ilove bonnie

Thursday, February 12, 2009

Roadside Mechanics Series #3

That is right, what Jen said. "Hell Yeah Mexico!" 

So how did that happen. Two days ago we were fighting a broken starter, and xs500 still did not want to start right with the kick starter. Yes it was running well, when it was running, and did a great job on our test ride to Boca Chica, but it was not perfect. 

So, yesterday morning our baffling motorcycle problem of extremely hard cold starts, and a screeching noise that occured on while the engine was starting was finally exposed. 

I was recommended to perform a cold engine compression check by Vanya, an "old motorcycle" mechanic whiz on advrider.com. The procedure for the test is the same as in our earlier blog post for the Roadside Mechanics Series #1, except the test is performed while the engine is cold. The idea behind performing this check was that our hard cold starts were caused by a small air leak that can only be present when the engine is cold. Therefore, the air leak did not effect the operation of the motorcycle at normal operating temperatures because as metal heats up it expands thus closing off the air leak. 

So first thing I did at 7 am on the day we are supposed to leave for Monterrey, Mexico, was pull the gas tank, remove the plugs and setup the compression tester. Our initial results for both sides were very poor at 90psi and proved Vanya's point. We had a cold engine compression leak.
Vanya suggested that if this test resulted in poor compression, I should check that the eight head bolts were tightened to 25 ft-lbs. This is where I discovered our  problem! The head bolts were just loose enough that they did not created a good seal with the cylinders until the engine was warm. That is why the warm compression check didn't expose the problem with our compression.

Through this process I have learned that discovering the origins of a problem in a troubled engine requires following a series of steps to make sure that the three main systems of compression, spark, and fuel are opereating properly when you perform a tuneup, and that to check them in that order. First, make sure the engine has good compression. The main check should be when the engine is warm, but if your engine will not start then a cold engine compression test could expose the underlying problem of why the engine won't start. Then, and only then, should you proceed onto checking the electrical system to make sure that a healthy spark is being delivered through the spark plug, and that it is being delivered at the right time. From there you should go onto check the fuel flow. All of these ingredients combine to make up a running engine, but the first systems have to be functioning flawless make sure that you are diagnosing the true problem.

After I tightened the head bolts I got a healthy reading from both of our cylinders.
The motorcycle started right up when I put it back together, and is a breeze to start with the kick starter when warm, all of our gear is strapped on, and Jen is sitting on the back of the bike. Hurray!!!!!

Benjamin
p.s. Now I am going to go through the other systems and make sure they are functioning flawlessly with the correct compression. Just another step in maintaining a 1976 Yamaha xs500 in Jen and the Art of Motorcycle Maintenance. 

3 comments:

Bikermom said...

Hi Ben and Jen, Good to see you made it to Mexico is it warm and beautiful.. It is cold, rainy and possibly snowy - but we still move forward.. I hope to ride Applegate on Saturday and run with Marty, Alfred, Mike and whoever elsse.. Jen good pict of the sleeping boy. Ben tell Jen how you slept through 2 big earthquakes!

Benjamin said...

I went through and check the adjustment of everything. The valves were a little loose, the Cam tensioner chain adjustment didn't result in much change that I could tell. I re adjusted the idle air mixture screws back to 1.25 turns out. The timing is reset again, and is all lubed up. We should be good to go for a long while now!!!

Motorcycle Mechanic job said...

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