The Stovebolt Site Forums The Shop Area Engine Shop 261 timing

What's the best way to go about timing a 261?
The shop I had the engine rebuilt had a former mechanic who liked to leave distributors loose for easy adjustment on the fly, but then when turning them back over to customers liked to forget them, and then a couple months down the line the timing works off.
How does one go about resetting the timing? I have it where it idles fairly smooth again just turning the distributor by ear, but would like to set it properly.
Is a vacuum gauge the best method?
Or is just moving it to where it runs and idles smoothly ok?
It's idling at about 475-500 rpm right now fairly smooth where I've got it.

If I remember right the factory spec is 500 RPM idle, at least on my 235 it is. Sounds like you've got it back in a good spot. If you're running points I use a tach and dwell meter to dial it in at 500 RPM.

I use an Innova "delay flash" timing light, and set the timing at around 2K RPM with the vacuum advance line disconnected. With the centrifugal advance maxed out, you're looking for around 35 degrees of advance. When the vacuum is functioning at light throttle cruise, it adds around another 10 degrees for good fuel economy, which goes away as soon as the throttle is opened up for hill climbing or passing. Most flywheels have the timing BB positioned at 5 degrees of advance, so dial in the flash delay to 30-32 degrees to account for the offset. Once the initial timing is set, find a long hill to pull at wide open throttle and listen for detonation for a final test. You might need to drop back a degree or two to eliminate any rattle on a hard pull.
Jerry

I see. They used to change the timing depending on the fuel right, so no harm in experimenting with it (within reason) or is it something you want to have dead on or it will cause damage or undue wear to engine?
Also, what do you mean by detonation?

Detonation = pinging.

Just asking here but what year is your 261 and what is the delco number for your distributor. Take the cap off, it’s stamped on the distributor plate. If it ends in 2403, you have the 56-63 distributor. These distributors are generally 12-14 degrees of mechanical advance which is 24-28 degrees at the motor. It’s always a question as to whether any particular distributor is actually getting to full advance.

Several months ago I had a nos 2403 distributor on my sun machine. Could not get anywhere near what it should have been. Took it apart and between a little surface rust and dried up hard factory grease, the advance weights were not extending and retracting properly. A little cleaning, polishing of the weights and fresh grease and it was back on track.

What I would do is figure out what year motor you have and see what the timing for that year is supposed to be. Some are Tdc others are 5 degrees before Tdc (advance). Set the octane selector plate at zero, set the distributor to Tdc or 5 before using the bb on the flywheel and the pointer on the bellhousing to set that up. Once it’s running, you can add a little advance by turning the distributor counter clockwise. If you have a dial back timing light, this process is a little easier. You add a little advance, adjust the idle speed. Take it for a drive. Add a little more, take it for a drive. If you hear pinging, take some timing out of it. It would not surprise me to see that motor take 8-10 degrees advance with todays fuel.

One last thing, you do not adjust the timing by loosening the octane selector bolt. Set that on zero and leave it alone. You set the timing by loosening the screw on the vacuum advance/distributor clamp.

What I do is static time the engine at TDC "U|C" on the flywheel with the Octane Selector set to zero. Then I dial in 5 deg with the octane selector and take it for a ride. That way I can see how many degrees advance I'm running at a glance -- well, a squint anyway. My problem with using a plain ol' timing light is that I can't get the idle down to 450 RPM which is theoretically just below where the mechanical advance starts.

Too much advance will produce pre-ignition that sounds like pinging and is when the flame travel is normal but is lit off too soon. It generally is not too harmful but robs power and fuel economy and is otherwise undesirable. It can happen at relatively light throttle, if say the vacuum advance doesn't retard soon enough (too weak a spring, some of them have springs that can be shimmed more or less). Recent repops at all crimped together and not adjustable. There is a spec for how much advance at what vacuum.

Detonation on the other hand is when the whole intake charge goes of all at once. This is a really bad deal and burns or knocks holes in pistons, breaks spark-plug insulators, hammers connecting rod bearings and is to be avoided at all costs. Most often it happens at full throttle or close to it. Often one can't hear it because of other engine noise like exhaust, intake "honk" from the carburetor, gear noise, wind and the like. Lean mixture is often a cause, along with excessive heat often caused by lean mixture. Airplane engines have exhaust gas temperature gauges to prevent this sort of thing and pilots are trained to watch the gauges. Red hot carbon deposits can also cause this; also spark plugs that are too hot a heat range, which can act like glow plugs in model airplane engines. Typically these problems aren't common in production auto and truck engines but when one starts raising the compression ratio it can be a problem with low-grade fuel.

Normal fuel burning is like the ripples caused when a pebble is dropped into a puddle- - - - -concentric waves of flames beginning at the spark plug tip and moving outward until they encounter an obstruction like the edges of the combustion chamber. Then the pressure wave is applied to the top of the piston to push it down the cylinder. Ideally, the pressure wave should encounter the piston right at the top of its travel so no energy is wasted trying to push the piston backwards, or chasing it down the cylinder if the spark happens too late. Since the flame front speed of a correct fuel/air mixture is pretty constant, the spark needs to happen sooner as the engine speed increases. The advance can be done by centrifugal weights, manifold vacuum, or a combination of both, plus the position of the distributor housing (initial timing) that determines when the whole process begins. Electronic ignition timing by computer is much more flexible than the old mechanical systems can be, sometimes altering timing several times a second. There is a practical limit to how much timing advance can be dialed in, and even one degree "too early" robs as much power as five degrees "late". I've proved that fact on dozens of dyno runs. Trying to nurse that last bit of torque out of an engine by tinkering with the ignition timing can cause serious problems. Volumes of information have been written on the causes and effects of preignition and detonation, and a soon as someone starts bloviating about it, someone else will dispute his findings. Don't get greedy about spark advance, or running a too-lean fuel mixture, and most of the problems will go away.
Jerry

Every Saturday in good weather i ride a 1960 Matchless motorcycle about 20 miles on my errands. It has a magneto ignition with a manual-only spark advance/retard. One doesn't forget to retard the spark on shut down lest it kick back unmercifly when kicked over next time. I have found that the timing doesn't matter much when running down the road. Holding the throttle steady and advancing or retarding the spark doesn't change things performance wise. What does matter is that it will begin to buck at idle and off-idle when riding through town unless i retard the spark about half way. Timing on this engine is a very inexact process involving a machinist scale stuck down the spark plug hole looking for 3/8 inch before top dead center. In the past when I got the timing a little too advanced I found that performance was good and more or less unchanging as I advanced the spark -- up to a point after that power fell off and the engine ran "hard." This is all to say I agree with Hot Rod's contention that too much advance is way worse than too little.

In many engines the advance can be constant over a wide RPM band because of such phenomona as squish and swirl in the combustion chamber that accelerate burning. Perhaps the hemispherical combustion chambers of the Matchless single is one of these. I suspect that the combustion chambers of a Stovebolt Six are not. These chambers are a sort of modified "bathtub" chamber that was used way back when because the design tolerated low octane fuel. I would guess that swirl is almost nonexistant so they require significant advance as RPM increases. Perhaps someone who has studied combustion chamber design more than I has some further comments. I'd like to know about bathtub chambers vs wedge chambers vs hemi chambers. What did GMC use? they're much better than Chevrolet of this era or so I'm told.

10-4 to all!

1 Ton Tommy, If all those engine designs get your head spinning, study the Honda Compound Vortex Controlled Combustion engine of the 1970s. It has two combustion chambers per cylinder and burned so efficiently and cleanly that no catalytic converter was required to meet emission standards.

I had one of those in college and got 35mpg with no computers and breaker point ignition.

Honda actually built experimental CVCC heads for a Chevy V8 and accomplished the same thing. That was after GM engineers claimed it couldn't be done on a full grown engine! Sort of embarrassed the entire US automotive industry!
Jerry

That sort of arrogance was a major problem with GM back then and in some ways remains pervasive in their corporate culture.

That's why they lost so much market share to the Japanese in the '70s. Once that ball got rolling the American auto industry couldn't stop it.

A girlfriend of mine had an early Honda Civic, maybe 1973. When almost new, we drove it up the freeway a hundred miles or so and back. As I recall it got nearly 50 MPG. It may have predated the CVCC head, I don't recall. I do remember thinking "What magic do they know?" BTW it was also fun to drive.

Yes, it would have been a CVCC. They came out in 1972. Mine was a '77 with a 5 speed. I was a lot of fun. Since it only weighed about 1500 lbs it was fast as hell.