odd problem....

[mr_bridger]

hey guys, got an odd one on our mini, could do with some thoughts on

if i hold the revs at about 4k, it starts to sort of hunt. and flicks up n down about 400rpm.

this is quite noticeable whilst driving, as it feels a bit jumpy. but seems only noticable on part throttle.

now ive remounted the crank sensor on a super thick bracket, fitted a new sensor, tried a new coil pack, a new edis unit, even another megajolt unit, and it still seems to do it.

any thoughts? advance related??. not fueling, as ive tried both a single HIF44 and twin HIF38's

i dont have a scope to check the signals etc.

any other ideas?

cheers
Ross

[brentp]

You're on the right track for diagnosing. Normally a flimsy bracket is responsible for misses and irregular running in the mid-rpm range. Plug wires and plugs? what's the gap between sensor and trigger wheel?

Regards,
Brent

[mr_bridger]

Hi Brent,

gap is about 0.8mm on the sensor., i posted a few weeks back about the plugs, running BPR6ES, they are new too....

leads are in good cond, only a year or so old. have another older set i can try, but its been doing this ages, i thought it was fuelling, but now ive tried new carb setup im sure its ignition.

anyone near coventry with a scope?

as a side not, all cabling is screened too and grounded at the edis pack..

[mr_bridger]

Oh, just thought....

although the cables are scfreened, the VR sensor wire is run in a loom that say the first 3rd is taped to the coil wires. could this cause a problem?

maybe i should seperate them?

[Gilesy998]

I might be wrong, but how have you mounted your trigger wheel? Is it on the outside of the crank damper, or possibly a machined damper? If so, the damper might be merrily doing it's job correctly - meaning the teeth of the pulley effectively "move" in relation to the crank itself on each power stroke, and causing the RPM signal the VR sensor sees to fluctuate, and thus advance or retard the timing accordingly.
Each firing event has a similar effect to tw@ing the piston crown with a FBH. That happening twice per revolution can set up a harmoninc enough to cause the crank to fail, even snap. The outer section of the crank damper is bonded to the inner with what is essentially a 1/8" thick band of rubber, allowing the outer to move relative to the inner, changing the natural harmonic frequency of the crank. Up to about 2° deflection if I remember correctly.

[mr_bridger]

yes, its a lasercut wheel mounted on the outside, on the rubber mounted bit.

i dont think 2 degs as going to make much odds when is pulling 35 plus degrees at part throttle without much load.

you dont notice it when on the power hard / accelerating...

dunno...