Speed Demon Tunning
Team Owner
Joined: Jun 2004
Posts: 20,216
Likes: 4
From: Smile, it's just the internet.
I never noticed a transfer slot on the secondary side of my 4150, but I'm known to overlook that which I'm not looking for...and sometimes that which I AM looking for (car keys, for example)!!
Maybe they were talking about the actual butterfly opening? (distance from edge of throttle plate to venturi wall) Only way to measure that would be with a pin gauge, though.
Lars has a track record of being right. I'd stick with his suggestions...
Maybe they were talking about the actual butterfly opening? (distance from edge of throttle plate to venturi wall) Only way to measure that would be with a pin gauge, though.
Lars has a track record of being right. I'd stick with his suggestions...
Pro
Joined: Sep 2004
Posts: 595
Likes: 0
From: Boston MA
Originally Posted by CGGorman
I never noticed a transfer slot on the secondary side of my 4150, but I'm known to overlook that which I'm not looking for...and sometimes that which I AM looking for (car keys, for example)!!
Maybe they were talking about the actual butterfly opening? (distance from edge of throttle plate to venturi wall) Only way to measure that would be with a pin gauge, though.
Lars has a track record of being right. I'd stick with his suggestions...
Maybe they were talking about the actual butterfly opening? (distance from edge of throttle plate to venturi wall) Only way to measure that would be with a pin gauge, though.
Lars has a track record of being right. I'd stick with his suggestions...
Thanks,
Joe
Team Owner
Joined: Jun 2004
Posts: 20,216
Likes: 4
From: Smile, it's just the internet.
I've got my primaries set up with about .020" exposed. The secondaries are open an arbitrary (never measured) amount to get idle speed to ~900. My cam/lash won't let it idle reliably below about 800, so I keep it around 900 to ease drivability. At a guess, I'd say the secondaries are open about the same as the primaries, so Holley's recommendations fit pretty close with the way mine is set up. My driveability is very good. Total timing is 36, all in by 2500. Initial timing is 18. I run vacuum advance, but never measured how much. I tune carb with an O2 sensor.
This is on a mild 427 with a Holley 850 DP and Weiand Stealth dual plane intake. Cam specs unknown, but somewhat lumpy and solid lifters.
Edit...I've got it jetted with 76s all around.
This is on a mild 427 with a Holley 850 DP and Weiand Stealth dual plane intake. Cam specs unknown, but somewhat lumpy and solid lifters.
Edit...I've got it jetted with 76s all around.
Melting Slicks
Joined: Jul 2002
Posts: 3,000
Likes: 22
From: Boulder Creek California
Speaking as a veteran of a Lars Tuning weekend...
You want primaries and secondaries open the same amount. The .020" figure is just a starting point. I didn't realize this after just reading about it and that's one of the things Lars helped me fix. I ended up a little more closed and solved my rich idle/fast idle problems.
If you don't have transfer slots on both sides you can balance primary and secondary openings by eye, look through the bottom and set them to have the same size sliver of light coming through.
In either case, adjust both the same amount and keep track of where you end up for future reference. Sight, sound, and (if you're really off) smell are all you need to tune an engine for the street
-Chris
You want primaries and secondaries open the same amount. The .020" figure is just a starting point. I didn't realize this after just reading about it and that's one of the things Lars helped me fix. I ended up a little more closed and solved my rich idle/fast idle problems.
If you don't have transfer slots on both sides you can balance primary and secondary openings by eye, look through the bottom and set them to have the same size sliver of light coming through.
In either case, adjust both the same amount and keep track of where you end up for future reference. Sight, sound, and (if you're really off) smell are all you need to tune an engine for the street
-Chris
Safety Car
Joined: Jul 2002
Posts: 3,770
Likes: 3
From: St Louis MO
Like Gorman says, I have the best luck setting the primary so there is no stumble then adjusting the idle speed using the secondary. I don't know how easy that is on BG carbs but I have a secondary idle adjusting lever on mine that allows you to adjust the idle from the top of the carb just like the primary side. It's a LOT easier than trying to turn that little screw on the bottom of the throttle plate (again this is on a Holley).
It's tricky getting the combination just right, it takes some trial and error. The 750 I have on the vette now had a bad stumble but I adjusted it out using this method. If you move the throttle very very slowly, not allowing the accel pump to shoot, and it runs rough, it'll be magnified on the street as a bad stumble. If you move the throttle slowly and you get a really smooth transition, you're there!!
Brett
It's tricky getting the combination just right, it takes some trial and error. The 750 I have on the vette now had a bad stumble but I adjusted it out using this method. If you move the throttle very very slowly, not allowing the accel pump to shoot, and it runs rough, it'll be magnified on the street as a bad stumble. If you move the throttle slowly and you get a really smooth transition, you're there!!
Brett
C6 the C5 of tomorrow
Joined: Nov 2000
Posts: 6,665
Likes: 2
From: Twin Cities Minnesota
I have a demon 750 and their tech support said it's better to drill out the idle bleeds to .076" than it is to put wire in the orifice...
Oh and definitly keep both transfer slots equal and as close to .020 as possible
Oh and definitly keep both transfer slots equal and as close to .020 as possible
Thread Starter
Pro
Joined: Sep 2004
Posts: 611
Likes: 0
From: Minnesota
I guess I don't understand how to set the total timing. I put the silver springs in from my kit and there does not seem to be any difference. I set the timing light to 36* and rev the engine until the timing mark stops advancing. I adjust it so it goes to the 0 mark. I then rev the engine and see that it hits the 0 mark at about 1500 RPM (the timing light is still set to 36* and the vacuum line is still removed from the distributor). What am I doing wrong? Should I put the gold springs in? I noticed that the weights are rusty. Would this cause a problem? Can I spray them with something like WD 40?
Safety Car
Joined: Jul 2002
Posts: 3,770
Likes: 3
From: St Louis MO
Originally Posted by 427V8
I have a demon 750 and their tech support said it's better to drill out the idle bleeds to .076" than it is to put wire in the orifice...
Brett
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Team Owner
Joined: Jun 2004
Posts: 20,216
Likes: 4
From: Smile, it's just the internet.
Captain, it sounds like you are doing right. The springs affect WHEN the weights move, which affects the RPM at which you see total timing level off. Lighter springs resist the centripetal forces of the rotating advance mechanism less than heavy springs, so the lighter springs let the advance come in sooner(lower rpm). If you are seeing full mech. adv. at 1500, you need heavier springs. Sorry, I don't know the spring colors. Of course, you need to ensure that the advancing mechanism and weights rotate freely... The AMOUNT mechanical advance is also adjustable and will determine your initial timing if you use Lars' method of setting your timing. The method of setting the amount varies by distributor. My MSD dist. uses a bushing that limits travel. I'm not familiar with the other distributors, unfortunately.
Last edited by CGGorman; Jul 30, 2005 at 09:38 AM.
Thread Starter
Pro
Joined: Sep 2004
Posts: 611
Likes: 0
From: Minnesota
What is wrong here? I put the soft springs in and set the light to 36*. adjust the timing mark to 0. I put the silver springs in (should be the next step up from the lightest). The light is still set to 36* and the timing mark is at 0 at idle.
