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I was bored and it was raining yesterday, so I decided to get my timing light out and try to determine some ignition timing values.

I have an 88 GL1500 and just now figured out I had an old ECM that was not updated IAW the Service Bulletin. I had redone the carb several times and finally replace the carb with a newer 97. That was 2 years ago, and I do not have a hesitation at all.

I had bought two extra spare ECMs off eBay very cheap. I think I paid ~$7 for one and ~$17 for the other. The 88 ECM can be one of the most expensive ($250-$450) GL1500 ECMs‘, so I wanted a spare for a rainy day, ha. One was old MN5, and one was new MT2.

The 88 hesitation Service Bulletin addresses a carb jet mod, but there is also an ECM replacement. With all the hype of a trigger wheel also correcting engine hesitation, I wanted to see if the ECM replacement changed the ignition timing.

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Setup: I pulled the front covers to expose the crank and cam gears. I did not have a timing wheel, so yes my measurements here are very crude. The timing marks on the crank are small and difficult to measure for comparison. So, I used the cam marks for reference measurements.

The cam is bigger and turns at half the engine rpms, and the markings are easier to view. There are 40 teeth on the cam gear, so each cam tooth represents 9 degrees. The crank would have to turn 18 degrees to observe the cam to rotate “1 tooth or 9 degrees“.

The ignition timing spec for a GL1500 fires at 0 degrees BTDC. I was a little surprised, but that is also what I verified with my timing light.

The ECM will advance the timing based on engine rpms and vacuum. The advance timing settings comes in pretty quickly at ~2,500 rpm. At higher rpms, there was a little more timing advance. Basically, full advance is before 5,000 rpm. All my check were done at +5,500 rpms. I would just rap the motor momentary and observe the cam timing marks for full advance. At 5,500 rpms, the ECM does see vacuum to fully advance the timing. (At high engine load, which I could not perform, the engine manifold pressure would be higher to retard the timing).

With my timing light on the cam reference marks, at 5,500 rpms the timing changed approximately 2 teeth. Actually, I estimate 2.2 (subjective) teeth shift. Like I said before, each tooth on the cam equates to 9 degrees, so this would equate to 18 - 19.8 degrees change on the cam. So, full advance in relation to the crank would have an ignition timing 36-39.6 BTDC. My timing light also has a little knob to alter and measure degree changes of the light pulsations.

When I pulled the vacuum tube off the ECM, there was no timing change at idle. At higher rpms with no vacuum on the ECMs, the electronic timing advanced was ~27 degrees BTDC. I also hooked up my mighty vac to draw vacuum for full advance, which added ~10 degrees.

My second and third ECMs appeared to have no change to timing in comparison to my first ECM. I could not determine any differences in timing at idle, full advance, or vacuum. But, I guess there could be other differences in the timing curve. At least I now know that all my ECMs work.

I ended up using the MT2 replacement ECM as identified in the Service Bulletin. The others MN5 ECMs will go back on the storage shelf. This MT2 ECM is only good for 88 & 89 GL1500 due to the connector(s).

I also had a 4 degree trigger, and I installed it after checking the 3 ECMs. I was hoping to find something different with the ignition system such as a slightly retarded system, but the ECM full timing advance of 36-40 degrees is very acceptable if not close to optimum, IMO. At idle, I would like to think 8-12 degree BTDC would have been better. Maybe this is where the 4 degree trigger helps with hesitation as compared to a set 0 degree idle timing. (I use to advance the Chevy timing higher to achieve the highest rpms, but I would make sure it was not too far advanced to hinder starting. Of course, I had no idea what the results would have been at high rpms. If an engine was pinging in the old days with unleaded gas, I would retard the timing without knowing the end affect of gas mileage).

Installing a trigger adds 4 degrees throughout the ignition timing. But, maybe it helps more at idle?

I checked the trigger with the timing light, and I could see a definite slight change in cam timing marks. With the light, I also dialed in 4 degrees, and it brought the timing mark back to “zero” on the cam at idle. Physically, the 4 degree wheel can be easily compared to the standard to visually observed the off set.

Note: With a 4 degree trigger wheel, full advance timing for a GL1500 would change to 40-44 degrees BTDC and idle would be 4 degrees BTDC. In general, some engine(s) do have a max timing at 41-45. Max power is typically achieved around 25 degrees BTDC. Further full advanced timing with vacuum is not for max power.

Lastly, the next day was a little chilly. My engine would normally have a hesitation after cold start up. The cold hesitation is now surprisingly gone, and I attribute that to the trigger. This is just an initial observation. Before the trigger, I did not have a hesitation after warm, and I have not test ridden the bike yet.

This is just some information I was always curious about. If you also think about this kind of stuff, next time you change timing belts, pull out your old antique timing light.
 

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Thanks for that well written document.

Added that to my bookmarks list for future needs.
 

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Right side, under fake fuel tank cover. It is strapped tothe plasticcover that goes over the fill neckunder the fill lid. Has theRed, Blue, White connectors.

I too forgot. Started taking the left side off with reverse level.:headbanger:
 
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