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Loss of power under load
My Ruby is in frequent use often covering 40 miles a day. Recently I noticed it was hesitant under load, normally towards the end of the 20-mile daily commute, sometimes popping on the overrun. It has gradually got worse and now fluffs /splutters between gear changes and struggles on hills. The first 5 miles of the journey are fine but this is gradually decreasing. Starts ok and idles fine, picks up from idle without hesitation runs well under minimal load.

I have changed the coil, condenser and rotor arm. Checked the points gap, checked battery voltage (6.4v). Plug leads are about 3 years old. Changed the exhaust manifold and inlet manifold gaskets, lagged the exhaust pipe, made a heat shield for the carb, cleaned the bowl, cleaned the emulsion block and checked the jets. Petrol is coming through clean and the tank has never had sealant in it. I've tried petrol from 2 different garages.

Each time out it gets slightly worse. What have I missed? Beginning to wonder if it could be a leaking head gasket? Any ideas appreciated.

Fuel pump diaphragm?
...or even corrosion in the pump allowing internal leakage, or a pump valve not operating properly. First take the top off and make sure its not full of crap.
A friend has had a very similar problem with a SV Anzani engine. Fine when cold but performance dropping off as it gets hot. A leak down test showed the transfer of compression between 2 and 3 as the engine warmed indicating a slightly warped head. Head now skimmed flat and, I'm told, so far, so good.

As an alternative scenario, does it have a Bosch distributor and, for whatever reason, the drive gear is wearing rapidly?



Edited to add, but I think I like the head gasket idea more.
To me, this sounds like an HT fault. The High Tension voltage that needs to reach the plugs and create a spark.

Under load the pressure within the cylinder increases. At higher pressure the voltage needed to jump the plug gap is a higher voltage.

If there is a crack in the rotor arm, or distributor cap, or the coil neck or even the spark plug, then the current will take the easiest route. At low revs and not under load the easiest route is likely to be to jump the plug gap and ignite the mixture in the cylinder. At higher revs, and under load the voltage to jump the plug gap increases so there is more chance of a short circuit of the HT through some crack somewhere.
Exactly that used to happen to me in a mark 1 Fiesta - it was carb icing, sitting for 5 minutes in a lay-by always fixed it for about 20 miles.
Has it started since the weather got warm?
Alan Fairless
Thank you for all your suggestions, very much appreciated. To answer them in order:

Reckless & Robert - a quick peek inside the pump revealed quite a bit of crud which I have swilled out as best I can. I'd forgotten there was a filter in there - it was clean. The problem is still there so next week I'll take the pump off to clean it out properly and check the valves. The diaphragm is only a couple of years old.

The head was warped Steve and I had it skimmed two years ago so I would expect it to be ok, although I have noticed a trace of oil around the centre stud recently which I thought was odd.

No Stuart, it's a DK4A dizzy with a good drive gear and no play anywhere so probably not that.

Re: HT Andrew, as I say the coil and rotor arm have been changed but not the plugs and cap. I will try these in order to rule them out but it isn't a distinct misfire, more a hesitancy.

The carb isn't icing John and it needs to stand for a good hour or so to cure the fault - but I do know that symptom from my Mini days.

And finally Alan, it doesn't seem to relate to the ambient temperature ... and I live in Inverness so heat is rarely a problem!

I will report back once I've had another go at it. Thanks again.  Peter.
Hi Peter,

My experience of popping and banging on the overrun, always suggests leaky exhaust or retarded ignition in my experience.

Good luck,

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