There has been some questions about bored carbs as of late and I would like to share my experience.
First off, I feel that the RD, RZ series engines do not have the port time/area needed to fully use any carb above 34mm to its full potential without suffering serious holes in the powerband and driveability problems. In maximum power applications with minimal regard for powerband, 34mm works fine. I have had MUCH better results using a modified stock 28mm carb body. The overall power is down only slightly with a much greater low/mid range power: A better overall choice for a balanced powerband.
I must add that I never developed my engines for maximum power as my home track was Sears Point, but my engines didn't suffer at anyone else’s hands at tracks like Willow Springs or Riverside either. I always believed that a balanced combination with smooth power was better than an engine that wouldn't pull its hat off its head below 8,000 only to wrench the wheel loose from the ground at 8,001 rpm's.
I preferred to use R-5 carbs when available as the air correction jet was aIready the proper size and eliminated one step needed in setting up RD carbs, but RD carbs are none the less fine. There are different models of 350 carbs, short top and tall top and this affects the choice of cables. I prefer the 350 carbs over 400 (certainly the Daytona) as the idle air bleed hole in the bellmouth of the carb is sealed with a ball that must come out before any of the air jet modification/tuning work can be done. Last that I heard, the junction boxes for the three cables was as rare as hens teeth, but if you pre-mix(HIGHLY recommended) you only need a two cable junction and these are easier to come by.
SPECS FOR JETTING
Needle Jet: 169 series, P-2, Spray shield cut down to 2mm in height
Jet Needle: 5J9 2nd, 3rd, 4th clip position
Slide cut-away: 2.5 (stock)
Pilot Jet: 30-35
Main Jet: 180-210
Air Correction Jet: 2mm
Carb bore at venturi: 29.5 mm
Idle air screw: 3/4-1 1/2 turns out from bottomed
Works best with at least RZ style reeds. I like metal reeds, so crucify me!
I would like to summarize and add something from my experience of running/boring carbs.
1). Carbs should be bored offset to the venturi centerline(see diagram) . A proper setup would have the boring bar JUST touch the area where the needle jet resides so as to remove as little material as possible.
2). This is done to prevent an excess of air flow past the slide at the idle position. If the carb is bored concentrically, too much material will be removed under the slide and the idle will be too high even with the idle screw removed. The reason that this is owes to the fact that the slide will rest on the body of the carb and still pass too much air. A high idle was always a dead give away of cheater carbs at the races.
3). Because of #2, the carbs should be bored on a mill to allow proper X/Y axis offset in relation the centerline of the venturi. Boring can be done on a lathe, but it takes a four jaw setup and someone pretty sharp with the lathe.
4). If the setup is proper, the end result is NOT an oval carb bore. Material is removed from the entire I.D. of the venturi, just more material in some spots(top) than others. CV carbs on 4 strokes are an example of "oval" carbs. 4 stroke carbs are done this way to give good low load mixing/response with the potential of high air flow during high load conditions. Kinda like a poor man's 4-barrel carb for bikes. Poor 4 strokes! They need all this help just to get out of their own way!
5). If boring an extreme amount, the manifold will need to be matched as well.
6).THIS IS THE MOST IMPORTANT PART, SO READ CAREFULLY! If you find that the slide does not raise high enough to give full opening with the new bore size, be reeeal careful trimming the top of the slide to allow slide to move farther up. If the slide is trimmed too much, the slide can lift over the slide guide pin that holds the slide from rotating(by way of the groove on the side of the slide). If that happens, the throttle will be stuck WIDE OPEN and no amount of fiddling with the throttle will shut the slide off at that point. Make sure your kill buttons work and only try full throttle in 4th, 5th or 6th so the bike doesn't got away from you. Just be real careful trimming the top and check and check and check again.
Great! Now We Have A Lot Of Air, But Don’t We Need Fuel?
The 169 series needle jet is a primary type needle and because of this, it needs a larger air correction jet. The stock needle jet was designed as a four stroke type needle and is known as a "bleed" type. It is primarily responsible for that hit in the seat of your pants feel at 6 grand. The primary type works much smoother. The passage that needs to be modified is easy to find. Look at the bell mouth of the carb and find the small hole at the bottom of the mouth that is in alignment with the needle jet(remove the needle jet for this operation) (see diagram). Purchase the 2mm correction jet from Sudco or Rocky. Buy a 4mmX.70 tap and a number 30 drill. Use the drill to drill out the stock .6mm air jet (see diagram). The drill should catch the air jet and spin it out. Use the tap drill to tap threads into the new hole. Stop when the tap gets tight as it should bottom in the hole. When finished, insert the 2mm air correction jet and marvel at how easy it was to turn a street bike carb into a carb with a racing heritage!
Before installing the 169 series needle jet, the spray bar(shield) must be cut down or the mid-range circuit will operate too late and screw up everything. With a pair of venier calipers set to 2mm, use the id measuring portion and mark off a line that is 2mm from the base of the needle that would be flush with the carb bore. Hold the needle jet in a vise(gently!) and position the scribe mark in such a way as to use the sides of the jaws as a guide for the blade. Using just a fine(32 teeth per inch) hacksaw blade, cut just barely away from the line, leaving enough to trim to length with a fine file(see diagram) .
The recommended main jet size may sound small to a lot of you but it is correct. The reason for this is due to the primary type needle, the air correction jet change and the way that the mixture is emulsified. The numbers are different but the dumb engine still sees the same fuel as when the stock carbs are used with a 280-310 jet. If you don't believe what I'm saying, feel free to start at 300 and work backwards.
The pilot jet I used was a 30. More modified bikes may need a 35. Air screw set to 3/4 turns out on my engines. If you feel that this is too rich, turn it out to 1 1/2. If you find your engine needs to have the screw out further, than lean out the pilot jet.
For a perspective, my engines were 375cc with long-rods, compression set at 175 psi, squish at .030", timing 1.8-2.0mm btdc, 106 octane, B9HS plugs, stock exhaust with modified baffles(more flow) and moderate amount of port work. They worked well enough that it took the FZR-400 series to finally start winning races from me so I think that this carb setup will work well for just about any RD from street bike on up to full race. I have used the same setup on long rod RZ-375’s for AFM Superstreet racing with great results. If your engine has an unmodified squish with cut heads, you may need more main jet as a larger amount of jet will be needed to cool an in-efficient engine.
The beauty of this is that the carbs with modifications can be had for a lot less than a set of GP Mikuni's and I always believed that it was smart to only spend money when needed and to stay away from trendy(38mm carbs?).
Part numbers that you need are:
002.165 2.0mm Air Correction Jet
002.296 5J9 Jet Needle
003.261 169 Series P-2 Needle Jet
These parts can be ordered from Sudco at 1-213-728-5407. Carb bodies can be had from the editor of this fine, informative newsletter, Doug Johnson.