Crazy RPM issue - Single Cylinder

[stevek]

Hi there,

Sometime back we started setting up a single cylinder Yamaha XT 600 to run with MS and a blower from a supercharged Ford Fiesta. We had a load of initial problems, relating to all sorts of things both mechanical and eletronic. We had to put the project on hold for basically 4 months and have just started up again, having fixed all the mechanical problems. Some of you *may* remember my posts, but I kinda doubt it considering the time lapse.

We now have a nice new 60-2 decoder wheel, MS V2.2 running MSnS-E (029s), MegaTune 2.25 and a VR sensor from a 90's model Chevrolet Omega (Lumina in the States, Commodore in Australia).

Here is the dilemma:
We setup the VR sensor so it is exactly 120 degrees BTDC, built a VR conditioner, tested it and hooked it up with a 2.2k 5v pullup resistor. We get a nice signal (all square like) but we cant match the RPM in Megatune to the motor. MT is setup with 1 cylinder, the trigger angle is set at 75+45 and the Trigger A is set at tooth 20.

If we set it up as a single cylinder the RPM shown in MT is approximately twice what we require. If we accelerate, it climbs smoothly without spiking, but appears to be - you guessed it - double the RPM of the motor.

Splitting it into 4 cyclinders gives an identical match at idle, but as soon as we accelerate the RPM in Megatune accelerates at 1/4 the rate of the motor.

So, one would think that setting it up as twin would be a good idea - same problem, except this time the acceleration is only 1/2 that of the motor.

Bugger.

The following trigger setups were used:
Time Based - no change
Trigger Return - no change
Trigger A 0, return 0 - no rpm
Trigger A 19, return 20 - same 'acceleration / division' problem throughout the rpm range (depending on how many cylinders were used)
Trigger A 2, return 57 - same 'acceleration / division' problem throughout the rpm range
Trigger A 19, return 20 - same 'acceleration / division' problem throughout the rpm range

Can anyone suggest what might be going wrong here? We are not worried about driving the coil (yet) we just want to be able to run the dang motor on a 1 to 1 rotation / rpm basis. Any clues? I'm confused!

Cheers,
Steve K

[stevek]

anyone?

[jsmcortina]

Did you try 2 cylinder? A single cylinder requires a tach pulse every 720 degrees. With a crank wheel the least you can get is every 360 degrees -hence double the rpm.

For spark accuracy I would:
-get trigger angle between 60 and 80 BTDC (by adjusting your trigger tooth number as well)
-pretend it is a 4 cylinder, set two trigger points and configure two spark outputs. Ignore the second output. This will help the code as it gets wheel position information every 180 degrees now instead of 360.

James

[stevek]

jsmcortina,

Thanks for your suggestion. We're not worried about spark right now, we want fuel. We can use the existing CDI ignition to provide the spark until we stabilize the fuel delivery.

We don't have space nor a position to attach a bracket to hold the VR between 60 - 80 degrees, so would I be correct in assuming that this will mean we are going to have problems at 120 degrees? (ie as it is the VR sits at 4:00 o'clock when the piston is at TDC)

Cheers,
Steve

[newtyres1]

I think you can change the effective position of the sensor by assigning a different tooth number for trigger A. I think what James is saying is to set it up something like this:
Trigger A = 9
(Trigger A return say 18 )
Trigger B = 39
(Trigger B return say 48 )
Enable Spark A and Spark B outputs - need to do this to keep the code happy
Set 4 cyl in constants
Set 65 degrees trigger angle - need to keep the wheel decoder happy

If your rpm is correct at idle it has to be correct at all rpm's. I'm assuming the gap in the wheel is lined up with the crank pin, but that won't matter at this stage if you're not using spark. If all that fails can you post your msq? Hope this helps.
Ian.

[stevek]

newtyres1,

Thanks - I'll give it a try.

Cheers,
Steve