I am at wits end with my car this year.I have been trying to fine tune it so it will run at its best...well it ain't working.Speed Demon 750dp..it's got a 355..Stealth intake..full MSD ignition..6AL...8361 pro billet (vacuum advance hooked up to ported)..blaster SS coil...8.5 super conductors..new Autolite 3924..all ignition is less than 5 months old...comp XE 274...AFR 195's...4 speed manual..3.73's.Motor has 3800 miles..compression test reveals 145-147 on all 8.Problem is when cruising...when I first take it out and cruise around all is well..after it warms up it starts to lean miss surge at cruise..was real bad with a 78-80...this best is with a 82 jet in the front..that seems WAY too large to me..stock for the Demon was a 76.I am running a 9.5" PV as my hot idle vacuum is 11.5-12".Less than two years old Holley Black pro series electric pump..Mallory 4309 regulator with return line setup..running two Earl's filters (one before pump..one after) that I cleaned less than 6 months ago.Car stays at a rock solid 6 psi at all times.Autometer in car fuel pressure gauge so I watch it constantly.Any ideas why I need such a LARGE jet?Please help..any ideas?

 

I doubt you have a lean surge. It's probably a rich surge.

Forget idle vacuum. That is the wrong way to choose a PV for street driving (okay for wild cam, though). You should chose a PV 1-2 inches below cruise vacuum.

I had a similar issue with an 8.5 PV. I finally went back to a 6.5 (what came in the carb stock), and bingo, problem gone.

 

I run 7 1/2 pounds of fuel pressure and 4.5 power valve

 

five7kid..I would say maybe to a rich surge..but afer 300 mile on the new plugs they are bone white..a light amount of carbon around the base of the plugs.They are just as clean as when I put them in.Only other thing I could thing of was am so rich I am washing them down..but the oil has no hint of fuel smell in it.I was thinkin of swapping back to a 6.5PV though..interesting you mention that.

 

Pappa smurf I only run a 4.5 power valve. I had to switch from the bigger one I had to get rid of the problem I had.



hey nice quote at the bottom of your msg's at least you admit to pretending to be fast!! muhhaaa j/k

Drew

 

4.5 gave me an off-idle flat spot, 8.5 the surge.

6.5 works, 7.5 or 5.5 may as well.

Yeah, try the 6.5.

 

After some debate..I am going to set the carb back up exactly as it came out of the box from Demon..then start the tuning process over again.I am beginning to feel like I am lost in a forest with this thing..maybe I tuned it in the wrong order.That will include swapping back to the 6.5 PV as you suggested five7kid.But one question that comes to mind is..If the PV that I have in there right now (9.5) is too large and I am running such a large jet (83) right now..how is the car running as well as it does?I would tend to think this thing should be spewing clouds of black,fouling plugs instantly,and loading the oil with so much fuel it would lock the motor up..yet it isn't.More of a curiousity question than anything.And if it is a rich surge..wouldn't it get better as the motor heated up..not worse.The other day it got so bad after awhile of cruising it was like I was lifting and mashing the gas repeatedly..but yet I was holding it at a steady throttle position.Bucking if you will.No clouds or anything.I guess what I am getting at is..why are my plugs so clean?Anyone at this point?Please.Drew..why did you swap back to a 4.5..what was your problem again?1FastZ..what is your vaccum reading..why the choice of the 4.5 also?What cam are you running?Thanks for all the help so far guys!

 

Time out, time out!

The number on the power valve indicates at what manifold vacuum the valve opens, not flow size (the flow allowed when the PV opens is controlled by the circuitry in the carb). That's why idle/cruise vacuum is important to know. You don't want the PV opening at idle or cruise (idle mixture screw control idle mixture - obviously, and main jets control cruise mixture).

 

I agree with you on that.But what I am saying is..let's say my PV is open at cruise..with an 83 jet..the PV is supposed to enrichen the circuit an equivelent of 8 jet sizes according to Demon..which would equal more or less a 91 jet size..holy cow.That we know is way to rich..but even if it isn't open at cruise..an 83 is still way to rich as far as I beleive at light throttle cruise for my combo.Thats what is confusing me so much..either way this thing should be PIG rich...and it isn't according to the plugs..they have no color at all.

 

Ok fast forward a week and I swapped to a 5.5PV as my idle vacuum is 11-11.5".Cruising a steady 45-55 I get around 17-17.5" vacuum..now I have a nice long flat spot till the PV opens.I have tinkered with every pump (right now it is a white) cam and have a 40 squirter on there so it should be enough fuel to cover the hole..a 42 made it real lazy..but yet still a flat spot.Rejetted back to stock 76/83 to start over.Now at a light to moderate cruise the car breaks up/misses like a bad ignition miss (it isn't though..read top post).Mash it and she screams without a miss..pulls damn hard.Check the plugs after a nice long cruise..and they're as clean as when installed.I say still lean.Two years tinkering with this damn Demon and I have about had it..looking to buy a used Holley 750dp??anyone?Gets aggravating when anyone else jets,tweaks the idle screws..and the such and their car acts perfect.I called Demon to talk to a tech about it and all the *** would say is the carb is to big.."big block only".We all know a 750 is not too big.Please anyone..help me out here.Demon rocks.I have tried/tuned everything and nothing has solved the flat spot nor the mid range miss.I am out of ideas after 2 years.Please?