Timing mystery

[Gordon]

I have an ignition timing mystery. I have built the Two Bit Twin as posted on Model Engine Maker. I have been having a problem getting it to run. I could get one cylinder to fire but not both at the same time. It would seem to fire on one cylinder and the other cylinder would misfire like the timing was too far advanced. This engine is a 50° offset so I figured that the cylinders had to fire at 410° or 310° (360 + or - 50°) Since I am using a wasted spark ignition and triggering the hall sensor off from the camshaft I thought that the magnets had to be 205° apart. After screwing around with all kinds of carb setting, valve timing etc I finally checked where the cylinders were actually firing. If I set cylinder #1 to fire just before TDC cylinder #2 fires 15° BTDC. I finally made a new magnet mount with the magnets set at 220° and the engine took off. I still have some tweaking and cleanup to do but the engine runs. I have no idea where the extra 15° came from but it runs.

 

[gbritnell]

By the looks of your engine it looks like the timing disc runs at cam speed (1/2 crank speed) if the right cylinder would fire first then the left cylinder would fire at 360-50 or 310 degrees after the right. That being said the magnets would have to be spaced at 155 degrees. Half of 310 degrees. Im not sure how your new disc can work at 220 degrees spacing

 

[Gordon]

By the looks of your engine it looks like the timing disc runs at cam speed (1/2 crank speed) if the right cylinder would fire first then the left cylinder would fire at 360-50 or 310 degrees after the right. That being said the magnets would have to be spaced at 155 degrees. Half of 310 degrees. Im not sure how your new disc can work at 220 degrees spacing

That is what it is doing. The pistons are on a common throw with knife and fork connecting rods so it is either360 + 50 or 360 - 50 depending on rotation so it is 310/2=155 or 410/2=205. I was trying to get it to run at 205 but actual measurement showed that it was firing at 15 BTDC on the second piston. I have no idea where the15 comes from but when I made a new magnet mount at 220 it started to run and it would not run at 205. I have no explanation. Your 155 is just the other side of my 205. 155+205=360

 

[Vietti]

Following this with interest. Building the Hoglet which is a 21 degree twin. The timing diagram calls for the magnets to be at 159 degrees apart. Not sure how to arrive at this timing except 180 minus 21=159

 

[Vietti]

I hope some others chime in here on this apparent mystery! I'd especially like to hear from some Hoglet builders on what degree spacing they used on the ignition magnets. Hope this does not hijack the original thread to much but may shed some light on that situation too.

 

[Gordon]

Following this with interest. Building the Hoglet which is a 21 degree twin. The timing diagram calls for the magnets to be at 159 degrees apart. Not sure how to arrive at this timing except 180 minus 21=159

You say 21 but that must be either side of center. I don't have the drawing available now but looking at pictures it has to be 42. That means that it would fire at 360-42=318 and the camshaft would be half of that or 159. That does not explain my 15 degree off from what should be 205 degree magnet position

 

[Vietti]

Gordon,

You are correct, must have had a brain cramp. Does not help your situation though. Any possibility of a measuring error??

 

[dsage]

Would the width of the magnets on your hall sensor pickup have anything to do with the actual angle the spark occurs? Depending on the action of the coil driver the spark may occur when the magnet LEAVES the hall sensor (not when the magnet first passes over the sensor). If the magnets are quite wide maybe that accounts for the angle difference?
Remove a spark plug and watch for the actual spark while measuring the angles of each cylinder.

 

[Steamchick]

Put a SIMPLE "Timing light" from the car shop in the HT side of the system and rotate the engine slowly - with a degree disc on the crank shaft and pointer set to show the crank position. The Timing light should flash when the ignition HT pulse hits the LED so you can see the timing easily... Thousands of car owners used to do this before electronics became common on cars, so the simple technology driven "by the spark volts" may help you understand what the magnets and electronics are doing. The only snag is that you may have to be doing over 1 rev per second (60rpm) to trigger the electronics. But on a Motorcycle engine (bigger than your model?) I just used a spanner on the crank and only rotated the crank around 90 degrees to get the strobe to show me when the system was actually sparking...
K2

 

[Gordon]

Put a SIMPLE "Timing light" from the car shop in the HT side of the system and rotate the engine slowly - with a degree disc on the crank shaft and pointer set to show the crank position. The Timing light should flash when the ignition HT pulse hits the LED so you can see the timing easily... Thousands of car owners used to do this before electronics became common on cars, so the simple technology driven "by the spark volts" may help you understand what the magnets and electronics are doing. The only snag is that you may have to be doing over 1 rev per second (60rpm) to trigger the electronics. But on a Motorcycle engine (bigger than your model?) I just used a spanner on the crank and only rotated the crank around 90 degrees to get the strobe to show me when the system was actually sparking...
K2

I have a RCEXL tester which beeps and lights up when the hall sensor is triggered. That can be triggered by just slowly rotating the engine. I used the beginning of the signal for the test even though the ignition actually fires when the sensor leaves the magnet. The result is that the tester showed that the sensor was activating at TDC on cylinder #1 and 15° BTDC on cylinder #2 using the magnet mount with the magnets set 205° apart as the calculations said they should be. I made another magnet mount at 205+15 and that works. I still have no idea why.

 

[Steamchick]

Magnets are lie people. Their fields and effects change when cost to others... maybe that is why you have a distorted effect? Or polarity sensitivity?
I know not why.

 

[Lloyd-ss]

Any chance that your particular sensor has a polarity requirement (many don't) and one magnet is backwards?

 

[Gordon]

Any chance that your particular sensor has a polarity requirement (many don't) and one magnet is backwards?

The fact is that the engine runs with the new magnet mount and both cylinders fire so the polarity has to be right or it would not fire. The hall sensor will not trigger with the wrong polarity. This magnet mount is the third one that I have made for this engine and all three act the same.

I recently completed a horizontal twin and that had the magnets at 180° as would be expected.

 

[Lloyd-ss]

Gordon, you are a smart guy and you know what you are doing, and the answer will come to you sooner or later. But I know how frustrating this kind of mystery can be.
We are counting on you!

 

[Bentwings]

Would the width of the magnets on your hall sensor pickup have anything to do with the actual angle the spark occurs? Depending on the action of the coil driver the spark may occur when the magnet LEAVES the hall sensor (not when the magnet first passes over the sensor). If the magnets are quite wide maybe that accounts for the angle difference?
Remove a spark plug and watch for the actual spark while measuring the angles of each cylinder.

Use a degree wheel for more accuracy.

The magnetic field is kinda elliptical so where it’s greatest attraction is where the system fires for most sensors it’s pretty small especially on small
engines if you mount a degree wheel and pointer you should be able to get the spot you want lined up. We go through this on our hot rods all the time. We use adjustable pointer to set exact location then we can set total advance exactly where we want it. It doesn’t take much to trigger Hall effect sensors.

 

[Gordon]

It is not really about getting the exact firing point, it is a question of why is the firing position 15° off from the theoretical firing point. The fact is that when I made a new magnet mount disk the engine would run and it would not run at the theoretical firing position. I still have no idea why that occurs and if it is actually 14° or 16° is not really important in trying to figure out why it runs at the position where it should not run and will not run at the theoretical correct position.

 

[Bentwings]

Would the width of the magnets on your hall sensor pickup have anything to do with the actual angle the spark occurs? Depending on the action of the coil driver the spark may occur when the magnet LEAVES the hall sensor (not when the magnet first passes over the sensor). If the magnets are quite wide maybe that accounts for the angle difference?
Remove a spark plug and watch for the actual spark while measuring the angles of each cylinder.

Wen stuff like this happens on hot rods we go bak to basics. Find top dead center install degree wheel . Locate where you pointer indicator should be then set degree wheel to either timing desired or 0 . Rotate motor to about 10 deg advance ( or backwards ( it now has to rotate toward TDC) before top center. This is about where most small engines start or run. Set you sensor some place easy. Rotate motor to 10 degrees or now TDC . Then set magnet so it just triggers ignition . If you ignition module has a built in regard or advance you need to know this so you can compensate. Now the motor should fire at 10-15 deg give or take. If you have points they should just be opening. Some electronic timing lights don’t work on model ignitions also check your light to make sure the dial back feature is off or setvto. 0. In sinker words you want the ignition to fire at about 10-15 degrees if you magnet goes past the sensor or trigger the id look there next . It should trigger no more than a couple degrees off depending on the place where the magnet closes the circuit usually if you get anywhere near a Hall effect with magnet it will trigger. It helps to have it very linear . It’s possible you have the cam in the engine set incorrectly so check that too.


Set

 

[minh-thanh]

I finally checked where the cylinders were actually firing. If I set cylinder #1 to fire just before TDC cylinder #2 fires 15° BTDC. I finally made a new magnet mount with the magnets set at 220° and the engine took off. I still have some tweaking and cleanup to do but the engine runs. I have no idea where the extra 15° came from but it runs.


It is not really about getting the exact firing point, it is a question of why is the firing position 15° off from the theoretical firing point. The fact is that when I made a new magnet mount disk the engine would run and it would not run at the theoretical firing position. I still have no idea why that occurs and if it is actually 14° or 16° is not really important in trying to figure out why it runs at the position where it should not run and will not run at the theoretical correct position.

I think your whole problem comes from an angle deviation from the standard crankshaft angle

As shown in the picture, you will adjust all the lobes according to the piston TDC position and it's fine for that, but ignition timing : It is a problem and it will deviate by an angle from standard.
Example: If you set cylinder 1 to ignite at TDC, cylinder 2 will have to ignite A degrees before TDC.. Even though the angles are the same

 

[Gordon]

I think your whole problem comes from an angle deviation from the standard crankshaft angle

View attachment 148155
As shown in the picture, you will adjust all the lobes according to the piston TDC position and it's fine for that, but ignition timing : It is a problem and it will deviate by an angle from standard.
Example: If you set cylinder 1 to ignite at TDC, cylinder 2 will have to ignite A degrees before TDC.. Even though the angles are the same

I am not sure of what you are saying but cylinder #1 does not care where #2 is when #1 fires and #2 does not care where #1 is but #2 is going to fire 50° after #1. I think that what you are saying may be valid if there were only one magnet but each cylinder has it's own magnet so basically each cylinder is operating independent of the other. The #1 magnet may fire #2 at the same time as #1 but it does nothing because #2 is on the exhaust stroke at that time.