lopey idle when vac advance is not hooked to maniofold vac

kalli · 08-18-2009, 05:54 AM

Hiya,

just wondering if that's normal, so I can take my mind off it.
Before I had Mallory distributor where you can adjust the vacuum advance canister.
I had that hooked to manifold vacuum and adjusted in such a way that at idle I wouldn't get more than 10 degrees advance from it. So car was idling at 22-24 degrees all plugged in.
I changed to the pertronix and the advance is set to 16 at idle and I can't change that.
As this is too much (jitters when driving slow), I changed to ported with the initial still being 14.
Since I did that the idle is way rougher than it was before. that normal?

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fakesnakes · 08-18-2009, 08:44 AM

Was the cam installed straight up or advanced? Can you measure your vacuum at idle? Also, what is your idle rpm? Can you adjust mechanical advance rate and peak?

You may need to play with it to get it to the right balance for your car. Try retarding the base timing and going back to manifold vacuum. Remember that with manifold vacuum it will retard as soon as you open the throttle plates, but the mechanical advance will start to kick in. You want to find the point where the transition keeps the engine in the power curve through a variety of gradual to wot accelerations.

Starfury · 08-18-2009, 11:30 AM

Manifold vacuum to the VA will smooth out the idle. That's one reason to run it on a car with a hot cam. As long as driveability is ok, I don't think there's a problem. Just make sure you check idle mix and speed.

kalli · 08-18-2009, 12:15 PM

it's driveable ok. just sounds completely different than what I was used to, so i got a bit worried. Idle mix and speed is already done. first thing when I slapped it on.

But since I have the pertronix distributor with different advance curve I definetly will have to re-check everything. I think I better start from scratch with adjusting the machanical advance and the idea to compensate with retarded initial timing seems good to me to make up a bit for the bit too much advance on full vacuum.

Good to hear it's normal that idle smoothes out when igniton is advanced on idle.

Will get back with numbers

67mustang302 · 08-18-2009, 02:45 PM

The rpm cam is a decent cam, it'll make some noise at idle. I idle mine at around 750-800 and when the fan kicks on it's at like 675-700 and a bit choppy.

kalli · 08-18-2009, 02:59 PM

Quote:

Originally Posted by 67mustang302

The rpm cam is decent cam, it'll make some noise at idle. I idle mine at around 750-800 and when the fan kicks on it's at like 675-700 and a bit choppy.

have it at 850 currently and don't want to go lower to keep the water flowing and fan running (mechanical). like the choppy idle, just not used to it. had to make sure first it's a normal effect

67mustang302 · 08-18-2009, 03:04 PM

Like I said, the RPM cam is a pretty decent sized cam for a 302, so it's gonna have an aggressive idle.

kalli · 08-22-2009, 07:05 AM

so ... i disconnected all vacuum and plugged and i timed the car to be 34 degrees advanced at 3500+ (it doesn't increase anymore there). that left me with timing of 6 degrees at idle. with the manifold hooked up idle at 22 degrees. that all resulted in peppery splugs, so ..

i changed the combination (weak and medium spring) with (heavy and medium spring).
Now I have about 13 initial with 34 at 3500. that sounds way more reasonable.
I have about 15" of vacuum at idle, which will advance by about 16 degrees when hooked to manifold vacuum, resulting in about 28' degrees at idle with all hoses connected. that usually gave me driveability issues (jitter at slow speeds).
So i plugged it into ported and it seems fine. It is unfortunate I can't adjust vacuum on the pertronix.

Anyway it's got more power than the combo before, even my lazy ass can tell. I'll check on the plugs now.
I can only hook up the LM1 or the Gtech at a time. so wondering what to do next.
But i think i'll do a few runs with the gtech and 8 degrees, 11, 13 and 15 initial to see what's best, then tackle the carb (which seems very good already since i changed jets and pump cam)

Kalli