Hi all

The bike is a Yamaha XT550 1983.

It starts perfect, idles perfect and runs normally most of the time.

Sometimes under load, accelerating, mid-high rpm’s the engine (I think) suddenly starts making a “hissing” sound and loses power. There is no “gray area” the hissing/powerloss it is there full-on or not at all.

1.If I shut off the throttle the engine stops making this hissing sound and returns to normal idling.
2.If I l let off the throttle a little, the engine stops “hissing” and regains normal power (for the low throttle) and the bike cruises smoothly at maybe 80-90 kmph.
3.If I hold the throttle when the engine starts “hissing” the engine will “hiss” on and lose power. The bike will slow down to maybe 50-60 kmph since the engine makes very little power.

The “hissing” sound itself is very similar to the hissing from an angry cat J or the ”sucking” sound when you grab the nozzle from a vacuum cleaner leaving a tiny opening. There are no squeaks or tic-tic sounds, the hissing is flat, even and kind of mono-tone.



What could it be?

1.If it is the con-rods big end bearing why does it only “hiss” SOMETIMES (not always) under heavy load? There is no hissing at idle or at moderate throttle. Usually at heavy throttle there is no hissing, it only happens now and then...
2.Could it possibly be the carbs intake boots? I can’t see anything wrong with them. Could an air leak show this symptoms?
3.If the head gasket is half-way shot and only leaks under max cylinder pressure, could the sound be mono-tone “hissing” and not “tic-tic-tic” like an leaking exhaust?
4.Some junk floating around in the exhaust?

What do you guys think??

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Yamaha XT600E '01, Honda XR400 '03, Yamaha XT550 '83

”sucking” sound when you grab the nozzle from a vacuum cleaner leaving a tiny opening. There are no squeaks or tic-tic sounds, the hissing is flat, even and kind of mono-tone.

I think you have answered it there,def sounds like an air leak. inlets possibly deforming and closing up when you give it the beans and air entering thru small cracks.

have you tried running with the fuel filler cap slightly open/loose? maybe vacumn being created in the tank?

Andy

I've known a loose spark plug do this, seemed like at certain revs the vibrations moved the plug to a posion where it leaked. It'll be something minor, but I'm afraid probably a *** to find. WD40/grease all the rubber bits one at a time, the temporary seal will be enough to tell you if you found the right bit to swap.

Andy

Thanks, good tips.



Threewheelbonnie, you speak the truth. It can be hard to figure out problems when they usually are not there :-)



I took a better look at the exhaust last night. The joint where the header meets the muffler is not that tight. I bet if the stuffing in the muffler is loose, and can block the path of the exhaust gas there will be a lot of sound when the gas is pressed through the joint at relatively high pressure. And the joint is right beside the engine. Maybe the sound wasn’t from the engine…knock on wood…

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Yamaha XT600E '01, Honda XR400 '03, Yamaha XT550 '83