Fevre

I am trying to tune my 750 dp and cannot get a happy medium for my a/f ratio at idle and just off idle, I have it pretty rich at idle and just before the mains are opened I get real lean. I have wires in all four idle passeges and am wondering if I used too big a wire diameter, .025 if I remember right. Does the idle circut work of draw meaning as more air flows through it draws more fuel? If so then I think I should reduce the size of all four wires or eliminate them in the rear circuts to allow more fuel flow. Without the wires in the idle screws where very insensitive and the a/f meter would barely change change when they where adj, now there are very sensitive and very small adj make it move.

Thanks in advance

GDaina

use a vacuum gague when adjusting the A/f ratio.....stop at teh highes vacuum reading, then do the other screw, and back and forth until you have teh highest vacuum reading.

Fevre

My problem is I get somewhat rich idle but it leans out just before the mains kick in. If I really richen it up at idle it runs fine just before the mains open but the idle smells like raw fuel. Seems I have the idle circuts to restricted but want some input on it before I go messing around too much.

Matt Gruber

According to Vizard 13:1 is best idle. :iagree:

Fevre

Ya that is just about where I am but it still wants to lean out just before the mains kick in, lean enough to crack and pop a little.

Matt Gruber

that sounds like too little pump shot.

Fevre

I am holding the throttle open at a point just before it opens far enough to draw on the mains, sorta like cruising around town at low rpm's, having no problems with it when I rev it off idle only when I hold the throttle open just a touch and before the main slots ore open. I actually have it way to rich on the mains, once it starts cracking and poping I pull the throttle open a little more and can see it go way rich and stay there when held at a constant rpm. Do you know if the idle circuts work off draw, more are flow draws more fuel? Seems they would and that I have them restricted to much.

The Dude

Sounds like you have two very common things working here--rich at idle and a lean bog when you get on it. Both are relatively easy fixes. The assumption is that you have the correct power valve and your timing is sopt-on correct.

(BTW, are you running a single-plane intake? Never mind.)

First, the idle issue. You need to get more air into the system so lean it out a bit. There are two things to try in this order. First, adjust your secondary throttle plates (butterflies) so that they're open farther at idle. You'll have to remove the carb to access the secondary throttle adjustment screw, replace the carb, test it, remove it, adjust it, etc. This screw is located on the passenger's side of the carb.

Second, you might have to drill some small holes in your primary butterflies. Start with about 1/16" inch holes centered in the front side. Check it. Enlarge in small increments and test again.

The first thing may help you but you may need some combination of both.

For the bog or hesitation when you stomp it, just experiment with larger diameter pump shooters. Just buy 3 or 4 sets a few numbers bigger and test it. You could also adjust the pump cam(s) to bring the pump shot in earlier, but that's not usually the problem. Just think, "I need bigger shooters" and you'll probably be OK.

Fevre

Just to make it very clear I do not have a bog or hesitation, it revs very nicely when you jump on it and the a/f meter reflects that, it is when you hold the throttle open just slightly that it leans out. I have the holes already drilled in the throttle plates and use the wires in the idle circuts to lean it out. I am using stagared pv, one at 12.5 and the other is at 6.5. My idle vac is about 16 to 17. I have tried to adj the throttle idle positon both at a lower and higher rpm level and still get the same leaning out when I hold open the throttle just a little.


[Modified by Fevre, 10:56 AM 10/1/2003]

Matt Gruber

the transition circuit is usually rich to prevent the lean out you describe. That is where the crummy mpg comes from with OD.

Fevre

I'll have to consult me refernce books to learn more about it, can you elaborate on the transition circuts a little? Would they fed fuel while holding the throttle open just a touch? If so then I might have them clogged or blocked somehow.

Anyone know if the idle circuts feed more fuel as air flow increases? That is my main question that needs answering.

Thanks

Matt Gruber

the idle circuit feeds until the throttle blades reach the transition slot. they take over until the main jets start. this can't be properly tested parked. as you drive the transition is quick and proper usually. But with OD it can be a mpg problem,

Fevre

Ya I am doing this in my garage but still it should not break up like it is when holding the throttle at a set postion. I can watch the a/f meter start at about 13 then as I open the throttle it start to move toward lean after the pump shot is used up then when I hold it open it leans way out flickering at 15 and that is when it starts the lean misfires. If I have it way rich on idle it does not lean out as bad so I am assuming there is still fuel feeding out of the idle circuts or why would me adjusting them have any affect on the reading once I have the throttle open and am holding where I am getting the lean condition. Once I have it open far enough for the mains to feed it goes way rich, I jetted up 2 sizes thinking it was the mains that where too lean.

Matt Gruber

it does sound like the wires are too big.
i don't know why a street car would need 4 idle screws(or 8 with norval's blower)
maybe a race car with a wild cam wth huge overlap, but not a street car. my 72 idles fine on one screw(#2 all the way in)

Fevre

I need to change the sec jets so I think I will just pull the wires out of those 2 circuts and see if that makes any diff. Off course I could always put my little 600 vac sec Holley on but what fun its that? :)

Thanks for the help Matt, I do appreciate it. :cheers:

Matt Gruber

Yes it is getting fuel from the idle and transition circuits as the mains are not flowing yet. So when you lean it out it is leaned out.
A cure would be to get annular discharge boosters, they are more sensitive and would fill in the gap, if they work as advertised.

norvalwilhelm

The normal size of wire to use is .012-.015 max. The idle circuit also continues to work even driving down the highway. Vacuum is always present sucking fuel through the idle circuit. Have a rich idle and it affects everything. The transfer slots must be closed at idle so the throttle blades must be closed. Turning the idle up opens the blades uncovers the slots and causes it to idle rich. I close the throttle plates and drill 4 .100 holes. One in each throttle plate. When drilling turn the carb upside down and drill on the side of the throttle plate that causes the carb to open when drilling. Hold it closed while drillling the hole. Drilling on the closed side is not right.
Get the idle screws that the car idles with the screws 1 1/2 turns out. That is how holley designed it.
I idle on all 8 screws set at 1 1/2 turns but I run 8 holes and 8 wires.

Fevre

Thanks Norval

I am going to yank the carb tonight and clean it then do some tweeking. I only have 2 holes drilled on the primary blades, don't recall the actual size, so I may have the transfer slots exposed in order to reach my desired idle rpm, I like it at 900 rpm for some reason but with the eng combo I have in it now I could probably go down to 600 or 700.

Brettmc

Where do the wires go? I know they go into the idle circuit, but where do you stick em when you have the bowls off?

Brett :thumbs:

Fevre

Yes you have to pull the carb apart and bend the wires in an 'L' shape. What I did was hold the metering block idle passeges face up and insert the wire into the passages then put the gasket over it and then put that side of the carb back together. I use the blue non stick gaskets and once you tighten the bowl down the wires will stick to the gasket so when you pull it apart again be carefull and the wire will be stuck to the gasket for reuse. If I remember I'll snap a few picks tonight, I don't always remember to do it so please don't hold me to it. :) It really makes the idle screws much more sensitive for tuning by not allow so much fuel to flow. DP are really made for racing so they are pretty rich out of the box. As you can tell I am not an expert but have learned a thing or two about them.