Hello!
I've just signed up to see if anyone can offer some pearls of wisdom to help see our SP250 Dart back to it's old self....
Basically we've been chasing a running problem for .... ages

The Dart starts from cold on the button and runs fabulously, very little choke required. You can drive off and enjoy all the fantastic delights of the engine, rev it through the range, small or large throttle inputs, snap on and off throttle... all sounding and pulling superbly. It's a real delight, with all the right noises........ until ... it gets warm

Then it starts to lack power and WILL NOT rev..... the higher up the rev range you go, the more it bellows out of the exhaust and sounds like it's strangling itself. If you open the throttle fully it just bogs and will not pull! If you use light throttle pressure it will rev right round the clock, but open the throttle any more and it starts to really complain....

Initially we thought it was an ignition problem and there did indeed turn out to be a problem with a coil. This was changed, but problem remained..... and on and on and on it went.... we've discovered all manner of problems along the journey but STILL not managed to fix the root cause.....

So far it's had:-
Timing checked
Numerous sets of plugs
Electronic ignition kit
Numerous coils
Plug leads
Distributor cap
Rotor arm
Carbs rebuilt (Burlen Fuel Services)
Valve clearances double checked
Conversion from +ve to -ve earth and new alternator
New earth leads
Another, different electronic ignition kit
Timing re-checked etc.

Yet it STILL won't rev under open throttle when hot.... Runs great when cold though!

So what are we missing?

ANY suggestions extremely welcome.... hair is officially being pulled out now...

Petrol evaporation where fuel pipe is near hot exhaust?
Petrol pump not giving full flow? (probably not this cause if it pumps a lot for a cold engine).

I had a similar problem with my saloon and fuel vaporisation was the problem. I insulated the petrol pipe with some material used to insulate hydraulic pipes on earth moving equipment where it passed near the exhaust. Another thing to check is the ventilation of the petrol tank. There was a recall in the 1960's to remove the spring loaded ball bearing in the cap, also check that the cap is not sealing on the rubber grommet between the body and the filler cap. A way to check is run until it starts playing up and then check if the there is a rush of air into the tank when you loosen the petrol cap.
Regards

Colin

Have you checked for your float chamber needle sticking closed or not opening fully?

I could understand fuel vapour lock if the engine is running hot. By that I mean hotter than it should be.
When the symptoms started, had any work been done bringing the fuel line closer to the exhaust?
Try a rotor arm from the Distributor Doctor or Barry Thorn. The Lucas ones from suppliers are as much use as tits on a fish!
I had exactly the same symptoms as you and spent a lot of time and money going down the wrong routes only to find it was the rotor arm shorting out. It was only while on a Dino tester when it shattered and I put the old one back on did the engine run as sweet as a nut.

Re shorting Rotor Arm . Simply hold the coil lead ( out of the cap ) to just above the rotor . flick engine and if the unit is faulty it will arc to earth through the Rotor.
steve

I had similar sounding issues on my saloon which I eventually cured by replacing the voltage regulator, it's a cheap enough part so might be worth a try

Thanks for the thoughts and suggestions!

There are no signs of fuel vaporisation - it exhibits these symptoms whatever the weather, even in the dead of winter when ambient temps are way too cold to allow vaporisation. There's also a glass fuel bowl / sedimenter in the engine bay just ahead of the carbs which doesn't empty when running.

Fuel pump is the correct SU type electric, is also new and appears to be working perfectly, certainly no symptoms of low delivery.

Carbs also effectively new (rebuilt by Burlen Fuel) so no sticking floats or sloppy needles etc.

Rotor arm is a thought..... I have a black one fitted which I think was a Lucas item.... I will test as described and see what it does.

Voltage regulator is also new and this fault has persisted from the old +ve earth dynamo to the new -ve earth alternator (it's actually a dynamater, but same thing) with inbuilt voltage regulator.

Just one more thought on vapouring, but I agree it doesn’t sound to be the fault.
Do you have the black insulation spacers that fit between the carbs and the inlet manifold? If not heat transfer is considerable and can increase the vapouring problem.

Are the carbs running too rich? You say it needs little choke on cold start up. The engine won't like a rich mixture when hot. Does it 'hunt' on tick over when warmed up? What colour are the plugs?