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post #1 of 9 (permalink) Old Feb 2nd, 10, 07:31 PM Thread Starter
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HEI ignition on an odd project

Hey guys. Im a huge GM fan and I have been working on a side project for the past couple months now. I had no idea where to ask my question so I figured this place looked good.

I have a 1983 Husqvarna 250cc dirtbike. (I can hear the laughing already). The flywheel/magneto ignition seized itself on to the crank and in order to remove that assembly I had to cut the end of the crankshaft. I was left with a crank stub and that is it. At this point buying a new flywheel/magneto wouldnt fix it due to the crank being too short. I had to search for other alternative ignition sources.

One morning I had an idea hit me: HEI ignition triggered by a hall effect sensor. Being that I work at an autoparts store, I went to work and leafed through the illustrated ignition guides writing down part numbers of pickups and reluctors I figured would work. In the end I found a pickup which fit nicely inside the housing on the bike and I ended up fabricating a triangle shaped arrow pickup which i screwed in to the end of the crank which was drilled and tapped

After wiring this all in to a 4 pin HEI Module and wiring everything to a small 12 volt power sport battery, i was ready...Or was I?

TIMING was one thing that stumped me. I physically looked in the exhaust port to see when the piston was up, and marked that point with a marker both on the crank and on the engine housing. Then I moved my pickup trigger to be dead on with the pickup however two things became my barriers:

1) The pickup does not sense the pointer when it is dead on, it sense it about 1/8 inch before they are parallel.
2) A degree is so small that when i move the pointer even a smidge, the bike has totally different behavior, whether it be intense low end and breaking up in the top end and almost taking your foot off while kickstarting (too much retard), or not even wanting to fire up/idle but having a major top end. (too much advance)

I have put a timing light on to try and see where my marks are lining up, and while i can see it, i dont have any numbers which I can correlate to my setup.

My question is this: If I buy a better timing light (as in one with a digital advance readout and so forth) will i be able to get a reading of the timing that I *CURRENTLY* have? Keep in mind I dont have a timing tab, only that rotating crank pointer/trigger. Do I have any options rather than a trial and error method?

Ive also heard of an HEI module used in some 1980 model Oldsmobiles with a 5th pin, which, when grounded reduces timing by X degrees (cant remember the number off hand). It was used to reduce strain on the starters back in the day. Im not sure using that would be of any benefit to me, just throwing that out there.

I appreciate any help! Here are a couple videos of the bike running. As I quickly found out behavior of the engine changes with load on it so after adjusting the trigger each time, there was no substitute for a test ride.

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post #2 of 9 (permalink) Old Feb 2nd, 10, 07:45 PM
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Re: HEI ignition on an odd project

yes. you have no advance mechanism to work with so this will be easy. Especially in a 2 stroke with the plug right on top of the piston.

You have to have accurate timing marks. Find tdc with a piston stop. Screw a bolt into the plug hole so it stops the piston maybe 1/4" down. Make a mark on the crank and the housing. Wing it around backwards till it stops again. Make a second mark on the housing where your crank mark stops.

Halfway between the 2 marks on the housing is TOP DEAD CENTER. Measure, make a mark, erase the other 2.

Degrees are degrees. Do some math based on the measured diameter of the end of the crank or whatever you are using, find the distance for 10 degrees, measure and mark 10 20 30 40 degrees before TDC.

NOW you can aim your timing light at the marks and measure your timing pretty accurately.

Next step of course is research. Exactly or approximately how much timing does a hot 2 stroke need? I have no clue. The ones I've timed had a single set of marks from the factory, if it was on the marks then it was right, they didn't tell you how much timing this was.

Still your marks should let you set the timing repeatably within a couple degrees.

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post #3 of 9 (permalink) Old Feb 2nd, 10, 07:53 PM Thread Starter
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Re: HEI ignition on an odd project

I GREATLY appreciate the bolt trick!!! I will definitely do that! Basically all I was going on to make my mark was looking in the exhaust and rotating the crank with the plug out and when it stopped going up, make my mark. That is not accurate though, which I knew but I didnt know of a better way. I have drawn on a piece of paper using a protractor to extrapolate what exactly a degree comes out to be. In other words a millimeter of movement in my crank trigger comes out to an inch of movement when you move away from the crank X distance. (simple logic)

The owners manuals ive downloaded says 3*BTDC is 'optimum' timing.

I believe, like you said, I need a reliable TDC marking to go from.

One other thing, will a 2 strokes timing change much depending on air temp and engine temp? The reason I ask, is I supposedly had this thing nailed down via trial and error in early november when it was about 75*F outside temp with engine warm. Nowadays it is 45 and 50*F outside and with engine warm and same timing setting, the low end is SUPER active and the engine has no top end unless in higher gears (more load). The pointer didnt move, so thats what lead me to the issue of temperature. Can this be?

Thanks again, JimM!!
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post #4 of 9 (permalink) Old Feb 6th, 10, 11:06 PM
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Re: HEI ignition on an odd project

Congratulations on having the ingenuity to cobble together an ignition sytem and get an old bike running again!

JimM gave outstanding advice on making an accurate timing mark. This is definitely the first thing to do.

One thing that needs some thought is timing advance or retard with rpm. Its been a while since I've thrown a timing light on a 2 stroke, but the few I've done have shown timing retard at higher rpm. I assume this is due to increased turbulence in the cylinder leading to a faster burn and therefore requiring less spark advance at higher rpm. I could be wrong here.

Most modern bikes are timed statically with a dial indicator through the spark plug hole. Position the piston at a specified distance before TDC, line up a mark on the ignition stator with a matching mark on the flywheel and you're good to go. Any spark advance changes that are different from this base setting are programmed into the ignition module.

This is not to say that you can't make your Husky run with a fixed timing setting but you will need to experiment to find the optimum setting. Your current timing light should work just fine. As you have already found, relatively minor timing changes have a profound effect on how a 2 stroke runs and there is no substitute for a test ride. Keep trying. High performance 2 strokes can be rather high strung and be a bit finicky on what they like. It may be that you find the optimum timing setting will vary from season to season.

67 Camaro, 96 LT1, 4L60E
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post #5 of 9 (permalink) Old Feb 7th, 10, 01:22 PM Thread Starter
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Re: HEI ignition on an odd project

Red67Camaro, Thanks for the reply. Im not sure I fully understand the dial indicator method. Whats throwing me off is the fact that there would be SOME area of 'sweep' where the indicator wouldnt move but the crank would. In other words the piston could be TDC but the crank-conrod wrist pin could be to one side or the other not dead on. That is why Im leaning toward Jim's piston-stop method. I ran this idea by a someone who also provided an additional tip: cut a wedge out of the bolt longways (to where it would look sort of like pacman looking at it from the end) so that cylinder pressure could escape and you wouldnt use so much effort to overcome compression, which when compression gives way, you could possibly smack the piston on the bolt if not careful.

You are correct though on it being finicky. But do you think *THIS* finicky? I mean it barely wraps up to [guessing] 6K before breaking up and jerking and sputtering-It has INSANE low end, where when starting out you have to be careful not to let that front wheel come up on you; have to burn the clutch for a good bit while easing into the throttle. I was wondering if, since advanced timing=more low end, I could dump octane boost in there with the gas/oil mix. Im wondering if that would negate the high timing some? I know, in a car if the timing is too high, and it starts detonation or preignition you can add some, but im not sure if it will work the other way around. lol just a little thinking out loud.

Also, about the high-end timing retard: How would that work on a stock 2 stroke magneto? The engine is spinning faster, so the magneto is sparking(in a way, not the magneto itself in reality) more. The only thing I am gaining by using an HEI module is the dwell adaptation, compared to the 'decreased dwell with increased rpm' on a magneto system. Any more thoughts on this?
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post #6 of 9 (permalink) Old Feb 7th, 10, 03:40 PM
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Re: HEI ignition on an odd project

You did good. I like it.

Can you run a die on the end of the crankshaft in order to mount a degree wheel to get your timing set? Bend a piece of heavy wire for a pointer just like we do for setting up camshafts.

I know you said you had to cut the crank off and I can see a stub remaining and a shoulder. You would have to check tdc before and after running to make sure the wheel didn't slip. Or if there is a keyway I'm not seeing you could make a slot in the wheel for it so the whhel couldn't slip under rotation.

Engine degree wheels are available fairly small O.D., as I have a little one(maybe 7 or 8" dia) that I have never taken out of the plastic. Prefer my 2 footer.

Old big stationary 2 cycle engines run at 3° btdc. Get them up over 6 or 8 and you know you've done wrong. Immediate like. These things have super high cr's.

And old stationary engines with mags used to have a device called an impulse coupling that retarded timing for starting purposes that kicked out as rpm rose. Reverse of what you are wanting.

quote: I ran this idea by a someone who also provided an additional tip: cut a wedge out of the bolt longways (to where it would look sort of like pacman looking at it from the end) so that cylinder pressure could escape and you wouldnt use so much effort to overcome compression, which when compression gives way, you could possibly smack the piston on the bolt if not careful.

good idea, or maybe drill a hole thru the bolt as a compression release if the bolt isn't too long for a bit.

Tim Smith
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post #7 of 9 (permalink) Old Feb 7th, 10, 03:55 PM Thread Starter
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Re: HEI ignition on an odd project

77Wolf10.85, Thanks for the ideas. Im not sure (as Ive never really built an engine or degreed a cam) So I wouldnt really know how to use a degree wheel. I have a few friends who have, but when I talk to them about my bike everything I say goes over their heads (which is understandable haha). 3 degrees before top is what the manual for the bike says, so that is what I am aiming at.

My crank does not have a keyway. just a hole drilled and tapped in the end for a 1/4-20 screw that goes thru the ignition pickup trigger. Just crack it loose to move the pointer to change the timing.
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post #8 of 9 (permalink) Old Feb 7th, 10, 10:06 PM
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Re: HEI ignition on an odd project

For making an accurate timing mark I would use JimM's piston stop method. Relieving compression pressure with a cut wedge or hole in the bolt is a good idea. I mentioned the dial indicator method as an illustration of how the timing is set on many bikes. I agree that using the zero point of the indicator isn't the most accurate way of determining TDC. When using a dial indicator to set timing, being dead on as far as crankshaft postion is not as important as long as the indicator reads zero when the piston is at TDC. The actual timing is set at a specified distance, usually several to many thousandths from TDC when the relationship between the indicator reading and crank position is more accurate.

As far as the high rpm timing retard, this is something I've seen on many 2 strokes and is something that I believe is built into the ignition module of said bikes. This is not a feature that you will have using a GM module. I mentioned it as something to think about when experimenting with your timing. It might be a factor in the optimum setting if you like to rev it out on long straights, big hills or similar situations.

I've played with the timing on my current bike, a 2002 Yamaha YZ250. Stock timing spec on this bike is .007" BTDC. At this setting it runs well but pings a bit on pump premium and benefits from blending in some race gas. I've retarded it incrementally as far as .002" BTDC at which it becomes very smooth, loses quite a bit of power and has no snap at all. I settled on .005" BTDC as the best setting. It calms down the pinging while retaining all of its performance, although it still likes a bit of race gas. I have no idea what my settings relate to in degrees. By eyeballing the stator position, I would guess the difference between .007" and .002" to be about 4 degrees give or take.

At this point your need for an accurate timing mark is to be able to make and guage incremental changes to the timing setting. In other words, knowing the timing setting that makes your bike run to your satisfaction and being able to return to that setting at a later time.

Keep trying and good luck. Keep us informed on how it goes.

67 Camaro, 96 LT1, 4L60E
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post #9 of 9 (permalink) Old Feb 9th, 10, 07:52 PM
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Re: HEI ignition on an odd project

You must realize that the flywheel is also a balancer for the crank/rod/piston? I agree with the others that it has definitely taken some (backwoods?) ingenuity to come up with that but I can't help but wonder what you are going to do to keep the ignition powered since the bike never came with a battery and now has no way to charge one if you put one on? Also it will be a short amount of time before the main bearings are hammered to death without the dampening effect of the flywheel (read "grenade effect"). Wear boots!!
Please don't take my comments wrong as I'm only curious if you are really pursuing this as just an experiment or to actually ride?
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