Walbro HDA

[Terry Syd]

I have to wait until my mate comes by to look at the work involved. I don't mind doing a few minor mods, but redesigning the transfers on this jug? I don't know if Will is that good of a mate. - Now if it was my saw...

I may have an HDA carb body to work on, but it is only a 15mm venturi. I'll pay the $20 he wants for it just to see if the mod is do-able. - If anyone in Oz has a junk 16.6mm HDA carb laying around, I'd be interested.

 

[w8ye]

I have a Zenoah G62 model airplane engine based on the 621 chainsaw engine. They come with a HDA48 (15.8mm) from the factory.

 

[Terry Syd]

I found a HDA-198A carb body for $20 that I can fix up. I can't access Walbro's site for some reason, but the venturi is the larger 16.6mm. Does anybody know what the HDA-198A came on. I see that the 198s were used on the 359, but that's the biggest saw I could find that used them. This Chinese saw is 71cc.

The 198 has a two large transition holes as compared to the Chinese carb which has a smaller first transition hole - so I should be able to get more low speed circuit flow from the 198. The 198 also has an extra jet before the venturi, kinda like a 'Power jet' on an old automobile carb. This is the first time I've run into the extra jet on a chainsaw carb.

Anybody have any tricks on setting these carbs up?

The extra main jet may allow me to get a flatter fuel curve, or not. I'll have to wait and see what it does.

 

[ncfarmboy]

I couldn't get on the Walbro site yesterday. Been looking for HDA larger than 16.66 don't think there is one at last I have not found one. AFAIK
198 is only used on 359.
Shep

 

[Terry Syd]

I don't think you'll find an HDA larger than 16.6, in fact, the way the venturi is constructed I wonder how they came up with 16.6. The venturi isn't circular, it elongated in a couple of directions. It appears they were trying to open up the carb and had to work around the internal passages.

The extra jet before the venturi may be a compensation for a less effective venturi. I'll have to wait until I pull the shafts out of the carbs to make the swap to get a good look at the jet. I expect it will have a flutter valve in it like the main discharge nozzle. If it didn't the engine would start sucking air back into the metering chamber at idle and the engine would stall (just like a when a discharge nozzle has failed).

If it has the flutter valve, then I doubt if I will be trying to drill the jet if I needed more fuel flow. Perhaps Walbro offers different size jets to fit the HDA.