Engine Vacuum?

According to the cam card, the lobe separation is 112.
Lobe centerline is 108/116.
Duration at 50 is listed as 259/266.

I would have thought that I should be pulling 12-15" Hg with that cam. I just don't want to spend time chasing problems that may not exist....

How big a motor and did you degree the cam??

I would think it would like 28 at idle and 38 all in?

and how low you trying to idle it?

505" engine.
I can't comment on the cam being degreed, the engine was assembled by a local (and well respected) Mopar/race engine builder.

Idle is around 8-900rpm.

Thanks!

If you have 16* initial timing and 800 idle speed, i dont think its very low.
Up timing to 22* and idle speed to 1000 and youll see 12" vac.

My apologies for the lack of info... I haven't really tuned this car in probably 2 years, and it's done probably 10-20 miles in that time. I did rebuilt the carb a year ago.

Plan for tomorrow is to check timing and valve lash, and to go from there. I seem to remember that the engine was once pulling 12-13" Hg, which could be somewhat confirmed by the fact that I put a 5.5" power valve in the carb last year.

I need to go over it all and get all the baseline sorted, before I get to worried about anything....

Cool I would definitely go with more initial advance and total 36 to 38 just turning it to 38 total may work wonders. so if you turn it to 24 that should give you 38 total.

Big enough cam you may want the timing to come in sooner it's a try and see from hear.

often it's tinker tinker tinker.

I still think you need more timing at idle.

but first I would check the power valve.

Then you can do this without driving start turning the advance up even as high as 32 or more at 1000 rpm see what it really likes for idle advance. Later turn it back till you can recurve the dizzy if need be.

That's a big enough cam even in a 505 and big cams like lots of idle advance.

I take it you have re set the float bowels?

Hi Dodgem,
I reset the float bowls last year when I rebuilt the carb (or re-gasketed the carb, as it were).
I thought I should double check the levels, but just ran out of time.
I'm somewhat lacking in the tuning area, mostly because I don't touch the car for a year, then forget what I learnt in the past. Indeed, took me ages to lash the valves, simply because it's been so long... and I forgot all my little tricks to getting the awkward plugs out to turn the engine.

And yes, I plan on tossing a different power valve in it.

Anyway, I'll get back to it on the weekend, unless I'm home from work early during the week.

Also, I found this in amongst my PMs from 5 years ago when this engine was first fired up, from the gentleman that spec'd the engine out:

"I would expect about 10-12 inches of vacuum on that combo. what are the idle rpm and timing set at? dominators don't have any vacuum ports so hopefully you drilled in a good spot. that combo should idle around 950 rpm with timing at around 18* btdc.
if those are correct i'd take a look at the valve lash. that cam is 259/266 @ .of with.587/.593 lift on a 112 lobe center. it should have plenty of vacuum if installed at 108* and the lash is correct. with the engine warm, lash should be .016 intake/.018 exhaust. aside from the vacuum issue, how does it run? throttle response should be crisp and power should be plenty to roast the tires at will. if it's not that way i'd say the tune is off considerably."

If nothing else, tells me I should be looking for more vacuum than I'm getting (which we already knew).

Keep at it!

Thinking about it over my lunch break right now, I think I had a brain fart when setting up the carb after last years 'rebuild'.... I don't think I set the idle through both of the idle speed screws. That probably means that the primary side butterflys are open too far, exposing too much of the transfer slot, and leading to all sorts of problems....

Yea i like to have the throttle cable off to start and count how many 1/2 turns it takes to close both butterflies the make them exactly even and always open and close the same after that and set all four mixture screws the same too! especially on a Dommy.

Been struggling to find time lately, but messed with it a bit yesterday.
Dropped the PV from 5.5 to 3.5, in case it was pulling a little fuel that it shouldn't have with the low vac condition.
Tried a few different things (different idle speeds etc).
After the last change, adjusting the idle mix again, I fired the car up and got excited because I saw the vac needle jump up to 15"! Then I realised that I was still in neutral. And that the car was idling at 1500 rpm.
Shifting to drive, it dropped to 1000 rpm and 10".
So I'm getting there, but I'm not sure how I've screwed up the tuning so that I'm 1500rpm in neutral and 1000 rpm in drive?

I need to investigate things a little more. I found record of a previous tuning set up where despite having larger IFRs etc, my idle screws were at about 1 1/2 turns out. Now they're only 3/4 turns out. I must have changed something somewhere (wrong IFRs or something) and getting more fuel at idle than I planned?
I'll go back to that set up and see what happens.

Cheers