RV8 spluttering under load

[RogerD]

Hi - I've a rover V8 - 4.3 on Eidlebrook carbs. She ran sweet as a nut when I got her, reving cleanly to over 6k without a hitch. Relentless power.

At one point she wouldn't rev over 5k and I found one of the spark plug HT leads was loose. Plugged it back on and all was ok again.

Recently I've done a few jobs on the engine:

Re plumbed the cooling system I had the pancake air filter off for a fair while during this)
Installed a simple gear change led strip which is attached to coil +
Removed the diff and had it upgraded to an LSD - which involved disconnecting the fuel line. I repelaced it with new identical ID piping as it was old and crumbling.

Since then she started spluttering (power coming and going) at around 5.5k. ONLY under load.

I looked at the dizzy cap and saw half of the pins were scored/gouged, so I got a replacement. Now the rotor arm hit half the pins - you could hear it - tap tap tap tap tap. New HT leads as well. She ran like a dog, misfiring, no power. I might not have put the cap on correctly - the new one didn't seem like a perfect fit. I put the old one back on with the old leads and back to normal. Clean but stuttering and loss of power at 5.5k.

I'm certain the pins were gougued from when I bougth it. I'd have heard it if it happened whilst I was driving it. It's very obvious. So I've assumed she's always run OK like this. Besides - she always revs cleanly over 6k with NO load.

Went out for an extended test drive. Ascertained I could go to over 6k cleanly at half to 3/4 throttle, but as soon as full throttle was introduced then I'd get the hesiitation / cutting-out at 5.5k.

I did some smooth, then some more aggressive acceleration. In the end I could, foor on the floor, get her to rev under load to 6k with a tiny hickup at around 5.5k (just a tiny hesitation "bump"), but otherwise cleanly. Thought it had "cured" itself. Somehow.

Last week I took a mate our for a blat and now she's unhappy from 4-4.5k onwards. If I floor her in too high a gear at lower revs (gives her a nice load) she'll hesitate and cough a bit - never used to do that - it's a V8!. I also assumed at this stage that it's misbehaving at 4.5k (rather than the previous 5.5k) due to having more load (100Kg of friend in the passenger seat)

Next up I replaced the plugs and coil. No difference. But she's also still misbehaving at 4.5k - so my mate (the extra weight) made no difference.

Here are my thoughts.

- I've had the carb filter off in my garage for a while - could something be in a jet? What would I check? Not sure where the jets are or what the look like on me eidelbrook.

- Fuel pressure - perhaps when I put the new fuel line on I allowed some gunk into the pump? I'm going to remove the filter inside the pump to take a look - though my engine bay mounted gauge shows 3psi - it this right? (Update - just checked the filter and it's clean)

Some notes: Tried with my gear change LED disconnected from the coil - zero difference. She ALWAYS revs totally cleanly over 6k in neutral. This leads me to assume the dizzy cap, whilst clearly a little worn, is not the issue.

[mgbv8]

What lifters does this engine have??

On my old engines the stock lifters would stop the engine revving at 5800-6000 rpm.

Has this engine got all stock internals?

Perry

[RogerD]

No idea to be honest. This is not mechanical. She pulled cleanly to over 6k until recently. The spluttering is now starting around 4.5k.

The engine is def modified. 250bhp at 6k on the dyno.

I noticed my battery was at 11.2v yesterday so charged it overnight . will test if I have an alternator issue today.

[RogerD]

Got it!

Metered the battery today at 13.4v on charge. 12.6v off charge. Full.

Started the engine. 12.35 volts. Revved her. 12.45 max - that's not right.

Went for a blat. climbed to 6k with a hint of stutter around 5800. Switched my lights on and spluttering at 5k started. Lights off, cleann to 5.8k.

Voltage never climbed above 12.45 - and it should go above 13v on tickover!

Got home, looked at the alternator, inspected the live lead and it fell off...



That's what's left of the spade connector - the rest is still on the alternator.

So I was basically lacking spark power - and all my plumbing work involved lots of starting and stoppong the engine, which lowered the battery, which increased the problem!

Sorted. Phew!

p.s. I'm still slightly baffled as to why an engine needs more power to the plugs under full throttle (as opposed to revs) - can anyone explain the logic.

Oh, and here she is a couple of months ago, cleanly revving to 6k - i get to do that at around the 40 second mark,

https://www.youtube.com/watch?v=w5MvKZd ... RR6RaVoYvw

[spratty]

are you sure it was a spark problem and not a fuel flow problem? if there wasn't enough power getting to the pump it might not have been able to shift enough fuel?

[DaveEFI]

ISTR reading somewhere that you do need a 'better' spark as the revs climb.
But that on a four-stoke, a two-stroke not.

Which seemed odd to me as two strokes often had magneto ignition which produces a 'better' spark as the revs climb, while four strokes normally coil, which if anything will get worse.

But I might well be just be having one of my common memory farts.

[RogerD]

Not as the revs climb. As you put your foot down. A bigger spark is needed when you open up the throttle, regardless of revs. That's why my engine revved cleanly in neutral but faltered under load.

I doubt it was a pump issue. The pump produces good pressure at 11v upwards.

I read an article I now can't find that said you need circa 18kv at mild throttle and 38kv at wide open throttle.

[RogerD]

Here you go, found it...

"ltage, in electricity, is like pressure in water. It’s a measure of how much push there is. In order to fire, a spark plug must be supplied a certain minimum electrical voltage, called the plug’s firing voltage. Firing voltage varies with plug design, how clean it is, how worn it is, combustion chamber temperature, how much load the engine is under, and how much fuel is mixed with the air. When the engine is under a full-throttle load, firing voltage goes way up. When your foot is off the accelerator pedal, firing voltage goes way down. It can vary from about 4000 to 18,000 volts. If the firing voltage is higher than available voltage that is being produced by the rest of the ignition system, the plug will misfire. It is like trying to squirt a stream of water across your yard. If the water pressure is too low, you just cannot do it."

http://www.autocorner.ca/pages/sparkplugs.html

Essentially, the denser mix of air and fuel needs a bigger spark.