3120 runs without spark!!!!!!!

[AUSSIE1]

Sounds like a coupla issues goin on with the high idle and at a guess too high comp causing a hot spot sounds plausible.

 

[wigglesworth]

Sounds like a coupla issues goin on with the high idle and at a guess too high comp causing a hot spot sounds plausible.

The high comp wont cause a hot spot, just pre detonation, aiding in the deiseling.

 

[dragrcr]

first off, it is possiblr, but i dont think you are getting the real story, i have to say BS, not by you but him.. if anything, it would run for maybe 2-3 secounds, but not on and on. there isnt enough heat in an idleing motor to keep a piece of carbon that hot, and with any dieseling, the timming is soo off that it will hardly run. mostly just sputters and pops.

 

[AUSSIE1]

first off, it is possiblr, but i dont think you are getting the real story, i have to say BS, not by you but him.. if anything, it would run for maybe 2-3 secounds, but not on and on. there isnt enough heat in an idleing motor to keep a piece of carbon that hot, and with any dieseling, the timming is soo off that it will hardly run. mostly just sputters and pops.

He's not the kind of fella to BS Drag.
Bit hard to determine without further investigation at this stage.

 

[dragrcr]

ok not BSing, but not knowing what he is talking about, that saw IS NOT idleing just fine with no spark, NOT HAPPENING.

 

[AUSSIE1]

ok not BSing, but not knowing what he is talking about, that saw IS NOT idleing just fine with no spark, NOT HAPPENING.

Yeah, when you see things like this in the flesh can have a different story.

 

[MCW]

I had my 7900 blow a base gasket and it took off, obviously running lean, and didn't turn off despite hitting the switch (that was working). Easily ran on past the limiter until it died from what I guess was the fuel not keeping up or it "blew" the hot bit of carbon off? Maybe 3 seconds at the most though. This could only be due to a hot spot somewhere in the cylinder.

 

[056kid]

must have huge compression,

























s0unds right fishy though...

 

[AUSSIE1]

I had my 7900 blow a base gasket and it took off, obviously running lean, and didn't turn off despite hitting the switch (that was working). Easily ran on past the limiter until it died from what I guess was the fuel not keeping up or it "blew" the hot bit of carbon off? Maybe 3 seconds at the most though. This could only be due to a hot spot somewhere in the cylinder.

Matt and Kid I'm thinking along these lines.
Two things the high idle and the running on.
He has said that the compression is high (by feel) with the base gasket not present.
I'm thinking he's getting the squish wrong for a start.
The cyl kit had to be replaced. Why?
I had a phone call from him this afternoon with a brief description and posted it here as a matter of interest.

 

[BobL]

He's got a piece of carbon that is getting hot enough to act as a glow plug.

What about a longish metal bur hanging off a spark plug or decomp thread? I'd be checking that out soonish.

 

[AUSSIE1]

What about a longish metal bur hanging off a spark plug or decomp thread? I'd be checking that out soonish.

Thanks Bob.
I'll pass that on.

 

[bejay]

would probably look at the idle issue first, and find out why its idling high it has to have an air leak somwhere if you solve that problem and get the carb adjusted it will likely shut off. just getting the correct mixture should cool the cylinder off quite a bit.
higher compression plus lean mixture is both going to increase the cylinder temps but dont think its a compression issue even if it was high enough for pre detonation, higher octane fuel could be used to prevent it.

 

[stihlboy]

premium or regular gas? i could see it with 87 octane and the wrong heat rated plug. was the cyl coated in oil on assembly? i have seen a "runaway" a diesel will do this with too much oil. just a thought