Identifying Walbro HDB by Venturi Size

[Chris-PA]

My McCulloch Silver Eagle 3420 (basically a PM605) is running well, but I've had a hard time tuning it. To get it tuned the L needle needs to be at 1/8 turn open. It's runs mostly OK like that, but it has an additional symptom in that it takes a long time to idle down after you release the throttle - maybe 2 or 3 seconds. This is not a big deal and it's been working fine, but it bugs me because I know it's not right. It means the basic mixture at idle is too rich, beyond what can be adjusted out of the carb (needles and seats appear fine), and probably the H mixture is better so it runs faster for a moment after you drop the throttle.

This was my Dad's saw, and at some point he replaced the carb, and I don't know where he got it. I suspect the original was a Zama C2S, but I don't know. This one is a Walbro HBD-6B.

I recently bought a used carb and new needles from EvilBay, and the carb that came with it is also an HDB, but the venturi is marked 48. Mine is marked 40. It's also an older casting and it's much nicer made and finished. And it's missing the inlet fitting, which I made a separate thread about. It has HBD cast in it but no other identifying marks that I can see.

I do not know how to associate the venturi sizes with the model, nor what is appropriate for the saw. I got to thinking last night that maybe this is the problem - if the venturi is too small for the displacement, even at idle it will be drawing more air than the carb was set up for - probably activating more of the main circuit along with the idle nozzles.

So does anyone know what venturi size a 3420/PM605 should have, or how to connect venturi sizes with HDB model numbers?

 

[Chris-PA]

I just looked at the carb I got again, and there is a "1" stamped in the body down by the H screw, so this is probably an HDB-1, which is a carb off a PM610. I would still like to know the relationship between model number and venturi size.

 

[SawTroll]

There isn't any logic relation that I know of - you have to look up every model on the Walbro IPL site.

40 and 48 are the venturi in 64ths of an inch.

 

[Chris-PA]

Thanks - I had seen that page but had not noticed it listed the venturi size up at the top. I got the inlet fitting out and I'm going to try the HDB-1 and see if that helps. I just have to fix the throttle plat spring first - it wants to pop out of its little slot.

 

[Chris-PA]

Well, it fired up in a couple of pulls and seems to like both the screws at about 1-1/4 turns, just like it should. Seems to run very nice. I've got a big red oak log to buck up so I can go tune it and see how it works under load, but everything is so wet right now!

It's curious - both the HDB-6B and HDB-1 were OEM for the PM610, which is 60cc. This is 55cc and the HDB-6B, or at least this variant, is too small for it.

 

[scottr]

My McCulloch Silver Eagle 3420 (basically a PM605) is running well, but I've had a hard time tuning it. To get it tuned the L needle needs to be at 1/8 turn open. It's runs mostly OK like that, but it has an additional symptom in that it takes a long time to idle down after you release the throttle - maybe 2 or 3 seconds. This is not a big deal and it's been working fine, but it bugs me because I know it's not right. It means the basic mixture at idle is too rich, beyond what can be adjusted out of the carb (needles and seats appear fine), and probably the H mixture is better so it runs faster for a moment after you drop the throttle.

This was my Dad's saw, and at some point he replaced the carb, and I don't know where he got it. I suspect the original was a Zama C2S, but I don't know. This one is a Walbro HBD-6B.

I recently bought a used carb and new needles from EvilBay, and the carb that came with it is also an HDB, but the venturi is marked 48. Mine is marked 40. It's also an older casting and it's much nicer made and finished. And it's missing the inlet fitting, which I made a separate thread about. It has HBD cast in it but no other identifying marks that I can see.

I do not know how to associate the venturi sizes with the model, nor what is appropriate for the saw. I got to thinking last night that maybe this is the problem - if the venturi is too small for the displacement, even at idle it will be drawing more air than the carb was set up for - probably activating more of the main circuit along with the idle nozzles.

So does anyone know what venturi size a 3420/PM605 should have, or how to connect venturi sizes with HDB model numbers?

Chris, I have a relatives Poulan 2000 that takes a couple seconds to return to idle and inspection revealed a chip of wood lodged under the edge of the reed valve .

 

[SawTroll]

Well, it fired up in a couple of pulls and seems to like both the screws at about 1-1/4 turns, just like it should. Seems to run very nice. I've got a big red oak log to buck up so I can go tune it and see how it works under load, but everything is so wet right now!

It's curious - both the HDB-6B and HDB-1 were OEM for the PM610, which is 60cc. This is 55cc and the HDB-6B, or at least this variant, is too small for it.

The 48/64 one seems quite large for a 60cc saw.

 

[Chris-PA]

The 48/64 one seems quite large for a 60cc saw.

Perhaps, and I'm running it on a 55cc saw at that! But clearly it was overwhelming the idle circuit with the smaller venturi. This is not a high rpm engine, so maybe port timing is set up for more airflow at lower speeds, and therefore it needs a larger carb than a comparable displacement modern engine.

Anyway - wow, what an improvement! I can adjust the idle properly now, and the larger venturi does not make it bog at all. Throttle response is good and it has a little more power in the cut. I can run it hard and drop the throttle, and it just goes to a nice idle every time, and it doesn't take a few seconds to do it. I'm very happy with it, especially considering I spent less than $20 on the parts. If I let it sit hot it doesn't flood easily like it did (because it was really too rich at idle).

 

[zogger]

Nice analysis and repair!