Intermittent misfire - Rover V8

[shed driver]

Rather annoyingly I'm struggling to track down the cause of a misfire in my 1973 P5B. It runs well at lower speeds, but going above 50mph for more than a few minutes causes a misfire and the car struggles to go above 1500 rpm - the speed drops and is quite jerky.

There's a new coil, electronic ignition and the plugs and HT leads are also new. The car starts almost instantly either hot or cold. Running at slower speeds there's no problem (touch wood!). Carbs have been balanced and timing is OK.

It feels like it's not getting enough fuel - it has the original mechanical fuel pump, but this seems to be working well.

I don't want to fire the parts cannon at it, but on the other hand I could put a low pressure electric pump in.

Any ideas before the show season?

SD.

[GreenV8S]

shed driver said:

for more than a few minutes

That's an interesting way to describe it. I suggest it's worth finding out more about how to provoke and clear the problem. For example, does it clear immediately if you return to a regime where it used to run cleanly?

[Richard-D]

If you drop the revs for a few minutes when it starts playing up, is it good again for a while? If so that could suggest that the float bowl is emptying at higher load but at lower revs the fuel pump is able to get ahead again.

[shed driver]

Richard-D said:

If you drop the revs for a few minutes when it starts playing up, is it good again for a while? If so that could suggest that the float bowl is emptying at higher load but at lower revs the fuel pump is able to get ahead again.

This sounds plausible, an electric fuel pump isn't too much of a big job hopefully!

SD.

[paul_c123]

How old is the fuel?

[shed driver]

paul_c123 said:

How old is the fuel?

It's always kept topped up with premium fuel (usually Tesco Momentum 99 RON) and has been through several tanks full, so I think I can discount that. I've replaced the fuel filters, and these are looking clean.

SD.

[TarquinMX5]

I seem to recall that they might have had a fuel tank breather, if yours has, is it clear?

[Belle427]

Its critical the ignition components are right, did it run ok on the electronic ignition when you fitted it?

[shed driver]

Belle427 said:

Its critical the ignition components are right, did it run ok on the electronic ignition when you fitted it?

It ran perfectly for about twelve months when the electronic ignition was fitted. I've got a new electric pump on the way, so when that's fitted I'll report back.

SD.

[Richard-D]

shed driver said:

It ran perfectly for about twelve months when the electronic ignition was fitted. I've got a new electric pump on the way, so when that's fitted I'll report back.

SD.

It's an interesting one and I'd like to know what it was. I would also have leant towards the coil from your initial description.

It'd be good to know if the misfire disappeared after idling for a short while but I suspect it wouldn't change your next action (putting a pump in it).

Another poster above suggested checking the tank breather. A partially blocked breather could easily cause this too. Have you tried removing the lank lid after it starts misfiring? A rush of air in would indicate this is your problem on a carb'd engine.

If only you could see the level of the float bowl without removing it.

[shed driver]

It's really difficult, because it only happens at speed or under load, for example going up a hill. I could try and half wedge the filler cap open and take it up the motorway.

Setting back to a slower speed does tend to make the misfire go away, which is why I'm thinking fuel.

SD.

[Belle427]

Coils can break down under load and heat but its tricky to guess at it, some brands are ste quality so dont overlook recently fitted components.

[shed driver]

Belle427 said:

Coils can break down under load and heat but its tricky to guess at it, some brands are ste quality so dont overlook recently fitted components.

It was a branded Pertronix coil, but it won't hurt to have a spare.

SD.

[rev-erend]

Check the carbon button inside the Dizzy cap is ok

[Belle427]

shed driver said:

Belle427 said:

Coils can break down under load and heat but its tricky to guess at it, some brands are ste quality so dont overlook recently fitted components.

It was a branded Pertronix coil, but it won't hurt to have a spare.

SD.

They are normally pretty good, I think I have a spare one of those somewhere but can't remember the resistance on it. Will check tomorrow, it was for their electronic module conversion and is black in colour.
Your welcome to it foc if it's the same.

[Metric Max]

It's well known that Ethanol damages older type fuel pipes, Is it possible that a rubber fuel pipe has broken up internally restricting the fuel flow?

[ARHarh]

Is it running stromberg or su carbs?

Stromberg I would suspect the diaphragm first, Su I would check the piston is moving freely.

Also check the advance and retard in the dizzy is working as expected. One that has caught me out before is the wire connecting the baseplate in the dizzy losing it connection when the base plate moves to advance the timing.

[rambo19]

Fuel supply issue for sure.

[bangerhoarder]

Our P5B is on electronic ignition, new coil and electric fuel pump. Fitting the new pump is easy enough and the old pump can stay. We used a Facet pump (and again on an MGB where the SU pump was leaking and weak).

Have you bypassed the 6v feed to the coil? That can cause issues if not.

Is the tank clean inside, have you checked the filters? It doesn't totally sound like that, but I've seen the effects on twin SUs of running with a rotten tank half full of water!

[shed driver]

Update - and good news!

New fuel pump fitted, all new filters through the system and it's running really well. Pulls strongly to 75 (ish) and no misfires or hesitancy noted on a 50 mile mix of motorways, fast A roads and city centre traffic. Fuel pump was slightly crusty and worn.

SD.