b_evans06

iTrader: (1)

I currently have a 650 speed demon mechanical secondary I had from a previous motor in my car. I now have a lq9 6.0 with a 235/240 649/609 cam, stock 243 heads, a gmpp single plane, 1 3/4" headers. Car weight 3200 and has 4.30 rear gears and I race 1/8th mile track. I have the current 650 on the car but it runs out of steam about 5800 rpms. I was looking into upgrading the 650 to flow more with either a bigger main body or baseplate. I am not real knowledgeable about building hybrid carbs like this so that is why I am asking. I would rather not buy a new carb as I do not have a lot in this one and want to make it work since I have had good luck out of it before. If anyone can offer some advice on what to do to my current carb to flow enough to support my current setup I would greatly appreciate it.

Z284thgen

iTrader: (46)

may i begin with saying at least a 850cfm to begin with

xpndbl3

iTrader: (23)

What do the plugs tell you right now? Runs out of steam? or is running lean up top?

b_evans06

iTrader: (1)

I didn't realize I would need an 850 for my setup. Can I modify my 650 to flow enough to support my engine? Right now it is jetted up some and runs rich because I don't think it is able to get enough air to match the fuel supplied. I did not want it to run too lean and I haven't driven it very much so far until I get the carb lined out. If I am wrong on this let me know as I admit to not being knowledgeable on carbs. It runs out of steam up top like it falls on its face after 5800+. It pulls hard down low and mid range but you can feel the power fall off and the motor doesn't want to rev higher. I think it is running out of air up top making it unable to pull to 7000.

Z284thgen

iTrader: (46)

not saying jump to a 850 right away but i bet you see a nice gain with a bigger CFM carb

got any buddys that you could borrow a bigger carb and try out

Pop N Wood

iTrader: (1)

Have to agree, you need a bigger carb. Pretty big cam and 6.0L needs more than what you have.

If you had a Holley you could buy a bigger mainbody and reuse the metering blocks and bowls. but I think your cheapest approach is just buying a second carb.

ZONES89RS

iTrader: (25)

I have a 750 main body and base plate from Proform and Quick Fuel and in reality it is probably a 800 CFM carb, a 650 is holding back, but it should still hit 6500 or so i think, your tune must be holding you back.

team39763

iTrader: (3)

Isn't the 650 demon wet flow rated, which would make it comparable to a holley 750 in reality?

rojs234

iTrader: (5)

Valve springs?? I know this is a dumb question, but I got'ta ask....is your RPM limiter in your MSD box set high enough? 5800 is just above where the box is set from factory. The size of the carb shouldn't cause it to "fall on it's face". I will sometimes disconnect the secondaries on a carb (750) while tuning with my wide band, either to check for a problem or to get primary set close. The engine will still pull easily to 6800RPM. It will show richer because of the limited air, but still pull strong....just takes a fraction longer to get there.LOL Just my personal experience. Ron

b_evans06

iTrader: (1)

I am running prc dual springs so I know I am covered there. It will go to 6500+ but it seems like it quits pulling as hard over around 5800. I have the rev limiter set on the msd to 7000.

rojs234

iTrader: (5)

That all sounds good. Have you ever hooked up a fuel pressure gauge that you could see on a full tilt run? Like maybe tape one to the windshield? If you show good fuel pressure (5-7 lbs) at the RPM where it craps out, I would maybe look for restrictions in the carb itself, low float settings, too small needle inlets, (some cheap rebuild kits have really small ones), anything that might restrict fuel flow at high RPM,