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Discussion Starter #1
On a comfortable (60 degrees) morning ride not long ago, I came into town to the first stoplight and found that my bike would not idle. It worked on one cylinder only unless I kept the revs up. I nursed it to my destination, ran an errand of approx. 10 minutes, came out and started it up to find that now it idled. I've never had any problems with it prior to this.

The next morning, I checked the valves, they were spot on in the middle of their adjustment range just where they had been for the last 11,000 miles. I changed the sparkplugs and went riding. Ran on both cylinders, but I found that the mileage had dropped by about 11-12 miles per gallon. It has stayed that way since that day. For no apparent reason, the mpg has sunk and it will not idle with any vigor although now it does idle. And the power is down needing a bit more throttle than before for the same speed.

It has never been over-heated. It doesn't burn oil. The valves have never been out of adjustment. It has spark, but I cannot tell by looking if it is proper. At idle, if I pull the rear sparkplug wire, it will continue idling at a lower rpm. If I put it back on and then pull the front cylinder plug wire, it will die so it seems that there is some problem with the rear cylinder.

Both cylinders have 160psi pressure when tested cold. I am allergic to messing about with hot engines. If there was a compression problem, there would be a difference hot or cold.

Is it possible for the timing to slip? Individually by cylinder?
Any ideas are appreciated.
I accidentally posted this in general bike forum. Maybe better here?
 

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Well then its not the carb. LOL Look for an intake boot to be cracked and leaking if it won't idle but runs fine when throttle applied.
 

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Discussion Starter #5
All looks good between carb and cylinders. Even though I cannot see a difference, is it possible that the spark is weak on the rear cyinder? Can a coil work but not provide full voltage? I always thought that coils are work/don't work with nothing in between. But I don't know.
 

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Sorry I'm at work and got busy. Get some wd-40 or similar and spray around boot and manifold when you're trying to keep it idling and see if it picks up. It doesn't take much of a leak to make it too lean to run at idle.
 

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Also check your plugs. What color and any significant color variations? A Coil can go bad and then good again, usually on the route to going bad altogether. Heat related, when they get hot they open and as they cool down the circuit completes again. If you get nothing on these try swapping your coils and see if the problem follows. Do these one at a time so you can find the hard fix.
 

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Discussion Starter #9
Thanks, Fulltilt1. The coil thing, going bad then seeming to revive, is interesting. I'll check when we get a slightly warmer day. No garage, so I work in a carport. Too chilly and dampish for the next few days.

I'll try the wd-40 on the intake manifold bits first. If no change, then I'll get serious about the coils.

The front plug looks perfect. The rear plug looks good but for a dark, carbony area on one side maybe like it's not burning hot enough to keep completely clean. Makes me suspect the coil.
 

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I'd try plugs first, the condition I mentioned for the coils is rare but not unheard of.
 

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If that plug is dark I doubt its got a leak then, should be light to wht. Swap coils see if it follows, maybe read resistance? How many miles? Could plug wire be bad?
 

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Sugar Bear is leaning toward what Fultilt1 is saying, It don't take much of a leak on the intake to cause grief, maybe on the back side, like hard to get to? Just a thought, had that long ago, and it was hard to figure out, hope you get it solved:) Ride safe.
 

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Discussion Starter #14
Hi, Guys. Well, the Seafoam didn't seem to make any changes. But, while wiping the bike down a bit, I got up close to the intake manifold, at least as close as you can get, hard to see clearly. I found many hairline cracks in the intake manifold, so fine that at first I thought I was looking at the texture of the intake manifold. But, the intake manifold has no texture. The cracks are clustered around the flange on both cylinders.

So I did the obvious. I took some tub and tile caulk and smeared a light coating on the hairline fractures I could see clearly (44 degrees, windy, raining, in a carport...). I know, the tube said something about warmer temps and squeekly clean surfaces, but I figured it would have SOME effect. The two days later, with less water flying about, went for a ride. The first thing I noticed is that the idle is back to perfect. This lends credence to the leaking intake suggestion. Mileage hasn't been tested in known conditions yet, but it has to be better. At least part of the problem is identified and thank you all for your suggestions.

Yamaha has been making the vstar 250 for about 20 years now. They have pared down the costs and increased the profit thereby. Just under 11,500 miles and the PLASTIC intake manifold is preparing to self-destruct. Very disappointed. Build it to a price, but don't compromise the quality of your name, Yamaha. I'm looking at Honda or maybe Royal Enfield for the next purchase in a year or so.
 
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