Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing What Octane Gasoline to use?

Nowhere in my owner's manual/service book does it say what type of gasoline (petrol) to use in the car. If there are any TR4A motored Morgans out there, can you tell me what octane gas you all use?

I have a few issues with my motor. My motor seems to be idling too high, it is around 1500 rpms, or so the gauge tells me so. Also, when I drive around for an hour or so in this warm sunny weather the engine is nice and hot. When I turn it off the motor keeps running for a little while. I guess I need someone who knows how to tune the carbs a bit better. I have found that the running on of the motor can be caused by a too high idle, or a too low octane of gas. I have been using mid-grade fuel.

Thanks

Unless the compression has been increased, the engine should be OK on mid grade fuel. I run my TR4 on 97octane but it is a higher compression engine. Could be that your timing is too far advanced, but if you want some reading to reassure you that running on is a common problem, try this link: (Post No 13 is helpful.)

http://www.tr-register.co.uk/forums/index.php?showtopic=35035

Having said all that you should try to eliminate it, and a higher octane fuel may help whilst you get it set up.

Malcolm

Main things to check are, Advance/Retard on the Distributor, also that idle speed needs to come down dramatically, check for air leaks into the inlet side !!

Running on can also be a result of the wrong heat range on the plugs. Won't be the idle issue though.

If memory serves me the handbook of the TR4 (1966) said 97 octane ("Super" on the Continent) and idle speed 800 rpm. I always had/have it at 1000 rpm (also on my 1500 Cortina) for smoother running.

Thanks for all your input. Very interesting issue. I'll let you know how it progresses.

Oh yes... the octane. It seems here in the States, we have Regular - 87 octane, Mid - 89 octane, and Hi - 92 octane. I wish I could find the 97 octane. That may be the culprit. I have a new tank of the hi in it, and hope it fixes the run-on dieseling.

I'll have to learn what the Advance/Retard is on the distributor and find out what proper heat range plugs to use.

Thanks

Careful with octane rating, there are several different standards. So your US octane could be different to the octane in your handbook.

For a TR4 engine, NGKs UK site says a BP6HS is the right one. Same heat range as my three wheeler!

A BP7HS would be the next colder running plug but I suspect it would not reach self cleaning temperature and foul.

Vitruvius, if your engine runs without pinking under acceleration on "mid" your ignition is retarded correspondingly. This could also be the source of the run-on. I'm afraid "hi" will not cure it if the ignition is not advanced a little at the same time. To advance the ignition is an easy task. Your mechanic will show you how in less than 5 minutes.

When I crossed the US in 1983 in a heavily pinking Buick I used Octane booster. It helped sometimes but not every time. As a last resort you could try this after advancing the ignition.

I think I need to explain distributor function traditional type ignition.

In the body of the distributor, attached to the shaft, are some weights and springs. The function of the weights and springs is to advance the ignition with revs and the springs to return the timing to the static figure when the engine slows down and stops.

On road car distributors, there is also a vacuum can which is aimed at putting more advance in when the inlet vacuum is high. An example of this would be accelerating to cruising speed. While accelerating, the can does nothing, but when cruising speed is reached, the driver lifts off the throttle, a high vaucuum is formed in the inlet manifold and the can further advances the ignition on what the weights have done. You don't need or get extra power, but you gain in fuel economy. Put your foot down to overtake, the vacuum disappears, the ignition goes back to the weight only setting so you lose the extra advance. This is fine as the weights and springs give the advance for load, and the vacuum can only the further advance for economy.

No where though does the distributor retard timing, it advances the timing with revs, and it advances the timing with vacuum.

If the balance weights stick, the most common symptom is the timing remaining advanced as you slow down, and if you then press the throttle, the car 'pinks' but doesn't do it from rest. Cure is usually as simple as a spray of light oil on the weight mechanism.

Calling the vacuum can an advance and retard device as many do distracts from its real purpose, an economy device.