17 replies to this topic

[subyguy]

Hi, just finished rebuilding a 1275 bored .060 over.  Crank was ground but not balanced, just a road car.  The motor sends a nasty vibration through the whole car, das, hood steering wheel bumpers all vibrate at idle and around 2000 rpm.  It's really bad as the rpms come down right before idle.  Its not bad while driving but will do it as changing gears or down shifting.  I have tried a different flywheel, pulled the harmonic balancer off, even ran it with the bolts out the engine mounts.  The motor has been tuned and timed properly.  It's in a 1975 so it has the solid mount subframe.  I'm at my wits end with this thing, don't know what else to do.  I hope someone can give me some advice.

 

Thanks.

[Captain Mainwaring]

Only speculation....rods and piston weights unequal or mismatched set?

 

Download one of the many free vibration applications for android or iphone - you'll read much more from that than listening.

 

Mini cranks have a harmonic at 3 and 6K - could be clutch perhaps?

[Naughty 40]

Are they genuine or pattern engine mounts? Friend had the same problem which disappeared after using genuine engine mounts

[Guess-Works.com]

It helps if you also have rubber engine steady mounts....

 

Did an engine a while ago, owner complained of a vibration in the engine bay, I changed the solid/poly engine steady bushes for standard rubber ones, vibration goes away....

 

Personally, I would check the balance of the flywheel, balancing of the crank, rods, piston is not really relevant for road engines, as long as someone has not been trying to lighten them or "making performance modifications" 

[subyguy]

Thanks for the replies. I don't know if they are genuine or not, got them from minimania in the States.  Also i'm using rubber engine steady bushings.   But I think it has been confirmed that i'll have to strip it down and balance everything for piece of mind.  I just wasn't sure if not balancing everything would show in the idle or not.

[mossy2a]

It may be wise to check the clearance of the exhaust downpipe, my mini recently had similar symptoms and I found it was due to the exhaust downpipe touching the tunnel. A standard exhaust should be ok but when you have a 2" downpipe it barely fits between the subframe and driveshaft

[ACDodd]

This is quite usual. The biggest rotating mass in the engine is the flywheel. Unfortunately the new clutches can be very far out of balance. I have balanced quite afew units for mlmotorsport over the years and in every case it has it has either eradicated the problem or massively reduced it. The other issue I have also found on a number of engines is a bent crank tail. This should be checked during the tears own and again rebuild stages of the engine build.


Ac

[Captain Mainwaring]

Just out of interest - what caused the crank tail to have a kink in it? Someone trying to lift by it?

[subyguy]

I put a DTI on the edge of the flywheel, turn the motor over by hand.  Only out .001.  I will still have the machine shop check it out.  The first thing I did was check if anything from the motor was hitting anywhere, all seemed clear.  Even when only one motor mount was touching the subframe it still did it.  Would just balancing the whole clutch and flywheel do the trick, or should I just do everything because of the new pistons and I don't know the history of the crank or connecting rods?

 

Thanks

[KernowCooper]

Heaviest mass is flywheel and clutch as AC Said so I'd do that before stripping the engine down

[Island Mini]

I know I'm stating the obvious here but the cylinders are firing in the correct order I can get mine to run say 2-1-3-4 but it judders as you state. As the others say I reckon your problem lies on the flywheel end though.

[Captain Mainwaring]

I know I'm stating the obvious here but the cylinders are firing in the correct order I can get mine to run say 2-1-3-4 but it judders as you state. As the others say I reckon your problem lies on the flywheel end though.

 

I think 2134 is the same as 1342, and for that matter 4213.....

 

Unless you change the camshaft, you can't get cylinders to fire in anything other than the right order. 

[Captain Mainwaring]

I put a DTI on the edge of the flywheel, turn the motor over by hand.  Only out .001.  I will still have the machine shop check it out.  The first thing I did was check if anything from the motor was hitting anywhere, all seemed clear.  Even when only one motor mount was touching the subframe it still did it.  Would just balancing the whole clutch and flywheel do the trick, or should I just do everything because of the new pistons and I don't know the history of the crank or connecting rods?

 

Thanks

 

 

Well I'm guessing you're using inches as your measurement unit - if so, for a centrifugal fan shaft expected to achieve G6.3, a thou TIR would be on the top end of run out to achieve the required standard of balance, It depends if your measurement was TIR or not.

 

I'd guess that it would probably be acceptable on a crank/flywheel taper - I don't know what you would balance to as individual assemblies - I'd think a clutch and flywheel on it's own would be OK at G6.3, but you won't get much more because the driven plate never goes back in the same place.

[ACDodd]

Hitting the flywheel with a BFH! Or aftermarket heat treatment that is not done correctly.

Ac

[subyguy]

It is .001 thou TIR.  So I don't think the end of the crank is bent at all, hopefully just a balancing issue somewhere .  I'm pulling the motor this week.