Duct size/HP observation and confirmation please

I have a 2 hp (rated by the seller) cyclone system with a 6” inlet. I have recently plumbed up the shop using 6” PVC and 6” to 4” Ys connected to 4” metal gates. All the joins are duct tape ( the real stuff) sealed because I’m moving things around so permanency isn’t there yet.
Today I had to make mods, so I took parts apart. I was astounded at how much sawdust was in the pipes.
I used to have every gate closed except for the one I was using, then read that I could optimize air and cleaning by having 2 - 4” ports open so as not to starve the system.
Is this normal to have sawdust remaining in the horizontal pipes?
There’s no appetite in the budget for a bigger collector.

The area of a 4" duct is less than 1/2 of a 6" duct. When you have one 4" pipe supplying a 6" main line the air flow is going to slow down and dust will settle out. If you are having settling issues with two 4" gates open the problem may be in the blower. "2 hp" doesn't mean anything by itself, what you need to know is whether your fan will pull sufficient air given the static pressure in your duct work. Your full scale mockup suggests not. You may need a more powerful blower or smaller diameter main lines.

"Budget" is a verb describing the process of matching resources to requirements.

There's a difference between "2hp" from Harbor Freight and Oneida for example. We need more information about what you're working with to be able to offer suggestions.

your pipe is too large for your dust collector, downsize to 5" mains.
Been there done that
Ron

Along the lines of what John Lanciani said, blower size and design play a substantial role beyond H.P. To exaggerate a bit, imagine two dust collectors of identical design. One has a 10 h.p. motor turning a 10" blower. The second machine has a 2 h.p. motor with 12" blower. Which one will move more air? My money is on the 2 h.p. 12" blower. I have a Penn State "2 h.p." conventional dust collector with cartridge filter and 6" pvc ducts. Machines have 4" ports so I had to 'neck down' the 6" ducting/hose as close to the machine as practical but my setup is far from perfect. The only time I've had anything accumulate in the 6" duct is when I was using a router table fence with a 2 1/2" port so 2 1/2" hose. I found that out when I was finished with the router table and used the 6" hose. There was a small rush of chips. The next time I used the 2 1/2" hose I cracked another blast gate so there was enough air to carry the chips and dust without their settling. And no, 2 1/2" hose with a dust collector is not optimal but ya use what ya got. I don't like emptying shop vacs.:)

I think before dropping a bunch of $$ on new ducting I'd try opening one 4" gate all the way and maybe just crack a second gate or even try just the 4" gate by itself and see if you get enough airflow velocity with your existing ducting.

I can see that your main is 6". Are your drops 6" as well? Or are the drops 4" duct? If they are 4" duct, you can definitely have air slow-down once it hits the main. Also, depending on where your open blast gates are, there may not be enough velocity in the 6" main duct to move the sawdust.

Ron probably has the best suggestion to downsize the main duct to 5".

Why bother to downsize? You fan is too small a flow to run 6" horizontal or vertical. Measure the open area above the dust. That is the ideal pipe cross section. I would leave the 6" and let it get enough dust to make it 5" diameter area, 15.7 square inches.. This will not work on vertical runs. Those need to be sized correctly to keep the dust moving upwards.
18.84 -15.7=3.14
I will guess the dust is roughly 3.14 square inches in cross section of the 6" duct.
Bill D