Walbro HDA

[Terry Syd]

A mate of mine bought a cheap Chinese Baumr-Ag chainsaw, 71cc, and I am going to do some mods on it. He got it new on-line for A$170 and I'm surprised at the quality, it will probably make a pretty good work saw.

It is an open port jug that likely needs more transfer area, but I can't do much about that issue. The muffler has two baffles in it and is going to need a fair bit of work. However, the carb is probably going to give me the most problems.

It is a Chinese copy of a Walbro and when I compare it to an HD that I have it appears to be configured as an HDA. It only has a 15mm venturi.

Does anybody know of an HDA carb that has a 17mm venturi?

If I can find an HDA with a 17mm venturi, I'll redrill it to increase the low end torque (it is quite gutless at the moment).

The piston is 51mm with a 10mm wrist pin - but it has a large DOME shape to it. This piston could possibly be used in other saws to increase the compression. I'll have to get a picture of it.

Anyway, back to the HDA - anybody know of an HDA with a 17mm venturi?

 

[Terry Syd]

I can't find a link to a Walbro document that gives the venturi sizes for the various carbs. I think I remember finding such a document some time ago, but bugger if I can find it now.

Does anyone have a link to such a Walbro document?

 

[HarleyT]

Find your carb's ipl, and it states the venture size at the top of each ipl.

http://wem.walbro.com/walbro/family.asp

 

[Terry Syd]

Thanks for the information - now I just have to find some carbs..

 

[Chris-PA]

It's probably the larger displacement version of the saw I did here, with some slightly different plastic covers: http://www.arboristsite.com/community/threads/porting-a-chinese-g621-clone.253359/

The carb works fine - I have looked at getting a larger carb but have not done it yet. HDA-137 is a larger one that should be a direct fit other than the impulse fitting, but they are not easy to find.

Is this the carb that's on it?

 

[SawTroll]

Find your carb's ipl, and it states the venture size at the top of each ipl.

http://wem.walbro.com/walbro/family.asp

Yes, that is my sourse on such matters as well.

Going by memory, I remember some HDA cabs with a 16.66mm venturi (HDA-87, 190, 191, 198 and 199), but no larger ones. I likely haven't looked up every one though....

If a larger venturi helps, depends on the intake, porting and muffler of course - and likely more as well.

The big question is if a saw like that is worth modifying at all.....

 

[Terry Syd]

Chris, thanks for the link back to your thread. Yes, this is the 71.4cc version of your saw. All they did was bore it out some more - and that is one of the problems with this mod. Take a look at the picture of your cylinder and notice how large the transfer tunnels are, then imagine boring it out from 62cc to 71cc and you can imagine the decrease in the volume of the transfer tunnels.

They increased the displacement at the same time they decreased the flow through the transfers. The transfers are ridiculously small, not only in the depth, but the width of the ports are the same as the 62cc model.

If I was building this as a 'sleeper' project saw I'd have to build up the outside of the cylinder with GB Weld and then grind much larger transfer tunnels in the cylinder. Essentially, the cylinder fins would keep it all together, LOL. I'm not that keen on doing such work on someone else's saw.

The muffler even has less fish gill vents than yours. The carb looks the same, but has a different name on the lid.

The reason I'd like to get a Walbro HDA is that the transition/air bleed ports on this Chinese copy are drilled vertically through the body. Whereas the Walbro has the forward port drilled at an angle and exits up on the venturi. I can mod the Walbro to flow a lot more through the low speed circuit and still get good throttle response with the Walbro configuration.

The muffler has two baffles in it that I can see. I've got to take it apart and see what I can do to get it to flow.

In US$ this thing cost about $135. With the price of saws in Oz being so high, this thing is dirt cheap.

I'm tempted to buy one myself for that money and do it up right. I could have a modded 71cc saw that weighs 12.5 lbs - and that has China written all over it. I could send it to Niko after it was done and he would finally have a hot saw to add to his collection - wouldn't his mates be impressed.

 

[matt167]

I have one of the 52cc china saws. Runs pretty good, but I'm gonna delete the base gasket and clean up the ports. I got it for $118 US. The 62cc runs $180 so I think these china saws are more expensive here

 

[Rudolf73]

This should be an interesting build. What size did the piston end up being?

 

[Chris-PA]

Chris, thanks for the link back to your thread. Yes, this is the 71.4cc version of your saw. All they did was bore it out some more - and that is one of the problems with this mod. Take a look at the picture of your cylinder and notice how large the transfer tunnels are, then imagine boring it out from 62cc to 71cc and you can imagine the decrease in the volume of the transfer tunnels.

They increased the displacement at the same time they decreased the flow through the transfers. The transfers are ridiculously small, not only in the depth, but the width of the ports are the same as the 62cc model.

If I was building this as a 'sleeper' project saw I'd have to build up the outside of the cylinder with GB Weld and then grind much larger transfer tunnels in the cylinder. Essentially, the cylinder fins would keep it all together, LOL. I'm not that keen on doing such work on someone else's saw.

The muffler even has less fish gill vents than yours. The carb looks the same, but has a different name on the lid.

The reason I'd like to get a Walbro HDA is that the transition/air bleed ports on this Chinese copy are drilled vertically through the body. Whereas the Walbro has the forward port drilled at an angle and exits up on the venturi. I can mod the Walbro to flow a lot more through the low speed circuit and still get good throttle response with the Walbro configuration.

The muffler has two baffles in it that I can see. I've got to take it apart and see what I can do to get it to flow.

In US$ this thing cost about $135. With the price of saws in Oz being so high, this thing is dirt cheap.

I'm tempted to buy one myself for that money and do it up right. I could have a modded 71cc saw that weighs 12.5 lbs - and that has China written all over it. I could send it to Niko after it was done and he would finally have a hot saw to add to his collection - wouldn't his mates be impressed.

I believe that the transfers of the 62cc have much too large a volume, so the larger bore should help in that regard and give a higher transfer velocity. The original G621 is an older design that predates the more recent emphasis on reduced case volume and increased transfer velocity. Regardless, while I don't have any other saws of similar size to compare it to, mine is a strong saw that didn't balk at all about cutting white oak with a 25" bar, so maybe see how it works first before getting into radical engine mods.

As to the carb - my saw has really nice throttle response so apparently the transfer circuit flows well, but it may not be the same carb.

I have seen that there are various mufflers used on these saws - mine was pretty free flowing but for the very outlet baffle. However, I had to open it up when that baffler came loose and I just removed it.

I have one of the 52cc china saws. Runs pretty good, but I'm gonna delete the base gasket and clean up the ports. I got it for $118 US. The 62cc runs $180 so I think these china saws are more expensive here

Yours is based on a different Zenoah design (G5000/G5200). There are Chinese clones of 62cc based off of both chassis, but the G5000 style uses a WT type carb. WTs are even smaller, which is why I chose the saws based on the G621 since they come with an HDA carb.

 

[matt167]

I knew mine used a wt style carb. I did not know there were 2 styles of 62cc saws. Mine is a 2 pull starter and pretty strong with good 34lg chain on it. Dollar value is excellent imo

 

[Terry Syd]

I believe that the transfers of the 62cc have much too large a volume, so the larger bore should help in that regard and give a higher transfer velocity. The original G621 is an older design that predates the more recent emphasis on reduced case volume and increased transfer velocity.

I don't know why people go on about 'velocity' on this forum, what counts is FLOW. Ya gotta flow enough mixture for the cylinder.

If you reduce case volume you increase primary compression, which pushes the maximum delivery ratio up higher in the RPM range. However, it doesn't increase the volume/mass of the flow. On the other hand, increased velocity might just alter the loop scavenging to send the higher velocity streams out the exhaust port instead of displacing the exhaust out of the combustion chamber.

The rear transfers are pathetic on this jug. I'd need to deepen them at least 2mm. The port window at BDC is bigger than the cross section of the transfer tunnel. (a combination that actually decreases the velocity of the flow)

Yeah Niko, this could be a real winner in your stable. If done up right and with a NK bar, the bloody thing could be a real 'sleeper'. I'm running a 9-pin on my 029, heck it's possible that this POS could pull a 10-pin. - You'd be the envy of Europe.

 

[Chris-PA]

I don't know why people go on about 'velocity' on this forum, what counts is FLOW. Ya gotta flow enough mixture for the cylinder.

If you reduce case volume you increase primary compression, which pushes the maximum delivery ratio up higher in the RPM range. However, it doesn't increase the volume/mass of the flow. On the other hand, increased velocity might just alter the loop scavenging to send the higher velocity streams out the exhaust port instead of displacing the exhaust out of the combustion chamber.

The rear transfers are pathetic on this jug. I'd need to deepen them at least 2mm. The port window at BDC is bigger than the cross section of the transfer tunnel. (a combination that actually decreases the velocity of the flow)

Yeah Niko, this could be a real winner in your stable. If done up right and with a NK bar, the bloody thing could be a real 'sleeper'. I'm running a 9-pin on my 029, heck it's possible that this POS could pull a 10-pin. - You'd be the envy of Europe.

Reducing case volume is a way to reduce the time it takes to move the volume of fresh mix into the combustion chamber. Otherwise if the case volume is excessive then as the piston pushes down on the case the gas just compresses, and when the transfers open it does not move rapidly. Ideally the angle between the intake closing and the transfers opening is not too large - like all things in these 2-strokes everything is a compromise and a balance.

Clearly the intent is not to sacrifice the ability to flow sufficient volume by increasing the velocity, but rather to have both as much as possible. If you have a smaller cross sectional area in the transfer runners, then if you can move the gas at a higher velocity you can still move the same volume into the cylinder.

While the issue of too much velocity blowing more mix out the ports is often brought up, I notice the strategy used to reduce emissions on the non-strato engines I've seen involves major reductions in transfer port area to increase velocity. Also, some of the engines I've ported with the most success have quite small transfer volume, especially when compared to older jugs I see.

What port area at BDC were you referring to? And which are the "rear" transfers?

This was where the timing ended up on mine:


The exhaust port width is 54% of bore (which was stock), intake I widened to 52%

 

[Terry Syd]

Reducing case volume is a way to reduce the time it takes to move the volume of fresh mix into the combustion chamber. Otherwise if the case volume is excessive then as the piston pushes down on the case the gas just compresses, and when the transfers open it does not move rapidly.

Like I said, increasing base/primary compression moves the point of maximum delivery ratio up further in the RPM range, that's because the mixture can be delivered in a shorter period of time.

Ideally the angle between the intake closing and the transfers opening is not too large - like all things in these 2-strokes everything is a compromise and a balance.

I like to have as much angle as possible between intake closing/opening and the transfers opening/closing (actually, between transfer closing and TDC). This is the 'swept' displacement of the engine - the actual displacement, not the cylinder displacement. That is one of the reasons I try to increase the time/area of the transfers by WIDENING the ports rather than raising them - I like to keep the swept displacement as large as possible.

Also, if you can get a good crankcase filling sooner, you can close the intake sooner, like on a rotary valve engine (say 65 degrees ATDC). The closing of the intake is related to crankcase filling.

Clearly the intent is not to sacrifice the ability to flow sufficient volume by increasing the velocity, but rather to have both as much as possible. If you have a smaller cross sectional area in the transfer runners, then if you can move the gas at a higher velocity you can still move the same volume into the cylinder.

Having a restriction in the transfer tunnels is not an increase in velocity relative to the loop scavenging. When the mixture reaches the larger port opening, the velocity will DROP. All you have done by decreasing the size of the tunnel is restricted the flow to the port opening. Think of a water hose as an analogy. If you have a restriction on the nozzle (port opening) the velocity will rise. If you have a larger opening relative to the size of the water hose the water velocity will decrease.

While the issue of too much velocity blowing more mix out the ports is often brought up, I notice the strategy used to reduce emissions on the non-strato engines I've seen involves major reductions in transfer port area to increase velocity. Also, some of the engines I've ported with the most success have quite small transfer volume, especially when compared to older jugs I see.

The factory sets the time/area of the transfers for when they want the maximum delivery ratio (for most intents, maximum torque). It is not an emissions issue related to 'velocity'.

What port area at BDC were you referring to? And which are the "rear" transfers?

Transfer port. The "rear" transfers are those closest to the intake. The front of the saw is where the bar is located, the rear of the saw is the handle you hold on to.

 

[Chris-PA]

Having a restriction in the transfer tunnels is not an increase in velocity relative to the loop scavenging. When the mixture reaches the larger port opening, the velocity will DROP. All you have done by decreasing the size of the tunnel is restricted the flow to the port opening. Think of a water hose as an analogy. If you have a restriction on the nozzle (port opening) the velocity will rise. If you have a larger opening relative to the size of the water hose the water velocity will decrease.

This is not a great analogy, as water is not compressible. As a gas the air moving through the smaller transfers will most definitely be at a higher velocity.

Having a larger case volume (which includes the transfer runners) means that as the piston comes down, more of the effective swept volume (after the intake port closes) goes into compressing the gas rather than moving it into the cylinder - lower pumping efficiency. Any mix that doesn't make it into the cylinder and remains in the case taking up volume there will block some the entrance of new mix the next time the intake opens (which happens at a lower pressure).

When drawing air into the case there is at most 1atmosphere (14.7psi) pushing the fresh mix into the case, although of course it won't be that high. The pressure pushing the mix up the transfers will be much higher than that, although it will be pushing against any residual combustion pressure left in the cylinder. It's a complex and interrelated problem, and it isn't possible to just compare port areas without factoring in the pressures and time (angle) as well - it must be considered from a dynamic point of view, not a static one.

My main point was that modern engine designs have shown that large transfer runner volumes/areas are not required and can be detrimental - the newer designs simply are not doing that any more but still make good power. It's not the kind of thing you can just look at, as it is interrelated with all the other timing and area parameters. Unfortunately when we modify engines we don't have access to the kind of testing data you'd need to really optimize it and so we have to guess and go by feel. So you might want to try it out even though it looks like the transfers are smaller than seems right to you.

I experienced this same thing on a Poulan PP315 engine. The design started out at 36cc and 40cc, was increased to 46cc and then to 49cc, all with essentially the same casting. The transfers in the 49cc got pretty small, but it ran like crazy.

 

[Terry Syd]

Chris, you can waffle on with your BS and try to baffle other guys on this forum, but I'm not impressed. I've been building engines for over 50 years. I expect that I'm the only guy on this forum that has actually had a technical discussion with Gordon Jennings (look him up).

I was also the technical editor of a motorcycle magazine for 8 years and had my articles peer reviewed by 10s of thousands of guys, including a bloke named Eric Gorr (look him up) who complemented me on the quality of my articles.

As far as porting is concerned, I learned a lot from Jerry Branch on his flow bench at Flowmetrics (look him up) . The guy was a freaking genius, a great gentleman and a true artist.

I've tried to point out a few things in the past, but your ego is too big to allow you to step back and try to understand. It's your loss.

 

[Chris-PA]

Chris, you can waffle on with your BS and try to baffle other guys on this forum, but I'm not impressed. I've been building engines for over 50 years. I expect that I'm the only guy on this forum that has actually had a technical discussion with Gordon Jennings (look him up).

I was also the technical editor of a motorcycle magazine for 8 years and had my articles peer reviewed by 10s of thousands of guys, including a bloke named Eric Gorr (look him up) who complemented me on the quality of my articles.

As far as porting is concerned, I learned a lot from Jerry Branch on his flow bench at Flowmetrics (look him up) . The guy was a freaking genius, a great gentleman and a true artist.

I've tried to point out a few things in the past, but your ego is too big to allow you to step back and try to understand. It's your loss.

Terry, your resort to personal attack and name dropping says more than I ever could. Which is a shame as we've previously had some good discussions, even when I've disagreed on technical points.

I will no longer be conversing with you.

 

[SawTroll]

I don't know why people go on about 'velocity' on this forum, what counts is FLOW. Ya gotta flow enough mixture for the cylinder.

If you reduce case volume you increase primary compression, which pushes the maximum delivery ratio up higher in the RPM range. However, it doesn't increase the volume/mass of the flow. On the other hand, increased velocity might just alter the loop scavenging to send the higher velocity streams out the exhaust port instead of displacing the exhaust out of the combustion chamber.

The rear transfers are pathetic on this jug. I'd need to deepen them at least 2mm. The port window at BDC is bigger than the cross section of the transfer tunnel. (a combination that actually decreases the velocity of the flow)

Yeah Niko, this could be a real winner in your stable. If done up right and with a NK bar, the bloody thing could be a real 'sleeper'. I'm running a 9-pin on my 029, heck it's possible that this POS could pull a 10-pin. - You'd be the envy of Europe.

A lot of that actually is correct- but I have my doubts about the end comments!

What you call primary compression may be much more important than many people believe....

 

[Terry Syd]

Nikko, if I ever blow up this 029 I'm going to get me one of these Baumr-Ag saws and fix it up right. The power to weight ratio will be great with the lightweight laminated NK Husky bar.

The problem with bumping up the primary/base/crankcase compression is that it also narrows the powerband. So it's a matter of 'horses for courses', ie: what you're going to use the saw for. For most chainsaw applications a broad powerband is desired, so the primary compression is kept down.

 

[CR888]

If l am gonna read a debate about the principles of two cycle combustion l'd rather hear it from Chris and Terry. Two clever educated cookies with manners to boot. l had a laugh thinking about Terry giving his mates new saw the JB weld tranfer modd. These china saw do seem very cheap here compared to other parts of the world. Subscribed.