Pull-cord kickback when starting a STIHL MS 660

[Mick Dempsey]

Just now, peds said:

Please boss, what's TDC short for?

Top dead centre, the highest point of the piston in its stroke.

[peds]

Ah no boss, it's because if it wasn't that short, you'd need longer arms to pull the cord.

 

But no, thank you for the knowledge. 

[kram]

Had a play with the 52cc chinese combi engine.

I measured the gap at 0.35mm, enlarging to 0.5mm has not made a difference so I will try again to 0.8mm. I am thinking the flywheel timing, or coil mount holes will need to be modified.

Someone mentioned variable advance coils, that retard below idle speed. Are there any that may fit this?

 

[GarethM]

The gap between what the coil and the flywheel, you want fag paper close to induce a magnet field.

[kram]

Yes, as close as possible would be best for a good spark, but it was suggested that a bigger gap could retard the timing.

 

I'm thinking a gap change wont be enough, I'll either have to adjust the coil mounting or the flywheel key. If I slot the coil mount holes that will give a couple milimetrrs adjustment.

[GarethM]

Don't think it works like that, larger the gap would just create a weaker spark or if electronic probably not at all.

 

For something that's spinning at what 3000+ rpm retarding the timing will probably make absolutely zero differences probably.

 

Plus being a two stroke it's probably even more useless as the microsecond timing is so infinitely small it's probably barely even noticeable or measurable.

[kram]

Having thought about it, my engine auger has the same issue of being a pain to start, hard pull or it hurts. Starting this combi is a real pain without the driveshaft fitted as theres not much to hold onto.

 

I got myself a timing light. Red marks are TDC, ignore the first blue mark (strobe was set +10). With the light set at zero, the clockwise most blue mark is roughly 24 degrees.

 

And just to help me avoid such stupidity in future I shall share my mistake. Do not start a clutched engine without the clutch bell/housing. I was lucky. A new spring is needed..

 

So is 24 degrees too much? Are there any variable coils I could fit for this engine?

 

The ignition is consistant across the rev range with no change at higher rpm, although it did seem to vary a little between multiple starts. I'll confirm that tomorrow.

 

Thanks.

Edited Monday at 17:22 by kram

[kram]

Could there be any other causes of this problem? Its a pain to turn over even with the ignition off, so perhaps I have gone down the wrong rabbit hole.

 

Add a decompression button?

 

A bit more testing and the recoil has broken, only retracts if you help it, but the spring is still very strong. Spring could be far too strong?

 

I made up a bracket to delay the ignition about 6 or 7 degrees. Not had a proper test but it starts - then the recoil went..

Edited Wednesday at 23:41 by kram