Understanding timing.

[n1265]

Hi Guys,

well I just re built the carb in my M880 and adjusted the idle and timing. My book says the '77 318 should be set at 2 Before for light duty, 2 After for heavy duty and 6 Before for high altitude with an idle speed of 750 RPM.

So now I am wondering, Whats the difference ? Is 4 deg really all that much and what do you gain / loose when you advance the timing ?

And where do you guys like to run your 318's and why ?

[712edf]

Advancing ignition timing usually gains power, up to a point. Then you get detonation if it fires too soon. This results in knocking because the burning mixture is pushing down on the pistons as they are trying to come up & compress the air in the combustion chamber. Advanced timing makes for harder starting/cranking possibly.

Yes 4 degrees can be critical. Each engine has its own personality & should be adjusted accordingly...... Two gals may be the same size, but a particular dress may only look good on one of them.

Bucky

[QuantumJo]

The rule of thumb that I use is to set the timing with a Vacuum Gage.

[n1265]

The rule of thumb that I use is to set the timing with a Vacuum Gage.

How do you get it to stay on the plug wire ?

[n1265]

Ok, So I just read a post on another forum about using a vacume gage to set the timing. and will try it.

My question is , when using a vacume gage and then double checking with a light, How much more timing would you expect to see from the book setting ?

I mean, If I use a vacume gage, then put a light on it and read 12b when the book calls for 2b, would this be an indication that I did not do it right with the gage ?

[712edf]

Not neccessarily. The ultimate ends to setting the ignition timing is too get the engine running as good as possible. The stronger vacuum reading is prefered over matching numbers in a book, should they differ.

A stretched timing chain will confuse timing readings too.

Running good & easy starting!!!!

Bucky

[n1265]

Ok, here is where I am at now, let me know if anything seem out of place.

Idle adjustment screws : 3 turns out
Idle rpm : 750
Idle vacuum : 18 in. and steady
Timing : 8 deg BTDC


The 3 turns on the Idle screws is the point where it does not increase rpms if going further out. Is this where they should stay or should I turn them back in 1/4 turn ?

When you snap the throttle, the vacuum gauge drops to zero, shoots up to 23in. then settles slowly down to 18in. is this an indication that I should advance the timing more to read 23in. ?

( the reading is taken off the lower port on the carb )


any advice welcome...

[MoparNorm]

Depending upon which vacuum port you use, the vacuum dropping to zero at acceleration is normal.

Adjust the carb to max vacuum at idle , then, when you have that set, adjust the timing to max vacuum at idle. You may have to retard it slightly for easier starts, but max vacuum is the desired setting for a running engine.
Throw away the timing light...*

Why does it work? Because the engine is simply an air pump. The more efficiently that you get air in (the carb and cam settings) burn it completely and get it out (timing and exhaust), the better and more efficiently your engine runs. That translates into power and fuel consumption.



*Timing lights and factory settings are worthless if you have performed any engine work. Such as re-bored, cam, exhaust, intake manifold, carb, etc.

[n1265]

Well, The best I can get it is 18in. vacuum at idle. The same settings as above except now I am at 10 deg BTDC.

Starts and runs pretty good except sometimes I get a hesitation when I go to take off. I got new vacuum hoses and air filter.

Any idea what the hesitation could be ?

[712edf]

Hesitation usually in the carb adjustment.

Bucky

[MoparNorm]

Hesitation usually in the carb adjustment.

Bucky

Possibly the accelerator pump.
Often too much fuel on acceleration will give you the same results and symptoms as too little fuel.

[KRB64]

This is good stuff.

[n1265]

The pump itself is new so I'm sure its not an issue with a worn or damaged cup.

I could try to re adjust the pump but before I do that I am wondering if the problem could be from having it leaned out to much, ( 3 turns on idle screws) and then having it hesitate when I take off.

Of all the adjustments I made on the carb, the one I least understand is the idle screws. I started at 2 turns out, turned them both 1/4 turns out until it did not increase the RPMs any further and left them at that point, which happens to be 3 turns.

Is this the right way to set the idle adjustment ? or am I barking up the wrong tree in thinking it could be the problem with the hesitation ?

another thing that confuses me is why the hesitation only happen sometimes ?

[QuantumJo]

What kind of carb do you have?

[n1265]

It is a Carter BBD , tag number is # 8146