I know this is an old thread, and there are posts all over asking these questions. But I just wanted to share some information, in case it is useful to anyone in the future.
1. The 357XP and 359 were introduced in 2001. Depending on the locale where they were sold, they either came with a Walbro HDA-174 or an EPA regulated Walbro HDA-175. The differences between these two carbs will become clear as an emerging pattern, as Walbro provides the 174 as a normal carb with a 16.66mm venturi, and the 175 as an EPA compliant carb. The major difference was the 175 had smaller jets, so to achieve faster acceleration it had a built in accelerator pump. Since the accelerator pump is such a crucial difference, it may be useful to understand what it is and does.
The HDA-175 also had adjustment needles with limiters and a different metering spring providing different pop-off pressure.
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Around 2004 the HDA-174 and HDA-175 were discontinued by Walbro, forcing Husqvarna to select new carbs for their product.
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2. In April 2003 Husqvarna updated their 357XP and 359 to use 4 new carbs! All four carbs still use a 16.66mm venturi. The particular carb choice is dependent on two qualifications: the first is if the saw came with handle heating (357XPG and 359G) or without handle heating (357XP and 359), and the second qualification is if the saw was sold in a locale requiring EPA regulation compliance, or not. Saws with handle heating received a Walbro HDA-190A, or a Walbro HDA-191A if they needed to meet EPA regulations. Saws without handle heating received a Walbro HDA-198A, or a Walbro HDA-199A if they needed to meet EPA regulations. Again, the major differences between the non-EPA and the EPA carbs are the restriction of jet size, adjustment needles with limiters and an accelerator pump.
The major difference between the heated handle carbs (190 and 191) and the non-heated handle carbs (198 and 199) was the inclusion of a plastic ring (537 04 53-01) at the top of the carb on non-heated handle carbs. This was Walbro's version of a compensating carburetor, similar to Stihl's Intellicarb idea. The principle was there was a passageway which allowed the backside of the metering diaphragm to have access to the pressure inside the intake track, after the air filter. The concept was that as the air filter gets clogged up, a more negative pressure develops and more fuel would be sucked through the carb without as much air, resulting in a rich condition. To compensate, the negative pressure was added to the top side of the metering diaphragm to counter-act the pressure difference. Scott Kunz (@Treemonkey) believes this to be a poor design, because the passage way is small. He tends to solve problems with HDA-199 carbs by making them like 191 carbs. The 191 carbs do not have a plastic ring at the top of the carb, because the ring thermally decoupled the top cover where the carb heater sat. Therefore, on heated handle (and carb) saws, the 190/191 carb was used, without a ring, and with a simple hole drilled in the top cover, and no compensation.
After 2005, EU regulations meant saws sold there received the EPA version carbs.
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Walbro has never provided instructions or parts to service the accelerator pump. Whether due to fuel quality or simply a limited lifetime, the accelerator pump would often get gummed up and stick, causing running problems. The easy fix was to replace the whole carb, as most small-engine technicians were not going to go through the trouble to diagnose, remove, service, and replace an accelerator pump.
3. In October 2008 Husqvarna introduced a service bulletin to change the carb on the 357XP and 359 again, this time to the Zama C3-EL42, ostensibly for better idling stability.
Regarding the use of these carbs on 346XP saws.
Any of these Walbro carbs will be a direct swap into a 346XP, however, little gain would be seen on a stock saw, while only modest gain might be seen on a gasket delete/muffler mod. To take advantage of the larger gains possible with these carbs would require full port jobs to a cylinder. Of all the carb choices, I would suggest given the choice between two carbs that the non-EPA, non accelerator pump carbs would be preferable. That means an HDA-174, HDA-190, or HDA-198. These carbs are, however, more rare, since less saws were produced with them. There is nothing wrong with using an EPA carb from a 357XP/359, although they too are becoming harder to find. The only downside is that the additional complication of the accelerator pump may mean they have a shorter service life due to a possible failure of the accelerator pump. I am also not certain if the possibility for failure of the accelerator pump would outweigh its benefit for a ported saw. It may be that the accelerator pump version carb performs better? Only side-by-side testing would show that.
The EL-42 carb will also work for a 346XP upgrade, however, it may not deliver enough fuel to a ported saw. There is a modification that can be made to drill out the jet,
which is detailed here, to increase fuel delivery.