Thin Sleeve Causing Seizures

Question: My 1987 CR125 has chronic piston seizure problems. The cylinder is bored one millimeters oversize. The lower end was rebuilt so I know it doesn't have a crankcase air leak. What could the problem be?

Answer: The original cylinder for your model bike had a very thin steel sleeve. Honda only offers one oversize piston. When the sleeve is overbored too far, the sleeve cannot transfer out heat into the water jacket efficiently. The heat builds up over the exhaust port, and the piston melts. You have two repair options: buy a new cylinder or install a new thicker sleeve in the old cylinder. Wiseco offers thick sleeves and forged piston kits.

Honda CR250 1988–91 HPP Problems

Question: My 1990 Honda CR250 is making me wacky. I tried to check the exhaust valve system, and I don't think it works properly. I removed the left-side valve cover from the cylinder, revved the engine and the valves hardly moved. They don't open fully when the engine is revved, and they don't close completely either. What is the most common cause of this problem and how can I fix it myself?

Answer: The problem is that the HPP mechanism isn't fully engaged, and the valves are just moving from the exhaust-gas pressure. The most common problem with the 1988–91 CR250 HPP systems, is the improper engagement of the governor control and the spindle rod that actuates the HPP valves. The following procedure may cure the problem. Remove the top right valve cover on the cylinder and the round-slotted access cover located under the water pump on the right side engine cover. Insert an 8mm T-handle through the access hole and onto the detent bolt that locks the governor control to the cam spindle, and turn the bolt 1/4 turn counterclockwise. Now, the bolt has disengaged the HPP system. Insert a straight-blade screwdriver into the slot in the top of the right-side pinion shaft (from the top right side of cylinder). Turn the pinion shaft counterclockwise 1/8 turn, and then turn the detent bolt (located under the right-side engine cover) 1/4 turn clockwise. It is important to release the spring tension from the pinion shafts in the cylinder to engage the detent bolt. This procedure also enables the HPP mechanism to be engaged without any chance of damage occurring to the fragile cam spindle.

Top-End Big Bore

Question: I have an old cylinder for my 250. The bore was ruined when the head gasket leaked, and there is severe erosion on the top edge of the cylinder. I read your article on top-end rebuilding and had an idea and a related question. I compete in amateur enduro events and the rules state that the displacement of bikes competing in the open class must be a minimum of 251cc. My question is, can I salvage this old junk cylinder by overboring the cylinder to fit a Wiseco piston kit and have the bore re-plated? If yes, will my bike be legal for the open class?

Answer: There are a number of companies offering cylinder repair services and replating. The way to fix the erosion problem is to heli-arc weld aluminum over the erosion and then re-face and bore the cylinder. WISECO and L.A. Sleeve make oversize piston kits and gaskets for most Japanese dirt bikes. The common overbore displacement sizes for 250s are 265, 285, and 310cc. After the cylinder is re-plated, the exhaust valves and the cylinder head must be matched to the larger bore size. This involves special metal machining and should be trusted to a qualified tuner or machinist. This type of mod will enable you to race your 250 in the open class.

Kawasaki Air/Oil Leaks

Question: My son and I are just getting started in dirt-biking. Over the winter I bought him a 1989 KX80 as a basket case. We are learning about dirt bike repairs by rebuilding this bike. It's a lot like model building, only the parts are old and greasy! We inspected the crankcases and noticed that there was some oil leaking from the three oval-shaped plugs that are spaced an equal distance around the main bearings. How can we repair this problem without buying new crankcases?

Answer: Every Kawasaki dealer's service department has a Team Green book with tips on how to repair common problems. Ask your dealer's service manager for a copy of the Team Green bulletin. It has photos and drawings of how to apply the epoxy over the crankcase plugs.

Top-End Seized After Rebuild

Question: I trail ride a 1989 YZ250. Last winter, I rebuilt the top end after reading your article in Dirt Rider. The bore was so worn that I had to skip to a one millimeter-oversize piston kit, just so the bore job would clean up a severely worn spot below the intake port. After I rebuilt the top end, I cycled the engine by letting it idle for three 15-minute sessions with adequate cool-down periods in between. When I first rode the bike, I heard some detonation noises but didn't think it was a serious problem, until it seized. What could be wrong?

Answer: Your problem is simple. When a cylinder is overbored, the displacement is increased and that boosts the compression ratio. Whenever a cylinder is overbored more than 0.010 inches or 0.25mms the cylinder-head diameter must be enlarged to the new bore size. Otherwise, the piston could contact the head or the edge of the head surface that extends into the bore could cause a hot-spot and pre-ignition. Also, the cylinder head's squish band must be narrowed by enlarging the combustion-chamber bowl. This also serves to increase the head's volume, thereby lowering the compression ratio. This work must be performed on a lathe by a qualified tuner or machinist. Average cost of this service is $50

Base Gasket Seeping

Question: I recently rebuilt the top end on my 1991 CR250. I was being as careful as I could be while taking the cylinder off, but the dowels were fused in pretty good and I had to pry it. Needless to say, I gouged the case a bit. I smoothed it out with sandpaper and reassembled the engine. The bike runs great, but a little oil seeps out of the cylinder-to-case mating surface. I assume this is transmission oil? Would it be OK to use something like a thin layer of Permatex Blue or Yamabond here? Would this make it even more difficult to remove the cylinder in the future? Should I just let it alone? The best price I could find on a new left side case was $215 and I'm sure it would be a lot of work and a lot of replacing gaskets along the way. Am I out of luck?

Answer: Air leaks can be very dangerous because the engine could rev independent of the throttle. An inexpensive way to fix your bike's problem is to draw-file the cylinder base and the crankcases. Then apply a thin coating of Yamabond or any other brand of non-drying sealer to both sides of the base gasket. The best technique for removing cylinders is to tap up on each side of the cylinder with a lead-shot plastic mallet. Remember to put a dab of grease on the cylinder-base dowel pins.

Frequency of Top-End Rebuilding?

Question: I have a 1990 RT180, and I don't think the rings or piston have been replaced. I don't know if the top end has ever been rebuilt because I bought the bike used. How long do piston and rings usually last on a two-stroke engine like mine? How often should the piston and rings be replaced, and should I replace them now?