By Dale Alexander
Not Another Boring Article!
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 it’s 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 hand 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 it’s hat off it’s head below 8,000 only to wrench the wheel loose from the ground at 8,001 rpm’s.
I preferred to use R5 carbs when available as the air correction jet was already 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 high top and this affects the choice of cables. I prefer the 350 carbs over 400 (certainly 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 connection and these are easier to come by.
SPECS FOR JETTING
Needle Jet: 169 series, P2, Spray shield cut down to 2 mm 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: ¾-1 ½ 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 was 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 to 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 have 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 reeeeal 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 slide3 guide pin that holds the slide from rotating (by way of the groove on the side of the slide). If this happens, the throttle will be stuck WIDE OPEN and no amount of fiddling with the throttle will shut the sl.ide off at this point. Make sure your kill buttons work and only try full throttle in 4th, 5th, or 6th so the bike doesn’t get 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 the 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 look at the bottom 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 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 jet, the spray bar (shield) must be cut down or the mdrange circuit will operate too late and screw up everything. With a pair of vernier 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 932 teeth per inch) hacksaw blade, cut just barely away from the line, leaving enough to trim with a fine file (see diagram).