|The MGA With An Attitude
Setting IGNITION TIMING (practical application) - IG-116A
On 1/3/2010, Bruce Mayo wrote:
"You describe ignition timing techniques and how one might set static timing at 7-10d BTDC. Then you go on to say that at low idle revs the mechanical advance will not have operated, but the dynamic advance setting can be set around 20d BTDC. How can this be? If the mechanical advance is not operating then should not the dynamic setting measured with a timing light and the static setting with the engine stationary, be the same thing?
These are slightly different circumstances. I recommend setting idle speed around 1000 rpm, in which case some mechanical advance is already happening. When static timing is set to 7d BTDC you should expect a dynamic timing reading to be a little earlier, like maybe 12d BTDC, unless you set idle speed abnormally low (500 rpm).
From way back in the late 60's I learned from a veteran MG dealer mechanic that it is a lost cause to push idle speed down in the 700-800 range, especially with the original heavy flywheel. Throttle response from dead idle would be sluggish (to put it mildly), and any fuel saving from slow idle would be minimal, not worth the bother, unless you spend lots of time standing still with engine idling. If so, then you could shut it off occasionally and save more fuel. So set idle speed in the 900-1000 range, and you may be much happier about the way it drives in stop and go traffic. One character to be aware of is that it may have a tendency to run on after shut down, but that's easy to deal with by brakes, 3rd gear and clutch to kill it immediately after you switch off (and no need to install an anti-run-on valve).
The thing is, with idle speed near 1000 there may be some mechanical advance already going, so the dynamic timing may be more like 12d. That's only 2-1/2d advance on the bob weights in the dizzy (but you might also have a little venturi vacuum advance sneaking in if idle speed is set a little higher). The difference then between 12d and 20d is a matter of pushing the timing up another 8d by rotating the distributor 4d, which then translates to more like 15d static timing.
This is not so far fetched an idea as it might sound. Factory spec's for ignition timing were originally set in light of variable quality of fuel in various parts of the world in 1955. Fuel in Mexico for instance generally sucks in comparison with fuel in the U.S. But the workshop manual does not specify different timing for different fuel grade, so it is intentionally conservative to accommodate poor fuel (which we seldom see here). When we have better fuel (generally higher octane) we can use more spark advance to good advantage.
There are other factors as a well, like a high mileage engine may be lower compression, and a well worn distributor may give less mechanical advance than a new one. If you change to a new non-original type distributor, all bets are off, and you get to start from scratch to determine how to set timing. So the original book spec's, and my suggestion to try 20d dynamic timing are all based on the stock distributor, and maybe a not-so-new distributor or engine.
You may have noticed many people on the net recommending 32d maximum advance at high speed (with vacuum disconnected). This is good advice, so keep it in mind when considering the rest of this message.
One more reference is the amount of mechanical advance in the stock distributor. There were several different distributors used in MGA. The common ones were:
40448 DM2 1957 MGA
40510B/D/F DM2P4 1956-1962 -- 12.25d max. mech. adv.
40797A 25D4 (replacement type) -- 17d @ 2500 rpm
-- 9d @ 1100 rpm
-- 2d @ 475 rpm
-- 0d @ 220 rpm
Going by the most common stock distributor, the DM2P4, that 12.25d mech. advance x2 +7d static would give 31.5d at high speed (without vacuum). If you like 32d max then the 7d static becomes 9.5d static, and strobe timing at 1000 rpm might be 15d. Depending on where the mechanical advance starts to ramp up, it might well have something close to 12d (or more) with a strobe light at 1000 rpm (even with 7d static timing). If you are trying to compare strobe light readings with static setting, the strobe light must be used at VERY low engine speed, before mechanical advance starts to move.
Somewhere in my Care And Feeding section I detail Tune Up the Casual Way, setting idle timing by ear without looking at the timing marks. Essentially with low engine speed (under 1000) advance until it shakes, retard until the shake goes away, and retard more until it slows down slightly (~200 rpm drop). This will give the best idle and low speed throttle response character. If all parts are standard issue you might stop there and enjoy the driving. As follow up, if it pings use better fuel or retard timing slightly. In my case, my engine cannot be made to ping under any circumstances, so trying to advance timing until it pings is futile.
If you want to fiddle and check things you can then drag out the timing light to check and record where you have set the idle timing, recording dynamic timing at 1000 rpm and also timing at very low idle speed (hoping to see no mechanical advance). Then using a setback timing light, crank it up to 3500 rpm, or whatever it takes to get maximum mechanical advance, and record the timing at high speed with vacuum disconnected. If you're lucky it might be very close to 32d. If it happens to be more than 34d, then by all means retard timing to about 32d (definitely not to exceed 34d) at high speed (no vacuum). Then you can go back and reset idle speed, followed by check timing at idle to see if idle timing and driveability is reasonable enough to keep you happy.
The shop manual spec for 7d static timing is based on the idea that common folk did not have a timing light in the 1950's, and mechanical characteristics of the distributor would limit total spark advance to a reasonable maximum without needing to check. If you have a timing light, you can approach the entire procedure from the other end of scale, set timing at 32d at high speed (no vacuum), and then check to see how it's doing at idle speed. If you can't get both ends to work out at the same time, then you need mechanical work on the distributor to change the amount of mechanical advance.
You might notice I keep saying "no vacuum" when setting timing. The vacuum unit has two useful functions. First it gives improved throttle response (quick advance) from slow idle so it will pull well off the line when the traffic light turns green. Second it gives improved fuel economy when cruising at partial throttle (significantly more than 32d advance then). But when you stomp the loud pedal full down for max power the vacuum drops and the vacuum advance disappears. This is when you get the 32d max timing at high speed under high load. With lighter throttle and lower combustion pressure the engine can tolerate more spark advance, and the vacuum unit does that. You can see what the vacuum unit is doing by watching the strobe light when you blip the throttle at low speed or connect/disconnect the vacuum line at high speed.
One issue with the vacuum unit is that it kicks in more advance at fairly low speed and light throttle. This can lead to too much advance and pinging in the 2500 rpm range. Some people get carried away with the idea that you can use the pinging signal to determine the best place to set timing, but most of the time that doesn't work. My engine for instance will never ping with any amount of spark advance.
For what it's worth, I have been using a Mallory Dual Points distributor with no vacuum unit for close to 20 years now. Without vacuum it can be a tad sluggish below 1500 rpm (similar to a quick camshaft), but I don't run it down there much anyway, the lighter flywheel helps compensate, and it's only a matter of getting used to the anticipation to poke the pedal a little quicker when the light turns green (or rev it up a bit before you launch it). At road speed I must admit that it probably comes up a bit short on fuel mileage (maybe 5% at cruise speed), but I like the Mallory anyway for its long term durability and reduced maintenance (now at 200,000 miles and still doing fine). Tune-up parts are also very high quality and last a long time. 10,000 miles at 25 mpg is 400 gallons or about $1000/yr for fuel. Vacuum on the dizzy might save $1/wk for someone who drives the car a lot, but it's not the highest priority in my life.
There is another nifty feature in the Mallory distributor. It is adjustable for maximum mechanical advance, so you don't have to change mechanical parts to modify that spec. I can set timing at idle to anything I happen to like for best low speed character, then adjust the max mechanical advance to give exactly 32d at 3500 rpm. It is nice not to have to compromise at either end of scale. When I changed engine spec's and camshaft several years ago I was able to reset the distributor to suit the new engine without needing to send it out to have weights and springs changed.
If that didn't answer all of your questions, feel free to ask more.