Carb tuning, big cam
 

Got the top-end together and trying to dial it in. About +12 deg at the crank. Im using a speed demon 750 vac sec, with the e-z idle feature, the screws are 1.5 turns out and the butterflies are seated with the.020 of the idle transfer slot exposed per instructions. I cant get it to idle any where close to 1000rpm. The cam is a mopar purple w/.484 lift. dont know the duration or overlap specs. Auto trans w/slightly higher stall. What do you folks know about drilling holes in the butterflies for more air with larger cams. All help is welcome, thanks guys!

I also have the same problem. I have a much milder cam but I can't get a reliable idle at anything less than +8. Stock should be TDC for my application. I assumed I would only have to advance to maybe +2,+4. I will be interested to see what the solution to your situation is.

Can you adjust how far the secondary butterflies are open? If so I would leave the primary set so the transfer slot is proper and try using the secondaries to get proper idle speed.

what kind of fuel pump do you have in the car?
what happens if the car idles below 1000?

Stock pump, not flooding or starving. Wont idle at all. I did a compression test today and came up with 130 psi in all cyls, 140psi when wetted with some oil. I need to back up for now and do some thinking. Thats kind of on the low side. I'm going to do a leakdown test this weekend cause I think I need to assess the general health of this motor before I spend any more time trying to tune it.

good idea on the leakdown test

I'm sure you have checked all this but:
- correct routing of plug wires and CCW rotation of Dist
- TDC of #1
- No leaks around intake or carb
- Good carb
- fuel pressure
- clean fuel filter

X3, but thanks.

I believe the cam you are using has int/exh duration of 241 @.050 lift and is designed for rpm band of 2200-6000. the lobe separation is 108 degrees.
large valve overlap results in poor idle.
high duration cams require more initial ignition advance at idle to overcome the valve overlap . Just listen to any big Harley.
most Hot rod pubs. suggest initial timing of:
10 -12 degrees with cam duration < 220 degrees.
14 -16 degrees " " " < 240 "
18 -20 " " " " < 260 "
You will need to limit total ign advance depending on how you run the engine..
Joe

I have 16 deg timming in the Dart with 610 lift ~250 dur w/ lobe sep ~110. Dial in a little more timing and possibly lean it out a little.

Thanks Joe and C4C! That jives with what I just read in Mopar action. that purple cam is as you described, and said that it was more of a track/hi-rpm cam due to the overlap and what-not, like you said. That cam is not a fit with the way I'll be using the car. too big. But I just need to tune and limp it through till next year. Thanks for the advice,and I'll hope to let you know how it comes out this weekend. Thank you guys.

an other method you can try is as follows:
1-put timing light aside
2-install vacuum guage to the manifold not a carb port.
3-set carb to the stock spec- idle and mixture with engine off.
4-start engine set idle speeed to 1000 rpm and rotate distributor for highest manifold vacuum/ reset idle to 1000rpm/ rotate dizzy again for highest vacuum reading. your are looking for 1000rpm with highest vacuum reading. (may be only 15in.hg. because of cam)
5-then retard dizzy till you reduce vacuum by one inch hg.
6-go back to carb and set mixture trying to regain the one inch hg. you gave up in step 5
7 - you may have to play with this for a while to get it close, but this is the ign advance your engine build wants.
8- hook up the timing light and note idle adv and total adv.
9- now comes the fun part-
depending how you run the engine you may have to reduce the total (not the idle) dizzy adv. to prevent hard hot starting / pinging/detonation on pump gas.
this can be done by stronger springs or disabling vacuum adv.
please let us know how you make out.
Joe