QuestionQUESTION: Hi,
I have a problem with my left cylinder fouling with oil.
I overhauled a 1988 Honda cd 200 twin (88000 km) recently (opened up engine and gearbox). Prior to reboring the cylinders to the second oversize (0.5) the left cylinder had slightly more wear than the right cylinder. Anyway, I bought original Honda pistons and rings, and after assembling everything, I noted that the left exhaust outlet was burning richer than the right. But since the motorcycle only uses one carburettor, both pistons receive the same fuel/ air mixture. (And both exhaust pipes are clean, it was not old oil in the pipe being heated up and smoking). Also, every time I remove the cylinder barrel I noted that the piston rings moved (is it possible to move in the cylinder at all, because it seems highly unlikely?). All the tolerances should have been correct as it is new parts fitted. I then suspected the cylinder head (of which new valve stem oil seals were fitted and the stem to guide clearance were fine) having a crack. I took it to a cylinder head specialist for testing. No problems were found on the head. Strange enough, the motor doesn't smoke (Only slightly when cold, but as soon as the engine heats up, no smoke, but one can note that when the engine is idling, and you open the throttle, that the engine first hesitates before revving. In this regard, the carburettor was cleaned in a ultrasonic bath, but to no avail).
Then, just a few questions:
The engine configuration is 360 degrees, and is banked 15 degrees (so I think) to the front.
Will this banking of the cylinders necessitate, when reassembling the engine, that you position the rings in a certain way to possibly prevent them from moving around in the cylinder barrel as I presume?
ANSWER: Edwin, While we didn't have that particular machine in the US, I am familiar with the design and general features.
If your compression readings are within a few pounds/kpa of each other and near factory specifications, the ring sealing should be fine. Rings normally are installed so that the gaps are all about 120 degrees apart to help negate possible blow-by if the gaps are all aligned.
However, be assured that the rings DO move around and the main cause is an out of round bore. I know you re-bored the cylinders, but unless they were bored on a torque plate to simulate the crush of the cylinder head and stud torque, the perfectly round bore job will become slightly oval-shaped as the engine heats up and the parts distort under heat and pressure. When the cylinder bore goes out of round, the rings, which are free to move obviously, will migrate around so that the open ends move to the point furthest away from the base circle.
The part-throttle hesitation is more than likely a sign that the engine needs a bit of carburetor metering work. With that many miles on the bike, the carb body may have worn so that there is somewhat of an air leak around the caburetor slide. The bikes are generally jetted lean from the factory to comply with emission standards, which gives that flat spot until the engine is really warmed up. If the carb idle screw cap is removed, that will give you the chance to change the idle mixture to where it idles the best. Then, a thin washer, placed below the needle clip for the throttle slide will cause some enrichening in the part throttle transition. If the air filter system and/or the exhaust has been changed from factory parts, then the engine will most certainly need a slightly larger main jet. Try different settings, look at the spark plug ends to read whether they are lean/rich and with some small changes, the performance should improve.
Before you start major carb rework, make sure that the spark advancer is functioning properly, if it still has points or has a mechanical advancer. The US bikes, back in the early 1980s still had points and condenser. Proper ignition timing and spark advance are critical to how the engine will perform at all engine speeds.
Bill Silver
---------- FOLLOW-UP ----------
QUESTION: Hi Bill,
Just the following:
My cd200 model (Roadmaster) were exported to South Africa (I'm from SA) and Pakistan. (I think the model you referred to was the CM200T, the American model).
Our idle screws didn't come out with caps as required by emissions regulations. I also think that the dealerships in SA jet all motorcycles upon arrival in SA, because I had a previous model which ran with the standard (or modified by the dealers)jetting with no troubles.
Further, my automatic timing unit (spark advancer) is completely overhauled and functions correctly (the advance starts when it is supposed to start, I checked with a timing light, and the timing is correct. Spark plugs are also correct as per manufacturers specs). I fitted a new carburetor, with the same results as previously mentioned. (Air filter and exhausts are stock standard, no air leaks from intake side of carburation system).
Then some questions:
Does it matter whether you position the rings 120 degrees apart or the two top compression rings 180 degrees from one another and the oil control rings at 90 degrees from the two compression rings?
If you break in a motorcycle correctly (I think I didn't) would it make a difference whether the cylinder barrels were bored incorrectly or not?
Can you tell me what way is the correct way to break in a motorcycle?
(I read an article on the Internet which said that to put a light load on the engine during break in is wrong, that you have to ride the engine hard (obviously when the engine first warmed up properly) for the first 20 miles) in order for the rings to bed in/ seat properly. Further, with what oil (engine car oil or wet clutch motorcycle oil) should the engine be breaked in? And should the oil be changed after the initial 20 miles?
Thank you Bill
AnswerEdwin, On older bikes with 1 piece oil rings, I put them 120 degrees apart. If you have 3 piece oil rings, you can put the two top rings 180 and then stagger the oil ring rails 180 as well. In the end, I don't think you are going to see much of a difference in ring gap placement. Piston clearances should have been set up about 1.5 thousandths of an inch.
Break in period is listed in owner's manuals. Normally, light load with varying throttle settings for about 600 miles, with no full-throttle running. The "Run it in Hard" idea is often stated for racing engines which have a short life. You need to gently allow the pistons/rings and cylinder bores to all become friends. Otherwise, you can seize the pistons on the high spots, which will drag material into the ring lands and lock the rings in place.
Always use 10-30 or 10-40 Honda or equivalent motorcycle rated oils, due to oil molecule shearing between the transmission gears. M/C oils have additional additive packages to help prevent it. Honda's first services were generally at 600 miles for oil change and readjusting of the valves and ignition timing.
You might want to check spark plug caps to see if their 5k ohm resistance values have eroded. The coil may be getting weaker due to age, heat and mileage, as well. Try just lifting the needle slightly with a shim to see if that changes the flat spot off idle.
Bill Silver
