Arrowstorm

Ok, so... I installed a tach on my 72 Vista Cruiser with an upgraded 350. It now has an aluminum intake, 4 bbl carb, HEI distributor and double headers.

Now, I have read that with conversions to electronic ignition, one should advance the timing a bit from stock 12º to 16º, and I have done so. Now how about idle rpms? Now that I have a tach, I can set idle speed based on rpms and not just vacuum pressure like I have been doing.

Stock manuals that I have seen show a 72 350 should idle between 575-600 rpms in park, but my 350 doesn't like that too much. It tends to run rough in park and even more so in drive at a full stop. However, if I up it to about 800-900, although it SOUNDS like a high idle, it runs much smoother. Is that bad?

My brother in law tells me that you don't want idle rpms too high because you can burn your transmission like that. But even with the idle up at 800, the transmission doesn't "clunk" into gear or anything. In fact, it's quite smooth.

Any help with proper idle rpms on a 350 with electronic ignition would be helpful. Thanks

Buz

TripDeuces

"I have read that with conversions to electronic ignition, one should advance the timing a bit from stock 12º to 16º, and I have done so."
Wow, total BS. Not to say upping the timing is a bad thing but it has nothing to do with the type of ignition used.

You haven't really done anything to effect the idle speed and smoothness of your idle so my thoughts are you have a possible vacuum leak and/or a poorly adjusted carb (IE idle mixture). For adjusting the idle mixture I'd use the vacuum gauge only trying to get the highest number you can. The idle speed will follow and get higher automatically. You should have to keep turning the idle speed down as the vacuum reading gets higher. Eventually turning either screw will do nothing better for you and you'll be done. If you can't set that idle screw in park to your desired RPM then you have another problem.

oldcutlass

It depends on which distributor your using as to what to set you initial timing to. If its a points replacement in your stock distributor then Trip is right, maybe a couple of degrees. If its an Hei then the statement is true because of the way the mechanical advance is setup.

Your idle quality is also determined on how your a/f adjustments. If they are set wrong it will idle like crap and raising the idle only masks the problem.

Octania

Keep in mind that if the idle is set high by opening the throttle... too much, then the off-idle port will be exposed and the idle circuit will be not much in effect, therefore, the idle adjuster screws may have little or no effect. Sometimes the factory or later rodders will drill a small hole like 3/32" into the throttle plates near the idle passages, in order to allow more air thru so that the butterflies can be set more nearly closed, therefore keeping the idle circuit active.

Arrowstorm

The set up is an accel HEI distributor with a 4bbl edelbrock carb.

The f/a adjustments have been made thus: Starting with passenger side screw, turned out until vacuum gauge shows highest consistent pull (19in on mine), stopping when further adjustment makes no difference. Adjust idle screw to "desired rpm" (this is where I am unclear). Repeat process on passenger side screw and again adjust idle screw to desired rpm. Then move to driver's side f/a screw and perform process to highest consistent pull.

And I know there's debate on this, but I leave the vacuum advance plugged in to the ported vacuum on the carb.

What I'm unclear on is the "desired rpms".

Fun71

I don't know what the advance specs are on the Accel HEI but the GM units typically have ~14º mechanical advance, so to get the total in the right range (34-36º) the initial is set to ~20º.
The factory points distributor likely has ~24º mechanical advance, so the initial is set to ~10º to get the total to 34-36º.

If you set the initial on a GM HEI to 16º then the total will be low at ~30º.

So, my recommendation is that you find out how much mechanical advance your distributor has and then set the initial to whatever is needed to have the total in the 34-36º range.

DoubleV

Just give manifold vacuum a try then. It will make your idle much stronger and smoother meaning you can back off on the idle adjustment.

jag1886

What he said works great. You also need to adjust the carb more than once. Get the highest vacuum, turn down the idle and readjust again, sometimes takes several adjustments to get it perfect.

TripDeuces

Your stock manual stated you want 575-600 in park. That was the desired RPM I was referring to. Since all the mods you made basically leave you with a stock engine it still should work just fine.
I should have stated that doing what I said does take a few tries to max it out. I've done it so often it's become second nature to me, sorry for the confusion.

Octania

Agreed on using manifold vacuum to the HEI
Try it, cannot hurt, easy to change back

"the desired RPM" is usually the service manual spec.
Ideally, you could idle at oh ONE rpm and use almost no gas to idle, but in real life the engine dies below a certain RPM. So, set it as low as it can tolerate and still idle OK. In Drive.

yankees

I just did the same conversion with stock points and bumped timing from 7 to roughly 14 or 15 degrees. Runs great and smokes the Mickey's easily now. Set park idle around 800 and 700 in drive. I assume my total is 38 or 39 degrees, if factory advance is 24 degrees.

rustyroger

When you are stationary and the transmission is engaged the torque converter slips to allow the engine to keep running, the clutch packs, bands etc will either be locked in engagement or freewheeling as they are designed to do. The converter is designed to do this, if it shifts smoothly into drive or reverse smoothly you have nothing to worry about.
Btw, I have always been advised to leave the transmission in drive while waiting for traffic lights or whatever, wear in the transmission mostly occurs when it engages gears.


Roger.