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My 1976 RD400 C is completely stock with the paper air filter, and a new clean one at that.  Please be advised that a paper filter is more restrictive than a foam filter in the stock airbox. This means the bike will need slightly richer jetting with an aftermarket foam filter.

That said, my bike now has 22,000 miles on the stock bores and it has stock jetting. Timing is stock at 2.4MM BTDC. Movable timing mark on stator has been verified and calibrated with a Sanginamiya (Suzuki) dial timing gauge.

Here is the foolproof test for determining if your pilot jet is the correct size: With a warmed up engine, screw in the pilot air screw all the way. If it does not kill the engine, go up 2.5 in pilot jet size until it will kill the engine when screwed in all the way.   Always start with the needle clip in the stock position or this test is not valid and if you try to tune the pilot jet with the clip in the wrong position, you will go around in circles and wind up dialing yourself out instead of in.

Now to tune the pilot air screw: Take off in 1st gear and notice if the bike grabs and pulls, or if you have to slip the clutch to keep it from stalling. If it wants to stall and you have to give it gas and slip the clutch, turn in the Pilot Air screw 1/4 turn and try again.   Once you are satisfied with the pull away from a dead stop, now go and try decelerating from 3rd or 2nd gear and listen to the pipes for backfires; a few  occasional pops is  about right, no popping is too rich, and a bunch of burbling and popping is too lean. Another test for this is to simply rev the engine in neutral and see how quickly the tach needle goes back into an idle. If it floats slowly back to idle with backfiring and popping, turn in the Pilot Airscrew 1/4 turn.  No backfiring at all and a gassy smell to the exhaust calls for opening the airscrew 1/4 turn.

You should never have to adjust the pilot airscrew more than one full turn either way, from 1.5 turns and if you find you have to, either the pilot is clogged, or the wrong size, or the needle clip is in the wrong position, or your fuel level in the float bowl is off.

RD400 C, D, E have a notch on the bottom of the piston skirt on the exhaust side to "free-port" the crankcase to the exhaust system to smooth out the "bucking" at part throttle. One way to reduce "surging/bucking" when decelerating is to set the idle speed as low as practical, like around 1000 RPM. Also having both slides set so they close equally also helps. If the pilot air screw is a tad lean, turning it in 1/8th of a turn will reduce surging. A good battery and proper timing is also key to getting good idle and reducing surging, they all work together and have interaction, so it's not just the carbs and jetting.

One other tuning note: It is far easier to tune a bike when it is all stock, because if it will not tune with the proper factory settings, there is a problem. Once you have it behaving, in stock trim, THEN go and modify and re-tune. This way you have a baseline because you know the bike will run properly if it is all stock. Without that baseline, you can chase your tail until the cows come home!

To remove the trim, you have to take off the little rectangular metal clips that are pushed onto the pins, look inside under the seat to see them.  Once they are all off, the trim will come off by gently pushing the pins  at the front of the seat out, and work back towards the rear.  Then once you have  the pins out the trim will be free and you will notice the pins have heads on them that slide in a groove in the trim. There are a couple places where the groove is wider and the pins can be slid to those areas and removed if you so desire to do so.

Once you remove the trim, then you can bend up the barbs to remove the old seat cover. With the new cover in place (do it on a warm sunny day to make the vinyl cover stretch easily) you can use an awl to poke new holes in the cover for the trim piece pins and then fit the trim back on and finally press the rectangular clips onto the trim pins( I use a 1/4" nut driver to do that).

I like the seat covers from Pit Perspective, very good fit, proper heat molded seams and vinyl texture pattern, good silks-screening of the logo on the back, and nice foam padding on the inside. IMHO, the reproduction seats with a large bulge at the back to imitate a rear cowl look funky and odd, they just do not match the look and style of the rest of the bike and seem out of place.  To me, they create a clash in styling that is distracting, but I'm a purist because I remember these bikes back when they were new and 1st came out, so others may not feel the way I do about it. .

Maybe 3/4 to 1 point, which is why I use  92 instead of 91 octane.
Running C98 (94 unleaded ethanol fee gasoline) costs $10/gallon, and VP is not always available.
Running C110, C112 is too high of an octane unless you mill the head and jack the compression way up.  110 octane just fouls plugs and wastes fuel in a stock RD/RZ.
One simple way to compenstate for lower octane fuel is to simply go up one jet size as a richer mixture raises the knock point.

However, dropping the fuel by 1 octane point is not significant, but what is significant is that removing the ethanol put the mixture back where it should be. Recall that if the fuel is 10% alcohol, with a fixed jet bike like the RZ, it makes it run 5% lean. That is a much bigger and problematic issue than dropping the octane 1%.

Of course, if you have milled your head 15 to 20 thousandths, or have a Koolhead with combustion chamber inserts that give you 135 to 150 PSI compression, you will have to use bigger jets and run C110 or C112 to avoid detonation and seizures.     

To answer the original poster's question, here is my experience:

The 1983-4 Canuck RZ pipes are great for what you want as they are lighter than stock USA caty-verter pipes, the produce more power over the entire rev range, and, they are quiet.  The other big bonus is they fit the bike properly and have the correct dimples and bumpers for the center stand and side stand.
In 1985, the Canuck RZ350 pipes featured new stainless steel silencers and look very sporty, plus they deliver a little more power than the 1984 Canuck pipes.

Of all the aftermarket tuned pipes, the original Spec II pipes ( standard and pro) were about the best.  They have decent power from about 5K up to 10.5K, definitely more power than stock, and hit hard enough to require new clutch plates and/or 3 stiffer springs to keep the clutch from slipping.  The Toomeys have too little loo late as far a practical street riding goes as all the extra power is past 8500 RPM, making them not very usable in day to day driving and traffic. DGs feel great but basically have less power than stock. Same goes for Lomas/Scorpion.

One problem we have run into is many of the aftermarket pipes do not fit USA RZ350s properly.
There are issues with them hitting the center stand pivot bolt/bracket, hitting the lower lip of the chin cowl, center stand and side stand hit the pipes. The Spec II pipes have always fit properly.   

In 2008 when our governor decided to double the vehicle registration fees in California, I non-oped many of my bikes.   Recently however, my twin sons have reached the legal driving age, so we decided to take the1975 Yamaha  RD200B out of mothballs and put her back on the road.

We drained off all the fluids, carbs were already dry but we cleaned them anyway. New gas, and oil, new battery, re-synced the carbies, new hand made air-filters (used Uni-foam sheet) a little top off on the oil tank with Motul 600(710) lubed the cables, lubed chain, set tire pressures, and then she fired up on the 2nd kick!

The grin on Ben and Buds faces was priceless as they took their 1st ride on a street Yamaha twin.  Both of them commented that they liked the feel of the power-band when it hits, who wouldn't?  Plus the smooth power delivery of a twin is a sensation unto itself, needless to say, they were really stoked. Only issue is the clutch slips a bit until the bike get good and warm, so we ordered new friction plates and new springs.  My boys have been riding dirt bikes since they were 5 years old, but street bikes are a little different animal.

Next project will be to awaken the  1971 CT1-C Enduro. We ordered a new petcock valve seal, battery, and it uses the same clutch plates as the RD200, so because the CT1-C was also slipping before it was put away, it will get a new clutch as well.

Am using 1 pint of water to 3/4 gallon of 92 octane E15 gasoline to remove all the ethanol. A drop of red food dye in the water helps you see it more clearly when decanting. Although a 1/4 tspn of anhydrous (dry) Magnesium Sulfate (Epsom salts) can be added to remove any cloudiness from the decanted gasoline, yuo have to then filter it to remove the salts. I found it much easier to simply let the gas/water mix sit for 24 hours after shaking it  or 5 minutes. When left to sit for a day, the gasoline becomes crystal clear and can then be decanted and used in your bike.  It may sound like a hassle, and it is a little trouble, but not near the trouble you have to go through  dealing with old fuel, clogged jets, and corroded carbs and rotten rubber components in the fuel system.  The bike likes it better too, because depending on how much ethanol is in your fuel (up to 20%) it makes you bike run up to 10%leaner than it should.