62Jeff

When my 65 with a 327/350hp engine is warmed up, the idle wanders from 650 to 900 rpms.

The car has the 350 hp bolt-ons, and the high compression pistons, but I do not know if the cam or heads are 350hp components. Heads are double hump with the right part numbers but I don't know what size the valves are.

What can cause this wandering idle?

Details:
Car sat for 4 or 5 years without being started, then I bought it in Feb 2006.

Over the last 6 weeks I've replaced the following things in the fuel and ignition system:
-Gas Tank
-Sending Unit
-Fuel Pump (AC Delco)
-Spark Plugs (AC R45, .035 gap)
-Plug Wires (AC Delco 508N)
-Distributor Cap (AC Delco)
-Rotor (AC Delco)
-Battery
-Battery Cables (Repro Spring Ring)
-and I put a kit in the 6 year old Holley 4150.

The fuel filter has not yet been replaced, the coil is an old Delco 202 coil.

Timing is 8 degrees, dwell is at 30. The idle wandered the same way before I replaced the plugs, wires, rotor, or dist. cap.

Thoughts?

Jeff

ffas23

A possible vacuum leak or carb problem.

Aflyer

I would also suggest a vacuum leak. Let it idle and try the vacuum line to the distributer see if that changes teh idle speed. You can try a can of carb cleaner, spray it around the base of the carb around the manifold. If the idle speed changes you have found your leak. Hope this helps.

George

62Jeff

I should add that the idle rises and drops almost in a rythm, as opposed to randomly idling at 650, then 900, then 650, etc.

62Jeff

Oh the idle definitely is adversely affected when I disconnect the vacuum from the distributor.

I've been leaning towards vacuum leak myself but wasn't sure if a poorly adjusted float would create the ebbs and flows of the idle.

Typically I start my search for vacuum leaks by disconnecting any vacuum connection but the carb to distributor and going from there. Tomorrow perhaps I'll disconnect the vacuum for the power brake booster (which was completely rebuilt by WhitePost 2 weeks ago) and the vacuum for the A/C system, and stick a vacuum gauge on the car to see what's going on.

yukon988

It may not idle as smoothly with the vacuum advance disconnected but I think the point was, is the idle rpm constant with it removed (and the side drawing vacuum plugged)? I would also lean towards a vacuum leak here.

62Jeff

With the vacuum advance disconnected and the hose plugged the idle is still a bit rough.

rgs

My first investigation would be a stickey float/needle valve. Take out the sight plugs and watch the fuel level. If it changes, even just a little bit, it will affect idle.

JohnZ

What's your idle vacuum reading, and which vacuum advance unit do you have (number stamped on the bracket)? If the vacuum advance unit isn't properly matched to your engine's idle vacuum characteristics and isn't fully-deployed against the stop at idle, it'll "dither" in and out and you'll have an unstable idle.

62Jeff

I'll check the idle vacuum and vacuum advance numbers here in a bit. The distributor has a stamped band on it that suggests it is a 250/300 horse distributor. I have no idea if this thing was rebuilt internally in the past to match my alleged 350 horse engine but I'll get those vacuum and vacuum advance numbers for you.

I did put a vacuum pump on both the vacuum advance and the A/C vacuum hose and both hold a vacuum OK.

67L36Driver

If you have a guage, post the figures (in. hg.) for 'starts to move' and 'bottoms out' of the vacume advance unit.

wombvette

As John says, you need a HP vacuum advance. It will stablize the idle and will help with off idle performance. It will stop falling on its face when you accelerate.

62Jeff

Well wouldn't you know it, after taking the car for a 10 mile run on Sunday, the warm idle is now rock-solid at 1100 RPM with the vacuum advance connected. No more idle wandering.

The vacuum is at 18.5 to 19" at that 1100 RMP idle speed and pretty darned steady.

I have not dug deep enough to get the numbers off of the vacuum advance or checked the start/end range with a vacuum pump/gauge. I'll do that and report back under a new thread later on. I would not be surprised to find out it's a low horse vacuum advance.

The car is like an old smoker when I first start it - it wants to clear it's throat a few times as it warms up and it never idles more than about 1100 RPM.

Performance of the car is much like all of my dates - all talk and no action. The car sounds like the chase scene from Bullitt when I run it through the gears but I'm pretty sure my 290 horse XK8 could easily walk all over it up to 60 MPH. My 455 vista cruiser would give it a good run to 60 as well, to give you an idea of how this car feels.

But I haven't given up, I'm sure with some tuning it will run a bit better and with your help and guidance, we'll get there!

Jeff

wombvette

I wouldnt consider 1100 an idle. That high and it should be fairly stable. get it down to about 750 and the roller coaster will return.

ctjackster

it shouldn't such a dog either - that L79 ain't too shabby off the line when it's set up right - should act more like those college gals, rather than that high school prom date

C2BOB

Try putting in 12 initial timing. Turn the idle back down to 600-700 rpm and see if it's improved. If it's not that then it's a lean condition. Lean mixtures hunt.