ImBatman

I have a DEMON 850 mech annular discharge. I have been trying to set the timing and tune the carb per the dticky thread and lars' paper but nothing I do to the AF mix screws seem to do anything. I have 10" of vac consistently and riht now I am a turn and a quarter out on all 4 screws. The car doesn't stumble or bog and really runs great but at idle it burns your eyes at the side pipe opening and blows a little smoke when reving at idle. I also get a strong smell of gas. Like I said turning the screws doesn't seem to decrease or increase the amount of vaccum or the idle of the motor. Any ideas, tuning tips, resources?

Thanks
Wade

wiseman79

I've had this problem before, mine was due to the power valve, went from a 6.4" to a 4.5" and worked fine. With 10" of vacuum though doesn't seems like this would be your problem. Have you checked to make sure your float level isn't too high?

MotorHead

You need to put wires in your idle feed restrictors to get the idle mixture screw adjustment back. It is way too rich because your idle feed restrictors are too big. If it is a Mighty Demon I think they have screw in restrictors, you can use a wire first to test

ImBatman

I thought about the power valve but like you said the 10" of vaccum makes me think dtherwise. Both the front and rear float bowls are set to right in the middle of the sight glass. Should I think about setting them lower?

Thanks
Wade

ImBatman

Uh huh???? Not sure I understand. It is a mighty demon. What kinda wire do a use to test and where do I insert them? And if this is the case this is something I can down size?

Thanks
Wade

MotorHead

My 825 Race Demon has .037" IFR's and I had to go down to .031". Your 850 Mighty Demon has .038" IFR's you need to go down to around .031" too

This is what you have to do to get it to idle and run correctly, I don't have diagrams of the metering blocks, there was another post here recently about exactly the same thing

http://forums.corvetteforum.com/show....php?t=1694550

ImBatman

Awesome I will take a look. Thanks!!

murukult

I had this kind of problem with my Demon when my secondary butterflys was not set properly(they was little bit open).

norvalwilhelm

An LM-1 is a wise choice, you are just shooting in the dark basically without one?

Motorhead. I am going to buy a wide band oxygen sensor for tuning but know nothing about them? which brand to buy? what is a good source? Price?
Any recommendations?
Thanks

I just ordered a wide band oxygen sensor. $360 with a dash mounted gage.

norvalwilhelm

I always thought that the power valve affected the idle mixture if the diaphram blew or the value was wrong.
I was wrong in assuming this. Regardless of the condition of the power valve is DOES NOT affect the idle mixture.
There is NO connection between the power valve and idle quality/mixture.

Brettmc

I had to add the wires to my 850 Holley. First you will need some type of A/F analyzer such as the LM1 or even a narrow-band O2 sensor plumbed into a collector and read with a digital multimeter. Reason being, different size wires will decrease the gas in the mixture a different amount. Be prepared to use different size wires. I found that telephone wire from Radio Shack was the perfect size for my carb. Hopefully, your metering blocks will have the replaceable IFRs, which will make the process much easier.

Although my BB has ~11" vacuum at idle, I found it ran much better with 4.5 power valves. Also, remember that jet changes will have a small effect on the idle mix.

Another very important item to watch are your transition slots. Start them very small and exactly the same. When you adjust the idle screws, adjust them both exactly the same amount.

Brett

MotorHead

Norval, sorry I didn't see your post untill now, I sent you an IM

Dash mounted are OK but the LM-1 http://www.innovatemotorsports.com/
logs the data and you can view it on your PC and it will show the A/F curve in relationship to your RPM and your throttle postion ( if you install throttle position sensor ) THis is much more usefull info than a gauge on the dash

ajrothm

Norval, the LM1 is the only way to go. Make sure you get the RPM converter which is seperate from the LM1 so when you log, it log RPMS. In reality you should get the LMA3 AUX BOX, then you can log 5 channels of data. That way you can log RPMs, BOOST, AFR, Fuel pressure(if you buy a transducer) and really anything that you can get a 0-5v signal from.

I use this to tune and log my turbo-bike, its the trick way to tune.

ajrothm

BTW stay away from those cheesy dash mounted ones from K&N or edelbrock, they are narrowband and virtually useless.

You must have a wideband with the heated 5 wire O2 sensor.

Motorhead was right about logging throttle position. I would definetly get a 0-5v TPS sensor rigged up so you can log TPS. Thats very important on forced induction.

gerry72

Power valve will affect idle (and everything else) only if it's blown. This is due to raw fuel leaking through the torn diaphragm into the vacuum chamber and down through the throttle plate. If the power valve is not blown, then even if it is open during idle, it has no affect on the idle since the power valve only affects the main metering circuit and can flow only through the boosters. This is why people trying to tackle a rich idle waste time by altering the jets and power valve. Doesn't do a damn thing for the idle.

ImBatman

Guys thanks for all the great info. I think I understand the wire mod pretty well now. Thanks for the info Wayne! My next question is how do I adjust the carb without one of these fancy gadgets????? $400 + is just not in the budget right now They didn't use these things in the "old days" and managed to get the cars running pretty good.

Carb is a stock MIGHTY DEMON 850 mech annular discharge.

What characteristics am I looking for when I start inserting the wires?

How do I best diagnose a blown PV?

Should I look at reducing the PV to a 4.5?

Do Holley parts interchange with my DEMON?

gkull

Brett - This is only a true statement if you have a perfect intake manifold that will deliver equal A/F to all cylinders. I tuned my 750 Demon using a sniffer in each pipe and the mixture screws were all different.

I would not dork with wires when it is so easy to drill a hole in the primary throttle blades. I had the same problem of the screws not being able to lean out the idle. drilling holes made it so about 1.5 turns out was about correct

ImBatman

gkull, could you explain drilling the blades in detail or point me to a place for info on this?

Thanks
Wade

Brettmc

That's true when you refer to idle mixture screws. However, I was referring to idle adjustment screws. I should have been more specific.

Yup, it's easier to drill the throttle plates but it's so irreversible if you go too big.

Brett

Brettmc

See post # 11 where it says narrow band O2 sensor and multimeter