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Old 03-18-2005, 05:07 PM   #16
GODSPEED
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Quote:
Originally Posted by inte
Lots of discussion over the LC4 vibes issue & a myriad of suggestions dealing with bar weight/filling fixes, but I remember somewhere seeing a post that indicated most of the symptom can be tuned out of the motor.

My 640 is running fine, but I noticed in the past few weeks it seems to be running a bit rougher than normal. Put a new plug in but no effect. Valves were checked less than 1k miles ago (one was a bit loose ).

It's well under 10k miles, but it was suggested a full valve job might be in order ... seems a bit early to me for that but I dunno...

I'm guessing a timing adjustment would do it, but I know there's got to be a thread someplace to the effect of "tuning your LC4".

Creeper or the "pre-9fiddy" Flanny must have posted something
Inte, Here is a vibration thread that deals with issues outside the engine.

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Old 03-18-2005, 06:56 PM   #17
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Quote:
Originally Posted by GODSPEED
Inte, Here is a vibration thread that deals with issues outside the engine.

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thank you , you quick-god-like-ghostly-helpfull sucker
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Old 03-18-2005, 08:31 PM   #18
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Originally Posted by Loadedagain
zoiks... i hope you get the engine manual allright... its 18meg.

if you don't get it let me know and i'll post it on ktmcanada.com
Sent you an email ... only have the "spare parts" manual ... if the other is downloadable ... super!
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Old 03-18-2005, 09:09 PM   #19
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My observation. Something to substantiate the jetting affect on vibrations. My used 1999 bike was originally a bit vibey, with everything stock. By this, I mean that my hands got numb and the mirrors were nearly useless. Then I did a bunch of things; fasstco bar inserts, main bearing, properly torqueing motor mounts, BST40 rejetting. Vibes gone, and I had thought it was the "big" things which worked.

A few months ago I installed an HSR42 carb. Couldn't find much jetting info so for over two months I basically tried every-single-freaking-configuration (pilots, needles, clips, mains, a/p nozzles, snorkel) until I got it dialed in. OMG you wouldn't believe how much jetting affected the vibes. Just a bit off, and it became a paint shaker! Had me worried for a while.

Anyways, once you get it tuned to the sweet spot it is very nice. I'm at the least vibes ever, though I still find myself making a compromise between smoothness and "rawness". The rawness giving a bit more grunt at the expense of slightly more vibes.

The vibes I'm talking about are..

a) What is felt in the handlebars (mirrors too), particularly at slower speeds.

b) Along the sides of the tank and the low spot in the seat, particularly at higher speeds.

Assuming you are jetted for best power, you might reduce (a) by leaning the pilot system, and (b) by richening your main. Interestingly, I have found that adding restriction (fewer discs) to the exhaust further reduces vibes on my bike. I run 3-4 discs on my ids2 (let's say 6-8 on stock). Feels good. Was originally running 8 on the ids2.
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Old 03-18-2005, 10:11 PM   #20
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timing?

Any "how-to" guides around on adjusting LC4 timing?
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Old 03-19-2005, 06:19 AM   #21
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Quote:
Originally Posted by markjenn
But the thing that bothers me a bit is that I don't see any technical basis for why jetting should affect vibration. As I understand it, vibration in a motor like the LC4 is 98% due simply to the masses being flung around in the motor. Whether the combustion occurs in at 13.5 a/f ratio or a 16.2 a/f ratio shouldn't make one iota of difference.
Again, I know many folks feel a difference with tuning changes and their opinions are certainly valid. I just think one needs to keep their expectations in check. These are paint shakers of motors and if someone is having trouble adjusting to it, I think that after the obvious things are done to try and help, the next best step is often to get a different bike.
- Mark
Of course air/fuel ratios can change the way an engine feels and its vibes. Turning the air screw alone smoothes the shakes at idle, just as the clip position changes the mid throttle chugging/chain snatch. When both are adjusted right, it WILL smooth things out quite a bit. The same goes for switching carbs. Going to a different carb will also change the resonant frequencies of the air flow (and the vibes that go with it)
Have you ever tried a bike/car (or anything else) of the same type/engine where one is carburated and one is EFI ?
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Old 03-19-2005, 08:10 AM   #22
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Sure, I've tuned lots of older cars before with mis-adjusted carbs and the like and they smoothed out, especially at idle. But we're talking V8s, V6s, and I4s, not singles. Crisp carburation is critical on a multi-cyl and mis-fire at idle will make an engine shake, but the type of vibration we're talking about here, is larger throttle openings on a single at higher engine speeds.

I fly aircraft with piston engines with manual mixture controls. Not particularly smooth either - big Lycoming and Continental flat sixes, both carb'ed and FI'ed. Except at idle, I don't notice any change in vibration with adjustments in mxiture.

Sorry, I'm still skeptical.

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Old 03-19-2005, 08:27 AM   #23
dagwood
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Vibration is a real non isssue. If you are bothered by vibes get a heavily counterweighted thumper like a DR650 or KLR. of course the counterweight robs power and slows throttle response but that's the price you pay for being thenthitive....
there are things(that have been beat to death) to smooth some of the roughness out but thats all just fine tuning that needs to done ANYWAY.
jeez.
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Old 03-19-2005, 10:04 AM   #24
inte OP
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Just found Creeper's valve adjustment guide over in "wisdom" - cool stuff. Same procedure on the 620 ... guess an LC4 is an LC4.

The guide specifies a .006 gauge & the shop here said .005 (same gauge as I had for the 620) ... just adjust for loose drag & call it good I suppose.
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Old 03-19-2005, 10:32 AM   #25
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Quote:
Originally Posted by inte
Just found Creeper's valve adjustment guide over in "wisdom" - cool stuff. Same procedure on the 620 ... guess an LC4 is an LC4.

The guide specifies a .006 gauge & the shop here said .005 (same gauge as I had for the 620) ... just adjust for loose drag & call it good I suppose.
Too bad vibration perception isn't as easy to come to terms with as feeler gauge feel... now where is that scotch tape?
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Old 03-19-2005, 10:39 AM   #26
inte OP
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Quote:
Originally Posted by creeper
Too bad vibration perception isn't as easy to come to terms with as feeler gauge feel... now where is that scotch tape?
Fortunately, I've made peace with the bike long ago ...

Only reason I posted the thread is becuase it started running a bit more rough than normal ... reminded me of my '70 Plymouth Duster when it needed a bit of a tune. Checked the enging mount bolts last night & they were pretty loose - that may have had a LOT to do with it...
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Old 03-19-2005, 10:51 AM   #27
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Quote:
Originally Posted by inte
Fortunately, I've made peace with the bike long ago ...

Only reason I posted the thread is becuase it started running a bit more rough than normal ... reminded me of my '70 Plymouth Duster when it needed a bit of a tune. Checked the enging mount bolts last night & they were pretty loose - that may have had a LOT to do with it...
Doesn't take much... had a couple little ol' 6mm mid-mount exhaust bolts disappear last year... I knew something had gone missing as the bike felt "unrefined" , took a few minutes to find that the bracket was no longer attached to the subframe. Now there is yellow paint on the heads at 12 o'clock, so I just give 'em a quick look now and then.
It's a handy thing actually... vibration change?... start looking for threaded holes with no fasteners in 'em and what still do have fasteners, give 'em a snuggie.
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Old 03-30-2005, 09:50 AM   #28
potatoho
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Quote:
Originally Posted by markjenn
But the thing that bothers me a bit is that I don't see any technical basis for why jetting should affect vibration. As I understand it, vibration in a motor like the LC4 is 98% due simply to the masses being flung around in the motor. Whether the combustion occurs in at 13.5 a/f ratio or a 16.2 a/f ratio shouldn't make one iota of difference.

As someone else mentioned there is the issue of the "character" of the vibration rather than just the amplitude - maybe this is the thing that changes, whatever the heck it is. And maybe ignition timing affects this.
Another data point regarding jetting/vibes. I just installed a yost power tube in my hsr42 and it further reduced the vibes. Not to be overly dramatic, but the entire rpm range is buttery smooth. The device helps to atomize fuel coming out of the main jet.

I suspect that my remaining vibes were due to inconsistent globs of fuel. This power tube helped a lot in this particular application, likely because the 42mm needs more air velocity than my motor delivers.
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Old 03-30-2005, 09:54 AM   #29
Surly
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Quote:
Originally Posted by potatoho
Another data point regarding jetting/vibes. I just installed a yost power tube in my hsr42 and it further reduced the vibes. Not to be overly dramatic, but the entire rpm range is buttery smooth. The device helps to atomize fuel coming out of the main jet.

I suspect that my remaining vibes were due to inconsistent globs of fuel. This power tube helped a lot in this particular application, likely because the 42mm needs more air velocity than my motor delivers.
what this yosty thing be? splain to me please
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Old 03-30-2005, 10:03 AM   #30
ChrisC
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Quote:
Originally Posted by Surly
what this yosty thing be? splain to me please
From a Hardley-Ableson site:


MIKUNI HSR 42 & 45 CARURETORS
Just like the other Yost Power Tubes, the Yost Pro Power Tube for Mikuni, by design, produces a very fine mist. This very fine mist allows the fuel to stay in suspension longer for better control of air and fuel mixtures and can be almost perfectly matched for any throttle position. You can expect an increase of up to five H.P., improved mid range performance, and an incredible throttle response that will be sure to impress. The Yost Pro Power Tube Kit comes with the Yost Pro Power Tube, a selection of five Main Jets, rubber washer and the Yost Float Bowl Drain Nut. The main jets in this kit are interchangeable with stock H-D CV main jets and can be purchased at any OEM or any aftermarket dealer. However, they are not interchangeable with Mikuni jets, the jet threads are different, even though the actual size of the jets are the same, IE; if you have a 175 Mikuni main jet installed originally then use the supplied 175 main jet in this kit. Carburetor Shown for reference only.
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