Author Topic: Poor hot idle and dat 'ole debil alcohol...  (Read 155 times)

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Offline berniebee

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Poor hot idle and dat 'ole debil alcohol...
« on: September 22, 2019, 11:56:40 PM »
We had planned to do an extended Saturday ride. My buddy on his silver '82 BMW R65LS and me on the (so far) trusty  brown '76 XS500. I'd done a few one hour shake down rides on it - it was time for a good long day ride.

But 25 minutes in, the highway traffic leading out of town slowed to a disappointing crawl. Must have been an accident up ahead. It was getting pretty hot, about 30C (86F). After 10 minutes of  stop and go, I stalled. But the XS500 restarted after a little extra time on the button. Five minutes later,  another stall - and this time it took several attempts to restart it. Two minutes later another stall and it was not gonna start. At the shoulder, Google told me that the slowdown was miles long, so I looked for an alternate route. Luckily we were at an exit ramp. After a few minutes, I started the bike and took off. Ok, runs fine. Five minutes later we were stuck in more traffic and this time, waiting for a traffic light, it stalled again. Though I was pretty sure that this was a hot-engine-in-traffic-only problem, I decided not to risk it. Besides, Google now told me that there was major traffic on the alternate route too.

We got home in light traffic, the bike performing flawlessly on a short highway run back home, but stalling several times on city streets. I'd had some rough idle/stalling issues in hot, slow going before, and noticed that the idle improved when I turned the headlight off. But this time it made no difference. It simply would not run under 2,000 rpm. My bike has yellow tinted transparent fuel lines. When I got home, I had a look. Bubbles! Not just one or two, but a stream of bubbles coming up from the carburetor into the fuel tank! The carb bowls were hot, the plastic air box was maybe even hotter. The starter motor cover beneath the carbs was a searing hot plate.

I got the answer from a vintage car enthusiast site. Ethanol. Most fuel these days has 10% ethanol mixed in. Ethanol has a lower boiling point than gasoline, so if you happen to have a hot running engine (like ours) and get stuck in traffic on a hot day...bubbles of ethanol vapour can result and your bike doesn't run right. When you get back on the highway, the bike consumes fuel more quickly, so of course fuel spends less time warming in the hot carb bowl before being consumed. And the airflow cools everything down a bit. No vapour. The vintage car site offered a few vapor separator solutions, but nothing applicable for a bike that doesn't have a fuel pump.

But. You CAN buy fuel without ethanol if you look for it. In my neck of the woods, unfortunately, only certain brands of high priced premium fuel are ethanol free. I'm playing with the idea of cramming a computer muffin fan somewhere under or behind the carbs.

Hope this helps someone.