much stock, then use the factory as baseline. They had figured out a thing or two about the timing curves.
I don't have a '66 book but do have a '67. I can tell you the mechanical advance for the 2 bbl looks noticibly different than the 4 bbl.
15* initial at 600 rpm would be about as much advance as would be wanted for a stock cam'd 383. So no, don't add manifold source for th evac advance on top of that. --
-- I understand that , but, why did the stumble go away ? -- What's the danger ?
 
Not really sure what you did there with the carb adjustments, but the fact it needed more pump shot suggests you made it leaner. I'll guess that adding timing to the 15* at idle made it a better match for the mix. How much timing was added? 5*, 10*, 15*? No real danger, just less efficient burn in relation to pushing the piston down the hole with the most leverage.

Its better to get the timing about right, and then adjust the carb. Even using the factory specs, there's some expected adjustment. Say its 10* at 600, well then 12 or 14* would still be in expected range. But 10 + 15* is well beyond the range. Yes it will run at 25*. And I don't know if its 15*, just throwing that out there as an example.

Timing is about firing the mix so the burn develops maximum pressure at through the most efficient position of the piston. How long a mix takes to burn and develop pressure varies with combustion chamber, compression and heat, as welll rpm (hence the mechanical advance) and mixture (hence the vacuum advance).
 
Not really sure what you did there with the carb adjustments, but the fact it needed more pump shot suggests you made it leaner. I'll guess that adding timing to the 15* at idle made it a better match for the mix. How much timing was added? 5*, 10*, 15*? No real danger, just less efficient burn in relation to pushing the piston down the hole with the most leverage.

Its better to get the timing about right, and then adjust the carb. Even using the factory specs, there's some expected adjustment. Say its 10* at 600, well then 12 or 14* would still be in expected range. But 10 + 15* is well beyond the range. Yes it will run at 25*. And I don't know if its 15*, just throwing that out there as an example.

Timing is about firing the mix so the burn develops maximum pressure at through the most efficient position of the piston. How long a mix takes to burn and develop pressure varies with combustion chamber, compression and heat, as welll rpm (hence the mechanical advance) and mixture (hence the vacuum advance).
 
No adjustments to the carb, other than a slight adjustment to the 2 idle mixture screws - very near to the edelbrock settings. - Timing is set at 15* at 900 RPM, vac disconnected. -- (1) Does all that I've done explain why the stumble went away ? -- (2) Should I put the vacuum tube back onto the ported side of the carb and be considering some changes to the carb instead ? -- The car is a 3,000 lb Street Rod - Stock Convertor - Auto Trans - 3.54 gears. -- Runs fine, as is, up to 80+ MPH - tempted to leave it as is, but don't want to cause other problems.
 
(1) Does all that I've done explain why the stumble went away ? -- (2) Should I put the vacuum tube back onto the ported side of the carb and be considering some changes to the carb instead ?
Not enough info to answer (1). It would be more guessing and speculation. Post #7 is as far as I'll go with that.
As far as (2), in general yes, but again not enough info. See post #5.
The following gives us the possiblility of 2 or 3 engine configurations.
I have a early (66) 383 Mopar, pretty much stock
and this suggests it may be none of the above.
is a 3,000 lb Street Rod

Timing set at 15 and 36 degrees.
To know the curve we need the RPMs and at least a few in between points. If its the original distributor that came with the engine, the specs are in the shop manual - so that can be estimated. But most guys who street rod put something else in. So who knows.

The motor had a flutter under slight acceleration between 1500 and 2200 RPM, with the vacuum hose connected to the "Timed Vacuum" port on the passenger side of the carb. -- I moved it to the "Manifold Vacuum" port on the driver side of the carb, and readjusted the idle, downward. I also moved the accelerator pump bracket from the middle hole to the hole nearest to the carb body (increased the squirt), and the flutter went away.. -- I moved the pump bracket 1st - slight improvement -- then switched the vacuum hose - presto, problem gone.
Changed too many things to guess what effected what. Very light acceleration should be fueled by the "idle circuit" which includes the transfer port. Pump shot helps cover the delay when the throttle is moved quickly while on the "idle circuit".

No adjustments to the carb, other than a slight adjustment to the 2 idle mixture screws - very near to the edelbrock settings.
Who ya kidden? :poke: LOL
You adjusted the base idle mix. "a slight adjustment to the 2 idle mixture screws"
Maybe did something with the thottle position "and readjusted the idle, downward"
Changed the pump shot twice "moved the accelerator pump bracket from the middle hole to the hole nearest to the carb body"

Timing is set at 15* at 900 RPM, vac disconnected.
Why is it idling at 900 rpm with a stock engine?

Runs fine, as is, up to 80+ MPH - tempted to leave it as is, but don't want to cause other problems
Can't tell whether you're interested in tuning, or just happy with it the way it is. If you're happy, enjoy. If you enjoy tuning, then keep working on it.
 
Changing the position on the accelerator pump arm usually is something that's done to an older carburetor to get it to function better. I had several stumbles and things due to the wrong size Jets and definitely due to not having strong enough Springs in the metering rods. The right weight spring and metering Rod helps a lot especially if they're is a performance cam or something in it.
You could have had a blocked passage or bad gas or something? But the accelerator pump is basically just for tip in. Not at the 1500- 2200 RPM Mark where you're talkin. Possibly in lieu of needing a bigger jet or smaller metering rod you're over use of the accelerator pump maybe a making up the difference?
 
-- _ _ _ and definitely due to not having strong enough Springs in the metering rods. _ _ _
-- I talked to Edelbrock Tec., and they said to switch to pink springs - currently yellow. -- I ordered a spring kit from Summit.
 
-- None around here. I checked them all. -- I'm in The Boonies.
I like the boonies! LOL Yeah our local Baxter's usually has quite a nice selection of jets metering rods and definitely that spring set.
 
I have a early (66) 383 Mopar, pretty much stock, and just installed a Edelbrock 1406 carburetor. Timing set at 15 and 36 degrees. The motor had a flutter under slight acceleration between 1500 and 2200 RPM, with the vacuum hose connected to the "Timed Vacuum" port on the passenger side of the carb. -- I moved it to the "Manifold Vacuum" port on the driver side of the carb, and readjusted the idle, downward. I also moved the accelerator pump bracket from the middle hole to the hole nearest to the carb body (increased the squirt), and the flutter went away.. -- I moved the pump bracket 1st - slight improvement -- then switched the vacuum hose - presto, problem gone. -- Timing, vacuum, and carburetors are something that I am not very adept at, so please don't shoot Me. There are many different opinions regarding where to hook the vacuum to the carb. -- Have I done something wrong, and just got lucky -- or -- ???
Depends on your Vcan, and when the 36 was in by; but I'm pretty sure the engine didn't like the total timing it was seeing, and you may have interpreted detonation as flutter.
For instance if the centrifugal begins advancing at 1000 and is all in by 2800, then it is advancing 21* in 1800 rpm, or 1.67degrees per 100rpm. Therefore at 1500, she would be seeing 15* plus 5x1.67 plus the can which could be as little as 9 or as much as 22. So that totals 32* to 45*at light throttle. By 2200 it looks like 15* plus 12x1.67 plus 9 to 22= 44* to 57*, at light throttle.
These numbers are probably too high for a stock iron headed engine

Bottom line is this; if you can change the severity of the flutter by changing the amount of throttle, or if it disappears with a defeated Vcan....... then the flutter was probably detonation.
And you want to avoid detonation at any cost, cuz detonation can break your pistons pretty quick.

It seems there are some guys here on FABO, that are Gung-Ho to throw timing at every engine they come across. EDIT;In your case, this might be a prime example of what not to do.
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The factory timings on a 69 383-2bbl automatic,The only FSM I have, were pretty aggressive. I see it at; 7.5 initial, and 2 to 12 at 1000,16 to 20 at 1500,34 to 38 at 4600, and with 21 to 27 in the Vcan by 13.5 inches vacuum. The 4bbl specs are a bit tamer
IMO, IMO,IMO, it is better to be 3 to 5 degrees retarded at all points, even 3 short at WOT, than even just 1 too many. On the street, at WOT, I can almost guarantee you,that your butt-dyno will not be able to measure perhaps even 3 degrees short of optimum.
So, again, IMO, don't be in a hurry to throw timing at it, a mistake can be very expensive. Sneak up on it.
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EDIT; I only tune SBMs, and only for street.
I'm not against throwing timing at some combos, cuz it can and does work.
SBMs,with automatics and hi-stalls,almost anything goes.
But I have come across many cases of stock type engines when throwing timing at it doesn't work.
And street engines, with manual transmissions is a special case, requiring discretion.
But you need to keep in mind too, that a lot of initial timing,tames the lope in the cam, and everybody wants to hear the lope.
So if you had a hi-compression360 with a stick, and a 270 cam, and you throw 22* into it at idle.......your precious lope will be as good as gone. And the carb tune you put into it, with a poor-choice of gears, will tend to make it very jumpy at low rpms, and parading it will be impossible. Whereas with an automatic, most of that jumpiness will be absorbed by the fluid coupling.
But if you were to back the timing down to 5*, you would hear that lope pretty good. And the jumpiness would be pretty much gone. This is because your peak cylinder pressure will occur when the piston is much further down the bore, and the strength of that pulse will be much reduced.
Of course, 5* is pretty lazy, but that's sometimes what it takes, especially for a VERY-high compression stick car with hiway gears, and he wants to parade it (me).
The trick then, is to bring the advance curve in hard and fast to play catch up by mid 2000, and still hit the power-timing by the early to mid 3000s. And take care of the Part Throttle timing with the Vcan., to make up the difference.
Without computer controlled timing, it is impossible to hit all the marks, so compromise is always tempering the choices. In a stick-car;I tend to sacrifice all power-timing below about 2000rpm, to favor part-throttle response. I also tend to sacrifice initial timing for PT response. Cuz face it; probably 95% of the engines life is spent in PT operation. I don't care about idle-lope, and with a hi-C 360 my butt-dyno cannot tell if the power timing is 3* short of what the engine dyno says is perfect. My tunes rely heavily on the Vcan.
But I can sure tell if the engine is;lazy or jumpy, or Snapppppy.
 
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Depends on your Vcan, and when the 36 was in by; but I'm pretty sure the engine didn't like the total timing it was seeing, and you interpreted detonation as flutter.
For instance if the centrifugal begins advancing at 1000 and is all in by 2800, then it is advancing 21* in 1800 rpm, or 1.67degrees per 100rpm. Therefore at 1500, she would be seeing 15* plus 5x1.67 plus the can which could be as little as 9 or as much as 22. So that totals 32* to 45*at light throttle. By 2200 it looks like 15* plus 12x1.67 plus 9 to 22= 44* to 57*, at light throttle.
These numbers are probably too high for a stock iron headed engine

Bottom line is this; if you can change the severity of the flutter by changing the amount of throttle, or if it disappears with a defeated Vcan....... then the flutter was probably detonation.
And you want to avoid detonation at any cost, cuz detonation can break your pistons pretty quick.

It seems there are quite a few guys here on FABO, that are Gung-Ho to throw timing at every engine they come across. Here may be a prime example of what not to do.
The factory timings on a 69 383-2bbl automatic,The only FSM I have, were pretty aggressive. I see it at; 7.5 initial, and 2 to 12 at 1000,16 to 20 at 1500,34 to 38 at 4600, and with 21 to 27 in the Vcan by 13.5 inches vacuum. The 4bbl specs are a bit tamer
IMO, IMO,IMO, it is better to be 3 to 5 degrees retarded at all points, even 3 short at WOT, than even just 1 too many. On the street, at WOT, I can almost guarantee you,that your butt-dyno will not be able to measure perhaps even 3 degrees short of optimum.
So, again, IMO, don't be in a hurry to throw timing at it, a mistake can be very expensive. Sneak up on it.
All true A/J.
But here's the thing with those '69 specs, they're for a CAP engines, that's why there is a very fast primary curve. All CAP/CAS curves look like that when compared to their non CAP counterparts. The CAP engines used much less initial and then above idle the mechanical advance quickly made up for this. (There's other threads on this, so if you want to discuss, lets take it to one of them).
Without knowing the info requested in post #11, we're just throwing examples out that may or may not be directly applicable. (The principles you state are totally valid - so there's no arguement about that.)
 
-- I talked to Edelbrock Tec., and they said to switch to pink springs - currently yellow. -- I ordered a spring kit from Summit. --
-- Got them -- put them in -- Vac tube on the passenger side, of carb -- didn't mess with the timing -- runs smooth. -- Thanks to All, for Your input.
 
Sounds like it just needed a little more fuel in the transfer.... Glad it feels better...
Lucky the car is light and not heavy, so its a little more forgiving...
Now.... Go burn some rubber !!!!!! And tell us how it really runs..... :)
 
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