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„Hissing“ sound and power loss under load – shot big end bearing?

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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 :-) 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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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 :-) 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??

Snake Stuck In Exaust. Gets Mad After Pipe Gets Hot & Hisses At You? Sorry Couldnt Resist...

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I'll give a couple of ideas but these are just shots in the dark. First, your #4. Is it possible that a portion of the exhaust pipe or muffler is so rusty and corroded INSIDE that pieces are sometimes moving around and plugging the exhaust flow? The only other thing I can think of is a valve sticking partially open from varnish/gunk on the stem.

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Thanks guys.

motoxvet, I took a better look at the exhaust. The joint where the header meets the muffler is not that tight. If the muffler gets blocked the joint will “hiss”. And the joint is right beside the engine, so I can’t say if the noise was coming from the engine or this joint.

So I opened up the end of the muffler to check for some loose stuffing (wool) or anything else that might block the muffler (see pic). As far as I could see, there was no stuffing in the muffler, and as far as I can tell nothing is loose in the muffler. The muffler had been “modified” to be extra silent, but I can’t see how that can block the muffler only sometimes and not always under the same engine load. Or how that can block the muffler in the first place. Restrict yes, block no..👍:thinking:

That said I went for a test ride last night after opening up the muffler. The bike ran normal for about 1 km test. I could not believe it, so I ran the bike until out of gas, maybe 30 minutes. Still nothing, the bike ran fine. Don’t get it, and certainly don’t trust that the problem has gone away. More testing to come before I do that..

exhaust_001.jpg

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