Misfiring 1985/86 RR V8

Did you replace the condenser as well as the coil?

Assuming the Head Gasket is good.....

I would look at timing, the Advance/Retard bellows and associated tube and the Dizzy Bob weights.

does sound like classic weak spark new plugs leads ,cap,arm ,coil usually are best replaced at same time

Thanks for the replies - couple of further points / questions:
This car has electronic ignition so there is no condenser or cap (capacitor ?) to worry about as far as I am aware.
Also are the Dizzy Bob weights the centrifugal weights in the distributor ? If so they are not shown in the parts manual for electronic ignition - if present are they in the same place as traditional distributor ?
Many thanks

Has the car been standing idle? Misfire and dying under load is a classic symptom of stale petrol, unleaded goes off very quickly.

what electronic dizzy is it ? opus? or has it an aftermarket fitted? or is it a later magnatronic fitted?

The electronic ignition is the OEM fitted one which was introduced about 1986.
Thanks

Yep the Bob weights are the centrifugal timing weights inside the dizzy.....the things that usually fall off the shaft when you change the rotor arm.....

Both my '88 and '93 Classics had them, and they were both electronic ignition.

On my '93 I had to change the ignition amplifier on the dizzy as I was getting a weak spark. The weak spark was caused by a leakage across the amplifier....the amp signals the coil when to fire, what was happening in my cause, the amplifier was telling the coil to fire pretty much continuously so it never had a chance to 'charge up' and produce a strong spark.

Also, just to add confusion, my LPG system had a second ignition amplifier to either advance/retard (can't remember which) the timing to fire the spark at the correct time for the LPG (as it has a different burn rate to petrol). When my Classic all of a sudden decided not to start, I disconnected this second amplifier and hey-presto the beast lived....I never reconnected the second amplifier, as when it was reconnected, I had trouble starting and when it did start it ran awful.

yeah but is that opus or magnatronic at that age? I ask becuase opus are know for being utter ****e so you would do better to put a magnatronic one in.

Saint.V8 said:

Yep the Bob weights are the centrifugal timing weights inside the dizzy.....the things that usually fall off the shaft when you change the rotor arm.....

Both my '88 and '93 Classics had them, and they were both electronic ignition.

On my '93 I had to change the ignition amplifier on the dizzy as I was getting a weak spark. The weak spark was caused by a leakage across the amplifier....the amp signals the coil when to fire, what was happening in my cause, the amplifier was telling the coil to fire pretty much continuously so it never had a chance to 'charge up' and produce a strong spark.

Also, just to add confusion, my LPG system had a second ignition amplifier to either advance/retard (can't remember which) the timing to fire the spark at the correct time for the LPG (as it has a different burn rate to petrol). When my Classic all of a sudden decided not to start, I disconnected this second amplifier and hey-presto the beast lived....I never reconnected the second amplifier, as when it was reconnected, I had trouble starting and when it did start it ran awful.

Click to expand...

The second amp should have been switching on for LPG and off for petrol. If it was permanently on, the engine would be far too much advanced for petrol running.

Sounds like timing or weak/lack of fuel.....

Weak spark certainly won't help....

Have you tried taking number one plug out....holding it with a decent large pair of INSULATED pliers ensuring the threaded or nut-like part is grounded against the engine rocker cover and get a kind assistant to crank the engine over....

The spark created should be a nice, big, fat, blue spark....and wimpy looking yellowish spark is far to weak.

From memory the classic engine ECU when you hit the throttle hard will pulse all 8 injectors to introduce a good amount of fuel for the upcoming increase in engine speed.

If the engine is dying with rapid pedal movement, this would indicate a weak mixture as you are introducing a large amount of air and the system doesn't have to required amount of fuel to sustain combustion....(like blowing out a match).

Also if the timing is out, when the engine speed picks up (due to the rapid pedal movement) it cannot fire the spark at the right time to keep up with itself.....this is usually accompanied by misfiring and possible backfiring before the engine dies.

Again, a weak spark won't have the energy to instigate a decent burn in a air rich/fuel weak mixture, consider a zippo lighter just filled with fuel but a naff flint...it sparks, but due to the excess fuel/air it doesn't carry enough heat to ignite the vapour.

Pressing the accelerator gentle allows the engine to catch up with itself due to either weak mixture of weak spark.

Just my opinon, but I would check spark strength and I think there is also a way of measuring injector output too.....goes along the lines of Place injectors into individual measuring beakers and turn engine over for 5 seconds, and measure fuel injected quantity.....BUT I would seek out the proper procedure, as you will be playing with injected fuel mist, at high pressure, next to electrical components.....EXTREME CAUTION IS ALWAYS ADVISED...and I only include this here to illustrate a point....it is in no way a proper or endored procedure.....

Now that has covered my butt.....probably best to get the injectos to a specialist to test if this is the route you decide to check.

dizzy a tooth out?

Saint.V8 - great reply which made a lot of sense only no injectors on this one. Will be checking spark strength and if there is any arcing over the weekend, many thanks.

Fett - how do i check if she is a tooth out ?

Thanks

Chris

Opps, my bad.....

If it is carb'ed I would check for timing and ensure there is enough fuel in the float chamber and (not to up on carb engines - but mini's had jet needles) check the jet needle is seated correctly...if it has one...!!!

From memory of my Classics (Injected though) the Dizzy was driven from a slotted drive, so it could only be either one way or 180deg out the other...BUT this slotted drive was driven on a helical gear from the camshaft.....if the helical gear is a tooth out on the cam shaft drive, the timing would be out causing difficult starting and crap running/acceleration.

Not sure how it would skip a tooth unless the heads/camshaft/timing gear have been removed and replaced incorrectly.....although with Range Rovers anythign can happen and maybe...just maybe if the dizzy drive gears are worn that *may* cause the gear to slip and knock the timing out....

Check for a good strong spark first, as this is the easiest thing to check, if you have a good spark, the next things would be fuel/air mixture or timing....

I still assume the Head Gasket is sound as a pound (as I mentioned previously), as rough running and lack of acceleration can be caused by lack of compression....if you have a compression gauge, it only takes 15 minutes to check all 8 cylinders for good compression.

its as siant said, should be rotor pointing to lead/plug no1 on cylinder one at tdc valves closed.

easy to just get it one tooth out as it slides round on the helical gear as you put it in.

do you know if its been out before car was laid up? it would explain a lot

Thanks for all the suggestions - progress has been slow and results even slower.
Got a new set of leads and a coil and started replacing the nearside bank leads one at a time, complted that side and checked in the manual to make sure the routing was OK. Very puzzled for a while until I realised that the leads had not been in the correct distributor cap port, at least according to the book ! They were all one behind, ie the one for cylinder 1 went to 8, 8 went to 4, 4 to 3, 3 to 6, 6 to 5, 5 to 7, 7 to 2 and 2 to 1. The firiung order is 18436572.
So I wired them up as documented and the car wouldn't even start !!
Put them back as they were and we start, misfire and die under load.

HELP please !!!

chrimcne said:

Fett - how do i check if she is a tooth out ?

Thanks

Chris

Click to expand...

distributor will be at end of adjustment

Fett - starting to feel out of my depth - how do I know the distributor is at the end of adjustment ?
Yours exasperated
Chris