Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing plus 8 carburator issue

Dear all,
my 1972 morgan plus 8 with twin SU carburetors stutters (really bad) on pick up from idle (including pinging and ratteling and finally killing the engine if not quickly off the throttle) but only when hot.
When not hot she runs like a charm. When hot, trying to accelerate from idle is impossible.

Have a fuel pump in the back on the tank and excess fuel is passed through the carburetors (so assume the fuel does not get too hot).

Ignition is at 6 degrees advance on idle.
New electronic ignition installed.

All ideas are welcome, thank you !

Tim

I would think it is most probably the fuel getting too hot and vaporising. Is the return to the tank working correctly? Is it quick enough to prevent excessive heating?

If not that then is air getting in post carb when hot? I’m thinking a gasket failing.

Have you considered if the ignition coil is failing when it is getting hot?

Is fuel pressure at the carb not more than 4PSI?

Lots of possibilities....hmm...? Pinking/pinging under acceleration is usually a sign of advanced ign timing and/or low fuel quality... though you typed that you have a NEW electronic ign system.... did the fault show up after fitting the new ign system..?

Some earlier Rover Engines were high compression and designed to run on higher octane fuel than is available at the pumps today which can cause issues...

From a poor memory I think my 85 +8 is set at 6 deg BTDC with a strobe timing light, and runs with the distributor vacuum disconnected. I did have an issue with poor fuel quality bought in a back road garage in France that caused pinking under load, it cleared when I filled up at a main line fuel stop...

If you have a weak mixture, that too can cause pinking and overheating, sometimes the engine runs on a little when the ign is switched off.. When the engine is cold the choke will be making the mixture richer.

Weak mixture/not enough fuel Rich mixture/too much fuel If you look at the spark plug electrode colours that might give you a clue ...?

I am confused re your fuel pump installation.... I have limited knowledge.... the fuel pump, stops pumping when the float in the carb closes it`s fuel supply (needle) valve, the fuel pump senses the rise in pressure and shuts off.

In an EFI system there is a higher pressure fuel pump in use that uses fuel to cool its self and such systems have pressure regulators that return excess fuel back to the tank via a seperate return fuel line...Was your car originally fitted with EFI...???

Yes this was My quandary also. Almost sounds like He has an EFI pump hooked up to His SU's.

Do you have enough/correct oil in the carb dampers? Where was your last petrol fill up? Correct petrol used? Was it OK before the new ignition? Something hanging up inside the distributor for the advance/retard.

'New Electronic Ignition"?

does that include a matching coil?, Had a similar problem with my stag and it was the coil overheating. To eliminate from the list swap the coil to a good know until as soon as symptoms start, if they disappear then you've found the problem.

Gary

Try to avoid using E10 and even E5 petrol if at all possible.
Is a ballast resistor on the coil necessary with the electronic ignition?
Each of the other suggestions are possible sources of the problem I'm afraid, so it becomes a process of elimination.

The purpose of a ballast resistor is to drop the voltage going to the coil once the engine is running. The design being that the coil`s operating voltage is perhaps as low as 6volts, thus the resistor drops the normal running voltage of the cars electrical system to circa 6 volts... However when the ign key is in the start position the ballast resistor is bypassed and the ign coil then receives whatever battery voltage is available to give it the best chance of generating a good spark.

Some such systems make use of a resistive length of wire to drop the voltage, so not always easy to spot and if in fitting a replacement coil a 12 volt coil is used, then in normal running it will be trying to operate in an under voltage situation..

The short answer is NO... (-:

Apologies if you already know all of that.

Again, happy to be corrected if I have made any mistakes..

Principles are fine but I think 9 volts more usual than 6.