Tech Tip #38 Honda CH250 & CN250 Camshaft Bearing Replacemen

Tech Tip #38 Honda CH250 & CN250 Camshaft Bearing Replacemen

Postby zozman » Tue Dec 17, 2019 12:04 pm

In previous Tech Tips, I have discussed several methods to diagnose engine noises by isolating the engine from the drive train. Bearing noises can be coming from the swing arm, final shaft, transmission, clutch, water pump, cam shaft or mains. Usually, when bearings fault, they exhibit a grinding, sometimes intermittent noise that may go unnoticed for some time.

My 1987 Honda CH 250 developed a noticeable intermittent engine noise that progressively got worse. By removing the drive belt, I was able to remove all the drive bearings except for the engine. Isolating the engine to from the drive train will tell you if that noise is internal to the engine. My sound was internal engine noise. But with only 6000 miles on the engine, I was doubtful the main bearings were bad.

I recommend that for any major examination of the engine or repair, the engine be removed from the chassis. For this repair, the oil pipe must be removed, and you just can’t get the front and the top bolts off. Secondly, unlike the Honda Helix, you just can’t get the cylinder head cover off without removing the gas tank above it and splash cover underneath it to do it.

For those guys that say they can drop the engine in fifteen minutes, I didn’t find that to be true. Honda’s flat rate manual gives a half hour to remove the plastic, another hour to remove the left side cover, air box, shocks, muffler, wheel and another half hour to disconnect hoses and lines and cables before removing the engine. I found the two-hour time to be accurate. Anyway, the engine can be worked on at your bench for ease.

In prepping the engine for removal, you must drain the coolant and engine oil. Upon draining, I discovered bearing races in the oil. One of the reasons Honda inserted a drain plug on the bottom of the right crankcase in 1987 was to allow more oil to drain and thus identify and remove metal shavings from the engine.

In removing the right side cover, the small bearings in the water pump were good, but more bearing races were discovered in the bottom. The bearing races were too small to be mains, which led me to remove the cylinder head cover. There I discovered that the inner camshaft bearing had disintegrated.

Repair Tip: When disassembling your engine, take progress pictures as you go along. Place all the large familiar body parts and frame pieces in a recycle tub. For the right side cover where the flywheel and water pump are, use a cigar box to place the parts in. For the cylinder head cover, use another cigar box. When assembling, if you get to the end of the job, there should be no parts in your cigar boxes.

There are two bearings on the camshaft, one inboard next to the sprocket gear and the other (91008-HB3-003, Koyo 16002) outboard held by a circlip. If I were lucky, the outboard would be the failure. That bearing is available and replaceable on the camshaft. I was unlucky- the inboard bearing failed and that one requires a new cam (14100-KS4-010). To remove the cam, loosen the cam chain adjuster on the rear side of the cylinder. Unbolt the two cam holder bolts, draw up the cam chain and remove the cam.

After removing the right side cover, there are markings on the flywheel for timing. On top of the flywheel, there is a zero mark and on the rim of the flywheel a “T.” The “T” is supposed to show through the timing hole cover when it’s removed. It does, but it is a lazy “T”-on its side. This mark is very hard to see. I placed some red fingernail polish on the side of the flywheel so when the left side cover is installed, I can see the red “T” through the timing hole. Use a new right side cover gasket upon assembly. (Reference section 10 manual)

On the backside of the cam sprocket, there is also a zero mark and a line running the diameter of the sprocket. You align the zero mark toward the rear of the engine case, so the sprocket line is parallel to the top of the head. This is to have the cam lobes facing away from the valves at top dead center. Install the cam chain onto the cam sprocket and the cam chain adjuster. Liberally squirt oil on the cam lobes and bearings. Use a new cylinder head gasket upon assembly. (Reference section 6-0 service manual).

The oil pipe bolts are distinctive and important. The top bolt is black, and the center bolt is silver. They are 6mm. The front bolt is gold and it is 12mm. All bolts are hollow to allow oil to flow through them. There are copper washers on each side of the oil line holes. Caution: mark the heads of the bolts with red nail polish and reinstall them correctly. Black bolt on the top, silver bolt in the center, gold bolt to the front. (Reference 6-15 manual)

Tip: With the engine removed, this is an excellent time to service the breather separator (Tech Tip #1), service the weight rollers and drive belt (Tech Tip #2), examine the driven clutch (Tech Tip #8), replace the air filter element, replace the muffler gasket, put on a new tire, check the rear brakes and replace the engine oil and coolant (Tech Tip #23).

Reassemble the engine in reverse order. Overall time from first bolt removal, receiving parts and reassemble: 6 days. Not easy but rewarding. Who knows what Honda would have charged for this even if they had done the repair?

Sidebar Giggles: I go to start the scoot after repair and it turns over, no fire. Now, I’m confident I have done the repair according to service manual specifications. I check the valve clearance—all good. I can’t get the scoot to start. The wife says “You sure you got gas? “Yep,” I said, “Tank’s full.” She pulls out a loose hose from the other side of the scoot and says, “What’s this for?” I look at it and realize it’s the vacuum hose for the fuel control valve. “Honey, winner, winner, chicken dinner!”

Randy Pozzi (07/2011, copyright 2019))
87 & 98 Helix
zozman
 
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