Engine misfire at high speed, full throttle

[Tim Proctor]

After 8 years storage, 2 years renovation, our Stag is back on the road An absolute delight with refurbished suspension throughout, the handling and ride are superb.

But, when hot, at speed, flooring the throttle results in bogging, with occasional backfire. It's a sharp misfire, instantaneous, so it would seem to be electrical rather than fuel. Backing off the throttle corrects it, and the engine can be pushed to full power with gradual throttle increase.

There are also occasional hot start hesitations.

We had a professional tune-up which included changing several plugs and a review of vacuum pipework, and subsequently, the car passed emissions testing, so I think we can assume plugs, points and other basics are good.

Ignition system is Newtronic.

Can anyone offer advice on chasing down the problem? We're not experienced in this, and reading around it on the internet-thingy gives such a profusion of possibilities we don't know where to start

Many thanks,

Louise

[mallardstag]

Hi,

Start with the easy cheap things - rotor arm, HT leads, spark plugs, distributor cap.

Simon

[Tdm]

I have a Newtronic system that's been in for 23 years. However at one time I had a problem and it turned out to be a small crack in the Newtronic disk that goes in the distributor, so replaced it with a spare that came with the system and problem solved.
Suggest you closely examine your disk for a crack, could be as simple as that.

[jakesmig]

Doubt it is ignition side by the sound of it, try and find out what fuel pressure to have, might not have enough flow at high speed. Damping on the despot should provide acceleration enrichment on throttle opening, if the piston is rising too quickly you will be missing this and the curling is weak, if it is rising too slowly the mixture is rich and you're flooding it momentarily.
Regardless IMO your problems are fueling.

John.

[Tim Proctor]

Thank you for all the suggestions.

John, the issue of dashpot oil came up several times in my internet research and was and flooding is an alternative scenario for the immediate bogging, as is the backfire, I suppose. The halt in proceedings of the engine is profound, though, it is easy to believe there is no ignition at all until the throttle is eased back. Would dashpot damping, ie, enrichment or leaning, produce this profound effect?

There was widespread confusion on forums about the effect of oil viscosity. I would have thought that on acceleration, the needle rising more slowly would keep the fuel aperture smaller, as it is more full of needle, and thus the mixture leaner. What am I missing?

Louise

[marshman]

Thank you for all the suggestions.

John, the issue of dashpot oil came up several times in my internet research and was and flooding is an alternative scenario for the immediate bogging, as is the backfire, I suppose. The halt in proceedings of the engine is profound, though, it is easy to believe there is no ignition at all until the throttle is eased back. Would dashpot damping, ie, enrichment or leaning, produce this profound effect?

There was widespread confusion on forums about the effect of oil viscosity. I would have thought that on acceleration, the needle rising more slowly would keep the fuel aperture smaller, as it is more full of needle, and thus the mixture leaner. What am I missing?

Louise

I used to think the same way, i.e. until the piston / needle rose up the mixture stayed weak, however the opposite is true. The following is a quote from the "Stromberg Service Bulliten".

"Acceleration
At any point in the throttle range a temporarily richer mixture is needed at the moment the throttle is suddenly opened. To provide this, a dash pot or hydraulic damper is arranged inside the hollow guide rod 17 of the air valve.
The rod itself is filled with suitable oil to within a iN of the end of the rod in which the damper 14 operates. When the throttle is opened, the immediate upward motion of the air valve is resisted by this plunger. For that short time the suction or depression at the jet orifice is increased and the mixture is enriched. For all normal requirements engine oil SAE 20 is suitable unless the car manufacturers recommend some variation from this. "

You have to think of the way it all operates and also consider the velocity of the air over the jet/needle.

So when you accelerate the mixture is made richer. No dashpot oil, would mean no damper effect and a weaker mixture.

I would say as long as there is oil in the dashpots the actual viscosity would not affect or produce what you are seeing. Has it only been like this since it was looked at in the garage. It could be that the mixture is just set a bit too weak and needs to be tweeked a bit. It it was OK and the problem has suddenly started then look at dashpots and also check out the diaphragms for splits. It could still be ignition related but will be on the H.T. side. Under acceleration the richer mixture needs a higher voltage to make the spark jump. This puts more stress on the Plugs, distributor cap and coil, any weakness will show up as a misfire.

Roger

[wilf]

Have the carb diaphragms been checked?

[jakesmig]

Thank you for all the suggestions.

John, the issue of dashpot oil came up several times in my internet research and was and flooding is an alternative scenario for the immediate bogging, as is the backfire, I suppose. The halt in proceedings of the engine is profound, though, it is easy to believe there is no ignition at all until the throttle is eased back. Would dashpot damping, ie, enrichment or leaning, produce this profound effect?

There was widespread confusion on forums about the effect of oil viscosity. I would have thought that on acceleration, the needle rising more slowly would keep the fuel aperture smaller, as it is more full of needle, and thus the mixture leaner. What am I missing?

Louise

I'll admit I mentioned dashpot oil a reasonable how amount in my earlier post, but I didn't really think I laboured the point that much, and my main point in the dash\oil scenario related purely to the abrupt 'stall' of the engine when you plant the foot. This is typical of poor enrichment fueling which is why I raised that point.
I also mentioned poor fuel pressure and fuel flow at full power. Bear in mind you probably require 4/5 times the fuel flow at high revs / full throttle that you require fiddling along at 60mph.
Whilst I've been buggering about with engines for 50 years I'm not a qualified mechanic, so as some would say "wtf does he know".

[flying farmer]

If the engine will rev to max rpm at light throttle settings but misfires above a certain revs at full throttle it is quite possibly a weak spark. The extra mixture density in the cylinders at full throttle makes it harder for the spark to jump the gap between the electrodes.

Might be a poor ignition coil, plug gaps that are too wide (new ones generally are set at greater than the recommended 25 thou) and possibly low voltage at the coil caused by the ignition ballast resistor (or resistor wire in the loom on the mk2)
The resistor wire in the loom caused me no end of grief on my mk2 until I found it was the problem and replaced it with a mk1 ceramic item. It particularly gave problems at low rpm when the lights or cooling fans were running and the low voltage caused misfires at idle as well as high rpm misfires.

The capacity of the float chambers on the Strombergs should give a few seconds of fuel supply before the level drops sufficiently to create the misfire if starting from low revs.
If fuel flow is the problem then flooring the car from a standing start should let it rev through the first couple of gears without trouble,(if it does this then the ignition is not a problem)
If the misfire then begins after a few seconds on full load it would suggest the float chambers have emptied. The backfiring can be caused by a mixture that is too weak to be fired by a spark plug being lit by the burning gasses in the exhaust manifold.
Neil

[Tim Proctor]

Marshman, thank you for the quotation about the needle damping, that makes much more sense of the situation

The carb diaphragms have been replaced. I think the diagnostics will start with plug gaps, as the problem *may* have begun after three new plugs were fitted. After that, the rest of the HO circuit, I suppose.

Oh, joy....

Louise

[Kithmo]

Marshman, thank you for the quotation about the needle damping, that makes much more sense of the situation

as the problem *may* have begun after three new plugs were fitted.

Louise

Why only three new plugs ? Perhaps you should replace the other five as well.