Proform 850 DP tuning questions

[chargerbr549]

I finally upgraded the 512 in my Charger from the 750 Vac Sec carb that I have had for years and installed the 850 proform DP carb, it definately has alot crisper of throttle response compared to the old carb but the idle is not as clean as it should be. Is there a good read online about  tuning these carbs or can somebody steer me in the right direction? I haven't messed with 4 corner idles and adjustable air bleeds before so I need to do some learning.
My 512 has the comp cams XE285HL cam with a 112 lsa, about 9.8 comp ratio ported 906 heads with 2.14/1.81 valves, holley street dominator intake, 1 7/8 hedders, has 20 initial and 36 total timing around 2400 rpm, 9.5 inches of vacuum at 950 rpm at about 3300 ft above sea level.
I had to lower the float levels to get the fuel level in spec and I adjusted the idle screws and it seems to run the best about 3/4 of a turn out, it reads the same idle vacuum as the 750 carb but its not as clean of an idle plus it smells a little rich out the exhaust so i'm thinking I might need to lean out the idle with different idle air bleeds, the spec sheet on my carb says 70's and 33's but I have no idea of what way to go to lean them out, I was hoping when I got the carb there would at least have been some instructions or tech line to call in the box as how to mess with some this stuff.

Kevin

[firefighter3931]

Kevin, did you adjust all 4 corners on the idle circuit ? I like to use a vacuum guage and adjust for maximum vacuum on each corner. You should be able to get by with the stock PF850 bleeds with your engine combination, inmo.

To lean out the mixture you need to increase the idle air bleed size.

Make sure you have good spark or it will idle "fat"


Ron

[chargerbr549]

I used a vacuum gauge when I adjusted all the idle mixture screws but they didn't seem quite as sensitive as my old 750 but I will mess with them a little more later this week and see if i can get them better dialed in.  I run the factory mopar style electronic ignition with the orange box and an MSD blaster coil with MSD helicore plug wires with .040 gap plugs, someday in the future I would like to goto a little more updated type ignition  box like an MSD digital box.

Kevin

[BSB67]

Try the idle air bleed restriction first as it is easy.  My experience is that it usually does not help much, and have found that decreasing the idle fuel restriction is more effective.   Try reducing the IFR size so that the idle mixture screws are 1.5 to 2 turns out.  The idle mixture screws should become a little more sensitive too.   

[firefighter3931]

Another thing to try is a quick blast of carb cleaner down into the air bleeds. Sometimes there is a little residue inside the circuit that can mess with the mixture.


Ron

[chargerbr549]

I did some more messing around with the carb this weekend, I opened up the idle air bleeds which seemed to help lean out the idle a little bit but I still only have the idle screws turned out between 3/4 to 1/2 turn out. I have been driving it a little bit and noticed it also starts to run a little rough between 2000 to 3000 rpm and I found out why, it starts to run fuel over the primary venturies and starts to run rough how do I stop it from doing that? I have the idle air bleeds at .076 now and I measured the idle restrictions and they are .037 which is on the big end of whats available (.029 is the smallest) and the fuel level is right on in the middle of the window but I also dropped the fuel level below the the edge of the sight glass and it still pulled fuel over the venturies. I also measured the fuel pressure and it runs between 7.5 to 8 lbs, is too much fuel pressure part of my problemo? Im getting tired of messing with this thing!

Kevin

[firefighter3931]

It sure sounds like excessive fuel pressure from what you're describing. Can you install a regulator to see what happens ? Increasing bleed size definately leans out the mixture.


Ron

[chargerbr549]

I reset the fuel pressure down to 5.5 lbs and I had to readjust the float level but it still acts the same as it did before, I think I will try and reduce the the size of the idle fuel restriction and see if that helps out or not.

[chargerbr549]

Do proform carbs use the same idle fuel restrictions and air bleeds as the holleys, quick fuels and demon carbs or do I have to order specifically proform stuff?

Kevin

[firefighter3931]

Do proform carbs use the same idle fuel restrictions and air bleeds as the holleys, quick fuels and demon carbs or do I have to order specifically proform stuff?

Kevin

They should, but i would check with Proform just to confirm. 


Ron

[chargerbr549]

I ended up putting in smaller idle feed restrictions and that helped a little bit but I am definately wanting to upgrade to a better ignition system with a fatter and longer spark, I am thinking about using an msd 6520 digital box with start retard ( so I can run around 20 degrees intial timing and not have to kill the ignition when its hot to be able to start it),and  an adjustable rev limiter. Can I run this box with my msd blaster coil and my factory chrysler style electronic distributer? I basically want to just replace my  mopar performance orange box, or do I have to goto a different coil and distributor?

Kevin

[71bee]

The orange boxes are junk. I fried 2 of them without even trying. go with the chrome box (P4120534). it will give you a hotter spark & can handle up to 8000 RPM.