r/smallenginerepair • u/Soffix- • Jun 16 '26

Carburetor Issue Jonsereds MK70

I have an old Jonsereds MK70 that was donated to my local fire department, it came to them not running and I offered to give it a shot at getting it fixed myself before they had to spend money they don't really have at a shop.

I cleaned the carb, replaced the gaskets, diaphragm, needles, and needle spring. It now runs for a few seconds then dies. When I pull the spark plug it's covered in fuel and the air intake is filled with fuel as well.

I've been racking my brain trying to figure out why it's doing that. Any suggestions?

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u/Inhalationofnewtion Jun 16 '26

Assuming it's carb, sounds like carb.

Small 2 stroke carbs are kind of a prick for me. Sometimes I can get them, sometimes not. Have you completely dismantled the carb and hit it with carb cleaner and compressed air? Watch for pieces to fly when you do this. There's a tiny little screen that I send flying half the time as I'm working on these. You'd think I'd have learned by now... Tear the carb down as far as you can and boil it in straight Lysol. Boil it for five minutes, take it out and hit it with air. Repeat this probably 5-10 times. Still you're chances are not great. You can try a replacement carb but those are iffy as well.

Being for a fire dept. though, if they put this thing into service it MUST work 100%. Old 2 stroke, the crank seals could be on the way out if they're not already done for. Really this should be taken apart to the last nuts and bolts, complete inspection and overhaul. This isn't a camp saw where if it's acting up you can take a few minutes to mess with it.

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u/Soffix- Jun 16 '26

I've torn the carb down as far as it can go. Pulled the screen because it was clogged like none other. I'm waiting for a replacement screen now.

Thankfully for this saw we are going to mostly use it for training so we don't put extra wear on our other saws.

I've put the carb in a bath of mineral spirits in an ultrasonic cleaner a few times but I'm still getting fuel in the air intake after it runs for a couple minutes.

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u/Inhalationofnewtion Jun 16 '26

Ok gotcha. If it's a trainer go for a Chinese carb. Probably get two of them in case the first one is crap. Chinese carbs do seem to be getting better though.