400/4 vibration

[Honsa]

Hi all, wonder if you could help me out with a problem that has surfaced on a 400/4 that has recently been returned to road worthiness. The bike had lain dismantled in the seller's garage for several years and been bestowed with umpteen faults by the previous owner(s). The bike now ticks over perfectly, responds to wrist action well, and gearbox and clutch action are good, BUT the engine vibrates almost continuously from 1000 to 8000RPM, not like a British vertical twin but certainly more than my other 400/4 which has similar vibration but only around 5000RPM. My suspects are the alternator rotor, clutch basket, and crank shaft. I would welcome thoughts and suggestions from forum members, i'm hoping for an easy fix but things rarely work out that way in my experience. Engine bolts are tight BTW. 
               Graham

[McCabe-Thiele (Ted)]

That sounds a hard one to diagnose without knowing what you have done to the engine during your build and experiencing the vibration first hand.
As a relative novice I would look for the easiest causes first.
Is the exhuast system catching on the frame anywhere when under load / accelerating it's amazing what effect that can have with a slight contact.
Are the 4 carbs properly synchronised.
As you have mentioned is the alternator tight & not fowling.

Then being a gloom & doom man were the conrods of the same weight and are the crank main bearings all as they should be.

Hope it turns out to be something easy - others here that know more about these bikes will know more.

[K2-K6]

Agree with Ted in that it's a difficult thing to give accurate diagnosis without experiencing it first hand.

Some initial thoughts;- geabox, clutch etc are significantly geared down from crankshaft speed and would seem unlikely as primary suspect.

Alternator rotor, security of etc worthwhile considering, checking to make sure nothing obviously amiss.

Rod, piston, weight matching would certainly be on the list if you can't find anything immediately that is causing it, both of which raise the question of your confidence in components, build etc.

Presumably you've looked at plugs to see if you've a comparative combustion difference between the cylinders  ?

Also,  is it vibration while driven along or run up through revs while static  ?

[Honsa]

Ted      Thanks for your input. I serviced the cylinder head but did'nt go lower than that as all looked good and it does'nt use oil, the engine has been split by someone at sometime but i don't know why. Conrods fitted to Brit twins seem to vary in weight from engine to engine but i had'nt thought about Honda rods needing to be matched in a batch of four, but why not, is the question. The engine idles happily at 1200RPM all day long so carb sync might not be 100% but possibly not far off.

K2-K6      Thanks for your input. Once i get a puller i'll pull the rotor and since i have a spare i can easily do a comparative test. Two spark plugs look as though the mixture is rich and two are ok so there maybe some work i can do in that area. Proper diagnosis of combustion by reading the plugs seems to be a black art from what i've read, but if a bit of tweaking changes the vibes for better or worse it's worth having a go. I can't answer your question about static versus in motion vibe levels so again that is something i can do. Would pistons from different batches have different weights, not sure about that one.

Although the 'gloom and doom' devil on my right shoulder thinks the crankshaft has been dropped and bent the fact vibration does disappear briefly at certain revs probably/hopefully suggests it is not the cause of the vibes. I was hoping to avoid a full engine strip but it's hanging in the balance at the mo.
Thanks for your replies guys and for your time and thoughts.
          Graham

[Trigger]

Everything need to be balanced and weighed. Pistons need to be within 5 grams of each other. Con rod weight is on the rod and all must match.
I have come across many bent cranks in my time and the crank needs to be balanced to the fly wheel or vice-versa. I have come across crank to fly wheel  problems many times on 750's as, the owner has cleaned the wheel up of rust and unbalanced the weight. Also changing the wheel will not match the balance on the crank anymore 

If there are drilled holes in the crank, then the crank has been balanced to wheel or drilled holes in the wheel means the wheel has been balanced to the crank.   

[Honsa]

Thanks Trigger, plenty of 'food for thought' there.
         G