out of balance crank

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has anyone ever run a crankshaft that got its flywheel turned around?

#2...an Atlas crank in a commando?

I am working on a 72 and despite everything it still has strong vibration up to 3000, by 4500 it finally begins to smooth out. I am using a single Mikuni from another 750 so I know the carburetion is correct. The plugs blacken fairly quickly, my theory is that the vibration upsets the carburetion which causes more vibration.
 
Re;” I am using a single Mikuni from another 750 so I know the carburetion is correct. The plugs blacken fairly quickly; my theory is that the vibration upsets the carburetion which causes more vibration.”
Something not right here, have you got the right pilot jet for a single carb?
What type of head steady have you got?
Have you rubber doughnuts become worn ?
 
I'm surprised that you didn't get the rotating and reciprocating parts balanced when you had them on the bench...

Anyway, your Atlas crank, as part of the rotating/reciprocating team, would have been set up at the factory with a higher balance factor than the Commando crank you swapped out, so the Atlas crank may not be out of balance, so to speak, but may be incorrectly balanced for the Commando application.

Mikuni carbs can tolerate a lot of shaking, so if the carb sat around for some time it may need a good cleaning and an idle jet replacement. As Seattle##gs asked, and I'll expand it a bit: what are the calibrated parts in the Mikuni, size?

Main jet, idle jet, slide, needle jet, needle, air correction jet???
 
What is the general though / feeling regarding balancing the crank rods & pistons?
Do folks actually feel a difference with regard to the "vibes"???.... is it worthwhile

Malc
 
Yes the difference is very noticeable. I had the whole assembly balanced last time in and the difference was stunning. The machinist said the oem balance was off quite a bit. Apparently the factory balanced the crank for up and down motion but not so much side to side.
'Pa' my trusty Norton is smooth as silk. (well OK not that smooth) and a joy to ride.
Ride On
Dave
 
Hi , all my Atlas crank had been dynamically balanced (most of them by basset Down) and my Seeley crank too (but JS parts , so comparison is unfair!!), and they are sweet as never..!
But My cdo ones were std , and no vibes too ........(-unfair too !)
 
this bike was brought to me a year ago to install a CNW electric start which is a very well engineered and very effective product. And very good after sales service, too.
It is a 72 and supposedly "all rebuilt" about 10 years ago and "bored to 850". Turns out the old .030 pistons were used with new rings and a so-so job of honing. And old dual Amals. In short nothing was right, everything is suspect. This was the first time it ran in ???years. I couldn't believe the vibration level on its first, very short test ride. rechecked the timing and went after the amals. Balancing the pair under these conditions is hard to do. So, to eliminate variables, I installed the 34 Mikuni from my own Norton using the Sudco/MAP jetting and my own which is a 35 pilot....6F9 needle.....2.5 slide.....P-6 needle jet.....230 main. No change in vibration. However it has power like a Norton should. If it wasn't for the excessive vibration and fouling plugs quickly it would be running great.
I removed the cylinders to look for any problems. None that I could see. I had they cylinders honed and fit new rings. I took the head to the local machinist to have it thoroughly checked out and new valve stem seals installed. He said it should be a very good head and ready to install. So I did. I decided to check the valve timing and discovered two things, 1) the chain was pretty well whipped so I replaced it with the JWIS timing chain from Comstock which is a very nice product. It is quite noticeably beefier than the stock one and reduced the amount of adjustment and the slack-tight-slack-tight business that goes on with an old chain. $40 well spent.
2nd) I discovered BY ACCIDENT that the oil pump had sucked up something and was badly damaged..chips out of the gear teeth. Luckily I had 4 used ones to replace it with.
Now, after all this I am only slightly ahead of where I started. My suspicions are that the crankshaft is the culprit.
 
Seems like you have little choice now, gonna have to pull the crank and have it checked for assembly and balanced just to put your mind at rest.
Make sure your engine isolastics have some play and put some new rubbers in the head steady (or replace that whole business with something that actually works) for the test ride.
Good luck, and please report back!
 
Also check there is nothing on the engine touching the frame, the rear of the primary case can be very close and if touching will transmit vibrations at all revs.
 
UPDATE...PROBLEM SOLVED....
After wrestling with this problem for months I discovered the real problem. The mufflers, which looked like any other peashooters, had a different sound, so as a last resort I switched mufflers with a known good pair and the intense vibration problem went away.
 
Saved an engine teardown!

I'm thinking that's the FIRST TIME I've heard of MUFFLERS fixing a vibration problem.
 
Me too. That's why it took me months to find the solution. Keep this in mind when everything else doesn't work.
 
Mass of mufflers can not influence vibration isolation threshold - unless too much strain in clamping to header attachments so transmit vibes into frame and shortening header life by cracking at sharpest bend. Glad you got vibes tamed but your logic on why does not compute - so nothing learned anyone else or you can use or make sense of. I think ya accidentally-ignorantly corrected some hidden tensioned contact between frame and power unit on changing muffler mountings. Carb tune will not hinder isolation above 2000 so only way to know ya forget to put a plug lead back on will only be felt as lack of zoom power not vibes over 2000ish. Shade tree fudge is grinding bevels in front cushion to soften as much as original designers intended, to may be able to flow with sane rpm traffic speeds w/o jarring hands and fasteners. Worst case I tested had 1/4" gap at front isolastic and unbalanced crank so isolated at 3500ish 55 mph/4th, which was about 10 mph faster than traffic in twisites so very annoying-scary to suffer vibes or go too fast for survival odds. Just more time/money to correct properly from crank/piston dynamic balancing outward. Sagging strained head steady binding can add rpms to reach isolation lift off threshold.
 
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All I can say is that I tried EVERYTHING to stop the vibration and nothing worked until I changed the mufflers which solved things by 95%. I did everything one at a time. My conclusion is the mufflers were the culprit. They will never get a second chance.
By the way, I just received the replacement mufflers today from Commando Specialties and they really look good. Seamless. good chrome. The sheet metal is a good thickness, not some ultra thin stuff that flexes under finger pressure. All for $202 which includes shipping. Will let you know how they perform.
 
Hobot is not disagreeing with the fact that when you changed your silencers, you solved the problem.

What he is disagreeing with, is that the silencers themselves were the root cause. He is doing this as there is no clear hypothesis to the fact.

He is suggesting that you accidentally and unknowingly addressed a different root cause, something touching something, something under stress or tension, etc.

I’m glad you have got rid of your vibration, but I do agree with Hobot in that it’s a mistake to assume you’ve correctly identified the root cause. I can’t think of any logical hypothesis whereby your silencers were really the root cause of your vibration issue.

To be fair, it’s really very common in problem solving to do something, elevate the symptoms, and thus assume that it was the root cause. But it IS only an assumption. I see this all the time, even in big companies, with highly paid engineers!

As always, I’d be happy to be proved wrong, as any proof, whether it supports or rejects ones hypothesis, is good learning.

Having said all of that, I’m kinda guessing that you might be thinking ‘I don’t care what the root cause was cos the problem has gone away!’
 
Check that the header on the LHS is not touching the stub for the sidestand, as this can cause vibration especially when the bike is leant hard into corners .
There will be witness marks on the header.
It may not touch at rest and on the centre stand, so may not show up until the bike is running and being used.
Regards Mike
 
Hehe when I'm checking this and that on patients seeming to ignore the area they came in for, some ask what I'm doing, I say trying to find whats preventing you from recovering so I know what I'm doing, to have some say, well I don't care Doc even if ya fix me by accident, ugh. Some L shoulder up back pains are heart related not over strained or local issue at all. Send em to cardiologist for new valve or bypass and thank me latter for alerting them. On other hand treating the heart condition alternatively can also resolve in house so to speak.

Glad lighter muffler smoothed the ride. Definitely a rare un-thought of vibe factor alert we all should remember to check. Mystery is why. Maybe heavier one bouncing on mounts was being felt, or maybe something wasn't as tight as should be till replaced w/o noticing what was corrected. I've have both muffler gut fracture attachments to bounce/rattle freely about an inch around for clanging noises even my buddy Wes said annoyed him when close at stops, yet did not feel them rattle only heard and seen bouncing looking into the puff ball exits.

Commandos are likely most mysterious simply complex cycle ever sold and consistently surprise us here on what is eventually found after opening up - after pages of listing causes that match symptoms. I'm seasoned enough to know we don't always find anything wrong yet problem solved on assembly, so didn't learn a thing anyone can use either.

BTW absolute Worse Vibe I've had hit suddenly felt-sounded like head going up/dn with pistons, turned out to be a tiny pebble trapped in iso mount crevice by the front skid-splash plate few ever mount.

PS seasoned enough to know while fixing one thing, its not uncommon to screw up something else w/o knowing right away, ugh.
 
MORE INFO...The new mufflers arrived from Commando Specialties and I am very happy with them all around. I would happily buy another pair.
I looked down the gullet of the problem mufflers and discovered that some moron in some factory somewhere decided to incorporate a baffle plate in the muffs, leaving a small hole roughly half the size of the final exit hole and offset to one side. Without further investigation I will conclude that this boneheaded idea was the culprit. Anybody else have peashooters with a baffle plate installed? I shall cut these in half and toss them into the garbage.
 
My 1st Combat had that sort of baffle but had mostly rusted through by my time. The whistler type were famous for performance and let off sounds and dedicated souls have investigated the type of baffle flutes shapes to find the best sounding and maybe performing were the crescent moons, not the v or square sort. When my 1st and other silencer baffles broke loose inside it was one end first which dropped the baffle cross wise to straight through flow but did not affect low isolation threshold. After both ends broke free the baffles settle for mostly straight through flow path w/o any notice on vibes, just rattle noise I temporary solved rebel red neck way. Assume any factoid needed but fact its still a mystery.

Send your crappy peashooters to hobot as sooner than later will be doing this again on Trixie Combat.

 
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