Vibration at specific rev range

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Hello All
Hoping you guys (or gals) can help out one of the few members from UK.
I bought a chrome tank Dirt Squirt a few months ago. It had stood idle for many years but ran well apart from a nasty vibration at a very specific rev range. The bike doesn't have a rev counter but the vibration kicked in from about 30mph in top gear but cleared again above 35mph. Whenever I hit those particular revs, in whatever gear, under acceleration or de-celebration, I got a really harsh vibration. Seemed to come through handle bars, seat and foot pegs. In fact it was so bad that oil was frothing out of the gearbox breather hole. Anyway, did a complete engine re-build with new main bearings, con-rod kit, re-bore and new piston and guess what? The vibration is still there - exactly as it was before. I would have thought engine mounts were the culprit but they seem to be perfect with bolts done up tightly. Has anyone suffered this sort of thing. I can't believe that a 98cc engine is meant to vibrate that badly and at such a specific rev range. Above the revs and below the revs it is as smooth as silk. Any thoughts?
Tim

[hodakamax]

First, Welcome to the forum. Strange indeed, my first thoughts lead me to the crankshaft ends which mount the flywheel and the clutch assembly. I would start with pulling both side covers and checking both clutch and flywheel with a dial indicator while rotating the crankshaft to see if they run out true. I'm sure the gang will have some other ideas on this problem.

Max

[Charlie R]

I'm with Max, STRANGE. Does it experience the same vibration when revved to that particular RPM when sitting still? If so, It would be safe to limit the search to engine only. Max is absolutely right about the crankshaft possibly being out of true could account for the vibration. If the crank was recently rebuilt and I'm assuming the rebuilder properly trued the crank assembly, I might also consider having a look at the flywheel. I would make sure it is the proper flywheel and it is properly seated on the crankshaft. It is possible that the flywheel itself has been damaged (dropped or pried on) and become bent or out of balance. Improper flywheel balance will account for a vibration at a specific RPM. Because the vibration was present before the engine was rebuilt and remained after, I would suspect the flywheel. I'm further assuming the same flywheel was used post rebuild?

[hodakamax]

Charlie, I think you're right, it does sound like the flywheel and the dial indicator would probably show that. Surely the crankshaft was trued during rebuilding which only leaves the clutch and flywheel suspect. I wonder, as you said, if the flywheel is not seated or damaged.

Tim, You might pull the flywheel and check the key and keyway and also check for damages to both the mating surfaces on the crank and flywheel. You might have a key that is too tall. It could be the wrong key or something is keeping it from seating. If the flywheel is tightening down on the key rather than on the mating surfaces it could cause a severe imbalance. That would also account for the specific RPM vibration. The flywheel would be both tipped and off center creating two vibrations that reinforce each other at a certain speed.
Charlie?

Max

[Charlie R]

Max, spot on info my friend. Please let us know what you find, someone else here may run into this problem. Good luck!

[hodakamax]

Ah, two brains are better than one, thanks for your input. These mechanical puzzles are really fun--Good for our brains!
Thanks

Max

[[email protected]]

Hey Guys
Really appreciate your comments. To answer a couple of questions. I trued the crank shaft after it was re-build and got 0.005mm run-out at the magneto end and 0.02mm at the clutch end. I was pretty happy with this. I used the same flywheel but I hadn't thought about the key being too tall - that's a very good point. I will check this and also run a dial indicator on the outside of the flywheel to see if it runs true. Will let you know the outcome.
Tim

[[email protected]]

Another message from 'blighty'.
I just checked a few things. Measured run-out on the outside of the flywheel and it was showing around 0.05mm. I then removed the flywheel and took the key out, then replaced the flywheel in roughly the same place to see if the key was affecting the run-out. Didn't seem to make any difference. Therefore the flywheel isn't running exactly true, but I wouldn't have thought 0.05mm would be enough to cause the vibration would you guys? The only other thing I can try is having the flywheel balanced. I'll see if I can get someone to do this for reasonable cost.
Any further thoughts please let me know.
Cheers Tim

[hodakamax]

Tim--Another thing you might try is lapping the two surfaces with the key out with an abrasive like valve grinding compound to see if you can improve the tolerances. I personally have never seen a flywheel out of significant balance. I'd try the lapping first. Darn, I would have bet money on it being the key!

Max

[[email protected]]

Hi Max
Another really good idea - hey, this forum thing really works doesn't it? I will try the lapping idea.
I'm now questioning myself over the vibration - maybe all Hodaka's vibrate like this? On the other hand I can't believe oil is meant to froth out of the gearbox breather. Also, under acceleration the vibration is horrible. The only way is to ride around it by dropping a gear and accelerating past the vibration spot. That can't be right.
Just to give myself peace of mind, I started the bike and put it into first gear with the clutch in and revved it to the vibration point. Vibration was just as bad as ever. At least I now know that none of the gearbox shafts are causing the problem, so it must be the crank or something on the crank.
Will update again shortly.
Tim

[Dale]

Tim,
I was also thinking crank when I first saw your post and think you are on track with this, but I am curious about the frothing of oil. It makes sense that if things are out of balance that the oil could get agitated enough to froth, but what if it is the other way around. What oil are you using in the gearbox and how much? The gearbox holds 20oz but you can only drain about 16oz from the bottom drain bolt. So it is easy to overfill when doing oil changes... When you check the oil level with the dipstick, it should be screwed in and then removed to view the level.

One thing is for sure. Your situation is not normal at all.

I have a 99 Road Toad that recently began frothing and feels and sounds sluggish when rev'd. I did not note any specific vibrations though. I have not torn into it yet, but suspect that the clutch side seal might have failed and is drawing in gearbox oil.
Dale

[[email protected]]

Hello Dale
Thanks for your post.
I am using the Belray Gearsaver as recommended on the Strictly Hodaka website. I put in around 550ml having rebuilt the engine, but I haven't checked it on the dipstick. Will do as you say and make sure I'm not over-filling it.
Thanks
Tim

[BrianZ]

I'm guessing this would have been noticed during the flywheel runout check, but make sure the flywheel rivets are not loose.

Assuming it's not the flywheel, the next thing to check is the clutch. Make sure the clutch pinion bushing is not worn out (common problem), and check to make sure the clutch plates are in good shape. The older style (all fiber) plates are known for breaking apart.

Brian

[Dale]

Tim,
That certainly eliminates any issue with oil type and oil amount. Brian's points are real possibilities. Again, your issue is certainly not normal and most likely, once the culprit is identified, it will be obvious.
Dale

[[email protected]]

Thanks for your points Dale and Brian.
I took the clutch apart when I re-built the engine and went through the 'blue printing' operation as recommended on the Strictly Hodaka website. All components looked to be in good condition but if I have no joy with the flywheel I'll have another look. Regarding the rivets in the flywheel, there are no obvious loose ones but I'll have another check.
Thanks for all your thoughts and useful comments. As you say Dale, it will probably be one of those 'smack yourself on the forehead' moments when the culprit finally reveals itself. I'm trying to take a leaf out of 'Zen and the art of motorcycle maintenance', and let the bike tell me what's wrong with it, but right now she's keeping schtum.
Cheers Tim

[dcooke007]

Might want to try removing the shifter cover, engine in neutral and start engine. Just wondering if you might be able see what the flywheel is doing with engine running.

Danny

[Zyx]

A two stroke single is not the most rotationally balanced engine out ther and they all vibrate, but what is described here is significant vibration at a certain speed. Not clear if this happens only when riding or also when sitting still and going up through the revs. Harmonics causes what you describe, so I am thinking it is not vibration per se but response to vibration. When the frequency of the vibrations hit a certain point, the bike amplifies that vibration to what you feel.

I would look for a cracked frame allowing harmonics to develop strongly at a given rpm range. Unless the engine is loose, I would think it is the frame allowing the dingle to get out of control at the harmonic frequency you experience. Does it happen in neutral. Does it happen in every gear if you accelerate slowly. Is it certain that it relates to engine speed not vehicle speed. To me, if it happens in neutral and each gear, it is not for example related to the wheels, but instead, the whole bike responding to frequency input. That sounds like a really loose steering head or a cracked frame. Or it could even be a loose gas tank, but I would not really expect a fuel tank to cause oil froth. I would inspect the frame closely.

[[email protected]]

Thanks Arizona Shorty
What you say makes sense. I can confirm, the vibration is related to the revs and happens in neutral and also in every gear at the same revs. I get the vibration with clutch in or out, in gear or out of gear. It even happens at the specific rev range when slowing down under engine braking. Steering head is an interesting idea. There definitely seems to be a lot of vibration coming through the bars. I will strip the steering head and re-grease to see if this makes any difference. There is no obvious movement in the steering head at the moment and I even tried revving the bike to the vibration point with the front brake on and pushing and pulling the bike to create tension in the steering head. Didn't seem to make any difference but I'm running out of ideas now.
Thanks again guys - I think we'll all learn something when we get to the bottom of this one.
Tim

[hodakamax]

Max still thinks it to be the flywheel. That .05 mm run out still seems to be excessive although I don't know what the specs are. Here's another long shot--it could be a flywheel from another brand that has the wrong taper (unlikely). There's probably not too many #903007 flywheels in the UK to try but if you could substitute another one from a running bike could be a worthwhile test. They are all the same Ace90 through your model. The lapping I mentioned earlier would show patterns of previous damage as you go through the process. I'm still thinking!

Max

[Zyx]

Let's assume it was an out of balance condition on the crank. Like an out of balance car tire or anything else not in balance, that degree of out of balance never changes, and the vibrations would increase in severity as speed increased and centripetal acceleration exerted a greater and greater force on the part out of balance. If this were the condition, the vibration would not come and then go away at a certain speed. It would be as out of balance at 8,000 rpm as it is at 1,000 rpm but would be the more violent in sensation at the higher revs. That isn't what is happening. As described, the vibration comes on suddenly and strongly at a certain point, then goes away once revs exceed the range at which the vibration expresses itself.

The only thing I know of that can do this is a harmonic response to frequency input, and as long as the vibration input is within the harmonic range, the amplified effect is the strong buzz you feel. Once vibration frequency exceeds a point, it apparently goes away. It is not that the vibration goes away, but that the bike can no longer vibrate in tune with the input frequency, and it appears to go away. If the engine could rev higher, the buzz might even happen at more than one point in the rev range, but would do it in multiples, such as 2,000 4,000 8,000. Here, if it doesn't happen until 5,000 rpm, it would not likely rev high enough to feel the next sympathetic frequency.

So while the engine is surely starting the problem because it vibrates, the strong narrow band response comes, I believe, from the frame. I would look at the motor mounts closely, as well as the bond between motor perch and frame tubes. Pull the tank and look at the weld lines between frame down tubes and steering head. Check all of the gussets. If someone jumped the bike and landed really hard, they could have broken a weld or fractured a tube at a weld junction. After that, the broken part becomes a tuning fork that responds to narrow band input. Or it could have been repaired badly, and the repair has let go. I would pull the tank and seat, loosen the motor mounts so you can wiggle the mounting ears, and check every weld for integrity. Look for thin lines of rust, a crack is going to rust and it will show.

[[email protected]]

Guys - I can't tell you how much I appreciate you taking the time to give me the benefit of your experience. This world wide web is a wonderful thing.
I have to say that what you're explaining about harmonics AS does make a lot of sense. Unfortunately I am away for a couple of days now, but come the weekend I'll go over the whole bike with a bright torch to see if I can spot any cracks or broken weld points. I've sent the flywheel off to a guy who can dynamically balance them, but to be honest I don't expect him to find to much wrong with it, but you never know. I'll also take the steering head apart to see if there's anything amiss there. With all the attention my bike is getting I really have to pursue this to the bitter end now don't I.
Thanks again and please keep giving me new ideas - I'll keep trying them out until I have a sweet running little Hodaka. I plan to do a few long distance trials on it in the UK, and I also write for a couple of the classic bike mags, so I can see this making an interesting story at some point.
Cheers Tim

[Dale]

I think that AZ's comments on that it could be resonating at a certain harmonic is a really good point. I have experienced a tank vibration that under compression would sound like the motor was going to explode. After removing the tank and installing foam underneath, the problem was resolved. This was way less consistent than your issue, but I can say that I would not have believed how bad of a sound it could make until it was fixed.

So thinking of harmonics. Does anyone know the history behind the top motor support that was added to the model 94 Wombats? What were the symptoms being corrected with it? Were any other models equipped with the top motor support? If so, I am not aware of them. I am also thinking that if it is a case of the motor support or even a broken frame, you should be able to have someone put pressure on the motor as you rev it and it should at least change the point at which it resonates.
Dale

[hodakamax]

Ah, six brains are better than one--This is getting interesting (and fun), Anyone wanna bet?--

And Dale, good thinking, you could isolate the engine with just furniture clamps before you did that other list of stuff.

Still thinking--

Maxie

[Zyx]

I would expect that one could damp or eliminate resonance simply by grabbing the vibrating part with their hand, so putting pressure here or there would certainly be likely to reduce, eliminate, or at least point to the part that is vibrating. The most frequent problem I have had with a Hodaka is the fuel tank buzzing. It always stemmed from loose front mounts without rubber isolators or the rear mount loose because that one is usually steel to steel. The instant problem appears to me to be the same sort of thing. Some part of the bike is free to vibrate when it shouldn't, and when subjected to its harmonic counterpart, it buzzes big time. Since this one will buzz in neutral, I would try revving in neutral slowly until you find that not-so-sweet spot, then hold it there while someone goes over the bike with finger tips to find the part that is vibrating most strongly. This might help to isolate the fault. Since the rest of us don't have this issue, I have to assume we can find and fix this problem.

If your steering head is not sloppy when the front wheel is off the ground, I would not think it is the issue, so perhaps I would not strip the head first, but see if I could find the culprit first. Steering head buzz can be damped just by putting your hand around the upper or lower bearing area and squeezing it. So if it is the steering head, you can find it or eliminate it as a problem quickly without taking it apart first.

[thrownchain]

What you are most likely feeling is a harmonic vibration. All motors get them at certain rpms. 4 stroke or 2 . That's what makes the cyl fins ring, and get the buzzing in the grips and pegs. Major reason they went to rubber mount for the handlebars and motor mounts. I'd be surprised you find much wrong outside of that.