How smooth should a 2MA / 1KT engine run?

[tzar250]

Hi

Coming up to 100 miles since a full engine rebuild including crank and rebored barrels, all new seals, etc.

She's starting 1st kick and generally running okay but there is a really intrusive and unpleasant vibration that starts at 4000 and continues to 5000 rpm.  I've not gone about 6500rpm yet but it seems to be less noticeable between 5000 and 6500rpm but not as smooth as it was before the rebuild.  It seems to occur whatever gear and even on closed throttle.  I've checked all mountings and it has had new mounting rubber bushes fitted too.  Carbs have been balanced to within an inch of their lives.

There was a bit of vibration at 4000 - 4250rpm before the rebuild but nowhere near as bad as it is now and it was over a narrow band.

Now I'm not expecting turbine smoothness from a parallel twin but it sounds like the engine is struggling in this vibration zone and its making it quite unpleasant to ride.  With a 40mph max speed limit and traffic here, its hard to avoid this rev range.

So, I'm wondering if this is a side effect of larger size pistons (0.75mm over) or should i strip it down to check further.

Any similar experiences or suggestions welcome!

[Mick]

unpleasant vibration that starts at 4000 and continues to 5000 rpm.it sounds like the engine is struggling in this vibration zone

Any similar experiences or suggestions welcome!

It could be nothing more than a flat spot you are experiencing..Is the carburetion stock,230mains?..
I once put 240s in and had a flat spot around that rev range,lowering the needles a notch or 2 sorted it out..

[teezer250]

I have had vibration noises on both my bikes at times.
It can be the fairings vibrating,fairings rubbing on the exhausts,radiator grille,even the gear lever vibrating due to the wave washer flattened out or missing,the left side 10mm damper mounting bolt that screws into the lower engine case underneath may be loose or come out(mine came out,loctited back in) completely.
Quite a few other parts are either rubber mounted or have foam padding and any of these missing could also cause the vibrations.

[Martin77]

The fact it does it even on a closed throttle suggests it's not a flat spot, but purely a mechanical one. I have no suggestions really.. maybe it is because of the heavier pistons accentuating the rocking couple.

[semaj]

This may help, slacken off ALL the engine mounting bolts inc. the damper bar fixings. Retighten the main engine mountings then the damper bar fixings. The two fixing points that have elongated holes should be tightened last so everything has pulled into its correct position. Remember one long bolt with a nut on holds the front side stand fixing so make sure the bolt is fully tight BEFORE tightening the nut The front support frame top bolts must be tightened before any others as there is no leeway in these fixings. If the damper bar is under any tension as it is tightened its not correct. When all is tight slacken and retighten the exhaust mounts so the exhaust will self align. Hope this makes sense! Also recheck the chain tension when you are sitting on the bike.

[tzar250]

Many thanks for all the suggestions..

It is running totally standard jetting but potentially some of the carbs parts are from mismatched pairs, so it may be worth swapping a few bits for new in case there is uneven wear on needles/jets.

I'm inclined to think it is mechanical and maybe down to the heavier pistons but I will try your suggestion first, Semaj before I start throwing anymore time and money at it.

In the meantime, if anyone is running oversize pistons and can comment on whether they are or are not also experiencing similar vibration, I would be grateful to hear as a comparison.

[teezer250]

One of my bikes has .25mm oversize pistons and has had some of the problems I mentioned in my earlier post.
I have checked and eliminated all the possibilities I have come across on both bikes but have not noticed it that much recently.

The vibration is more of sound rather than feeling through the bars or footrests.

[tzar250]

Well I have followed Semaj's suggestion (thank you) and in the process discovered that the front mounting bolt for the bar that the damper rod fixes to was only finger tight.  Not sure if I forgot to fully tighten it when assembling (entirely possible) or if it had vibrated loose.  Anyway, I've put some thread lock on it now.

I was hoping this would make a dramatic difference and indeed, the vibration does now seem reduced but still present and still enough to be annoying and definitely worse than it was before the engine rebuild with 0.75mm oversize pistons.  Vibes through the bars and footrests from 4000 to 5000 rpm.

I think I will carry on as it is for now to continue with the running in process and check the mounting bolts again in a few hundred miles.

If the vibration is still present - I have the original set of standard bore barrels which I hope are within tolerance enough to refit with new pistons after a deglaze and see if it resolves the problem.

[teezer250]

On a 2ma I owned from 1994-1997 I had to rebore the barrels and ended up with 0.75mm pistons and that did not have the problems you are having regarding vibration.
Have you put the 2 small engine dampers(they look like washers with a bit of rubber on) on the rear right and front left engine mounting bolts between the engine and frame?

[tzar250]

Yup - fitted those damper washers in the correct location - new - as were the mounting bushes.  Although oddly, when I dropped the engine out of the frame for the rebuild, I discovered that a previous "owner" had fitted both the damper washers to the rear engine mount only - one either side.

[Mick]

Last night I changed the head gasket on my Track 2ma(use it on the road too,daytime MOT).It's the 1st time I've touched engine in 5 years of owning it,pistons(both) are stamped 2.00..Ive never come across the vibrations you are encountering.

[teezer250]

  Carbs have been balanced to within an inch of their lives.

With the engine switched off,have you opened up the throttle and checked that the small dots on the slides are visible and aligned at the same time through the small windows on the carbs?

[evan_calgary]

Also have not seen any vibrations in the last 3 I built.  All quite smooth.  Also coming from RD350 YPVS so that may have somehting to do with it...

[tzar250]

Thanks for all the advise, suggestions and feedback on your own engines.  Useful to know that other oversize engines do not appear to have the same problem.

After a bit more running in, I've felt happier to rev it a bit higher and with that, the vibration seems to have lessened somewhat.  Still something there but more of a buzz now.  Although finding the not fully tightened bolt on the torsion bar arm has probably made the most difference.  In time, I will probably replace the torsion arms rubber bushes for new.

Unfortunately, one of the windows on the carbs is too fogged to see the white dot on the slide, so I usually use the "lollipop" method (albeit with plastic glue spreaders instead) to balance the carbs.

However, I decided I would give the carb balance one more check this evening, so stripped the tank and airbox off again.  This time, I went by checking the bottom of the slide against the top of the carb opening with the throttle nearly fully open.  It took about quarter of a turn on one adjuster to bring them both to exactly the same level and I can't imagine getting them any closer balanced than this.  So, I'll give it another ride tomorrow and see if there's a difference.

Annoyingly with the tank off, I noticed that the coolant level in the expansion tank has continued to drop (I posted previously about this) and I can't imagine it's still trapped air escaping any more.  I'll see where the level is after a couple more rides, but if it keeps dropping, it looks like engine out again.  There are no external sign of leaks.

[teezer250]

Just a thought.
When you rebuilt the engine,did you put the 2 "o" rings on the lower casing just in front of where the crank sits as these seal the passageways to allow coolant flowing up to the base of the barrels and beyond?
It has been known for these to be forgotten and coolant leaking into the cases.
You can see one recess for the "o" ring just to the left of the conrod.