Running woe #2: Hovering high RPMs
The bike still wasn't running just right.
When I'd rev the engine up and then close the throttle, the engine would hover at high RPMs for a while before dropping back down to a more sane level on their own. It was not only annoying to sit at traffic lights with the engine screaming away at 4500 RPM for 15 seconds, it also made riding dangerous because I couldn't smoothly roll off of the throttle in curves.
My first thought was that the problem was that the carbs were out of balance - That one carb was trying to keep the engine at a high RPM, while the other was lugging it back to idle. I double-checked that the carb balance, in case I messed it up after I cleaned out the carbs, but it was still perfect.
I knew that a lean mixture could cause an engine to hover at a high RPM, and that lean mixtures are often the result of vacuum leaks. To test this out, I sprayed WD40 all over the carb bodies, petcock vacuum line, and intake manifold, but I couldn't detect any change in how the engine ran. However, on closer inspection, I saw that the rubber intake manifold bodies that appeared to be in fine shape were in fact riddled with cracks and covered in black RTV rubber. Also the hose clamps on the intake manifolds had seen better days.
I got a couple of NOS intake manifolds from EBay and replaced the flakey Yamaha hose clamps with heavy-duty industrial hose clamps. However, the problem persisted.
I was a little nervous to mess with the idle mixture screws on the carburetors...I left them screwed out the same number of turns as the previous owner had them. However, all signs pointed to a weak mixture, and I knew it couldn't be caused by a simple vacuum leak.
I ended up screwing out the idle mixture screws on each carb about three turns, and the problem immediately disappeared. Over the course of the next few days, I tweaked the mixture screws until it was running really well. I knew I had it too rich if the engine was noticably down on power. I knew it was too lean if the engine RPMs didn't drop crisply when I closed the throttle. I could pull the spark plugs and look at their color after a few days of running to make sure I was still on track.
After fiddling with the idle mixture screws, the bike ran beautifully, and I drove it for several months that way.
I still had the annoying problem of the idle slowing down. When I'd close the throttle, the idle would immediately drop to about 1200 RPM, but as I sat at stoplights, the idle would get lower and lower until the bike stalled out after about 10 seconds, if I didn't continually blip the throttle. Turning up the idle adjuster screw, even just a quarter turn, would cause the engine to idle about about 3500 RPM. I resigned myself to blipping the throttle at lights, drove it, and continued to scratch my head about what could be causing that problem.
When I'd rev the engine up and then close the throttle, the engine would hover at high RPMs for a while before dropping back down to a more sane level on their own. It was not only annoying to sit at traffic lights with the engine screaming away at 4500 RPM for 15 seconds, it also made riding dangerous because I couldn't smoothly roll off of the throttle in curves.
My first thought was that the problem was that the carbs were out of balance - That one carb was trying to keep the engine at a high RPM, while the other was lugging it back to idle. I double-checked that the carb balance, in case I messed it up after I cleaned out the carbs, but it was still perfect.
I knew that a lean mixture could cause an engine to hover at a high RPM, and that lean mixtures are often the result of vacuum leaks. To test this out, I sprayed WD40 all over the carb bodies, petcock vacuum line, and intake manifold, but I couldn't detect any change in how the engine ran. However, on closer inspection, I saw that the rubber intake manifold bodies that appeared to be in fine shape were in fact riddled with cracks and covered in black RTV rubber. Also the hose clamps on the intake manifolds had seen better days.
I got a couple of NOS intake manifolds from EBay and replaced the flakey Yamaha hose clamps with heavy-duty industrial hose clamps. However, the problem persisted.
I was a little nervous to mess with the idle mixture screws on the carburetors...I left them screwed out the same number of turns as the previous owner had them. However, all signs pointed to a weak mixture, and I knew it couldn't be caused by a simple vacuum leak.
I ended up screwing out the idle mixture screws on each carb about three turns, and the problem immediately disappeared. Over the course of the next few days, I tweaked the mixture screws until it was running really well. I knew I had it too rich if the engine was noticably down on power. I knew it was too lean if the engine RPMs didn't drop crisply when I closed the throttle. I could pull the spark plugs and look at their color after a few days of running to make sure I was still on track.
After fiddling with the idle mixture screws, the bike ran beautifully, and I drove it for several months that way.
I still had the annoying problem of the idle slowing down. When I'd close the throttle, the idle would immediately drop to about 1200 RPM, but as I sat at stoplights, the idle would get lower and lower until the bike stalled out after about 10 seconds, if I didn't continually blip the throttle. Turning up the idle adjuster screw, even just a quarter turn, would cause the engine to idle about about 3500 RPM. I resigned myself to blipping the throttle at lights, drove it, and continued to scratch my head about what could be causing that problem.
posted by veloandy at 12:51 PM
1 Comments:
Hi there my name is Steve. i also have a xs400 with the same problem as you describe. I have done all obvious things to rectify the problem, but as yet to no avail.
Just wondering if you ever discovered the cause of that particular problem?
regards
Steve
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