First post with a question

[Va boy]

Hello all, I happened upon this forum while searching for an answer to my current problem.
I'm working on an older model Stihl FS46 trimmer. Initial issue was that it would not crank. This is my neighbors trimmer that he asked me to look at for him. I rebuilt the carb, it had been sitting and had the typical ethanol build up, little particles throughout it. Put it back together, no go. Tested the plug, no spark. Disassembled outer case to get a better look. Contact points between magnets and ignition coil were corroded/rusted. Took ignition coil off, cleaned contact points of it and magnet surface, gapped @ .012" assembled everything back.
Well....now it fires right up, but, it's stuck in fast idle, trigger has zero effect. I have set carb screw settings to factory, as well as trying to adjust, no change. I checked throttle cable, it's not holding carb at WOT, does this even with no cable attached. I even tried another carb that I KNOW to be good, and same thing, high idle, way too high. Of note, I also disassembled the trigger assembly, it all seemed fluid to me.
I'm wanting to think possibly a vacuum leak, but where? Carb to block interface looks tight? Am I missing something obvious? I've read A LOT of entries on the internet before settling on this site to ask my question, appears to be some sharp guys in here.
Thanks for reading my short novel, I just wanted to include any and all clues that may help with my issue, I'm not too proud to admit, I'm stumped!

Thanks,
Jason

 

[bertsmobile1]

Crankshaft seals would be my first guess
Two ways to check
1) put color tune in place of plug and look at the colour of the cylinder flame.
If it is very light whitish blue = air leak
If it is deep blue to yellow = carb problem

2) do a vacuum test with the piston 1/2 way down the stoke so the transfer port is closed by both rings.
Note this is not the same as a leak down test because the seals will oft hold pressure but still suck in air on the up stroke.

And while not trying to be insulting remember that the needle closest to the cylinder is the low speed jet and the needle furthest away is the high speed jet.
People ( including me occasionally ) get it wrong way around.
Both of these are fuel jets so in = lean & out =rich. Always gets me because I have been playing with Amals for 45 years where the jets control the air so adjustment are inverse.

 

[Va boy]

bertsmobile1,
Thanks so much for your informative reply, excellent thought. I'll do further testing today.
Have a great weekend!
Va boy