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Walbro HDC-44 carb question.

fossil

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Here's a pic of an HDC44 carb with what appears to be a vent with a screen over it. It doesn't seem to go anywhere as it directly in the path of the throttle rod and doesn't pass air either way.

I've seen it on other carbs and was wondering if anyone knows what it's for?


Walbro HDC-44 carb vent.jpg
 

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Is it a detent ball and spring?
 

fossil

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Is it a detent ball and spring?

No. It's a screen with C clip.

After I put the post up I thought I would look a little harder. The passage is closed as long as the throttle is closed.

As the throttle is opened the passage becomes open to the venturi.
Nothing on the IPL showing it.

Without pulling the throttle shaft, it looks like a flat on the shaft (looking onto the venturi) rolls into the hole and air passes into the venturi.

The issue I am having with the saw is it wouldn't run worth a crap. It's on a little Homelite XL-1 (SEZ family).

The OEM Homelite manifold gasket has three holes in it which allows the case pulse to go into the pump side of the carb body and into that passage.

I went through everything that could be wrong on the saw and all is in spec.

I made another gasket to block that passage off and I'm betting the saw will run now.

Damned if I can figure what it's for though.
 
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fossil

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Well, despite my hopes that that particular HDC-44 would work I finally gave up on it. I have gone through it several times to no avail.

I did have a spare so I swapped out the kit from the troubled one to the spare and the saw runs like a top now.
 

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Here's a pic of an HDC44 carb with what appears to be a vent with a screen over it. It doesn't seem to go anywhere as it directly in the path of the throttle rod and doesn't pass air either way.

I've seen it on other carbs and was wondering if anyone knows what it's for?


View attachment 278616
Hi Tim.

That HDC looks like the version that had the accelerator circuit integrated into the idle transition ports. I have run into a few. It applies impulse into the idle circuit upon transition of the index in the throttle shaft. I suppose it was there to allow the L to be set rather lean and still have enough fuel to elimate bogging when transitioning through to WOT. Here is a link to a diagram/post I tried to make on Another Site. I find them rather finicky to get running correctly. But, I see you also mention it vents directly into the Venturi. I have not seen one with the index in the shaft that does not lead directly to the idle circuit through the brass jet. I wonder if they eventually started blocking parts of that circuit with brass plugs and gaskets while they used up that run of carbs?

https://www.arboristsite.com/community/threads/homelite-chainsaws.101147/post-6916068
 
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fossil

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Hi Tim.

That HDC looks like the version that had the accelerator circuit integrated into the idle transition ports. I have run into a few. It applies impulse into the idle circuit upon transition of the index in the throttle shaft. I suppose it was there to allow the L to be set rather lean and still have enough fuel to elimate bogging when transitioning through to WOT. Here is a link to a diagram/post I tried to make on Another Site. I find them rather finicky to get running correctly. But, I see you also mention it vents directly into the Venturi. I have not seen one with the index in the shaft that does not lead directly to the idle circuit through the brass jet. I wonder if they eventually started blocking parts of that circuit with brass plugs and gaskets while they used up that run of carbs?

https://www.arboristsite.com/community/threads/homelite-chainsaws.101147/post-6916068


I've generally just inadvertently blocked them off with the gasket on previous saws. The SDC-44 pulled fuel fine but it wouldn't keep the fuel flowing. The check valve passed several suck/bow tests but I will visually check that as well.

I noticed this one as I couldn't get it to run despite trying everything I could think of.

I think I'll pull that carb apart as far as I can get it and see if anything is apparent.

The SDC -44 I replaced the bad one with did not appear to pass any air at any throttle position.

Looking at the carb that won't run, there is a cup plug in the hole where that brass plug with the hole in it is.

The screen on the check valve was dished so I put a new one in.
 
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