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Ported km85 can't rev past 10200

Redline

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Not chainsaw related but I figure this is where the porters and modders hang. I have a km85 that I rebuilt from a bunch of parts. The whole works. Bearings, seals, rings, new carb, fuel lines and filter, ported, muffler mod, squish 015, timing advance. It 4 strokes with the high turned out almost 2 turns and when I lean it out, it maxes rpm at 10200 and then it seems to hit a limiter in the coil, sounds the same all the way in to likely too lean... Tried 3 different coils, (same pn) and another carb. I mostly use it for formal hedge trimming so more rpms and power are good to have.
Is it a limited coil?
Can I mod the coil somehow?
Anyone know if a different coil might fit? Would an aftermarket coil be unlimited - cheaper to produce?
My other km 85 unit maxes out about the same rpm with a muffler mod and no advance but my hybrid km74 will go way higher. FS 74s are hard to come by these days. Could I put the chainsaw attachment on and tune it in the cut by leaning it past the limiter to max power so it's not running so rich?
 

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Nutball

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Can you post a video of a several second rev starting 2 turns out, then lean it in all the way to max rpm and hold it a few seconds?

If the coil is limiting it, it could be to protect certain parts from breaking or too much bearing heat. I know string trimmers, if it can accept such an attachment, often are able to function just fine at a near idle rpm. Some sound like they don't really even gain much if any rpm between idle and full throttle. One old 2 stroke weed eater I have cuts best near idle, much faster and it always breaks the string.

So, maybe they intended it to not overrev certain attachments: string trimmer, blower, hedge clippers, edger, tiller.

I would focus on port timings that maximize torque at around 9000-10,000rpm
 

Redline

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Thanks nutball!
Time to get the degree wheel out and maybe cut cylinder and squish band? I do want more rpms though as this one is my dedicated hedge trimmer powerhead. More rpms when trimming hedges is great. Line head in photo but not often as I only use that head for my personal yard.
 

Redline

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I posted this before I even ran it and just got in from knocking some foot high wet grass... Peak torque around 9700. Leaned it out a tad and it lost power so put it back where it should be. Barely slowed down with 6 inches of line! More power than my mm powerhead of the same make and model.. I think I got the gains I was looking for. These little engines are fun to work on. Good to practice porting and mods on. I usually get them from the scrap bin at my local dealer for 40-60 bucks.
 

Nutball

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Base gasket delete and ignition advance probably helped adjust the timing (port and electrical) to have more power around 9000-10,000rpm. 25cc engines are sensitive, and from what little experience I have with a 25cc Echo chainsaw, just adjusting the ports around .5mm can make it loose a good bit of it's low end torque while possibly only slightly increasing its peak power. Not that such an adjustment was bad, but one had an advantage of cutting faster in more wood types with its broader powerband. Overall, the max torque range jumped around 2000 rpm over stock, which could be what you are experiencing.
 

DND 9000

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These ignition modules used on the 4137 series are limited to 10500 rpm. It is possible that the limiter kicks in a little bit below or above that speed.
 

Redline

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I ordered an aftermarket coil from hlsupply and... O ya. Unlimited past the red line. Still 4 stroking past 12000. I'll keep it just under 12 and see how it slices tomorrow. Problem solved. I put a different flywheel on it as well because it was pretty hot the other day, maybe too advanced... I'll progressively advance it as I use it.
 

smokey7

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Good deal so a aftermarket coil was unlimited?
 
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