850 MK III Warm Up

My MKIII is a bit tired, consuming some oil so its time for a top end job. I am looking to improve performance yet still be able to run reliably on pump gas. (It doesn't have the snap of my "72 Combat). Anyone have any experience with good results? Increase compression? Mild porting, etc? I am not looking to snap the chain just enough to know I did something after I am done. I don't want to split the cases.

0u812 said:

My MKIII is a bit tired, consuming some oil so its time for a top end job. I am looking to improve performance yet still be able to run reliably on pump gas. (It doesn't have the snap of my "72 Combat). Anyone have any experience with good results? Increase compression? Mild porting, etc? I am not looking to snap the chain just enough to know I did something after I am done. I don't want to split the cases.

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Gooday Ou812,

I found the RH10 head the best, stock cam, just up the compression to about 9.5 or 10:1. Electronic ignition and good Amal 32's and it keeps with my combat easy. Runs best on about 95ron. I do not know what gas you can get.

If you have the RH4 head someone else will have to help you.

Cheers

0u812 said:

My MKIII is a bit tired, consuming some oil so its time for a top end job. I am looking to improve performance yet still be able to run reliably on pump gas. (It doesn't have the snap of my "72 Combat). Anyone have any experience with good results? Increase compression? Mild porting, etc? I am not looking to snap the chain just enough to know I did something after I am done. I don't want to split the cases.

Click to expand...

Combat Commando with higher compression ratio, 2S camshaft and approx 30kg less weight! Replace the 'black cap silencers' for 'reverse cone silencers' get rid of the electric starter/gear and you will be surpriced.

Where is there 70 lbs of extra weight in the mk 3?

I can see maybe 25 max in the starter and assembly, but the engine has 15 % more torque than the 750 (combat 49, reg 750 48, 850 56 ft lbs)
If the engine is put into good shape, it should feel just as strong as the Combat, perhaps even a little stronger midrange.
But the stuffed up silencers must go. I dont know what Norton was thinking about with those.

I fitted a thin head gasket to mine, this bumped the compression to about 9 to one, same as a regular 750.
So with that you have an engine with the same cam and compression as a regular 750, but Combat sized ports and carbs, (rh4) only a better porting job(Dave Comeau) , and the extra cubes plus stronger cases to go with it.
JS sells an ultra thin head gasket, I think it will give you about 9.5 to one, perhaps he will comment.

Glen

worntorn said:

But the stuffed up silencers must go.

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Well, most will have already been replaced with peashooters years ago.

worntorn said:

I dont know what Norton was thinking about with those.

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The introduction of the annular discharge silencers and black plastic airbox became necessary due to noise emission regulations introduced in certain countries from 1973-on that peashooters were not capable of complying with. Apparently, the annular discharge silencers were carefully designed and not all that restrictive.

My Mark 3 still had the black caps when I purchased it 5 years ago. Top speed was an indicated 95 mph. I changed to open peashooters then ran it down the same stretch of road at an indicated 115 mph, no other changes.
Since then Ive installed the thinner headgasket which did make acceleration sharper, havent tried for top end.

Glen

0u812 said:

My MKIII is a bit tired, consuming some oil so its time for a top end job. I am looking to improve performance yet still be able to run reliably on pump gas. (It doesn't have the snap of my "72 Combat). Anyone have any experience with good results? Increase compression? Mild porting, etc? I am not looking to snap the chain just enough to know I did something after I am done. I don't want to split the cases.

Click to expand...

A top end job will warm it up considerably all by itself. Keeping things stock and standard is what I would suggest for you. With the barrel off, you can get a good look at the cam to see just how far you need to go.

If you want to keep fuel options limited to pump gas, you better leave compression, head work and cam close to stock. If going for broke, modern valve inserts can be had.

That being said, Intake and exhaust options have grown considerably. An old school option would be a pair of Amal Mark 2's and an exhaust upgrade, This should and will potentially give you all you can handle. Drop a tooth on the gearbox sprocket for an added nudge.

Top end job first, though.

Who makes a good 9.5:1 piston for 850's these days? I think I can get by fine and they would up the punch a little for sure. Thank you for the reply.

All (?) 850 pistons are flat topped - you change the compression by the head gasket, fitting a different RHxx head - or skimming material off the head. Read the above posts again.

BTW, what is the consensus on doing a 'top end job' without checking the bottom end - bigend shells, mains, sludge trap. For many engines (and mechanics), this is a recipe for bottom end problems ??

Here are some head gasket options.

http://www.jsmotorsport.com/technical_compression.asp


The thinnest will give you 9.5 to one, which is about as far as one might push things for a road bike on 92 octane.
I installed an .020 fire ring head gasket, in my 850. According to the info on the JS site, an. 020 head gasket will give compression ratio of 9 to one. It definitely added some punch.

I guess 9.5 would give just that little bit extra.

Glen