Demon tuning problems, need some major help. - Third Generation F-Body Message Boards

86camaro383 · 05-09-2009, 09:00 PM

Heres my set-up, 383 (500Hp 510TQ) t-56, L/Ts and 4 inch exhaust.

Carb is a 750 cfm mighty demon.

Problem, 2 years ago, I built the 383 for it, had it dyno tuned, and it ran awesome, no hesitiation what so over although that was with a automatic.

I let gas sit in the carb over winter, although Id started it a couple of times during winter.

Last year it had a major bog, you would floor it, it would hesitate until about 2500-3000 RPMS, then slam you into the back of your seat. I called barry grant tech. support, they said its probally gummed up, so rebuilt it, so I ordered up a rebuild kit and did it. I started from scratch with jetting. Ran a little worse. So i got the dyno sheet, and returned the jetting to what they did (Jets are :P=78 S=87)

Now when its idleing, and you wack the throttle, it revs nice and sounds like its all there. When in gear, it does the same thing, hesitates until it reaches 2500-3000 RPMs and then its got tons of power. It makes 11 LBs of vaccumm at idle, but has a 3.5 Power valve in it. Could that cause it?

More specs: squirters are 31's Ive regasketed it, put ned accelerater pump diaphrams on, ...I just dont know what to do... HELP!!!!

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86camaro383 · 05-10-2009, 08:01 PM

noone?

TAGTASCOTTY · 05-10-2009, 08:26 PM

Try a "four corner idle adjust" I had a similar prob with my holley setup. Basically take off carb, and adjust the secondaries open a hair, helped me out more than everything I had tried.... Godd luck man

86camaro383 · 05-10-2009, 09:21 PM

yeah we tried that. We are thinking more along the lines if the power valve, and squirters....

383roller · 05-10-2009, 09:25 PM

i had to go up a few jet sizes for my Demon, it ran rich at idle and stumbled at low engine speeds

EvilCartman · 05-11-2009, 04:11 AM

Messing with the idle feed restrictors and air bleeds got mine to where it drives great at low engine speeds. Just need to work on the high speed air bleeds now and a couple other things.

86camaro383, did you clean out all the passages when rebuilding it? Sometimes there's metal shavings that might be left behind that could clog up something. Also, check the accelerator pump arm play, there shouldn't be any slack.

GOATS2QUICK · 05-11-2009, 05:34 AM

power valve seems wrong for that setup. why a 3.5? you running a monster cam?
i would go with a 6.5.
and maybe jet the secondary down a couple.

86camaro383 · 05-11-2009, 07:40 AM

Quote:

Originally Posted by GOATS2QUICK

power valve seems wrong for that setup. why a 3.5? you running a monster cam?
i would go with a 6.5.
and maybe jet the secondary down a couple.

Im running a fairly decent cam, Motor makes 11lbs of vaccum at idle.

I doubt I got anything in the passages because I when I rebuilt it, it did everything twice. cleaned it with carb cleaner, blew it out with shop air, cleaned it out with brake clean, and blew it out again. Ill have to check the accelerater pump arm. The squirter size is 31, my friend was saying we should try moving it up to a 35 on the primary side, anyone else agree?

five7kid · 05-11-2009, 03:24 PM

The power valve number is what the manifold vacuum has to drop to in order to open the PV. If the number is too low, you'll get a stumble off-idle. You only want the PV low enough to keep it closed during cruise conditions, and you want it high enough to prevent that off-idle stumble.

Try a 6.5 PV first. Putting a larger squirter on would be a band aid for the real problem, and will empty your accelerator pump sooner.

86camaro383 · 05-11-2009, 07:28 PM

alright, Should have the 6.5 PV tomarrow, ill let you guys know how I made out!

86camaro383 · 05-13-2009, 09:14 PM

UPDATE: well I never got a 6.5 PV they ordered the wrong one and got me a 7.5, so i slapped that in...Made a HUGE difference, Felt like I gained 50HP. It was getting to dark to see what color the exhaust was, But felt like it had a little miss, I know it ran rich for a while when I tuned it, so im gonna throw another set of plugs in and go from there. Wish me luck!

86camaro383 · 09-07-2009, 03:33 PM

well its been much much better now, so I havent really messed with it, But i decided to finally get a 6.5 power valve in. It seems like it runs alot better at WOT with a 6.5 power valve instead of a 7.5, but has a light lean bog when I 1st stomp on it. You guys think I should but the 7.5 valve back in and call it a day, or should I increase the squirter size a little bit and see what that does.

onebad82z · 09-11-2009, 04:28 PM

What is your cruise vacuum? I set my PV to that before idle vacuum. I idle at 11" and run a 10.5" PV in my Demon.

Also will question what your timing curve looks like? The more idle timing I added the better my car ran. I have 30 right now as that is what my combo liked best. Get the timing dialed in 100% before ripping into the carb.

Can't understand the 6.5 making a difference over a 7.5 at WOT. Either one would be open (at WOT) and if you haven't changed the PVCR's or primary jets there is no additional fuel.

86camaro383 · 09-13-2009, 06:20 PM

idk about WOT. I know after I went to the 6.5 PV, it just drives nicer all around except the off idle stumble. I dont understand why you have a 10.5 PV in yours when yours idles at 11. So as soon as you tap on the gas, it would open the PV and operate insanly rich. Timing wise its timed at 38* with the mechanial advance. (total timing)

onebad82z · 09-13-2009, 06:29 PM

I set my PV 2-3" below my cruise vacuum for best drivability. Cruise vacuum is 15". Could go higher but they don't make one higher than 10.5" AFAIK.

38 is good at WOT, but what do you have at idle? Big cams like ALOT of idle timing. Vacuum advance is a good thing to have as well on ANY stret driven car. You run it?

onebad82z · 09-13-2009, 06:40 PM

What is the setup on the carb right now? Pump cams? Squirters? Float Levels... all that?

86camaro383 · 09-13-2009, 08:52 PM

dont run vaccum advance....its mechanical advance. pump cams are whatever demons have stock, squirters are 31's, float levels are in the middle line of the sight plug.

onebad82z · 09-13-2009, 09:19 PM

Quote:

Originally Posted by 86camaro383

dont run vaccum advance....its mechanical advance. pump cams are whatever demons have stock, squirters are 31's, float levels are in the middle line of the sight plug.

You have the MSD 85551 dizzy? When is the timing all in? Pink cams are stock, try a blue #427 pump cam in the #2 hole on the primary side.

onebad82z · 10-09-2009, 10:16 AM

Did you get anywhere with this??

CamarosRUS · 10-09-2009, 10:59 AM

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