Vibration at specific rev range

[hodakamax]

Hmmm---A bit more thinking on this. All along, this foaming of the oil is not fitting into my way of thinking. It's hard for me to imagine that a vibration could be strong enough to cause this. I suppose that during a rebuild one could have missed the fracture and the bearing still could be reasonably tight if the fracture was migrating around the outside of the bearing contact surfaces. All speculation on my part now, but a simple crankcase pressure test could test this new theory.

Still thinkin' (and thirsty)

Maxie

PS--Still time to change your bet!

[Bullfrog]

Tim:

As I recall, you did inspect the engine mount brackets of the frame and found them to be OK and the engine mount bolts tight. However, I've not heard of any close inspection of the frame itself looking for cracks/breaks. (Since I have a beer coming if you find a cracked/broken frame, I'm hoping you'll do that inspection.)

Ed

[hodakamax]

My new theory of the 6204 bearing loose in case and the flywheel related problem now comes with odds. One beer will get you two if the frame is broken. I bet 10:1 it's not the rear tire pressure. There's still time to change your bet. Think about it.

Maxie

[thrownchain]

Going back to the original article, it states that the vibration comes in at a certain rev range and then goes away a short distance up the range later. If it was the flywheel as some have speculated, wouldn't the vibration start and then never stop ? Like an out of balance tire, it wouldn't go away with the increase in revs.

[hodakamax]

Good question, If the flywheel is not seated it could be both off center and tilted each with there own vibrations that re-enforce each other (and cancel each other) at specific RPMs. What I like about my new theory of the 6204 bearing loose in the case (on the clutch side) is that it supports two clues; vibration and oil frothing. Still time to change your bet!

Maxie

PS---I thought I was wrong once but I was mistaken.

[Zyx]

While I would not say that it definitely is not a cracked bearing seat, I would think that if it were cracked to the point of inducing jarring vibration, it would also not provide a positive seal against oil intrusion. So I would expect it to smoke. If not all the time, at least when it vibrates.

As far as a clutch or magneto being mis-seated, maybe, but I doubt it. If a key way was not aligned, the mag would not go on far enough to get thread contct and it would be pretty obvious. Clutch side is splined. Don't see how it could be misaligned and still held on with a nut.

Also, to create jarring vibration, the thing oscillating has to be fairly large or it would not have the mass to create a jarring vibration. A clutch spinning can create a lot of force, but only if far off balance, and we have already speculated that an out of balance clutch would vibrate all the time in an escalating manner.

I stand pat, right or wrong.

[hodakamax]

Wow, this is getting complicated. Let's think together for a moment. If the crankcase seal is compromised ,not the rubber seal, but the entire seal, by a crack between the crankcase and the gearbox I would think that when the piston comes down, pressurizing the crankcase, that the pressure would leak through the crack and also pressurize the gearbox.(blowing oil out the vent). But when the piston goes up creating a vacuum it draws in a fuel/air mix from the intake much easier than drawing it from the gearbox. It doesn't seem to me that it would be drawing much oil from the gearbox creating a smokey exhaust. This is why I'm starting to lean towards my new theory of a loose bearing in the case that also provides a crack to cause pressure in the gearbox. These are the two main clues that we have. But of course it could still be the flywheel or (shudder), the frame. All fun and I'm looking forward to a couple of beers on the house.--
Still thinkin'

Maxie

PS---and as always, still time to change your bet!

[[email protected]]

OK Guys - time for a quick update.
To answer some new posts, I replaced both main bearings when I did the engine re-build and I didn't notice any cracks around the housings. In fact the bearings were quite difficult to get out so I put the cases in the oven at 140 degrees c and then tapped them out. Replaced with new ones in the same way, putting the bearings in the freezer first, which has always worked well for me in the past. Bearings appear good.
I was getting a bit fed up waiting for the guy to balance the flywheel, so I took it back from him (temporarily) and tried carefully lapping the taper surfaces. I also removed some oil from the gearbox so that the dipstick, when fully screwed in, now shows oil halfway between the bottom of the dipstick and the mark.
Took the bike out and there was no noticeable difference in the vibration, but this time no oil coming out of the breather. Maybe I just over-filled it? I put in 550ml after the engine re-build.
Question - is the vibration on my Dirt Squirt no different to any other Dirt Squirt? I wish one of you guys could just drop in and try the bike out. I tried revving the bike whilst stationary and grabbing hold of the engine and various other parts to see if I could change or reduce the vibration. Didn't work. The only thing that was apparent to me again, was that vibration is at least as bad, if not worse, when slowing down? If I get up to about 35 in fourth and then shut off the throttle, at the particular vibration point everything shakes and rattles. Once below the vibration point it all smooth's out again.
Can't see any signs of cracking on the frame so am none the wiser. I think I'll send the flywheel back to the balancer to get it done this time, as it's the only thing I can think of doing. If this doesn't work I'm thinking I'll have to just put up with the vibration. Maybe at some point something will break and then I'll know what was wrong.
All the best
Tim

[Charlie R]

Tim,

Could you post a picture of your flywheel?

Charlie

[hodakamax]

Great minds keep gravitating back to the flywheel. Go Charlie! Are you in on the bet? I think AZs buying--

Maxie

[Bullfrog]

I'll bet I know what you are thinking Charlie. Are you looking for rivet fretting tracks? Good thought. If that is at the base of the issue, the flywheel could pass a concentricity test . . . once.

Ed
PS: I'm still standing pat on a broken frame.

[hodakamax]

Oops---I forgot about Ed, let me rephrase that. Get in on the bet Charlie, I think AZ and Ed are buying!

Again--

Guess, Hey it's April Fool's Day!

PS--and seriously, Ed you could be right on the concentric test, the engine was not running when the test was done. It could be coming back to being concentric when not running. Just thinkin' once more. Hmm--that might mean the flywheel IS out of balance but seated. Tim, back to your flywheel person, you might have been right in the beginning.

[BrianZ]

Any chance a broken ignition coil mount could cause the vibration? I have seen several frames with broken coil mounting brackets, and I am thinking that if one side is broken, the coil could possibly resonate.

In the off chance that I am right, I like a nice cold apple cider, in the tall cans.

Brian

[hodakamax]

Brian, that falls into the broken frame theory which has 2:1 odds. If true you would receive two Cold Apple Ciders in tall cans!

Maxie

[[email protected]]

Hey Guys
I was going to insert a picture of the flywheel but I can't work out how to do this? Any pointers? It looks OK to me with no damage to rivets which all look and feel tight.
Interesting. When I did a runout measurement of the outside of the flywheel, I got 0.07mm this time. Remember that the crank is almost perfect at 0.005mm. I'm guessing the outside of the flywheel isn't going to be a perfect surface, but that seems quite a lot of runout to me. What do you think?
Could it be that the taper was made off-centre? By lapping it I may have made the problem worse?
All the best
Tim

[[email protected]]

Me again.
Just found out something else.
I measured runout on the outside of the flywheel - this time I got around 0.1m? I then took the flywheel off, removed the key and put the flywheel back on again with the key slot on the opposite side to where it should be. Runout this time was only 0.03mm. So, the flywheel isn't running true on the crank. My brain is beginning to fade. Can this be corrected? Do I need a new flywheel? Just for my own peace of mind I re-measured runout on the crankshaft itself and got 0.01 on the polished surface and, by carefully going backwards and forwards so as to avoid the key slot, I measured around 0.02mm on the tapered part of the crankshaft. So this tells me that the crank itself is pretty good but I have a dodgy flywheel. Am I right. Does anyone have a spare flywheel for my Dirt Squirt?
All the best
Tim

[Bullfrog]

My knee jerk reaction is that the key is standing too tall.

The woodruff key serves one purpose and ONE purpose only -- which is to assure that the flywheel goes back on the crank in the proper rotational position. It has NO role in preventing rotation of the flywheel relative to the crank during normal operation.

If the woodruff key stands too tall . . . it will not allow the flywheel to properly seat on the taper . . . which will result in a wobble/eccentricity.

You might do another concentricity check of the flywheel on the crank with the woodruff key slots aligned reasonably well by eye (no key in place).

Um, I still haven't heard about an inch by inch examination of the frame for cracks/breaks. Can you help me out here? I've got a beer riding on the concept that you will find one.

Finally, are there dark, rust colored, radial lines emanating from the edge of the flywheel hub and going out toward the OD of the flywheel?

Ed

[hodakamax]

OK, I hate to say flywheel one more time. I had a case in the shop many years ago where the flywheel had come loose. A new washer and re-torquing did not cure the problem. It continued to come loose after a short period of time. I don't remember if damage had occurred to the mating surfaces or the flywheel tapered collar had cracked but it wouldn't stay tight. Cranks are tough and it's always the flywheel that suffers it seems. I've already addressed the key being too high early on and trying another flywheel. OK, I've said flywheel five more times, Need I say more?

Max

[Charlie R]

I started with suspecting the flywheel and to this point I've read nothing to change my suspicions. I'll buy in on the bet and stay the course with the flywheel theory until this is resolved or more info becomes available. I sure would like to see a picture of this flywheel tho....for a few reasons. And I drink unsweet iced tea only these days....

If it comes to it, I would send a known good flywheel and key Tim. I would ask that you cover the postage cost.

Max, I sent you an email and a PM. If you have any interest, send me your off SH contact info. Otherwise, it goes to auction.

This has been an interesting topic guys!

[[email protected]]

Hello Ed/Max
Sorry, should have made it clearer on my last post. I measured runout on the outside of the flywheel twice, both times without the key being in place. The first time with the key slot lined up (by eye) with where it should be. This gave 0.1mm runout. I then moved the flywheel around to where the key slot was opposite to where it should be and this gave me around 0.03mm runout. In fact I found one spot where the runout was only 0.02mm. Anyway, each time I tightened the flywheel bolt, so the flywheel is definitely not running straight on the crank when the key is in place, but how do I correct this? It makes me think that if I could take about 0.04mm off the runout on the high side then the flywheel would run fairly true. Can this be done? I don't want to make it worse.
Looking back at my previous posts I see that I measured the runout on the outside of the flywheel at about 0.07mm before I lapped the taper surfaces. I think I just made things worse. When I did the lapping I tried to keep gentle and even pressure on the flywheel while my son turned the back wheel back and forth in gear. Each time the crank made about one turn each way. If I find where the low spot is on the flywheel and mark this, would it work to do the same thing but just to put a little gentle pressure on the flywheel at the low spot? What do you think?
One more thing. If anyone has the capability to measure runout on the outside of their flywheel I'd be really interested to find out what the measurement is?
All the best and thanks again for your consideration.
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

Repeat, might be able to see run out with engine running and the flywheel under load.

Danny

[[email protected]]

Hello Guys
To answer a few more questions.
I have been over the frame with a powerful torch and can't see anything amiss. I tested the engine mounts when the engine was out of the frame and they seemed solid enough. I even changed the engine mounting bolts but it made no difference.
Regarding running the engine with the side cover off, I did this but it was difficult to see any movement in the flywheel. As soon as the revs rose the flywheel appeared to be stable?
As regards the loan of a known good flywheel (thanks Charlie) that sounds like a really good idea. There is a guy in the UK who has a few Hodaka's (some of you might know Dennis Price) and I'm trying to track him down to see if he has a spare I can borrow to try, which would be quicker. If I don't have any joy then I'll come back to you Charlie. My email address is [email protected] so maybe you want to send me an email off-line so we can swap addresses and I can organise paying for shipping.
Finally, what do you guys think of my idea of lapping the flywheel whilst keeping gentle pressure on the outside of the flywheel at the low spot. Would this correct the runout and give me a true running flywheel do you think? On the picture attachment you can just see a faint black mark at 3.00 o'clock on the flywheel. My plan would be to rotate the back wheel and keep gentle pressure on this mark, which represents the low spot on the runout of the circumference of the flywheel. Let me know your thoughts.
All the best
Tim

[dcooke007]

"A picture is worth a thousand words". Looks like the wrong flywheel. The model 94 and 95 all use the same flywheel. The one you have looks like a 100 cc. This is what your flywheel should look like.

Danny

[[email protected]]

Hey Guys - I think we may be homing in on the problem.
Just to be clear Danny, my bike is model 96 Dirt Squirt with chrome tank. Made in 1973. Is mine the correct flywheel? I assume so.
OK, with the key removed (doesn't seem to make any difference to runout on the flywheel if the key is in place or not), I positioned the flywheel with the key slot in the correct place on the crank - just by eye. Tightened the bolt and took the following measurements at the positions shown on the picture.

Position 1 - 0.14mm
Position 2 - 0.07mm
Position 3 - 0.03mm

I then moved the crank around by 180 degrees so the key slot was opposite where it should be and got the following measurements.

Position 1 - 0.02mm
Position 2 - 0.015mm
Position 3 -0.005mm

So, my flywheel is clearly not running true but how do I correct this? I don't know whether it came from the factory like this, but you can see that the variance in runout would, in my opinion, be enough to cause vibration at specific revs, being that it's tilted on the crank. The tapers on crank and flywheel look to be in quite good condition, especially now that I've lapped them, but in doing so I fear I've made the problem worse. Because the runout is different when I rotate the flywheel it makes me think that both crank taper and flywheel taper are wrong/damaged. Short of trying to source new crank and flywheel can anyone think of a way to correct this? Please help - beginning to think I've bought a dud engine here, and on top of that spent a load of money on new bearings, re-bore, big end kit etc.
Look forward to hearing from you all.
Tim

[dcooke007]

Hey Tim, yes you have the correct flywheel. Some how in all theses posts I thought you had a mdl 94 Hodaka. Sorry.

I had to convert metric to inches to have a frame of reference. You largest total run out .14mm \ .0055 inches. Perfection would be nice but I am not sure that small amount of run out is cause for concern if the source of the run out is the flywheel and not the crank. Lets see what others have to say.

Danny