Dwell variation of 6 degrees at 3500rpm - is dizzy stuffed

[ozscott]

Hi all - my 95 3.9 has started playing up. I replaced all primary and secondary ignition components about 18months to 2 years ago and done about 35000 miles in the meantime. The vehicle is now missing over about 4000rpm slightly with some cold start issues. It misses on both LPG and petrol. Its way past my limited knowledge now and has been with the stealer.

They reckon that by peening the rotor button they closed the air gap down and increased the KVs nicely and it ran better but still had the upper rev range slight misfire.

They reckon they have checked the dwell variation and its 6 degrees by 3500rpm and that it could be the cause of the problems...is that from shaft wear/pickup plate problems? Does it mean new dizzy or should I still be suspicious about the new Lucas Amplifier module/New Lucas oil coiled coil?

Cheers

PS. I would go Mallory Unilite or similar with its own HEI amp module anyway if I replaced the dizzy so the last part of the Q might become academic....but I dont want to replace amp if I cant then use it with the new dizzy

 

[p m]

How do you measure dwell without points?

 

[MUSKYMAN]

Peter is correct in that Dwell as a term comes from the time a set of points is closed or the period of time the coil is being recharged or saturated.

Dwell is also used to describe the period of time between signal spikes in a hall effect electronic distributor. Just like a set of points a Hall effect distributor produces a square wave or a open/closed signal that charges and discharges the coil.

the dwell varience I believe they are telling you about referes to the spacing between the square wave signals the distributor produces. now because the trigger wheel on the distributor is a fixed piece and the hall effect pick-up is fixed one would think this cant change. But what you need to consider is that the shaft it fixed to can rotate forward or back relative to its driven shaft due to the advance weights and the stator plate that the Hall effect pick-up can be rotated by the vacume advance. If the mechanical advance has issues such as a broken spring or the return spring in the advance diaphram has failed then you will get a high varience in the effective dwell and the engine will miss at higher RPM's.

so in a nut shell...yes the distributor has issues:rofl:

your issue sorta sounds similar to when someone pulls up on the rotor and breaks the mechanical advance to me, has it been tuned up just recently?

 

[ozscott]

thanks matey - yep it had issues suddenly one day and I took it to the stealer and they said they would get the mech advance springs and dizzy generally checked and they sent it off and it came back with a sound bill of health. They have now sent it to another place that reckons its got issues....so I think it might be safer to replace instead of rebuilding....i would like to take it as an opportunity to get rid of the prince of darkness altogether with a Mallory unit with light pickup and its own amp and a Mallory coil....I wouldnt mind betting its goodby problems that plaque this bloody ignition system.

 

[MUSKYMAN]

yeah thats a good plan...the mallory is a nice piece for sure compared to the stocker.

if mine took a dump I would be going the same way

 

[ozscott]

Cheers Musky

 

[ozscott]

How sweet is this - an Australian land rover owners site put me onto Performance Ignitions in Melbourne. For 25 years they have done only dizzy re-builds and race car dizzys. They do one for a rover V8 - Hooray!!!

They use generic Bosh Ford (Aust) 6 cylinder Dizzy with a GM Holden (Aust) cap and a Ford (Oz again) 6 cyl button and a generic Bosch amplifier module (which module the breakdown patrols carry at $50 a throw!!!).

They put it all together and use the drive gear off the old Lucas for $550 Australian and that includes them machining and drilling for the drive gear and the amp on the side. THey have sold hundreds and never had any back.

I love it....in Australia (much like Chevy in the US I guess) Ford and Holden Dizzys last forever and the cap pretty much lasts forever also. The buttons get replaced but only a high mileages.

The vac advance (if it ever goes) costs about $20 to replace (unlike $300 for the rover one).

Sweet - I can see keeping this vehicle for a long time; if I do need a cap or button I can get one anywhere, even service stations for a song.

Cheers all