Engine out of balance

[duckspeed]

My mates 1150 has a very marked vibration at around 4,500rpm I have experienced it on most other 1150's but his is definitely much worse. We have had a dig round in it's warranty history, to discover it had both pistons replaced to correct a high oil consumption issue.
I got to thinking that during production someone would match the weight of a pair of pistons for each engine, where as the dealer would just install the pair of pistons that landed on the bench.
I am considering stripping the motor, having the pistons machined to match them, and a careful balance of the rotating masses of the crank, flywheel, clutch etc.
Any experiences or suggestions gratefully received.
I have done a search but nowt of any use
Should mention it is a single spark, and other than the vibration goes like fcuk

 

[Timolgra]

Have you balanced the throttles correctly?

 

[duckspeed]

Have you balanced the throttles correctly?

Thanks for the reply, yes throttles balanced using Carb-balancer from Motorworks, done it two or three times as paranoia soon sets in

 

[Timolgra]

If not done already try balancing throttles at 4k rpm

 

[cookie]

the pistons are generally balanced to each other & not the crankshaft

 

[Timolgra]

Is the vibration there in every gear?

 

[boxer]

Give it to someone else and have them balance the injectors through the rev range to rule out incorrect method on your part.

Make sure nothing loose.

 

[Greg Masters]

It's very unlikely that the factory tolerances in piston weight would cause a vibration issue at 4-5000 rpm.

 

[littleredrooster]

I'm fairly certain that pistons will be size-coded, but not weighed at the factory. I cannot think that the minute variation in weight would cause a noticeable vibration.

As others have said, it's worth balancing the TBs under load at mid-range revs. I use the vacuum gauges bungeed to the bars and a quiet bit of road...

 

[cookie]

i'm not 100% sure about oilheads, but airheads and other nicasil coated engine barrels use pistons size graded to match the cylinders.

IIRC weight of pistons should be within 5g of each other, so watch out using odd pistons made by different manufacturers as bmw use.

 

[MikeO]

Has it got the original Eprom chip fitted? I had one of those 'upgraded' ones from eBay - made my Adv rough as hell around 4000rpm...

I think you need to think a bit wider about the possible cause - to connect it to a piston change might be reasonable if it was fine just before the change and rough directly afterwards, but otherwise any connection is a bit of a leap...

I'd agree a dynamic TB balance would be a logical first step...

Mike

 

[Steptoe]

i'm not 100% sure about oilheads, but airheads and other nicasil coated engine barrels use pistons size graded to match the cylinders.

.

Graded in A,B and C. Which is stamped on the base of the barrels.

 

[duckspeed]

Thanks guys, more throttle body fiddling to follow then.

 

[PaulRS]

Hi Guys.

My name is Paul, long time lurker and first poster.

On the subject if it pays to balance an engine, I'd say yes.
If you have an engine apart for some reason, do it, you won't be sorry.

I did so on my old Guzzi BotT racer and the Beemer engine in my hack, and it sure makes a difference.

In this case the pistons where replaced under warranty, I guess?
But since Herr BMW allows a 10 grams difference in pistons, that difference will introduce an unbalance and the vibes that come with it.

On one R1150 engine I had apart, I had the whole balanced, all the rotating masses checked out ok, crank, flywheel and clutch all were in specs, conrods weighed, nothing much to win here either.

Pistons were 8 grams off, though.
Some grinding and filing sorted that out and the engine ran much smoother.

On another engine I had the cylinders off for gasket repair, I took the cheap and dirty way and weighed the pistons while connected to the rods, again 4 grams difference.

So, long story short, even a megafactory like BMW don't mind, if in specs, it is in specs, done.

The only BMW engine I know off that is build by handpicking parts and balancing is the HP MegaMoto.

Now for the mandatory pics;

Remove and polish the casting edges, 2 grams won.

Drilling dimples, not holes, on all four sides next to the wrist pin, in the webbing, an unstressed part of the piston, an other few grams gone.

Paul.

 

[duckspeed]

Hi Guys.

My name is Paul, long time lurker and first poster.

On the subject if it pays to balance an engine, I'd say yes.
If you have an engine apart for some reason, do it, you won't be sorry.

I did so on my old Guzzi BotT racer and the Beemer engine in my hack, and it sure makes a difference.

In this case the pistons where replaced under warranty, I guess?
But since Herr BMW allows a 10 grams difference in pistons, that difference will introduce an unbalance and the vibes that come with it.

On one R1150 engine I had apart, I had the whole balanced, all the rotating masses checked out ok, crank, flywheel and clutch all were in specs, conrods weighed, nothing much to win here either.

Pistons were 8 grams off, though.
Some grinding and filing sorted that out and the engine ran much smoother.

On another engine I had the cylinders off for gasket repair, I took the cheap and dirty way and weighed the pistons while connected to the rods, again 4 grams difference.

So, long story short, even a megafactory like BMW don't mind, if in specs, it is in specs, done.

The only BMW engine I know off that is build by handpicking parts and balancing is the HP MegaMoto.

Now for the mandatory pics;

Remove and polish the casting edges, 2 grams won.

Drilling dimples, not holes, on all four sides next to the wrist pin, in the webbing, an unstressed part of the piston, an other few grams gone.

Paul.

Thanks for that Paul, superb first post, I am liking the way you think.

 

[cookie]

I had the crank, rods, pistons, flywheel and clutch balanced on my old GT850, and it was like an electric motor compared to any other Guzzi I've ridden