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MG MGB Technical - stage 3 head

I have a 1975 bgt. The previous owner fitted a stage 3 head but, so far as I can discover, left the standard cam in. It was suggested to me today that this might mean that all the petrol might not be burnt consequently risking premature bore wear. This does not seem likely to me since it is the cam, is it not, that determines the lift and the amount of fuel delivered.

Anyone any advice?


Rod
R E Merrall

Rod,
there's a few things at play here and it's not that simple to confirm what the effects will be but here goes....

A stage 3 head should have bigger valves and should be polished and ported. How much the ports have been opened up determines the air/fuel mixture speed on it's way from the carb to the cly. Too big and the fuel can "drop out" at low speeds and cause a rich mixture at higher speeds when the airflow increases.

You're correct the cam has an impact on the amount of mixture entering the cyl, however it's the carb that determines the amount of fuel for a given flow of air.

The only way to see the what's really going on is to get a rolling road session to measure the Air Fuel Ratio across the full operating rev range.

I'm sure others will be along with better/fuller explanations and if PB drops by... listen as that will be the gospel..
Over to you PB :-)

Best of....
MGmike


M McAndrew

Exactly what Mike says, get it on a rolling road and ensure it is running as best as it can. Do you have some sort of problem leading you to write the post?
All I would add about stage 3 is what does that mean? There are no laws governing what mods constitute what or even guarantees of performance increase. The only way of knowing what spec head you have (unless you can find from the po, who did the head to what exact spec) is to remove the head and strip it for examination.

Peter
Peter Burgess Tuning

Rod,
You don't mention whether there have been any commensurate improvements to other aspects of the engine's breathing; i.e., air filters and exhaust system or indeed any fueling adjustments, namely SU needle change.
Allan Reeling

"all the petrol might not be burnt consequently risking premature bore wear"

That seems highly unlikely to me as well, unless the mixture was grossly rich, which would have other symptoms. A gas analyser should tell you if it is putting out unburnt fuel at some rpms, I would have thought.

Not a cam supplier that told you that, perchance?
Paul Hunt

Thanks for the comments. I have had no problems in running, the engine pulls well etc. The po had the head fitted by Osselli about 10 years ago together with HS4 carbs, electronic ignition and richer needles. An MG specialist in Fife said that I might have the problems with unburnt fuel and that prompted the post. MOT is due next month so see what the reading suggest. Don't know of a rolling road near here.


Rod
R E Merrall

Rod,
Cardenden or Lundin Links?

I'd be very surprised if it was Cardenden ;-)

Google shows 3 rolling roads in the Kingdom but I've no recommendation or knowledge of them or their classic capabilities.
Let me know if you do book a session as I'd like to tag along for the experience...

best of
MGmike
M McAndrew

Mike

Thanks. Specialist was in Cardenden! I might well try a rolling road but probably not until the spring. Car is eligible for historic tax in April so will be off the road in March. If you email me on rodmerrall70@sky.com i'll keep in touch.

Rod
R E Merrall

Anyone used M-Tech rolling road in Westbury Wilts, or any recommendations near Bristol?
c cummins

Rod,
"richer needles". WHICH???? Just "richer" doesn't mean a great deal, except they are smaller than standard after the first 1/4", metering out more fuel, but whether it's giving you the optimum mixture through the rev range is unknown.
A good rolling road will be able to tell the mixture at different revs and varying loads. Then you might stand a chance of matching the mixture delivery for your particular engine.
Allan Reeling

It isn't just a richer needle but it's profile along the needle stations. My car was finally sorted on the rolling road at Aldon using a needle close to AAA and adjusting it with emery paper. A 20th century equivalent to remapping. The original B head was the bottleneck in gas flow so portbwork will always help even if you do nothing else
Stan Best

The trick is finding a rolling road with a good selection of SU needles, as I'd imagine Peter B has. Mr Cummins - that may be the issue with M-Tech in Westbury, unless they do old BMC stuff. If you have some success, please let us know - it's not a million miles from here! There is (or was) a rolling road in Gillingham, Dorset that set up a colleague's sprint/hillclimb Mini way back when. Of course, one can always kick off with the SU needle chart and some graph paper to give one a rough idea before getting the professionals in!
P A Allen

This thread was discussed between 02/02/2016 and 28/02/2016

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