72 350sl idle spec & stumble

[knarf]

According to my Haynes manual the idle specification for my 1972 350sl is 700 - 800 rpm at operating temperture, timing set to 5 atdc with the vacuum retard connected.
Does this mean that it's ok for the idle to fluctuate between 700 & 800 rpm on a properly tuned engine as mine now does or should the idle settle dead on a number within this range?
I'm trying to diagnose an off idle hesitation and stumble between approximately 1100 and 1500 rpm at normal operating temperatures.
At idle and over approximately 1500 rpm the engine stumbling is not evident at normal operating temperatures.
Fuel pressure 2 bar, vacuum 17cm/hg, trigger points clean, throttle switch new, plug wire resistance 6.10 - 6.45 ohms.
Before the engine warms up no hesitation or stumbling is evident in any rpm range.
I'm suspecting bad fuel injectors.


Any and all thoughta and comments appreciated

Knarf

[psfred]

What do your plugs look like? Are they all the same color? If you have dying injectors they are either staying open or not opening properly -- hanging open will make that cylinder run rich, not opening will make that cylinder run lean.

Idle speed should be pretty steady. May float a little, but wandering constantly would suggest to me that something is wrong.

If your problem is only evident at certain throttle openings, check the throttle switch -- it must cycle as it opens and closes. If you have wandering idle problems, it probably isn't the throttle switch.

Off idle hesitation and lumpy running can be a vacuum leak, believe it or not. If the hose to the MAP sensor is bad, you can have good vac at the manifold and variable vacuum at the MAP. Leaks around injectors will also make some cylinders run lean. Leaks at the various hoses on the vac side at the idle circuit can also drive you nuts, as can leaks at the intake manifold plenum (there are 8 rubber rings between the manifold halves). Check with some brake parts cleaner or propane -- if the idle changes as you spray or jet propane on a spot, you have a vac leak.

You may also want to check the temp sensors -- air temp sensor (in air cleaner snout) 300 ohms at 20C, 210 ohms at 30 C, 105 ohms at 40 C (105F, should be close at body temp). Coolant sensor 2500 ohms at 20 C (68 F), 1200 ohms at 40 C (105 F)325 ohms at 80 C (175 F).

The book says that running too lean at idle will make the engine run even leaner hot, so you may want to check the CO and then bump the idle CO up a little (or just turn the adjustment screw one click richer -- clockwise, I think).

I'd also check the timing chain stretch -- if the valve timing is getting too far off (right side will be worse) it may be time for a new chain.

Peter