Ignition settings for cb750 4 cyl wasted spark 2 tach inputs

[750essess]

The motor is a 76 honda cb750 with dyna hall effect pickups at 180 deg apart. Have the pickup for 1,4 wired to the opto in with pullup resistor and the pickup for 2,3 into the vr 2 with pull up also. The triggers happen 35 deg before top dead center. I have set the offset to -35 and the second offset to 0. The trigger is rising edge, normal charging, skip pulses=3, time mask =0, percent mask=15, spark output = going high(inverted). Under advanced options, I have wheel set to 0 and dual spark set to dual tach inputs. I get a good tach signal and spark at the plugs, but engine will not run (runs ok with dyna ignition box). Cranking it over with starter and with a timing light and can mess with the offsets and get it to fire on the marks, but it will still not run. It acts like its 180 degress out or something. Anyone have any ideas what I'm doing wrong. Thanks in advance.

[Matt Cramer]

Try setting the offset to +35 instead of -35.

[750essess]

I think I did try that at one time, but I will try it again tonight. Thanks

[750essess]

Ok, with offset at 35, 1 and 4 fire correctly, but 2 and 3 are advanced 35 degrees. If set trigger capture to falling edge instead of rising it flip flops. Is there maybe a problem in the code that it doesn't apply the offset to both channels on dual tachs? I'm using 2.883b firmware. (edit) Just found the 2.883i firmware, will try that

[grippo]

When the first sensor is passed, you are at 35 btdc for the first pair of cylinders, and that sets the timing for those. The assumption in the code is that when you pass the second sensor (90 deg past the first one), you will be 35 deg btdc for the second pair of cylinders. I believe you are saying this is the case, so it should work. But if it isn't, then you can try changing the Odd angle offset in the advanced ignition menu. This will add a timing offset to only the second channel in addition to the trigger offset which applies to both of them.

[750essess]

The second sensor for 2 and 3 is 180 degrees past the first. Both are sitting at 35 degrees before TDC. Because of this, I need crank trigger to be set to calculated.. right? So it looks at the advance table for cranking timing. I set the entire advance table to zero's. The timing light reads zero advance on channel one, but 35 deg advance on 2. If I put 35 deg in offset advance for channel 2 it adds 35 degrees more advance, and it won't allow me to put a negative number here. Just to experiment, I put 145 offset in and it fires at zero, but on tdc for 1and 4. If I could move the 2 and 3 sensor retarded 35 deg, it would work, but there isn't enough adjustment.

[grippo]

I have looked at the code again, and it should fire the second output the same as the first, so with a +35deg trigger offset in the regular ignition menu (and yes use calculated rather than trigger rise), it should wait 35 deg and then fire at TDC in both cases. Since it doesn't, I would try setting the angle offset of the 2nd output negative. I looked at the MT ini and as you said it doesn't allow negative, but it should. Also, in the version I looked at it was not the proper variable type. Let me check this with the latest version tonight and I will give you a version that will allow you to put in -35 deg, so it should fire.

[750essess]

Thanks. I'm glad you found something wrong, thought I was going out of mind It seems to me that it isn't using the advance table for channel 2 either, it didnt change no matter what I put in the table. Maybe it will when the other problem is fixed.

[grippo]

Now you are scaring me. The advance table is applied to both channels and should work equally on both. The only exception would be if you used trigger rise instead of calculated. Then the table advance won't kick in until past cranking rpm. Also I didn't find anything wrong in the MS2 code by looking at it. The only thing "wrong' is that the MT ini you have won't let you put in a negative advance offset. As I suspected the latest ini has the correct word size for the variable. The attached ini will let you put in a negative offset for the 2nd channel. I'm not sure this has ever been tested, so it might not work, but it's an easy thing to try. Another thing to try is to set the 2nd channel advance offset to 0. leave the trigger offset at 35 deg, but try trigger rise instead of Calculated. This will eliminate any possible problem with the engine jerking during the ignition and slowing on the exhaust cycle. This should fire the plugs the instant the sensor is detected by the processor.So both units should fire 35 deg BTDC if both sensors have the same position relative to TDC.

[750essess]

I'll try it tonight. I don't think the motor will start with full 35 degree advance, if the first option doesn't work.