Question about Piston Slap

[cywelchjr]

Ok, I know what piston slap sounds like, and I know how it behaves on water cooled engines, both in cases of very minor slap and with pretty severe wear.

That said, can anybody describe what piston slap sounds like on these engines and how it behaves on an air cooled engine? I know it pretty must just gets quieter on a water cooled engine, and it makes sense since the temps are rather fixed, they can only get so hot as long as the cooling system is working. But might it be that an air cooled engine having a wider temp range (especially after a high speed run that turns into slow speed driving in a short distance) can work differently, such as getting louder at higher temps due to possible higher expansion of the cylinder vs the piston?

I ask because I'm getting a rattle that sounds just like piston slap as I have heard it on a couple of cars I have had, but it doesn't behave the same. Under normal conditions I only hear it first thing in the morning and then it goes away till the engine gets very well warmed up (this takes at least 15 miles of riding to get there) and stays pretty constant unless I do a high speed run followed by surface street riding (it gets louder then for a while and then quiets back down as it cools back to normal temps) and when it cools back down but not completely cold goes completely away.

It also is only apparent from about 3k to about 5k under light load, and gets quieter as I crank harder on the throttle. In over 3000 miles there has been NO apparent change in either the volume or quality of the sound, and the engine doesn't use any oil (well, very very little, it's gone from the top level line to right about between the two lines in 2000 miles) to speak of.

Any ideas? What could it be if not piston slap? Adjusting the cam chain adjuster has made no difference and the valves are in spec as best I can tell. Oil pressure seems to be good as well with 20/50 Valvoline.

[trbig]

What kind of gas are you running and how many miles are on the bike?

You might try running higher octane. If it goes away, then do some tankfulls of Seafoam to clean up the carbon.

Also, you might try a different oil next time. Not trying to turn this into an oil thread.. I know some people will swear by valvoline.. but I also know it makes some people's motor top ends rattle like that. You can pull up an oil thread to see what most use.. or try something different.. even a different weight like a 10/40 or something.

I have taken a couple motors down that had some slap going on. Over time, the bores of the cylinders get egg shaped front to back, as well as some wear on the piston skirt that opens the tolerances between piston and cylinder. The wear on the skirt is pretty noticeable.

If it's not getting worse, I wouldn't sweat it yet. Any time a motor starts making any knocking sounds now, I automatically start assuming crank bearings since I've had such a problem with them.

Hope it's something simple.


Tod

[XSokieSPECIAL]

There is this also.....

.....I thought I had a problem too. ( Double click on the doc so you can read it)

[tarzan]

I had that same problem on my old SG I sold last summer.
It was the vacuum advance.It sounds worse with a fairing.
It won't hurt anything it is just annoying.

[skids]

The primary chain can slap but I wouldn't think it could continuously.

[trbig]

Yeah, at idle and the carbs needing synched, you can hear some primary chain. But under load you shouldn't.


Tod

[petejw]

you could try putting a stethoscope or screw
driver next to the barrels and hear if the noise
is coming from there or down near the sump
to hear if its coming from the bearings. jat

[BossMaverick]

I also have that annoying vacuum advance fairing combination thing going on. I wouldn't worry about it because as you said, it goes away if you hit the gas in that rpm range. Your oil use is fine and your not complaining of low power or anything so I think your fine.

[cywelchjr]

you could try putting a stethoscope or screw
driver next to the barrels and hear if the noise
is coming from there or down near the sump
to hear if its coming from the bearings. jat

I'm not sure how I would do that since the only time the noise happens is under light to medium load from about 3 to 5 k. Even in first gear that's a little fast to be trying to get a stethoscope against it to hear.

[cywelchjr]

I also have that annoying vacuum advance fairing combination thing going on. I wouldn't worry about it because as you said, it goes away if you hit the gas in that rpm range. Your oil use is fine and your not complaining of low power or anything so I think your fine.

I would agree except it only gets quieter, it doesn't go away. I'm changing the engine anyways, because I got a nice engine that's already painted a nice black, and so far everything looks really good on it. I'm in the process of sanding an polishing the edges of the fins (don't want to polish them too much though as I intend to put HT clear on them to keep them from having to be polished all the time. I want it to look nice, but I want to ride, not polish it all the time.

If this engine makes the same sound, I'll accept it as normal, but right now, I'm not sure. The TSB states that the knock is right around 4k, and this starts a lot lower. I'm not worried the engine is going to grenade on me, but I'm not feeling like I can trust it for almost 1000 miles for the California Rally this June. Maybe I should put a trailer hitch on and carry a spare engine just in case? Naw, I wouldn't want to try and change one out with only hand tools. My air tools make the job so much easier.

I was really hoping that someone had some insight on how piston slap behaves on an air cooled engine. It would be nice if someone who knew what the TSB noise sounds like could ride Brutus and tell me for sure if it's the same sound, but there just don't seem to be any other members close enough (especially one who knows the sound). I would love to get a second opinion on it, but don't have a way to accurately capture the sound or have someone who knows these engines who can ride it and tell me what they think of the sound.

Frankly I'm a bit jealous of the guys in the midwest (not of the short riding season though) as they have so much help close by. I know there are some really knowledgeable guys down south, and at least one over in Reno, but I don't think any of them are coming up or over this way any time soon so it limits my options. I'll have to see if I can figure out a way to mount my camera on the bike and get some video along with the audio and maybe I can capture the sound well enough for troubleshooting.

[bikerphil]

Got the same knock on my 79SF. I got it to quiet down a lot by limiting the travel of the vac. advance unit to about half it's usual draw. My 79F doesn't make that noise at all. Make sure the vac. advance unit is not "wobbling" during operation.

[trbig]

Is there any way you could put it on the center stand, start it, put it in gear, then put a load on it for a little bit with the rear brake? You can take the left engine cover off when you do this and see if the vaccum mechanism is vibrating wildly.

Another thing to watch for with that sidecover off is to see the timing plate spinning true. If the crank bearings are going bad, you can actually see it wobble a bit. I've had the timing plate mounted on the crank wobble enough to smear a couple sets of pickup coils. If it gets bad, it will start seeping oil out of that seal. It can't seal against that vibration.

ANOTHER thing.. is to watch that the vaccum mechanism is actually moving. The bearing it rides on sometimes gets rusty and freezes up, not allowing it to have the right timing.. and the engine will get pre-detonation like the gas is bad.


Tod

[Larrym]

Acoustic Anomalies??

Cy,

For entertainment or enlightenment, you decide:



Thing is that the camera is less than 3 inches from the left cover. At higher speeds the wind noise drowns out that clicking sound.

Yes, the vacuum advance is connected to the correct port on #2 Carb. I had the cover off last night to adjust my CCT and the timing plate is spinning true without rubbing against the ignition pick-ups.

The noise is what one would hear if you had your face down there during a ride. The low rpms mean that the vacuum pulses from #2 carb aren't fast enough to smooth out the action of the vacuum advance. I'm sure it evens out at the higher rpms/speeds. I don't hear it all that much but then again I don't have a fairing which acts like an acoustical bell/reflector to make the sound appear louder as I ride. Makes me wonder if adding a dampener in the vacuum line might be a good idea after all.

So I'd ask if the noise you hear appears to be louder on the left or right side of the bike as you're tooling down the road trying to stay between the lines. As far as that goes, I'd recommend extra caution if you decide to do a left/right side listening test while riding: moving your head/body to one side or the other of the centerline of the bike may inadvertently cause the bike to steer in that direction. We wouldn't want you or your nice ride to cross the centerline of the road and end up in a head-on collision.

Then at the very least we'd have an awkward pause in your posts here on the forum while you're on the mend in a hospital bed somewhere.

[cywelchjr]

Cy,

For entertainment or enlightenment, you decide:

Thing is that the camera is less than 3 inches from the left cover. At higher speeds the wind noise drowns out that clicking sound.

Well, that sound is not at all like the sound I'm getting.

Yes, the vacuum advance is connected to the correct port on #2 Carb. I had the cover off last night to adjust my CCT and the timing plate is spinning true without rubbing against the ignition pick-ups.

I've checked that on mine also, and there is no wobble to it at all. Straight and true, the edges are crisp and clear at all speeds, both with and without the timing light involved.

The noise is what one would hear if you had your face down there during a ride. The low rpms mean that the vacuum pulses from #2 carb aren't fast enough to smooth out the action of the vacuum advance. I'm sure it evens out at the higher rpms/speeds. I don't hear it all that much but then again I don't have a fairing which acts like an acoustical bell/reflector to make the sound appear louder as I ride. Makes me wonder if adding a dampener in the vacuum line might be a good idea after all.

My vacuum advance bounces around just up off idle also. It does however smooth out as the throttle advances and RPM increases. My concern with adding a damper is that the ignition retard from quickly opening the throttle would be slower, and more likely to cause pinging. I'm thinking that the jitter at low RPM's may be what truly causes the problem with the wires, as the fast repeated vibrations like that can't be good for those wires.

So I'd ask if the noise you hear appears to be louder on the left or right side of the bike as you're tooling down the road trying to stay between the lines. As far as that goes, I'd recommend extra caution if you decide to do a left/right side listening test while riding: moving your head/body to one side or the other of the centerline of the bike may inadvertently cause the bike to steer in that direction. We wouldn't want you or your nice ride to cross the centerline of the road and end up in a head-on collision.

I have been trying to get an idea of where it's coming from, and best I can tell, it seems to be lower than the valve train, but higher than the crankshaft. It's hard to tell going down the road of course, but the directionality of the sound seems to be just slightly lower than the valve train noise. I've been wondering if I would even notice the noise without the fairing? I'm thinking about putting some sort of sound dampening material which I hope might help.

Then at the very least we'd have an awkward pause in your posts here on the forum while you're on the mend in a hospital bed somewhere.

It would be rather awkward for me as well.

[TopCatGr58]

Hey Cy,

Simple test, surprised someone didn't already suggest it! DISCONNECT The vacuum hose going to the advance unit, just plug it so you don't have a vacuum leak, and then ride it to see/hear if you hear the noise!

If you don't, then you've discovered the cause of the noise, but if you still hear the noise, then it won't be the vac. adv. unit, and it could be piston slap?
T.C.