Misfire, high revs, full throttle

[Tim Proctor]

The engine runs well, but at full throttle and above 4000 revs, fluffs badly.

Where should we start the diagnostics process? Neither of us have much experience with this, nor the tools.

We have a Newtronic electronic ignition system fitted.

Louise

[barkerwilliams]

Louise,

If the car runs well with very hard acceleration up to 4000 revs then probably the fuel supply, carburation is good and I would concentrate on the ignition.

Is this a problem only when the engine has been running for some time and everything is very hot?

First check that the narrow vacuum line from the engine to the distributor is connected. Disconnect from distributor and with engine running check with finger on the end that there is a vacuum present on the line.

Secondly would be a new rotor arm - cheap easy and often causes problems - replace with a red rotor arm. ( the carbon black in black rotor arms is conductive)

As you have electronic ignition check that the unit has a very good earth.

Ignition timing. An adjustable timing light (about £30) if you don't have one will allow the timing to be adjusted correctly but more importantly will show the distributor weights and vacuum advance as the engine revs increase.

Then set of plugs.

Then distributor cap and leads. ( borrow a set from a friend?)

After that it starts to get more complicated. Ignition coil can start to break down when hot at high revs. Electronic ignition may have issues.

It is easy to buy a cheap set of electronic ignition, coil, rotor arm spark plugs and leads for about £85. Whilst they may not be the best they can offer a great help in diagnosing a problem by substituting items one at a time. It just depends on your expertise.

If you do substitute an item and it does not clear the problem replace the original item before moving on (apart from spark plugs) otherwise there is the strong possibility that you will introduce a second fault making it harder to find both and muddying the waters.

Hope this is a start,

Alan

[Tim Proctor]

Louise,

....

Is this a problem only when the engine has been running for some time and everything is very hot?

.....

Hope this is a start,

Alan

Thank you for all this, Alan. Looks like a good plan to follow.

I don't know if it does this before getting hot: my husband complains if I welly it before everything is warm!

Louise

[mole42]

I don't know if it does this before getting hot: my husband complains if I welly it before everything is warm!

Louise

Leave him at home!

[flying farmer]

My favourite would be an inadequate coil.
Had this a few times in the past on 8 cylinder engines so only ever use high output coils on anything with 8 cylinders.
Neil

[Stagdad]

You might just be running out of fuel due to a clogged in line fuel filter, an easy job to swap.

[Tim Proctor]

My favourite would be an inadequate coil.
Had this a few times in the past on 8 cylinder engines so only ever use high output coils on anything with 8 cylinders.
Neil

The coil was one of the few items in the HT side that has been changed in the course of this renovation. It came from Rimmer's, but I don't know where they source from, or what quality they are.

Louise

[TR5convalescent]

The coil was one of the few items in the HT side that has been changed in the course of this renovation. It came from Rimmer's, but I don't know where they source from, or what quality they are.

Louise

Louise is it a Lucas Sports coil? If it's silver it's just an ordinary one. I had similar issues with contactless electronic ignition until I changed the coil for a Lucas Sports (gold coloured). I think that the OE coils just do not have the oomph for some electronic ignitions. This is probably why Pertronix and Alden sell matched coils.

[StagJonno]

Check also the voltage markings on the coil. Unless a PO has seriously modified the circuit, it should be rated at 6 Volts with a ballast resistor in series to mop-up the other 6V (for the car's 12V system). During cranking, a 12V feed comes from the solenoid to bypass the ballast resistor so that a good spark is produced with the depressed voltage caused by the high current demand of the starter motor.

Ballast resistor in early models was a wire wound ceramic chunk mounted adjacent to the coil. On later models, the resistor was incorporated in the wire feeding the coil +ve (coloured Pink/ White, though the pink can fade significantly).

If a 12V coil was fitted, most of the time it will be underpowered, delivering only a weak spark.

Also worth checking the connections to the electronic ignition module. I don't have details of yours, but I'd expect that the power feed needing to be connected to a 12V ignition-controlled supply. It is not unknown, especially on Mk2 with the "hidden" ballast, for the power to be incorrectly taken from the coil +ve if the fitter hasn't appreciated that this isn't 12V! If yours has a separate ballast resistor, it's OK for the +12V to be taken from the non-coil end of the resistor.

Jonno

[236572J]

Hi, I’ve also had a problem with the coil on my Stag Mk1.5. The car what fitted with an Illumition Mk17 electronic ignition. The coil was an unmarked type and there were no markings on it or the bracket.
Anyway, about a month ago the engine cut out, about 50m from our house. As I found that there was no spark, I changed the coil and the car started right up. I used a new Lucas Coil DLB102 that came with the car when I bought it.
2 weeks later, within 5 meters of where it cut out last time, it cut out and would only start with another coil.
I know that the car should have suitable coil for the electronic ignition, so I bought a Megaspark 6- primary coil resistance 2.8 ohms and secondary coil resistance 5.77 k/ohms - (which doesn’t need a ballast resistor). This choice might have been a mistake!
First time out in the car, right outside the garage, as I was putting the car away, it cut out again. Temporary coil fitted.
I checked the voltage at the coil and found it to be about 8v and I know that the coil wont work very well with this.
I checked the wiring diagram and found that there are 2 wires at the Starter motor solenoid but one has been disconnected and taped back. I checked the continuity of this and it is one of two that is doubled up at the coil supply. The other will be the ballast supply.
I’ve (now) watched the Moss Youtube Ballasted Ignition System videos (4) so I understand it a little bit more.
The disconnected wire should be connected to the coil as the starter motor spins, to help during cold starts, I guess.
Before I knew this but after the Megaspark 6, cut out, I ran a wire directly to the battery to the coil and through a relay, operated by the double up coil supply wire.
The engine works lovely and there is plenty of power, something that has been missing for a while.
But, am I going to do any damage to anything. Clearly I am bypassing the ballasted wire? And should I move house to make the car more reliable?

[flying farmer]

If you are using a coil designed for 12 volts and feeding it with 12 volts from the relay you should have no problems. I don't know if the MK1.5 has the ceramic ballast resistor next to the coil like the mk1, or the in loom ballast resistor like the mk2, but I have had both sorts fail on various cars and the only cure is to bypass the original resistor and use a 12v coil, or replace the resistor and continue to use an appropriately rated coil

[Phil S]

I agree with the above, but KISS, start with the basics fuel filter, plugs , timing, then move to vacuum advance, rotor arm, dizzy cap and leads. Always try to start with the things that don't cost anything to check or change.
if possible borrow bits or try them on another car if available.
Phil

[Richard G]

The engine runs well, but at full throttle and above 4000 revs, fluffs badly.

Where should we start the diagnostics process? Neither of us have much experience with this, nor the tools.

We have a Newtronic electronic ignition system fitted.

Louise

I don't often rev mine to 4000rpm!

[236572J]

Thank you for your advice. I wanted to check with others with greater knowledge and experience that myself and the forum is just the place to do it. I have quite new (4K miles) plugs, leads, distributor cap, rotor arm, etc. I will change the fuel filter tomorrow. The fuel pump is new. We went to the Sahara in October and the car lost power just at one point, on the last night, driving up a steep hill just before we got into Tetuan. I changed it when I got back home as I had a spare. I have checked the timing and it is spot on at 12 deg BTDC but I might try changing it just a little. I have checked the vacuum advance and it was a little loose so I wrapped a little bit of tape around the distributor end. We will use the car on Friday, so we will see how it goes and if we get to lunch and back! The car has the built in thick wire resistor (loom) of the Mk2. I did not realise this until after I bought the Megaspark coil. I really do appreciate your comments.

[KOY 23]

Driving the Stag in the Sahara and revving above 4K. That’s how they should be driven.
Hope you find the problem soon.