I got a bug up my %&*$ today and decided to get some real world electrical readings. There has been some discussion of the decompression control and how it works. I took some measurements today with the following results.

Bike is a 1997 Savage, stock engine as far as I can tell. 29k miles. The cables were good. Only about two tenths of a volt between the battery positive post and the starter terminal while cranking.

When 12 volts is applied from the starter circuit (green/yellow wire):

A.      12 volts is sent out the red/black wire to the decompression relay to open the exhaust valve. This signal lasts for approximately one second.

B.      12 volts is sent out the yellow/black wire to the starter relay after approximately a half second delay. The delay ensures that the starter won’t have to fight the engine on the compression stroke.

C.      After one second, the voltage is still applied to the starter motor but the decompression solenoid is deactivated.

I used a Simpson 260 analog meter and watched the needle move to obtain these results. For dynamic actions, you can’t rely on a cheap digital meter to respond quickly enough. One could always hook up an oscilloscope but that is really overkill in this case.

Current readings were also taken with a clamp on ammeter.

The decompression solenoid draws 15 amps.

The starter draws 40 amps approximately while spinning the motor over with no spark plug in.  This is with the decompression relay not activated. Since there was a big hole in the combustion chamber where the plug was supposed to be, it spun up smoothly as there was no effective compression built up. I would think a bare starter motor would draw maybe 35 amps by its ownself.

While cranking the motor over with the plug in and the decompression solenoid deactivated, the current varies from 50 to 80 amps as the motor spins the engine through the intake, compression, power and exhaust cycles. I didn’t measure the starter current draw with the solenoid activated, that would yield no useful information and put too much of a strain on the starter gears and battery.  

Obviously, when the starter is first activated there is quite a large inrush current drain.  I think I saw a flash reading of maybe 170 amps. I didn’t try to start the bike with the decompression solenoid disconnected and the spark plug in, as I didn’t want to kill the battery.

Next post: battery voltage readings with three different batteries.

John in Kalifornia

Thanks for the readings,  This will help me in spec'ing out the transistors I need for building a solid state decomp controller.  I couldn't find my amp meter so I couldn't take a reading of the decomp solenoid.

LouSiana wrote on 09/04/12 at 15:23:32:

Yes I know you can easily start it without the decomp,  but in doing this your adding much more wear and tear on the starter.  Mine also starts right up without it atm since it's bypassed until I make a new decomp controller.  

R.F.

"The starter will start it fine w/o the decomp if you rotate the engine off of the compression stroke every time.

Easy way to tell is to operate the decomp lever.
you can easily operate it when it's on the exhaust stroke.
otherwise, you can hardly operate it by hand."

I was surprised at the 15 amp draw of the solenoid. Guess you need a strong pull to fight the valve spring pressure.

I'm gonna go with the manual lever that RpNSht sells. Simplify the wiring without the decomp solenoid and controller.

I was afraid that the small AGM battery would be marginal and occasions could occur requiring push starting. I might still try to adapt a handlebar compression release lever like some kick start singles have, just in case. Hard to work the clutch and compression release with the same hand if the CR lever is mounted on the head.

John in Kalifornia