Strange vibrations

[moggiegeek]

I've been trying to track down a strange noise which occurs at around 2000rpm, not below 1000 and gone by 2500. This following fitiing 12g940 head, HIF38, maniflow inlet and exhaust to a 1098 engine that was in excellent condition - about 10k after complete rebuild.
Disengaging the clutch makes no difference and the noise relates to engine speed not road speed.essentially you can turn up the idel speed and it appears reliably!
The noise seems to locate around distributer, it's an oscillating, rubbing kind of noise but is very dificult to locate precisely. I'm wondering if the vaccum advance assembly can be the culprit. The distributor is new, only a couple of thousand miles old- could it be this?
I havehecked exhaust for rubbing/vibrating, bonnet stays etc etc. Only thing thta seemed to help was retightening the distributor pich bolt but the improvement may be imaginary....
Help?

[alexmcguffie]

How has the head been fitted to the block? Any modifications to the block to stop the valves potentially hitting it...? I assume you have a standard Minor cam fitted still?

[moggiegeek]

Yes standard mods to exhaust valves, ie cutting back seats, no high rise cam - standard 1098 cam fitted, standard ratio rocker gear - sintered , refaced 15thou clearance, copper head gasket.

[katy]

Is it possible to install the distributor w/the drive dog 180º out? Is there enough slop to allow this? If so maybe that's your problem?

[bmcecosse]

It doesn't matter if the dizzy drive is '180 degrees out' - makes no odds. Maybe slack timing chain?? Sure it's not just 'resonance' in the carb throat? Try EP90 in the carb damper!

[alexmcguffie]

I've never tried it but didn't think it was possible to fit the distributor 108 deg out. If by chance it could be done then I wouldn't imagine it would be a happy fit!

Is the noise metalic etc etc? flapping fan belt?[frame][/frame]

[moggiethouable]

Procure a long wooden handled screw driver, touch the dizzy etc with the metal end and place your ear to the wooden end, listen and you may be able to pin down where the noise is emanating.

[katy]

The noise seems to locate around distributer, it's an oscillating, rubbing kind of noise but is very dificult to locate precisely. The distributor is new, only a couple of thousand miles old- could it be this?

Try replacing the distributor w/a different one.

[moggiegeek]

I will try all three suggestions and report back.

[moggiegeek]

Have tried all suggestions - thicker oil in Dashpot helped throttle response. Finally, in despair, leaning on alternator the noise stopped - turned out to be slightly undersize aftermarket bolt chattering in bracket for alternator. Thanks guys.

[bmcecosse]

Well done ! And thanks for coming back with the result - a new one on me !!