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  #1  
Old Dec 23rd, 12, 05:11 PM
Boondie Boondie is offline
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Mark
 
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Default BBC timing advice

Guys I have a friend coming over to help tune my motor, any advice or tips ?
468 Howards hyd roller 290 296 237 243 .634 .640 112 108 part number 120265-12, Brodix heads quickfuel 750 dp.

Any help would be appreciated

Cheers
Merry xmas
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  #2  
Old Dec 23rd, 12, 07:36 PM
Eleanor's Nemesis Eleanor's Nemesis is offline
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Michael Gekko
 
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Default Re: BBC timing advice

I'm guessing this is a street car.....make sure you have a vacuum advance if it is, and use manifold vacuum as opposed to ported vacuum.
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  #3  
Old Dec 23rd, 12, 11:02 PM
Boondie Boondie is offline
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Default Re: BBC timing advice

Yeah going to hook up a vacuum gauge to the manifold, dont have any vacuum hookups at all, and yes street car
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  #4  
Old Dec 24th, 12, 02:21 PM
Steptoe Steptoe is offline
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Default Re: BBC timing advice

Dial in the secondary butterflys so the primary idle speed screw and mixture screws are 1 1/2 turns +/- 1/2 turn at about 16 deg advancetart with getting the AFR s dialed in ball park...
Set AFRs in ball park
idle in 15s with about 16 deg advance
cruise open road 15s with about 40 deg advance
full load above 3500 rpms mid 12s about 35 degs
lock the timing to do that.


Asuming have no EGR or pollution addons...manifold VA.
Then set the timing to ball park..
intial approx 8 to 12 degs
idle (intial +VA) 14 to 18 depending on cam
total (intial +cent) all in about 3200 rpms between 32 and 36 degs
All up...intial+cent+VA do not go above 42 degs unless tuning with a knocksensor.
Setting the cent start over advanced... 36/38 degs the work backwards till power economy drops off ... will do so notiacly withing 2 to 3 degs ans advance up a couple degrees only... DO NOT start under advanced and advance up

The VA must be all in at no more than 1" above idle vaccuum with no VA conected but mixtures adjusted at the intial advance to best idle (that is a temp setting to establish the top vaccuum spec for the VA only)
Generally ball park a VA working between approx 5 to 7 inches to 11 to 13 " is what u will need with around 7 to 9 degs in it...

Do not confuse "tuning" and "dialing" an engine in
Dialing an engine in is establishing the tuning specs, like u see in the handbook/ manual of a car....
tuning is setting the engine up within the narrow established dailed in specs
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  #5  
Old Dec 24th, 12, 03:40 PM
tillman tillman is offline
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Default Re: BBC timing advice

I have almost same setup
18 deg initial and 38 total on timing
check vacuum with car in gear and idling
take that vacuum # divide in half and add .5.This will be your power valve size.
Remove carb and close off rear transition slot and square off the front(or open .020)
Open mixture screws 1 turn
If your car wont idle then you will need to drill holes in the primary butterflies or purchase a Barry Grant with the idle ease screw.

I only had 5in of vacuum so I purchased the Barry Garnt vacuum secondary demon with idle ease.(works great)
My engine idles at 800rpm in park nd 700 in gear.
Hope this helps
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  #6  
Old Dec 24th, 12, 04:40 PM
Z15CAM Z15CAM is offline
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Default Re: BBC timing advice

290 Degree Intake is a BIG Cam for anything under a 10.25 Static compression. I would not be surprised if your looking at a 21 Degree Initial Timing. As far as Total Mechanical it all depends on Tranny and Gearing.

18 to 21 Degree Initial can be a beggar to Start, in cold weather, without an Initial Timing Retard Device. And guess what It will not start will anything less the 16 Degrees.

Personally for the Street I prefer a Solid Roller Marine Cam something in the range of around a 278 Intake Duration (288 Max) in the New Age Profiles with say around a .640" Lift in a MKIV with around a 9.5 to 10.25 Static Compression, M20 and 3:55 Gears.

By the Way Merry Xmas and Happy New Year and have fun with your BBC - They can sure Suck GAS ;o)
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Last edited by Z15CAM; Dec 24th, 12 at 05:04 PM.
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  #7  
Old Dec 24th, 12, 05:03 PM
Boondie Boondie is offline
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Mark
 
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Default Re: BBC timing advice

Thanks for all the advice and help , will let you know how it goes.


Cheers
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  #8  
Old Dec 25th, 12, 11:00 AM
Steptoe Steptoe is offline
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Default Re: BBC timing advice

Quote:
18 to 21 Degree Initial can be a beggar to Start, in cold weather, without an Initial Timing Retard Device.
Which is the VA is used in the idle advance ie VA on manifold vaccuum all in at idle vaccuum idle advance is intial+ VA. low intial takes load off the starter.
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  #9  
Old Dec 25th, 12, 11:26 AM
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bcm66 bcm66 is offline
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Bryon / Big B
 
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Default Re: BBC timing advice

Quote:
Originally Posted by Boondie View Post
Yeah going to hook up a vacuum gauge to the manifold, dont have any vacuum hookups at all, and yes street car
So you don't have a vacuum advance on the distributor?
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  #10  
Old Dec 26th, 12, 11:37 AM
Steptoe Steptoe is offline
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Default Re: BBC timing advice

OH and never drill holes in butterflies... that was an old school 'trick' from the 60s when carb design was different. Refer to my post above as to setting secondary butterfly heights....which is why they make them adjustable...instead of a random sized , gess work hole, increase accurately, while the engine is running , the gap around the butterfly.
Quite simple realy.
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  #11  
Old Dec 26th, 12, 07:24 PM
Straight-line-69 Straight-line-69 is offline
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Default Re: BBC timing advice

A few tips,..

1) Don't tune anything until the engine is at operating temp.

2) Then set the floats.

3) Yes run the VA to manifold vacuum but (against the advice above) run a can that's "all in" BELOW your idle vacuum reading. For instance, if you're pulling 14hg at idle, find a can that's "all in" by 12hg. If you pick a can that's still adding to your advance curve above your idle's vacuum reading, your idle speed (rpm) will always be in flux. Many don't understand this and find themselves 'tuining' all sorts of stuff that never fixes the problem.

4) Also (against the advice above) forget initial timing and simply time her to 34 degrees BTDC at 3000 RPM and let the initial fall where it falls. Adjust from there but you won't be more than 2-3 degrees off.

5) I never understood the notion that your PV should be half idle vacuum. Without hesitation, I'd recommend your PV rating to be 2-3 hg less than your idle reading. For instance, if you idle at 13hg use a PV with a rating of 10.5 (not 6.5 as some would advise).

6) A 750 cfm carb is a bit small for a healthy 468 so prepare for a bit of a stumble or hesitation off idle or coming out of turns. You'll cure that with bigger squirters and/or a different pump cam(s),..perhaps the "blue" cam in the two position would be a good call.

7) I think you have four-corner idle mix adjustment. Adjust them in rotation for best idle speed. You may have to go in a circle two or three times. This is important. A carb is still using (to a degree) the idle circuit up to 2000 RPM.

8) Your jetting should be very close out of the box. But confirm by checking 2-3 of the spark plugs.

Good luck!
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  #12  
Old Dec 27th, 12, 10:31 AM
Steptoe Steptoe is offline
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Default Re: BBC timing advice

Quote:
3) Yes run the VA to manifold vacuum but (against the advice above) run a can that's "all in" BELOW your idle vacuum reading. For instance, if you're pulling 14hg at idle, find a can that's "all in" by 12hg.
The "no more than 1" above..."
The VA starts to work below the idle vaccuum, and need to be all in at the VA attached idle vaccuum..as the VA starts to pull and advance a couple degrees, the vaccuum increases, which pulls a few more degrees, which takes a non VA idle vaccuum from say 7" to 12 " with VA attached....generally if more than 1" above non va idle, as I describe above, the VA will not pull fully in causing , often a flucutation in idle speed

Quote:
Also (against the advice above) forget initial timing and simply time her to 34 degrees BTDC at 3000 RPM and let the initial fall where it falls. Adjust from there but you won't be more than 2-3 degrees off.
Most modern dizzies are set up for egr etc, which means little cent , more VA...non egr/ polution engines , setup CORECTLY have , usually over 20 degs in the cent....which means modern HEI dizzies require modifucation.
So the choice is quick setup and compensate (which means stuff is not right.. "compensate" ) or do it right.
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  #13  
Old Jan 6th, 13, 10:35 AM
JohnZ JohnZ is offline
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Default Re: BBC timing advice

Quote:
Originally Posted by Straight-line-69 View Post

4) Also (against the advice above) forget initial timing and simply time her to 34 degrees BTDC at 3000 RPM and let the initial fall where it falls. Adjust from there but you won't be more than 2-3 degrees off.

5) I never understood the notion that your PV should be half idle vacuum. Without hesitation, I'd recommend your PV rating to be 2-3 hg less than your idle reading. For instance, if you idle at 13hg use a PV with a rating of 10.5 (not 6.5 as some would advise).
4. Don't stop at 3000 rpm - many curves don't stop advancing until 5000 rpm; rev it until it stops advancing so you know how much centrifugal advance is in the distributor.

5. In that example, a 10.5 PV will be open almost all the time; the slightest load will drop manifold vacuum below the 10.5" Hg. opening point.

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