Startomatic operation mystery

[DirtbikePilot]

Howdy all. I'm working on an original startomatic generator set for my parents. I decided to design and build my own control circuit for the generator set that has more functionality than the original controls. I decided to compare my design with the original design to see if it would give me any ideas for how to improve mine and it reveled a total mystery when I examined the schematic and operational description. An original plant starts cranking with the starter motor when a load is detected. There is a delay between the time the starter motor begins cranking and when the fuel control solenoid engages to free the fuel rack and withdraw the decompression pin. According to the schematic, which can be seen with the following link, the starter motor contactor and the fuel control solenoid are both powered by the same circuit. Contact P/2 closes and completes the circuit for both simultaneously.

http://www.stationary-engine.co.uk/ListerData/SOMMan23.htm

Nothing explains the delay that can readily be seen in the following video. The click can easily be heard and the movement of the solenoid can actually be seen at the same time.

http://www.youtube.com/watch?v=aT_exyuLSpk

What gives? Does anyone knows what causes this delay? My design has a delay incorporated for the FCS as well as other things. Mine is all torn apart right now so I can't find the cause until I get it put back together. That will be a while.

[Bottleveg]

When the SOM turns off the ‘pin’ of the de-compression lever engages under the exhaust tapped. The engine freewheels and eventually stops at the point where the pin is trapped under the tappet.
When the SOM starts, the pin won’t release until the engine has turned and removed the pressure off the pin.
The idea is to give the genny a ‘swing at it’ before the engine goes onto compression.
The pins wear over time so they pull out prematurely and can stall the genny. The cure is to remove the pin and turn it round or, if this has already been done, make another pin.
   Mark.

P.S.
I notice on the video you posted that there is quite a delay on that unit. Is this your SOM? I wonder if the rack is a bit stiff?
Another way to achieve the same thing is to fit a timer delay relay into the circuit.
An adjustable relay can be set to how many seconds you wish to delay it for.

[DirtbikePilot]

I thought of that idea with the engine needing to turn to release the pin, but like you noticed, that one has a large delay that can't be explained that way.

I thought they all had a longer delay like in the video above, but after studying two other videos of a startomatic starting, it looks like the pin must be held in place by the force from the valve springs. I wouldn't have guessed that would be enough time for it to spin up and actually make it past compression. I guess it is though.

The controlling circuit that I'm making does use delay relays. They control the glow plug, starter solenoid, fuel control solenoid, the overload alarm and the overload shut down system.

[millman56]

Could the delay be due to that when accelerating the engine from stopped, the start windings drag the nominal 24 volts down to a level that does not supply the solenoid with enough current to overcome inherent friction,  when the engine is spinning freely the current draw will drop allowing the solenoid to operate ?
                                                                                                                                                                                                                                            Another possibility,   the economy resistor limits the current available to the solenoid when the engine is running, so if the limit switch or wiring which controls this  is faulty then the solenoid may not have enoiugh current to operate as it should.

Mark.

[Bottleveg]

I’m not aware of any other delay mechanism that Lister fitted. It’s probable that the one in the video has a slightly rusty pin. This would hold it for longer in its sleeve until vibration and the two rack springs open it. Or the owner may have fitted a delay relay.
 A good unit will drive the engine through the compression stroke but the pin avoids an immediate load. 
Personally I prefer a delay relay to take the load off the batteries and start windings.
  Mark.

[Bottleveg]

Could the delay be due to that when accelerating the engine from stopped, the start windings drag the nominal 24 volts down to a level that does not supply the solenoid with enough current to overcome inherent friction,  when the engine is spinning freely the current draw will drop allowing the solenoid to operate ?
                                                                                                                                                                                                                                            Another possibility,   the economy resistor limits the current available to the solenoid when the engine is running, so if the limit switch or wiring which controls this  is faulty then the solenoid may not have enoiugh current to operate as it should.

Mark.

Hi Mark,
Yes that’s a possibility. I did think ‘ those batteries aren’t too good’ when I first watched the video.
Not sure about the economy resister. I guess it’s possible to wire it ‘back to front’ but the solenoid coil would burn out if it was run on ‘high load’ in general running.
On a similar note, it’s possible to loose half of the starter winding on some of these gennys. The unit will still work but only at half the power.
  Time to get my hands oily again! I have a mis-behaving Kroll to attend to.
    Mark.

[millman56]

Hi Mark,

What I meant was, that if for some reason the feed to the solenoid is going through the  economy resistor at both start up and run, then as this is at a reduced current,  it would struggle to pull the core.

Good luck with the Kroll.

Mark.

[Bottleveg]

Hi Mark,

What I meant was, that if for some reason the feed to the solenoid is going through the  economy resistor at both start up and run, then as this is at a reduced current,  it would struggle to pull the core.

Good luck with the Kroll.

Mark.

OIC. It’s possible but I don’t think the economy circuit would have any chance of operating the solenoid. I seem to remember I’ve had this on one unit, wires wrong in the top box, I think.
I must start writing things down, and then making a note of where I wrote them!
What is it with getting older? Brain gets smaller and fingers seem to get bigger. Ears get bigger but hearing gets worse
Kroll sorted (touching wooden desk).
   Mark.

[listeroil]

I have to totally concur with what Bottleveg said at the beginning of this thread.  There is no electrical delay only mechanical delay caused by the valve lifter that is lodged under the exhaust cam follower flange.  A friend of mine had exactly the same problem with his 8/1 SOM just this week  the decompressor was pulling the engine onto full compression and stalling on the starter motor when it hit compression and blowing the mainfuse. Just out of interest on the initial turn over the voltage at the decomp solenoid was 16 volts on a 24 volt system and the voltage will probably increase as the engine spins faster.

What should occur is the engine should remain decompressed for the first or second engine revolution until the camshaft lifts the exhaust valve and the solenoid should then put the engine on compression Sometimes the rod that goes under the collar on the valve lifter gets worn and the solenoid operates to soon.

The remedy for this is to rotate the valve lifter rod 180deg as Bottleveg suggested.  This was done yesterday on my friends unit and it is now working perfectly.

Mick

[DirtbikePilot]

Thanks for the replies. I'd also agree this could be explained by the voltage being too low at first to operate the solenoid due to the starter motor taking all the available juice until it gets going some.

[somian]

My 8/1 set certainly had no delay when I received it. I have made a start sequencer, primarily because I have fitted a glow plug, but whilst I was at it I put in a delay for the pump/lifter solenoid. Now, after operating a load, it waits for the plug to warm, pulls in the start contactor, and later energises the solenoid. Works a treat!

Ian