killervette666

I'm running a 650 cfm Speed Demon carb, with a performer rpm intake, L-48 engine, aftermarket cam (summit cam, don't remember the numbers but its a bit lumpy). Recently took off the carb to try to tune it and adjust the throttle linkage, put it back on, set the idle, everything seems fine, we set it to idle at 900 rpm, when we put it in gear the rpms drop to 500. What could cause this? We played with the idle-ease thing forever and it didn't make any difference, the only thing we can do is to raise the rpms using the idle adjustment hex key on the right side of the carb while in gear, but then when we put it back in park the rpms end up being way high, around 1250, so basically no matter what we try the rpms are much much lower in gear than in park. Any advice?

gingerbreadman1977

its normal to drop a few revs when going from neutral to gears. i am running a 750 demon and adjusted it to idle to 800 when in park. when i go to drive it drops to 600 or something like that. are you doing the adjustment with a warm motor. every now and again my car has a spack attack and idles at 1100 at the lights or even sometimes low at 500 but its rare. i think thats just carbs in general and a fuller than normal float bowl or vacum variation etc etc that does it.

kevinator80

Tuning a carb is much more than setting the idle speed. Start with a vacuum gauge and set the mixture screws for highest vacuum. Also, don't forget the timing part of the equation. You will have to find what your engine likes.

On my 650 sd, the hex head screw (on the passenger side of carb) adjusts the transition slot on the secondary side of the carb. I think you should be adj. the primary side (screw on the drivers side) and try to keep both about equal.

442olds

how much vacuum do you pull at idle in gear. One thing that will help is if you have a vacuum advance on the distributor. Hook it up to constant vacuum, not ported vacuum. If you have say 15 in hg at idle and the vacuum advance add's 5 degrees over base at idel you can reduce the drop in idel going from neutral to in gear.

I use a megasquirt efi computer to control my timing. It will bump up my timing in gear since my idel vacuum is 3 in hg. Idel in nuetral is 850~900, in gear it drops to 700. The computer kicks the timing up to 23 degrees at 3 in hg when between 650 and 800 rpms. It simply amazes people that I can run a cam that big on the street.

Get your tranisition slots set back up to the proper location (take the carb off and adjust the idel screw and the secondary idel screw to the proper location. Then set the base timing to whatever, adjust the idel screws for max vacuum, hook up the vacuum advance and see what happens. You'll want about 20~25 degrees of advance in gear to keep the rpms from dropping more than 300 revs from neutral to in gear.

gkull

442 explained a possible cure.

In gear is loading the motor down. So the answer is to make as much HP as possible at sub 1000 rpm idle speed. You have to set your idle mix with a vacuum gauge and recurve the distributer advance. so that you get 16 - 20 initial advance without exceeding the 3500 rpm max mech. advance of 38 with iron heads.

You need to use ported vac on the carb so you don't have added advance at idle

Matt Gruber

500 rpm is about perfect in D.
i'm not sure what the problem is?
900 in N is not bad.

jetnoise

Demons do seem to like a lot of initial timing

Your unknown cam specs and converter might be influencing it as well