Why does my motorcycle keep dying
May 28, 2007 10:40 AM   Subscribe

Help me diagnose my motorcycle problem. (fairly lengthy explanation follows)

Soon after posting this question, I ended up with a 1982 Yamaha XV750 Virago. It had only 5200 miles on it and was impressively clean for a 25 year old bike, so I bit the bullet and got it. One problem the previous owner told me about was the choke -- the bike needed the choke to be on all the time or else it stalls. He told me that it didn't really bother him and he just lived with it. Since it was my first bike, I really wanted it to be perfect, so following the advice of a coworker, I added some Sea Foam to the tank (around 1-2 fluid ounces). The theory was that there was some kind of clog somewhere in the fuel line and this would clean it out. That's when the real problems started.

The day after adding Sea Foam, the bike would not even start any more. The engine would turn over but it wouldn't stay running more than about 5 seconds. The spark plugs were badly fouled (carbon). I replaced those and it ran again, for about 5 miles, then it stranded me. Same problem - it would crank but not stay running. I duck-walked it to a nearby parking lot and it sat there for a week until I towed it to a cycle shop.

The shop rebuilt the carburetors and replaced the rubber intake manifold boots which were badly dried out, worn, and brittle. I got it back Saturday morning and it was running beautifully.

I put about 30 miles or so on it. Toward the end of those miles, it was starting to backfire (? not sure if I'm using the term correctly - basically it was making occasional "pop" sounds). Soon after that I started heading home. It was getting less and less responsive to throttle. And then finally, in the middle of an intersection, it just died and would not start again. So I walked it to a nearby gas station, got a ride home, and came back and cleaned the spark plugs as best I could with a toothbrush, rag, and WD-40. After some struggling, it started up again, and I nervously rode it home.

This has been a nerve wracking and frustrating experience so far. At least I'd like to make a learning experience out of it. What the hell is going on with this bike?

I am of course going to follow up with the shop when they open tomorrow. But I'm looking for the metafilter second opinion.
posted by jclovebrew to Travel & Transportation (8 answers total) 1 user marked this as a favorite
You'll have more luck with this on a motorcycle-related forum like the Virago Tech Forums.

You need gas, air, compression, and spark to go. Cleaning the carbs was a good bet; the popping on deceleration, blackened spark plugs, and loss of power are all symptoms of fuel not making it through the carbs. But that doesn't mean it's still making it to the cylinder. I don't know the maintenance procedures on a Virago, but if the valves aren't letting fuel-air in you're not going anywhere.

Air: Is the air filter clogged? Is it home to a starling or a family of mice?

Compression: If the piston can't compress the air (the valves are letting the air out, or the piston seals are letting air by) you're not going to go. Compression testing is a pain in the ass, though, so that's probably the last step.

Spark: You know the plugs fire. You know the battery can crank the starter. But there's more to the electrical system than just those. Are the plugs firing on time, hot enough? Is the alternator firing the plugs or the battery?

If this sounds like a million things to check, it's because it is. That's why you want to check with Virago experts who know what things are most likely to go on that bike. 5200 miles on a 25-year-old bike is over the "good" side of the curve back into "problem" territory, though, because that means the odds are very good that the bike just sat unused for years. (Or maybe the cluster failed and was replaced. In fact I'd probably lean towards that rather than only 200 miles a year on a popular workhorse bike like that.)

Finally: What to do when your motorcycle won't start.
posted by mendel at 11:15 AM on May 28, 2007

Just to add a few more details:

I think air is ok and compression is ok. Compression was measured 155/160 (nicely within range) and the air filter was replaced during the carb service.

Don't know about the valves. They haven't been touched by me or the shop.

Million things to check, huh? Maybe I'm in over my head.

I will check those links, mendel.
posted by jclovebrew at 11:21 AM on May 28, 2007

I would take the tank off, drain it completely and clean the insides. Since it ran fine for a while after the carbs were rebuilt then slowly died, I would suspect bad gas or debris (rust? water?) in your gas tank. I would also take the gas petcock off and make sure they are clean also.
posted by BillsR100 at 11:58 AM on May 28, 2007

You can learn a great deal about the condition of a 4 cycle engine by reading the spark plugs. From your symptom description of fouled plugs, I'm guessing your plugs look like pictures 1, 2, or 3 on the linked page.

There are only a few reasons plugs foul so quickly. Either your fuel mixture is so rich there isn't enough oxygen in the mixture to burn all the carbon contained in the gas charge delivered to the cylinder, or extra carbon is in the cylinder, in the form of oil droplets, when combustion occurs. If your carbs were just rebuilt correctly (I'm assuming that the people who did it would've checked your jet sizes, metering rods or needles, venturis, and throttle bodies as well as replacing gaskets and floats), I'd assume your fuel mixture is within range of being right, so that excess fuel isn't your major problem.

That leads me to look for reasons why excess oil is making into the cylinder, past the piston's oil control rings, or down from the valve train. The most obvious reason for plugs fouling so quickly would be that something is pumping the oil to froth condition in the crankcase. Oil from the valve train can foul plugs, too, but it generally takes a lot longer, perhaps a couple hundred miles, worst case, due to the smaller amount of oil, and the mechanical action of the valves. That's why I'd look for the bottom end causes before investigating the top end.

Perhaps your crankcase vent is plugged, or if you have PCV, your PCV valve is clogged. Or, perhaps you've got an oil overfill condition. Or, perhaps you've got a mechanical problem, such as a bad oil control ring, or scraper ring, or rings whose end spaces are out of spec, or whose end spaces have somehow lined up. These latter issues would not necessarily be revealed by a classic compression test at cranking speeds, simply because not a whole lot of gas will escape to the crankcase on each cycle, once the first few pumps of the cylinder charge it.

You could open the oil filler, and try running the engine a bit with the filler cap/dipstick removed. You shouldn't get massive amounts of discharge out the filler cap neck, but you might get a little with a normal functioning engine. If you get big whooshes of gas, or frothy oil, you know something is wrong mechanically in the lower end. This could range from a cracked piston, to a bad ring, to a scored cylinder wall.

But start with a careful reading of the spark plugs. You'll get the best information from them.
posted by paulsc at 12:19 PM on May 28, 2007

Based on my experience (ok, only one bike) I'd suspect the values. I had more or less the same symptoms on my bike because the valves constantly required adjustment. But the best advice you have is to go hit a motorcycle specific forum.
posted by chairface at 12:32 PM on May 28, 2007

I propose a slight variation on BillsR 1 0 0's answer.

You have some kind of contamination in your tank, I think, but not water or rust, because Sea Foam would have helped a water problem and not worsened rust. I'm guessing you have a layer of oil in the bottom of your tank which the Sea Foam stirred up and got even more of than usual into the gas. If you can reach into the tank with a piece of cotton (not synthetic!) cloth tied to a stick (securely) and scrape the bottom of the tank, you might be able to see any oil on the cloth.

How would oil have gotten there? Well, it doesn't have very many miles on it, and I am imagining someone wanted to take it out but didn't have any gas for it, and so added some from a convenient can hanging around in the garage, but that can actually had 2 cycle fuel for lawnmowers instead of 4 cycle fuel for your motorcycle. 2 cycle fuel has a considerable amount of oil mixed with gasoline. Whatever problems they had when they were riding it, or didn't, they brought it back and let it sit and the oil just settled to the bottom of the tank in a layer, from where it has been getting into the gas ever since, causing you to need to use the choke and fouling your plugs.
posted by jamjam at 2:25 PM on May 28, 2007

Clever guess, jamjam -- this would explain everything if it turned out right!
posted by gum at 12:11 AM on May 29, 2007

If a shop re-built carbs without discovering the crappy fuel mixture scenario jamjam mentioned, I'd stay away from that shop in the future. And if there was enough oil coming through from the gas tank to foul plugs in 30 miles, the bike should've smoked like a wood stove from the time it was started.

But it's an easy theory to check, and I hope for jclovebrew's sake it's right.
posted by paulsc at 2:00 AM on May 29, 2007

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