Discussion on point gaps and timing

David G

Well-known Member
I see a lot of discussion on point gaps and timing, so here are my assumptions:

Point gap on a magneto must be set so the points open when the magnetic field is strongest, so it is critical.

Point gap on a distributor based system is the best of both worlds for being able to open/close correctly through all the RPM ranges. It seems like .020 is the industry standard. Initial timing should not change between subsequent times points are being installed, and should be considered a double check that all is correct. If you set the point gap consistently initial timing should be exactly the same each time when new points are installed. Timing should ALWAYS be done after points are installed or cleaned up. If timing is different, something is worn or set wrong.

If point gap is too close, timing will be retarded, if too loose, then it will be advanced. I use this on small engines with battery powered coils to adjust the timing a little of I want to.

Initial timing should be advanced enough to allow for the spark to occur at the right time when full RPM, but not kick back on starting, period. The distributor has a mechanical advance in it to move the timing from the initial value to full advance. It does the best it can with only being able to look at one variable which is RPM. A computer can control timing much better, because it can set initial timing at 0, it can look at RPM, load and temperatures to determine advance.

The manuals for the tractors which were built in the 40s-50s assumed the tractors would be hand started and fuel was low octane. The optimum timing can be achieved by ear, and is usually a couple of degrees more advanced than the books have it. I run at full throttle, then adjust timing to max RPM. I then check it with a light as a double check. You will want to back the timing off a little if it kicks back when starting hot, if you work it really hard to prevent pinging, or if there is a flutter at idle due to timing too advanced there.

Timing too advanced does NOT cause run on, it is 90% caused by idle set too high.

If timing bounces, then you have an issue with advance mechanism, worn gears or worn bushing, probably worn bushing.


Please post your comments, always great to have good discussion.
 
Agreed for the most part with the following exceptions:

Point gap set too close in a Kettering ignition system results in advanced rather than retarded timing, all else equal.

Accurate ignition timing cannot be set by ear.

Retarded (not advanced) timing can cause run on due to excessive exhaust temperature but agreed that the most common reason is engine idle speed set too high.

Dean
 
I believe that firing happens when the points open, if gapped wider, then they open sooner (advance), if closer later (retard). Please tell more on that theory.
 
Hello David G,

Ignition/engine timing is just another specification the engine manufacturer recommends, just like cap screw torque.
Timing is usually checked at a specific engine R.P.M. Having a good knowledge of engine timing, you stated that it can be adjusted by ear. Why? A timing light and the right specs is all you need. I personally like to use a vacuum gauge, and in conjunction with engine R.P.M.'s is what I use to set the timing. Least amount of vacuum at a given engine R.P.M., least amount of power, and the best fuel economy. Most amount of R.P.M.'s, most of the engine power. Using a vacuum gauge is sometimes called power tuning,

Guido.
 
Timing will retard with wear. Vehicles with timing chains will be more prone to wear than gear driven camshafts which will increase backlash with wear. Retarded timing can cause run-on both due to increased heat but primarily because the throttle plate will be opened more to maintain a specified idle speed due to the slow down of retarded timing. Widening the point gap will advance timing.
 
Dave is correct, Imagine a setting of .0005" (I know but go along with it) The cam lobe comes along and only the very tip of the lobe hits the rubbing block (very late, no dwell) If the point gap were .40, the cam would just get the points closed, and then they would open again (Early). Geometry.
I agree that timing cannot be set by ear. In today's cars timing is however set by ""ear"". the knock sensor listens to the engine for the initial indications of spark knock (at a level so tiny we could never hear it) and sets the ignition timing algorithm to stay at that tiny knock level through operating range from running idle to full power from cold to full temp. The best we can do with a 1950ish tractor would be to apply a knock sensor, run the tractor under full load with the fuel of choice, and adjust the timing to just barely begin knock, and lock it down. Springs and weights would need to be tailored to meet the real world curve, but that is pretty high effort. Getting some advantage of today's fuels probably does allow more advance. But the timing still needs to be where it will start without backing up against starter rotation. Jim
 
Not that point gaps are meant to adjust or change timing but reducing the gap retards timing, increasing the gap advances timing.

Gap too small and the coil may not have proper time to discharge the through the secondary, this will also cause points to wear faster. Gap too long and the coil may not have proper time to energize the primary, this may not be noticed so much on a low RPM tractor but would still have a weak spark that gets worse the higher the engine speed gets.
 
My first car was a 69 ford v8. I became a master tune up specialist by necessity. A narrow gap will retard the timing and cause doggy rpm. A wide gap gives hotter spark and more "bang" for the buck (plugs too). To wide of a gap will result in poor idle and high rpm miss. No two engines tune the same. A always roughed tuned by ear and fine tuned with light and dwell meter. The point gap will wear to the narrow due to the fiber piece that rides on the lobe of the distributor shaft. The plugs will wear to wide as the anode burns away.
 
My first car was a VW beetle. I quickly because a hippie who could rebuild an engine on a shoestring budget and R&R an engine for a quick repair in minutes.

I learned everything from an air cooled "guru" but much applies to old tractor engines. He always taught me that the only timing that matters a hoot is the total advance. To this day, when I have a distributor out of a VW I set it close, start it and wind it up to 3000 rpm, and twirl the distributor to no more than 32 degrees advance. In the case of those engines you use a timing light and place the TDC notch at the point where the belt meets the pulley.

Adjusting on something like a Farmall is easy, but those old Fords with the front distributor can't be done like that. In that case you use measurements to get the points in just the right spot.
 
The one big difference between cars and gas engine tractors is rpms. Cars , especially today routinely see 6000-7000 rpm . Tractors operate more in the 1000 to 1500 rpm range depending on year. You do not need as much timing advance at 1300rpm as you do at 7000rpm for spark to occur at the same given degree point in engine rotation. Computers may give you more control over engine performance but the old principle of keep it simple worked pretty well for the designers of the 20s 30s 40s 50s etc, as a lot of those tractors survived and operate as designed 70 or 80 years later . Computers are constantly changing and replacements for failed units become unavailable . Meanwhile the old magneto ignition or point condenser distributor and carbureted tractors keep plugging away . Pretty simple to diagnose points and condensers and carb equipped engines for the average farmer , not so much with computer controls. They both have their place but for a tractor I much prefer simplicity.
 
I have always used dwell to set points, the dwell being the optimum time the points are closed to saturate the condenser. This will also decrease the amount the points arc. Using a distributor machine will teach you lots about dist. You can custom adjust the timing advance to be what you desire. Almost a necessity for a dual point system, at least to get the most out of them.
 
I am sure you meant saturate the coil, as the condenser is shorted to ground when the points are closed. Jim
 
Got me wondering now. I really think it charges the condenser when points are closed. when you load a condenser to check mfds you ground the condenser and charge it with voltage, am I not right?
 
"It does the best it can with only being able to look at one variable which is RPM."

Older two-stroke outboards didn't even use that. The stator plate is rotated to advance the spark. As the plate rotates, a cam on it opens the throttle. So when you twist the tiller handle you are actually advancing the spark manually, which in turn opens the carburetor. It doesn't know anything about RPM. You pick the propeller size to make the motor run at the rated RPM under your expected load.
 

I always set the points on the wide side and let them come to me... Points are only perfect the day you set them up from then its all downhill.

Timing, I always total it if it cranks and idles good I could care less about base timing... Different engines have a different total numbers its always best to use book totals as a starting point... If you are going to just ride around in it sometimes you can fudge on the totals as much as 10 degs for the best performance. If you are going to work it hard I normally don't stray away from book totals.

There is the rare case were you have to bump the base and totals up to get it going I don't have a problem with it as long as TDC is confirmed and TDC aligns with the timing marks...
 
You are a bit off on the RPMs for car engines. While some HIGH PERFORMANCE SMALL DISPLACEMENT engines may run at up to 7000 RPMs, most ordinary automotive gasoline engines are most comfortable in the 2000 to 5000 RPM range. Average engine speed at 65 mph is generally thought to be most efficient at around 2200 RPMs.
Most tractor engines are designed to make their peak power around 1500 or so.
 
OK, a few points of information here....
First, the CORRECT way to set points is DWELL ANGLE. Period. Gap is simply a near proximity. Ideal dwell is generally 27 to 32 degrees.
Second, changing points requires resetting dwell and timing because points are NOT a precision manufactured part. Tiny differences in rubbing block size and placement will make a difference in dwell and timing. Timing is generally reset after points replacement due to variations in manufacturing tolerances of the points. Therefore, your reasoning on the timing is reversed. Timing check is not a check to verify that all is installed correctly.
Third, timing setting by varying point setting is more exclusive to small single or twin cylinder engines. Not so much for the automotive type of engines in most tractors.
My comment on running on, also known as dieseling is that it can have multiple causes. High idle is a common cause. Carbon buildup in cylinders is another. This can be caused by mixtures too rich as well as continual lugging of the engine from running at RPMs too slow for the loading condition.
Lastly, there are no "one size fits all" cause/effect remedies for every circumstance. There are always those that do not follow the "rules" so to speak. After about 45 years of working with and on engines, I have found that there are really no short answers, and the best method is to roll up your sleeves and do the diagnostics when there is a problem.
 
A lot of revving and swearing we're involved tuning the ford. I always wondered why ford distributors didn't have the little slide up window and screw adjustment to set point gap/dwell like a Chevy.
 
You are correct in that wide gap opens points earlier (sparks earlier) but at the expence of less coil saturation. Closing of the points just decides how long the coil gets saturated. DWEll means how long points are closed saturating coil. Point gap is just a close estimate if you don't have a dwell meter. Both measurements are designed to give the best coil saturation and nothing more. Then set initial timing for best start by turning dist. When setting up the distributor in my "66 GTO, I first set dwell and then hook up tach. and timing light. Apply timing tape to crank damper. No vacuum advance (race only) By varying mech. advance springs/weights. I run the rpm up to 2500 and set max timing at 34 degrees, then back down by 500 rpm increments to verify a proportionally even gradual drop. Done with different shaped weights/springs. Then initial timing is always set for best starting. Forget what the manual says , the engine knows best what it wants to start the best and that's all initial timing is for. This is for drag racing only. You want it to start good and accelerate at wot best. Acceleration wants a lot of advance and in early. Steady speed driving not so much.
 
Condenser is charged from the back EMF of the coil which can be close to a hundred volts. Condenser doesn't get much charge from battery voltage.
 
The wire going into a condenser is insulated from the case. Therefore the condenser is not grounded out and therefore cannot and is not "charged" when points are closed. The coil windings are saturated and when voltage is interrupted (pts open) the saturation collapses into the core and out the coil wire to dist.
 
Right. With the points closed, current is flowing & the primary coil is producing a magnetic field. When the points open ,flow stops & the field collapses. Collapsing field is induced into the secondary windings which produces the high voltage to fire the plug.
 
They still need a proportional amount of advance, I can really hear it when I zero out the advance to check TDC on an engine turning just 600 RPM. I would guess an engine will need at least 28-30 degrees of advance at 1500 RPM to run correctly.
 
Agreed.

Should have had a cup of coffee before typing.

Was thinking of dwell rather than gap.

Dean
 
several models will have shift points of 6500-7000rpm on WOT and I am talking normal cars , not Corvettes Mustangs etc. And lots of direct injected turbo charged engines. Tractor engines make torque , car engines horsepower.
 
You have some pretty good assumptions. Lots of folks aren't so lucky. I would add that the most important part of setting points is knowing how to FEEL a feeler gauge so it can be used properly. Most guys that I know don't have a clue. Common perception is that whatever feeler you can force in is what it is. Learn the proper drag and you can use a feeler almost as accurately as a dial indicator.

Just a hair off on the lobe or the feeler drag will change the timing but if you are consistent with your procedures timing will be verified with a light and it will stay dead on.

I will agree you can properly time by ear if you are familiar with the machine. In my younger day's I did it all the time & would be within 2 degrees when I put the light on.
 
The condenser. IIRC the condenser accomplishes two things: 1) it speeds up the collapsing of the coil thereby increasing it's secondary output, and 2) prevents the points from arcing (can't remember the details on that part).
 
Most of my old engines and vehicles seem to run fine with timing by ear and point setting by sight. I'll run a feeler gauge or dwell tester once in a while just to see how close I am. Sometimes a little work with the points file and clean up with newspaper is all they need.
 
(quoted from post at 17:48:03 09/27/15) You are a bit off on the RPMs for car engines. While some HIGH PERFORMANCE SMALL DISPLACEMENT engines may run at up to 7000 RPMs, most ordinary automotive gasoline engines are most comfortable in the 2000 to 5000 RPM range.

HIGH PERFORMANCE SMALL DISPLACEMENT engines run up to 20,000 rpm.
 
(quoted from post at 14:07:53 09/27/15) I see a lot of discussion on point gaps and timing, so here are my assumptions:

Point gap on a magneto must be set so the points open when the magnetic field is strongest, so it is critical.

Point gap on a distributor based system is the best of both worlds for being able to open/close correctly through all the RPM ranges. It seems like .020 is the industry standard. Initial timing should not change between subsequent times points are being installed, and should be considered a double check that all is correct. If you set the point gap consistently initial timing should be exactly the same each time when new points are installed. Timing should ALWAYS be done after points are installed or cleaned up. If timing is different, something is worn or set wrong.

If point gap is too close, timing will be retarded, if too loose, then it will be advanced. I use this on small engines with battery powered coils to adjust the timing a little of I want to.

Initial timing should be advanced enough to allow for the spark to occur at the right time when full RPM, but not kick back on starting, period. The distributor has a mechanical advance in it to move the timing from the initial value to full advance. It does the best it can with only being able to look at one variable which is RPM. A computer can control timing much better, because it can set initial timing at 0, it can look at RPM, load and temperatures to determine advance.

The manuals for the tractors which were built in the 40s-50s assumed the tractors would be hand started and fuel was low octane. The optimum timing can be achieved by ear, and is usually a couple of degrees more advanced than the books have it. I run at full throttle, then adjust timing to max RPM. I then check it with a light as a double check. You will want to back the timing off a little if it kicks back when starting hot, if you work it really hard to prevent pinging, or if there is a flutter at idle due to timing too advanced there.

Timing too advanced does NOT cause run on, it is 90% caused by idle set too high.

If timing bounces, then you have an issue with advance mechanism, worn gears or worn bushing, probably worn bushing.


Please post your comments, always great to have good discussion.

Outside of mags I have no idea why anybody tolerates points? Possibly the fear of electricity?
A well designed engine needs less timing advance than an average engine.
It's not how much timing advance you can dial into the engine, with more as better. It's a matter of just enough advance to produce peak HP and peak torque on a dyno.
 
(quoted from post at 20:01:34 09/28/15)
(quoted from post at 17:48:03 09/27/15) You are a bit off on the RPMs for car engines. While some HIGH PERFORMANCE SMALL DISPLACEMENT engines may run at up to 7000 RPMs, most ordinary automotive gasoline engines are most comfortable in the 2000 to 5000 RPM range.

HIGH PERFORMANCE SMALL DISPLACEMENT engines run up to 20,000 rpm.
ould you like to step out of the shadows & give us the make & model of those "ordinary automotive gasoline engines" that run 20,000 rpm or were you just trying to demonstrate what you don't know? Oh, your excuse is that you didn't read right!
 
(quoted from post at 13:10:37 09/28/15)
(quoted from post at 20:01:34 09/28/15)
(quoted from post at 17:48:03 09/27/15) You are a bit off on the RPMs for car engines. While some HIGH PERFORMANCE SMALL DISPLACEMENT engines may run at up to 7000 RPMs, most ordinary automotive gasoline engines are most comfortable in the 2000 to 5000 RPM range.

HIGH PERFORMANCE SMALL DISPLACEMENT engines run up to 20,000 rpm.
ould you like to step out of the shadows & give us the make & model of those "ordinary automotive gasoline engines" that run 20,000 rpm or were you just trying to demonstrate what you don't know? Oh, your excuse is that you didn't read right!

Well, no where in your post do you say "automobile".
I consider a race car to be an automobile with a HIGH PERFORMANCE SMALL DISPLACEMENT engine.
Hondo, an automobile maker since you're stuck in the past, runs a F1 engine up to 21,000 rpm. F1 is a race series that runs mostly in Europe.
 
(quoted from post at 21:55:07 09/28/15)
(quoted from post at 13:10:37 09/28/15)
(quoted from post at 20:01:34 09/28/15)
(quoted from post at 17:48:03 09/27/15) You are a bit off on the RPMs for car engines. While some HIGH PERFORMANCE SMALL DISPLACEMENT engines may run at up to 7000 RPMs, most ordinary automotive gasoline engines are most comfortable in the 2000 to 5000 RPM range.

HIGH PERFORMANCE SMALL DISPLACEMENT engines run up to 20,000 rpm.
ould you like to step out of the shadows & give us the make & model of those "ordinary automotive gasoline engines" that run 20,000 rpm or were you just trying to demonstrate what you don't know? Oh, your excuse is that you didn't read right!

Well, no where in your post do you say "automobile".
I consider a race car to be an automobile with a HIGH PERFORMANCE SMALL DISPLACEMENT engine.
Hondo, an automobile maker since you're stuck in the past, runs a F1 engine up to 21,000 rpm. F1 is a race series that runs mostly in Europe.
Get reading glasses,Joker! "most ordinary automotive gasoline engines". Yeah, sure, all the world see a Formula One race car as an "ordinary automotive gasoline engine". :roll: :lol:
 
(quoted from post at 18:28:28 09/28/15)
(quoted from post at 21:55:07 09/28/15)
(quoted from post at 13:10:37 09/28/15)
(quoted from post at 20:01:34 09/28/15)
(quoted from post at 17:48:03 09/27/15) You are a bit off on the RPMs for car engines. While some HIGH PERFORMANCE SMALL DISPLACEMENT engines may run at up to 7000 RPMs, most ordinary automotive gasoline engines are most comfortable in the 2000 to 5000 RPM range.

HIGH PERFORMANCE SMALL DISPLACEMENT engines run up to 20,000 rpm.
ould you like to step out of the shadows & give us the make & model of those "ordinary automotive gasoline engines" that run 20,000 rpm or were you just trying to demonstrate what you don't know? Oh, your excuse is that you didn't read right!

Well, no where in your post do you say "automobile".
I consider a race car to be an automobile with a HIGH PERFORMANCE SMALL DISPLACEMENT engine.
Hondo, an automobile maker since you're stuck in the past, runs a F1 engine up to 21,000 rpm. F1 is a race series that runs mostly in Europe.
Get reading glasses,Joker! "most ordinary automotive gasoline engines". Yeah, sure, all the world see a Formula One race car as an "ordinary automotive gasoline engine". :roll: :lol:
ith points!!!!!!!!!!!!LOL
 

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