Timing issue 911SC 3.0

[StillySC]

Does anyone have any ideas about issues with ignition timing, ive just rebuild my SC 3ltr basic repairs strip down to block, on rebuild i cannot get the timing right as the strobe set at 5 deg advance off, the timing marks are jumping and not static (about 6 deg of jump). Ive read articles on the net where similar problems have occurred resulting in strip and clean of dissy, i'm also led to believe that the bob weights in the dissy may cause this too - any thoughts would be greatly appreciated.

Nick

[tr7v8]

Does this have points or is it electronic?
Could be sticky bob weights although 6 Deg isn't a lot when the overall mechanical advance is around 15-20Deg or so. Might be worth stripping everything off you can & giving it a good wash with WD40, but make sure it's ALL out before you try firing it up as it's inflammable . Also check the rotor arm is secure. On another thought lift the dizzy (mark it so you can easily repace it) & check you don't have loads of wear in either the shaft or the drive gear/tongue.

[StillySC]

Jim thanks for your response, i intend to 'have a go' this weekend and strip the dissy, its points not electronic ignition, should the timing jump around this much on tick over ? Thanks for all the advice.

Nick

[EL Gordo]

points? on an sc, surely you have the cdi system? or is it an old marinelli distributor set up how old is your car?

Gordon

[colt45_gto]

nick pm me your address i'll come and have a look at it.

[StillySC]

The cars a 1980 SC, i'm sure its a Marinelli dissy - And your right there arn't any points, but its not electronic ignition either is it ? Not 100% sure what i'm talking about as you can tell !!! Just dont want to shell out for a new dissy until ive exhausted all the alternatives, as i left the dissy in with-out the cap when cleaning down the engine on strip i'm just assuming that some dirt has got inside causing the problem as it was fine before, i'm also experiencing exhaust backfires when letting off the throttle so the timing must be out. Should the timing jump around 5/6 degrees on tickover ? Thanks for all your help.

Nick

[EL Gordo]

ok you should have the bosch distributor and the bosh cdi system, yes it is electronic in the sense that the dizzy collects the signal from the cdi and distributes it, are your leads and stuff new? You wont have the marinelli if everything is stock fittings. Also how is the coil?

no I dont think you should see that amount of jump at idle or at all. Dizzy could have stuck/ sticky centrifugal weights
( this I know as I am dealing with a problem quite similar at the moment. and I have been talking to a man who knows such things)
apparently they can have a "shoulder" appear over time and this can be removed very patiently !!!!

could also be overfuelling causing the backfire on the overrun by the way but dont get bogged down with that just yet.

[StillySC]

Just removed Dissy, yes its a bosch, rotor arm was siezed on and corroded so first point of call is to replace and see what that does, the shaft of the dissy seems to have minimal play so not going to strip any more down at this point, i'm told that the overfueling may be down to the timing so hope to get this sorted first. I fitted brand new leads when engine stripped as they seemed to be breaking down a little under load.

Gordon thanks again for all your help

Nick

[EL Gordo]

the timing is 5 degrees plus or minus 1 degree at tdc 950 revs and cruially 23 deg plus minus 2 degrees at 4000 rpm all vacuum off by the way !!

[EL Gordo]

scaring myself with this technical stuff now

[Brett928S2]

scaring myself with this technical stuff now

Hi

If only you knew how to use it lol

All the best Brett

[EL Gordo]

oh i do !!!!!!

[StillySC]

SUCCESS !!!! Timing issue resolved, the 'jumping' of the timing on tick over and subsequent missfire all down to a worn Rotor arm (Thank God !!)

Thanks

[colt45_gto]

see i told ya didnt i? well guess not but glad you sorted it mate