Jonsered 451 Carb needs help

LivinTheDream

LivinTheDream

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Carb keeps flooding after I disassembled, cleaned it, and installed new gaskets. I've tried adjusting the angle of the needle valve control lever several different ways, including setting it level with the diaphragm. I can sometimes get the saw to start and run at full throttle but after a couple minutes it dies out and there's fuel in the air intake.
> From attached photos, does the angle of the needle valve lever look right?
> Is it worth buying a new needle and lever, or is it likely the seat is worn?
> I haven't been able to find a replacement carb for the Jonsered 451 (vintage 1976-1983 I believe).
Suggestions?
IMG_0415.JPGIMG_0416.JPGIMG_0417.JPGIMG_0418.JPG
 

Nex

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I've had a similar issue on my 450, the successor to yours. For me the issue was the tank vent filter that was clogged. If they're built about the same you should have a line coming out under the chain-brake cover with a sintered brass plug in it. Make sure to clean that plug and try again. On the 450 atleast the fuel pressure in the tank is pretty substantial when the saw is hot (as in boiling fuel kinda pressure if you've been working the saw hard, even in winter)

Great saw though, built like a tank!
 
LivinTheDream

LivinTheDream

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Thanks, Nex. It does make sense that if fuel pressure got high, it could push past the needle and flood the engine.
But I looked at my 451-EV and couldn't find a line with a sintered brass plug in it. I didn't see anything that looked like a tank vent. If it's something you could take a photo of, that might help me.
 
LivinTheDream

LivinTheDream

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A few thoughts (keeping in mind that photos can be deceptive):
the needle looks worn in the pic where it's held in fingers
the lever looks high in the last pic but angle could be misleading
a pressure test would help diagnose problem
Thanks.
When I took the photos (on my iphone), I didn't see any defect in the needle, but as the photos got enlarged when posting here (and I'm viewing on my PC), I see what you mean.
I had the lever positioned several different ways but outcome seemed the same.
Last night I ordered a carb kit which includes a new needle and lever (and more gaskets).
From what I can tell, there's no "seat" per se for the "float" needle; just the metal of the casting? Or is there supposed to be some sort of seal?
 

Nex

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Here's a pic of what I'm talking about. Again, I'm not sure your saw is built the same, but here's the culprit in my case. Easy to miss if you don't know it's there :)

Best of luck, and hope it helps!

20211001_003158.jpg20211001_003222.jpg
 
LivinTheDream

LivinTheDream

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Great Photos; Thank You!
Mine looks a little different (see below), but maybe I need to do more disassembly? Maybe it is under that red bump-out with what looks like a black tube above it?
IMG_0492.JPG
 

Nex

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Yea, that looks quite a bit different. But yea, I would probably tear it down and give it a good cleaning anyway :) I'm sure you'll find the problem!
 
LivinTheDream

LivinTheDream

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I replaced the needle, lever, and spring. It runs great now!
Photo shows old and new parts (new ones came with a shorter axle which I didn't use).
I think the combination of the new needle and a stronger (longer) spring helped seat the fuel inlet.
First time assembling the new parts, it ran and then flooded out. I disassembled, bent the lever arm a little lower, and success.
 

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