Norton 750 Detuning

In my Norton restoration book by Roy Bacon, It states that vin # 211110 was the first detuned Norton engine. Was this a 1972 or 1973 Norton 750. I found a Norton Vin [HASHTAG]#211170[/HASHTAG] and the owner states it is a 1973. Is this correct? Also what did Norton do to make it a detuned engine?

I believe the reference is to the bikes NOT being made as Combat motors. The word "detuned" is a little confusing as 750s made prior to Combat tuning would have been what? Pre-tuned?

nortonisthebest said:

In my Norton restoration book by Roy Bacon, It states that vin # 211110 was the first detuned Norton engine. I found a Norton Vin [HASHTAG]#211170[/HASHTAG] and the owner states it is a 1973. Is this correct?

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On the next line of your Norton Restoration book it should say "1973....212278".

The actual date of manufacture is normally stamped on the frame plate?


A model manufactured in 1972 may very well be 'titled' as a 1973 as it would've taken several weeks for bikes to reach US dealers and then they may not have sold immediately so it's not uncommon for bikes to be titled later than the actual calendar year of manufacture.

nortonisthebest said:

Also what did Norton do to make it a detuned engine?

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This applied to approximately 1500 Combats that had already been manufactured and returned to the factory and involved reducing the compression ratio, initially by fitting a thicker head gasket, replacing the Combat camshaft with the standard camshaft and fitting a larger gearbox output sprocket.

rvich said:

I believe the reference is to the bikes NOT being made as Combat motors. The word "detuned" is a little confusing as 750s made prior to Combat tuning would have been what? Pre-tuned?

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They would be 'standard' tune. Standard tune models were also made during the 'Combat' production period.

Post Combat 750 is slightly more complicated:

30mm carb, standard comp. head.
32mm carb, RH5 'low' (standard) comp. head.
32mm carb, RH6 'high' comp. (not as high as Combat) head.

I forgot to include to date stamped on the head plate. Oct-72. Engine [HASHTAG]#211170[/HASHTAG]. Could this be a Combat motor?
The first detuned combat was 211110.

nortonisthebest said:

I forgot to include to date stamped on the head plate. Oct-72. Engine [HASHTAG]#211170[/HASHTAG]. Could this be a Combat motor?
The first detuned combat was 211110.

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I think the term "first detuned Combat" is misleading.

According to one factory Service Release, Combat production ceased from 211110 therefore, 211170 shouldn't be a Combat. If a Combat is 'detuned' then it's no longer a 'Combat' as this was a state of tune rather than a model type.

Check for the "C" stamped on the top of the head. (It may be under the headsteady). The serial numbers aren't always the best source.

My stock Trixie plate sn 210563 date (Oct.) 10/72 still has all the warts/quirks/features/trills of factory issue Combat Bombs, minus alu or composite gasket that pistons stick up through .050". Trixie does not have turn signals. Not detuned as lifts front in 1st/2nd passing 6800 rpm, a few times tested. Who knows what was in the works by then as the Bomb shells had already gone off with warrenties bad PR sucking Norton reserves down to back peddling newer 'improved' models. My 1st Combat sn 206XXX March '72 (pre-Peel) was essentially identical to Trixie but had decent composite FlameRing gasket so was a bit more responsive when acquired as slightly massaged-tuned up-dialed as demo bike for sales shop maintaining it. Both Combats came in tradtional black/gold paint, Trixie Roadster the other Interstate tank. Added rear brake safety spring to Trixie as major non Norton upgrade. If detuned version enjoy any ole electronic ign curve but short changing grin cracking lip parting w/o contact breakers leap onto 2S cam rear tire wear.

OK, So this wouldn't be a combat but a standard 1972 750 commando. It still wouldn't have all the 1973 750 final improvements being a 1972 engine # 211170 Oct 1972.

nortonisthebest said:

OK, So this wouldn't be a combat but a standard 1972 750 commando. It still wouldn't have all the 1973 750 final improvements being a 1972 engine # 211170 Oct 1972.

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Theoretically, 211170 shouldn't be a Combat, but without more detailed information it can't be absolutely ruled out.
Unless it was a Hi-Rider originally, then a US apparently post-Combat model is likely to have the 32mm carbs, 'high' compression 'RH6' head and disc front brake.

I think you answered all my questions.
Thank you all for your comments.

Now ya know where to set ign timing and octane requirements. Ball park way if assembled is compression test w/o the leak down risk bother. IIRC good Combat usually significantly over 175 psi but tricky conditions skew interpetations.

Likely this was a late '72 MkIV 750 exiting the factory with post combat spec. The '73 750 model year incorporated MkV specs and had a few differences compared to the MkIV. As above mentioned MkV came in two tunes - RH5 head was low compression and RH6 high compression. Neither had the combat cam nor the shaved head "C". The 750 MkV had body parts used on the new 850 MkI, such as black long speedo/tach cans, stainless fenders, square Lucas tailight. Generally assumed the MkV engine number sequence started 220xxx (does not include the mysterious 235xxx series).