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Bill,
I was just given a 1985 Honda Shadow VT500c. The bike ran great for the first two weeks. I had the oil changed and the bike washed. three days later, i was cruising at about 60 mph and the bike sputtered and cut off. when i got off the road, i saw that the bike spit oil from the right side exhaust from the back cylinder. Now the bike will not idle but it will try to run, but only with throttle, no idle at all. I have no idea where to start. Suggestions to me were the timing being knocked off bad, a warped valve, and a broken seal on the piston. Please give me any ideas you may have.
Josh
Answer -
Josh, first you need to pull one of the plugs from the back cylinder and stick a compression gauge in there to see what you have for readings. Should be around 150-175psi with wide open throttle, engine off (use KILL SWITCH to OFF position when cranking). If compression is way down below 150psi then probably the bike has holed a piston. Check the looks of the plugs when you are doing the compression test. Dark black, oily plugs are usually signs of either compression loss past the rings, hole in piston or sometimes a blown head gasket, in certain circumstances. Low compression will require removal of the engine and tearing down the top end of the rear cylinder.
You also need to ascertain why the problem happened. Lean mixtures due to clogged carbs, air leaks in the intake tract, fuel restrictions and low coolant levels which cause overheating will damage the pistons.
If the compression is close to the same as the front cylinder, then other things are possible. With fresh plugs, inserted into the plug wires and laid against the engine, crank engine (KILL SWITCH in ON position) and look for spark at front and rear cylinders. No spark can be from a bad spark module or pulser coil failure for the rear cylinder.
If the bike was pressure washed and water got into the fuel system, then you may have water in the float bowl of the carburetors. This will block fuel going past the idle and main jets. Drain the bowls with the small screws at the bottom of each carb and try to capture the fuel in a cup to see if there was water in the bowl or not.
There is a small sediment bowl on the bottom of the fuel valve (petcock) that should be checked for signs of water or contamination. If the vacuum line comes off the fuel valve or the diaphragm gets a puncture, then the fuel flow is greatly reduced and you have a vacuum leak, as well.
You might round up a repair manual from a dealer, on-line or from www.helminc.com
Bill Silver
Reply
Bill,
The Spark plugs were dark black and had oil on them. I'm going to run the compression test Sunday and see how it reads. There is fire from the spark plugs. but from further inspection, it seems as if the head gasket had been replaced, there is some extra gasket material between the cylinders. Where is the best place to get another and what would be the easiest way of redo this ( me myself or the HOnda dealer?) Thank you very much for you help and advice.
Josh
AnswerJosh, I don't know your level of expertise on m/c repair. You need to understand the concepts of camshaft timing, at the very least. The motor has to come out of the chassis, as far as I know, but you can verify with a manual.
If someone either made a gasket..(hard thing to do) or didn't install it correctly (didn't torque the bolts/nuts properly or installed it upside down or some other problem like leaving out an o-ring or locating dowel), then that may be your only main problem. If that does prove to be the source of the engine issue, be sure to inspect the surfaces of the head and cylinder for being flat and clean before reassembly.
Depending on the mileage, you may need to replace camchain tensioners/guides while you are in there. If one set is worn, you may as well replace the ones on the front cylinder, as well. A lot of this depends on the mileage on the bike. Check the rubber sleeve around the tensioner spring. They can harden over time and interfere with the function of the tensioner.
Check your intake manifolds for cracks and damage that can lead to an air leak.
Some Honda dealers won't work on bikes older than 10 years, so you may have to check around to find a good wrench, if you can't manage this yourself. This could be a $500+ repair bill...
Bill Silver
