With my LM-1 wideband sensor, I'm detecting a lean condition at WOT (or under power), and rich at cruise. Cruise is 12:1, and power is about 16:1. I've got the idle down to 14.7 with 10" vacuum. With my last combo (same as now but 292 hyd. flat cam, ported #492 heads), it was rich all the time, about 12.5:1. Never got a chance to play with it.

So far, the 825 Mighty Demon is box stock, which means 76/84 jets, and I put in a 4.5 power valve. Timing specs are 36 total, 16 initial with 12* vacuum (28* total at idle).

What do you guys think the first move should be? I know if I increase the jet size I'll probably cure the WOT lean-out but I'll make the cruise REALLY fat. Opinions on where to start?

Maybe the powervalve isn't opening at WOT? If this was the case you could put in a PV with a higher opening point and jet down on the primary to reduce your mix at cruize. You're pulling plenty of vacume to run a higher opening PV IMO. What is the vacume at cruize? TTT Bob

This doesn't sounds like a tuning issue but more like there's something wrong... Hook up a fuel pressure gauge and check pressure at WOT. I have a feeling your fuel system is inadequate.

That's possible, but I wouldn't think so. It's got 7.5PSI at idle, with a Carter 172GPH pump with 1/2 lines all the way. Still possible though.

Well, maybe not, because it doesn't need to be at high RPMs or full throttle for it to happen. If I just roll on the throttle, it will lean out.

sounds like some sort of power valve issue on the lean power thing, not reading the low vacuum, passages are plugged, Pv just plain no good, etc.

the rich cruise will most likely require smaller jets. I'd get the PV issue fixed first.

Tom

You also want to be sure this isn't just related to the secondary side of the carb too.
It sounds like it is going lean when you get into the secondaries the way you describe it.
Your carb should have come with a phone number of BG Tech.
Give them a call & you might get lucky & get someone that can help.
My experiences have been,,, "mixed" :(

Originally posted by Wolfplace:
You also want to be sure this isn't just related to the secondary side of the carb too.
It sounds like it is going lean when you get into the secondaries the way you describe it.
Your carb should have come with a phone number of BG Tech.
Give them a call & you might get lucky & get someone that can help.
My experiences have been,,, "mixed" :( I hear ya on the tech support, Mike. I have the same feelings.

You are correct, the more throttle I give it, the leaner it gets. Doesn't need to be full throttle or high RPMs, it leans out regardless.

Ok, I just talked to Demon:

First, he said the carb was WAAYY to big. He said a 650 would be a better choice. Now, I know that the 825 is large, I knew this when I bought it...I bought it for so cheap I couldn't pass it up. However, their website says it should work on somewhat radical 383-415cid engines. :rolleyes:

His reasoning was that there simply isn't enough vacuum to pull enough fuel. I'm not sure about this, unless if he's saying there isn't enough air velocity at the upper RPMs to pull the fuel from the boosters (since vacuum at WOT = 0). If this is so, why did it work so well with my previous heads and cam?

He was also convinced I should have idling issues, like the mixture screws would have to be like 2 turns out (from the 3/4 turn recommended), which it isn't. I've got it burning at 14.7:1 with the screws 3/4 turn out.

Ugh, I dunno.

I'm confussed? do you mean 12:1 at wot and 16:1 at cruise? 12:1 is pretty rich or I that is what I though was rich? 12:1 to 12.7:1 is usually the range where max power is made. What determines max hp is quarter mile times?

Nope, you read it right the first time. 16:1 at WOT and 12:1 at cruise.

12:1 is too rich for cruise, it should be around 14.0-15.0, depending on the motor. For WOT, 12.5-13.0 is good, again, depending on the motor.

Usually the MPH determines the HP.

it sounds to me like BG Tech either doesnt know or is feeding you a line of crap, maybe both. your car/combo is not mild by any means, that carb should run fine on the car.

i have no experience with demon carbs, but i have read where people bought them and they had aluminum shavings/debris in the passages and/or air bleeds out of the box.

if it were me i would rebuild the carb and inspect/clean EVERYTHING.

Have you verified the float level on the secondaries?

Brandon,
I agree, it should run fine. It ran GREAT with my previous heads and cam, and my motor is more "radical" than it was before (bar far), so it should be a better fit than it was before. I once instance of some aluminum shavings in the carb, but I didn't totally check all the passages with a fine wire.

Zwede, no, not yet. I will do that for sure, since I'll probably pull the carb apart.

It seems like something is "wrong" rather than it being out-of-tune. If anybody has any other suggestions, keep 'em coming!

Yikes, that is way off! And, yes, mph does determine hp. 12:1 at cruise is eye watering rich. That carb is perfect for your combination (cfm wise). I believe a too small carb wouldn't be able to get enought velocity through the boosters and couldn't pull fuel out of the bowls. You have a strange problem. The main metering system determines its air fuel ratio based on throttle opening, less throttle means less pull on the boosters, and load determines vacumme, which is an on/off system. Is there a difference between power valve fuel flow say between a double pumper and a vac secondary. Does your carb have jets behind to power valve to alter the fuel flow? If you could change those you could richen up the wot af ratio and lean out the main jets.

Hey silver69, Do you have the correct carb main body to metering block gaskets?