Won’t Rev with full throttle past 3,000 rpm

[Ozecho]

Hi all,

ive got an issue with my ‘77 original (Strombergs) stag - at full throttle in any gear, at 3000rpm or so the engine falters and won’t go any further.

However, if I back off on the accelerator and accelerate gently it will easy go up to 4000 rpm (not game to take her much further).

I thought it it might be fuel filter which I replaced and it felt like it got better (might just be I n my head).

Could it be the fuel pump? (Looks original)

Thanks in advance for your help!

[72BLUE]

Might be worth checking the vent pipes for the fuel tank, could be that the air can't get into the tank fast enough (to replace the fuel being used) so you could have a vacuum effect.

Try running the car with the fuel locking cap undone, if that cures it, you probably have a vacuum........


Cheers
Keith

[davidf]

Two suggestions
Many years ago I had similar issue on a Spitfire - it turned out to be a fuel line starting to rot on the inside (yes even in 1973 on a '68 Spit), and a flap inside the line was acting as a flow limiter.
More recently, my Stag was misfiring under load, so only up to certain revs on full load, but could carefully creep up to higher revs on light throttle. That turned out to be a coil breaking down under load and heat. It was Ok while cold. Eventually proved it by carrying a spare coil, and doing a quick swap when the engine was hot and mis-firing under load. I left the original coil in place (to keep it hot) to prove that it mis-fired again when swapped back. I had tried a number of coils, but they all suffered the same (this was after changing to a different electronic ignition). Finally got a 'Flame Thrower' from Demon Tweaks.

[022Dave]

It's worth checking the dizzy weights and plates, to see that they move to advance the ignition.

Dave

[Kithmo]

It's also worth checking the diaphragms in the carbs, one (or both) could be split.

[marshman]

As a first guess I'm with Kithmo, check the carb diaphragms first - quick and easy. Had similar symptoms a few years back.

[richardthestag]

Hi all,

ive got an issue with my ‘77 original (Strombergs) stag - at full throttle in any gear, at 3000rpm or so the engine falters and won’t go any further.

However, if I back off on the accelerator and accelerate gently it will easy go up to 4000 rpm (not game to take her much further).

I thought it it might be fuel filter which I replaced and it felt like it got better (might just be I n my head).

Could it be the fuel pump? (Looks original)

Thanks in advance for your help!

welcome aboard, let us know who you are and what you drive

like ^ said diaphragms are a very easy first check. hole in diaphragm will slow the carb damper rising and as a result the engine will be running way too lean on that carb.

on a healthy stag v8 the fun starts above 4k rpm, hairs should be standing up on the back of your neck at anything over 6k rpm

[flying farmer]

I usually find these problems are ignition related, typically a weak spark. 8 cylinder engines need a good quality ignition coil.

Make sure you are getting proper voltage at the coil, if it is an original ballasted coil you should have about 7 volts when it is running, if it is down to about 5 volts the in loom ballast wire is probably failing. I had this happen on my mk 2 stag a couple of years ago and cured it by bypassing the original ballast resistor and fitting a mk1 ceramic one instead.

It is also worth checking the plug gaps, new plugs often come with far bigger plug gap than the 25 thou specified and this just causes the spark to blow out at full throttle due to the extra pressure in the cylinders.

[Ozecho]

ok- thanks all ... lots of suggestions for me to take a look at this weekend!

I suspected the fuel pump as not delivering enough fuel, as under load (when going up a hill) the "starvation" happens earlier in the Rev count (2,500). Ive tried to measure the fuel delivery after the filter into a jug and got about 500mls in 60 seconds. Sound about right?

Ive looked at the dizzy and it does move ...

Diaphrams will need to wait for the weekend.... can I do this with the carbys in place?

Voltage at the coil is regular - ie. 12v. Ive had the car for 12 months, and it came with new wiring and fuse, supplying direct voltage from the battery to the coil. I seem to have plenty of spark out of the coil.

thanks again for any comments...

[Goldstar]

I had a similar issue and it turned out to be the coil - modern coils are of variable quality, it seems, so I got a flamethrower and bypassed the (MK1) ballast.

Good luck, be interesting to find out what it is

[Ozecho]

Took a look at both Diaphragms today... they were a little “sooty” but no holes or rips or tears...

tried the tank open idea, but it still happens.

Im starting to lean towards a coil or spark leads or plugs issue. When it’s idiling a can hear an occasional “snap” - sometimes multiple in a row, sometimes not. It sounds like a spark going awry. Can’t see anything though even in a dark garage.

Any other ideas before I start replacing leads and plugs?

[Lingen]

Ozecho.. Try running the engine in the dark with the bonnet open. You may be able to see where the spark noise is coming from! If you can't, it may be coming from inside the distributor cap if there is a crack in it.
Mike.

[nigele]

Also check the rotor arm, especially if it is the black one with a rivet. The red ones from the Distributor Doctor are much better.

Nigel

[flying farmer]

My Stag engined estate started with a full throttle misfire problem a couple of weeks ago. I replaced the plugs and leads which were probably all getting on towards 20 years old and all is well again.

I was amazed by the build up of crap on the spark plugs, the engine used to run on efi until I put it in the estate and the plugs never used to foul, but I suppose that's the difference between efi and carbs.

Come to think of it, fitting the new efi system probably increased cylinder pressures to a point where the old knackered ignition system could not cope!

Neil

[JeffW]

i had a misfire at around 2800rpm-3000 and it turned out to be two faulty plug wires (which were only 2 years old) having high resistance. I also replaced the coil with a Bosch Red and all is perfect now, although I believe the coil replacement wasn't needed!

Jeff