macdarren

I can't get the car to run well at at an idle below about 1750. If I set lower it will seems to pick some different random RPM between 500 and 1500 every time it returns to an idle. The cam is a bit long meaning I would not expect it to idle well below 900-1000, and in fact below 800 or so it will lope badly and die. If I set it to 1000 it will idle until I run it around or rev it some then frequently it will decide to idle too slow and die. If I set it way high like 1750 then of course it is racing, setting it to something lower however means that the next time it returns to an idle it may decide to drop to below 800 and probably die. I had an older nicely worked over holley 750 (choke milled butterflies drilled etc.) on the motor (427) that seemed to work better. When I had the motor on the dyno the holley sounded smoother and was nice when accelerating the motor however at the top end it looked a bit too small. Dyno pulls showed that the Demon made more power from 3k to 7k with the most gain above 6500 of course, but I didn't really spend alot of time with it at idle and never noticed this won't idle well issue until the motor was in the car.....

I have been told that the Demons don't have as 'nice' air flow characteristics especially hen not at WOT compared to the holleys....or maybe this is just related to the relative sizes. I have played with the idle-ez and it is hard to say if that has made any difference.....I am tempted to go back the the holley but the Demon looks cooler and does make that extra HP across the power band at WOT, but if it won't idle it is rather hard to drive. (I have to keep blipping the thottle until it picks an RPM it can idle at.)

Both carbs are mech secondaries the holley a 750 with work, the Demon a box stock 850. Both have been jetted properly. I believe the idle screws are also set right but it is a little hard to tell with the idle all over. I can set it and all seems well then drive it around the block and it will either be racing or dying a minute later. There doesn't seem to be a binding issue anywhere. The throttle comes back to the stop it is just that the engine seems to want to change idle speeds and I seem to end up adjusting it up or down to keep it running or from racing. This happens with or without the air cleaner. I suspected a timing issue but it seems stable and there is no vacuum advance. I also suspected maybe a vacuum leak at the carb but have been unable to locate any, and I have put in new felpro gaskets. Aside from moving back to the Holley to see if the problem goes away, does anyone have any thoughts?

TIA
darren

Dicecal

http://lbfun.com/Corvette/Tech/vette...up%20Paper.pdf

macdarren

Thanks I'll check that out!

1coolC2

Darren,
Post this over on the C3 forum. Lots of guys with Demon carbs that have had to work through all kinds of power valve and idle circuits issues. Good bunch of guys with allot of experience tuning carbs.
R/
Jeff

aaronz28

Darren,
You'll need to give your cam specs (at .050_) and let us know which model Demon you have..

Barry Grant specifically designs the different models to work with different cam durations at certain cubic inch displacements...


while this might sound bizarre... I can tell you from experiance that I tried a Mighty Demon 750 on my DZ302 and it was very rough... I called BG and they said that was because the 750 was far too big for a 302,, I then put a Mighty Demon 650 on it and couldn't be happier and I didn't have to touch it.


if you put the wrong Demon on your motor, STOP, youll never get it to run as it should.

Thanks

Aaron

macdarren

Aaron,

Sounds to me like maybe I have too large a demon as well. I had planned to go with the 825 mighty demon but when I had it on the dyno all that I could get was the mighty demon 850...given the modified holley 750 was a bit small (I don't recall the flow checked number after it was modified, as it is has been a long time but I thought it was in the 800 range) (it was an excellent carb on a previous incarnation of this motor), maybe the 800 or 825 demon would indeed be a better choice, but my past holley experience didn't lead me to think that the difference from 800 to 850 would cause this sort of problem, but then the demons are different...

My cam specs are as follows: .672 gross lift intake, .638 exhaust
with 262 degrees intake and 269 exhaust duration at .050 lash is .020/.022 respectively, lobe centers are 110....displacement as I mentioned is 427ci and static compression is just under 11:1 (actual is probably a point lower given the cam overlap) Until I can figure out this idle problem I have been running a race/street fuel mix that works out to about 100 octane.

Other factors might be the carb spacer (Wilson) matched to the victor jr intake and the carb.

This combo was fine on the dyno but then I wasn't trying to get it to idle at any particularly slow speed. It did not seem to have the tendency to die that it now has in the car. First thought was float levels being off since the dyno is more level than the motor mounted position but I have adjusted things the little bit needed to make them correct in the car.

Any help would be appreciated.
Darren

macdarren

ttt

PAmotorman

i think deamon carbs come with a idle speed adjustment inside the carb. you get to it by removing the air cleaner stud. the idle transfer slots so be open no more than .020 and after you set that you set the idle speed as i posted above. check here,http://www.barrygrant.com/fromBarryG...e-Addendum.pdf