RACING TIPS

Props
The K&B 3.5 air-cooled engine either stock or modified runs the fastest with an Octura 1440 with the trailing edge cut back and slightly cupped.

The stock K&B Pro air-cooled seems to like an Octura 1450 with a slight cup. The old style water-cooled modified engines run best on the same prop with the trailing edge cut back. Many racers use this prop on the Pro as well.

The Stock K&B 11cc has been raced with a variety of props. The favorites seem to be the Prather 235, the Octura 1455 cutback and cupped, or even a stock Octura 1450. The Modified 11cc engine can run those props as well, but is faster with an Octura 1655 cutback and cupped.

The trailing edge cut back is usually between 15 and 17 degrees measured from a right angle to the shaft axis. Try different cups to get the best performance, then make a "prop duplicator" so you can reproduce the prop.

Shafts
Outboard shafts need good lubrication to keep from breaking. The standard Teflon tube is all that's required. The 3.5 and 7.5cc engines do well with a good grease such as Mercury Quicksilver 2-4-C with Teflon. A little of your favorite oil (I use automatic transmission fluid) in the lower unit oil hole between heats is good insurance. Disassemble the engine and grease the shaft after each day at the pond. Shafts will often last several seasons with this care.

The same system can work for 11cc engines as well, but I prefer a pressure system for my 11cc mod outboard. I installed a stainless steel tube (brass would be better) and a 2 ounce tank. The tank is filled with ATF and pressurized from the tuned pipe. It feeds into a fitting tapped into the lower unit oil hole. Add oil after every heat and shafts hold up much better than with grease alone. The tank should last all day. If not, restrict the oil line.

K & B Pro Problems
The K&B 7.5cc Pro engine is a great advance in outboard performance. However, it has two problem areas in its stock form; the carburetor and the crankshaft. All the early engines broke their crankshafts at the front of the intake window after a short time. Tempering to a lower hardness seems to have helped, but this remains a problem especially on modified engines.

The huge carb also gives some people trouble. Though the fuel draw is marginal, the standard carb is fastest, but you MUST use the needle valve that came with the engine. The old needle valve is too small. The engine loves to run rich, so don't be afraid to open the needle.

Modified engines often won't draw fuel with the big carb, so the old size 7.5 carb is much easier to set up. Bigger is still faster though, so some experimentation is helpful. Joe Monahan and Rod Geraghty have a selection of aftermarket carbs. See links.

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Richard Hazlewood's XT-120 Hull Mods

The Picture 1 shows the areas to be modified. The non-hook pads are sheeted with 3/32 balsa with the grain running from side to side. See Picture 5. Make up two identical sponson bottoms from 1/32 plywood. They should be 2 inches wide, measured along the bottom, from the transom to the rear of the non-hook pads. They will overhang the origional sponson bottoms as shown in Pictures 2 and 3. Taper the sheet width by drawing a smooth curve in the outer edge from the rear of the non-hook pads to the origional sponson width at the bow. Add 3/32 balsa sheet the size of the origional sponson bottom to the RIGHT sponson with the grain running bow to stern. (With the hull upside down as in the picture, it will be the left side looking toward the bow.) Sand a uniform angle in this sheet so it tapers from full thickness on the tunnel side to nothing at the outer edge. Use a long sanding block. See Pictures 2 and 4. Taper the sheet thickness to the origional sponson bottom near the bow. This deepens the inner edge of the right sponson (the inside sponson in a turn) 1/8 inch. Add the plywood bottoms to both sides, Fill the overhang areas with Bondo or your favorite filler, and sand all edges sharp. Seal with your sealer of choice and you will have a hull that can race with the best.

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