Combat to standard conversion

Status
Not open for further replies.
Hi Bill,

Too late for not butchering the cases, already done by the PO. I guess this might preclude using the original breather location, although the Comstock/CNW breather valve could make it work anyway?
The 2S cam in the bike is worn out and the owner would prefer to use a genuine Norton item, unless convinced otherwise. This is however something we will discuss thoroughly before placing an order.
I will change to the later cam bush system, the twin bush Mk3 looks good I think, though anything is better than the helpless original mess.
What were they thinking etc...
Skimming the pistons should work, the crown is around 5 mm at the thinnest, the cut outs not withstanding. Cutting off 1 mm/.040" should be OK I guess, but it does reduce the margins regarding detonation and things like that. Have you tried this out yourself?

Food for thought, thanks Bill.

SR
 
Combat cams were not scrolled originally and the cam bushing was X-ed for oil grooves which came with the two tabbed washers. Design changes to remove the large oil sump plug filter, moving the oil pickup forward and this cam bush change....What could go wrong. :shock: Murphy comes knocking at your door. Hind sight is 20/20 and I just touched on some of the things that gave the Combat a bad rap.
If the new cam is scrolled then a cam bush change would be good. Just be aware there are 2S new old stock still around. I have a new 2S cam that is scrolled. My combat bike is un-scrolled. A combat if it is well sorted is a nice bike. Keep the compression at 10:1 if you stay with the combat cam.
Cheers,
Thomas
 
Hi Thomas, thanks for your interest. We're most likely shopping for "genuine" Andover Norton parts, the bike's owner is adamant about this. From their website it looks like their 2S cam is scrolled. I wouldn't run a non-scrolled journal in a plan bearing (though I do think it would work, hot oil spreads as easily on the inside as the outside!) but a scrolled bearing on a scrolled journal should work fine. The scrolled drive side bearing on this engine is fine so I don't want to change it, but the original timing side one will be replaced.

SR
 
Hi Tri-man,

When I bought my Combat, several decades ago, it had been de-tuned by our local guru for a previous owner. He simply installed a standard factory cam and two copper head gaskets to drop the compression. Sounds a bit crude by the methods of today, but that's how people used to do it. This bike ran great, lots of low and mid-range torque, and idled better than any Commando around. I have long since changed it back to Combat spec, which is as easy as it gets, change cam, install one head gasket. I have never found the combat engine to be particularly lacking in the mid-range, it just wakes up noticeably at around 4k and keeps on pulling. There is nothing especial about my engine, stock bore, has super blends and the typical crank case mods, all from before I owned it, Boyer, re-sleeved Amals, that's about it. Doesn't have to be fancy to work.

B
 
Hi Interbak,
Thanks for your input. The engine in question is a well used and abused old thing, I delivered the crank to the grinder today, I'm not sure it will clean up with the second grind, may need a third, there's evidence of butchery everywhere and I spent a few hours removing nicks and dings from the rods already. High revs aren't on the meny anymore I'm afraid. The more I look into this engine, the less I want to stress it, so I believe we'll go with Plan A which is a standard cam and lower compression, just like yours was.
I too have used two head gaskets on occasion, never had any trouble with it either, but there must be thicker gaskets available out there somewhere, like with Jim Schmidt?

SR
 
tri-man said:
I delivered the crank to the grinder today, I'm not sure it will clean up with the second grind, may need a third, there's evidence of butchery everywhere and I spent a few hours removing nicks and dings from the rods already. High revs aren't on the meny anymore I'm afraid. The more I look into this engine, the less I want to stress it, so I believe we'll go with Plan A which is a standard cam and lower compression
there must be thicker gaskets available out there somewhere, like with Jim Schmidt?

SR
Click to expand...
tri-man
As per section C20 of the Norton workshop manual, you do realize that the crankshaft can't be ground with the flywheel center and crank cheeks as an assembly. the stone also needs to have a radius with a 0.90" rad is very important on both edges. Especially if you are on a third grind. Hopeful your machine shop grinder knows this and is familiar with Norton Commando Cranks. If you already know this procedure as old news then pardon me, but I am just checking.
Cheers,
Thomas
 
tri-man said:
I too have used two head gaskets on occasion, never had any trouble with it either, but there must be thicker gaskets available out there somewhere, like with Jim Schmidt?

SR
Click to expand...


Yep, JS do copper gaskets in various thicknesses. I used a 60 thou head gasket to reduce the compression when I returned my race bike to the road.
He also does copper base gaskets which will lower the compression a bit more, but more importantly seem to seal better than the genuine paper ones.
 
Hi Thomas,
I appreciate your concern and one should always be grateful for good advice. I gave the machinist the sheet from the manual. We talked it over to boot, I feel confident. By the way, he's a brit biker too, and has done a number of BSA and Triumph cranks before.
The flywheel is removed.

Hi John,
Very useful info, thank you. I will have a look at the JS stuff today.

Regards
SR
 
The compression ratio should not alter the torque characteristics, so unless you have a fuel problem, I would not lower it - and if the inlet ports are tapered back to about 30mm at the valve guide , that should be good. The cam determines the width of the power band and where in the rev range it occurs, however a well-designed 2 into 1 exhaust will often take the top off the power- band and add it to the midrange. If you go down this path, you might need a vernier adjustment on the cam so you can optimise. If you are changing the cam, more lift rather than more radical timings might be the way to go. If you advance a standard cam a few degrees combined with a 2 into 1 exhaust system, you can get more torque. With a combat cam, I would advance it about 6 degrees.
 
Hi acotrel,
What you are proposing here is good stuff.
I'm sure you''re mods would work well and give a solid mid range and an eminently usable bike, but this is a customer's bike so I need to go with his wishes rather than mine. The bike itself is a very nice standard bike with two pipes and Peashooters that he intend to keep. He'd like to keep the engine manageable and easy to live with.
This engine has been USED to its fullest Combat potential over a long time, second re-grind needed on the big end journals and evidence of the mains rotating in their bosses etc.
High revolutions is a killer, I believe a standard cam with a sensible CR, combined with sympathetic use, might give this tired old Norton a new lease of life before retirement. I'd like to keep it as simple as I can.
 
8.7 : 1 worked well with a similar spec . But 32 Mk II Amals .

BSA advance unit gave good bottom end , with beefy Adv Springs , Full Adv at 3 or 4.000 .

Softer at the bottom with the Combat Cam , megaponitus at 3.200 . Over 3.200 it was acelerateing .

Std Cam it was accelerateing from 1100 rpm , as the Clutch was IN .

23 T gearing isnt really suitable for he City , as 80 mph is over the limit . The ' sweet spot ' .
So 22 T would get top whack , and be a little more tractable .

You Just leave a car length in front , getting underway , in traffic .
Hit Neutral if their Brains Abscent , Then youve a choice of 1st if you have to slow ,
or 2nd if they are actually getting out of the way .

Maybe theres better set ups now , but the Junction Box , particularly the short plastic one , isnt good for balance or smoothness .
A dual Cable Twistgrip was preferable .
 
Std Glasspac opriginal ' mufflers ' with the glasspac long gone , are actually ' reverse conee ' or ' doble come ' Megas with perforated tube end to end . Interstae & Roadster .

A Flat spot or stutter , only pronounced under W o T from Low Revs , like when you saw a Honda , Cruising in top gear . :p three blats ( noise ) and it'd clear , if present .
 
If you have got little bottom end power under 3000 RPM and your overall gearing is high, you at least need a smooth transition from nothing to a lot of power. Megaphonitis is often unreliable. Especially with a road bike in traffic, the power has to be there when you need it. The answer if you get into trouble is to slip the clutch, however you might already be struggling to stay alive. You can probably tell from what I've said, that I don't ride a motorcycle on public roads - on a race circuit is much safer.
 
tri-man said:
Hi acotrel,
What you are proposing here is good stuff.
I'm sure you''re mods would work well and give a solid mid range and an eminently usable bike, but this is a customer's bike so I need to go with his wishes rather than mine. The bike itself is a very nice standard bike with two pipes and Peashooters that he intend to keep. He'd like to keep the engine manageable and easy to live with.
This engine has been USED to its fullest Combat potential over a long time, second re-grind needed on the big end journals and evidence of the mains rotating in their bosses etc.
High revolutions is a killer, I believe a standard cam with a sensible CR, combined with sympathetic use, might give this tired old Norton a new lease of life before retirement. I'd like to keep it as simple as I can.
Click to expand...

Your customer wants a pre combat conversion. The quickest way is camshaft and head change as already mentioned. My combat developed a terrible pinging problem due to poorer fuel. What I found was a previous owner changed to the standard cam while leaving the high compression head. Less overlap from the standard cam profile creates more cylinder pressure, mine was at 182 psi both sides. For several years it ran fine, but as more ethanol was introduced it became a problem. I've owned this one for 20 years.
 
Hi David, and thanks for your input.
My recommendations to the bike's owner, after considering all information given me on here, is to stay with the Combat head, install a standard cam and lower the compression ratio to 8.5-9:1
The Combat head has fine ports IMHO, tapering down from 32 to 28.5mm as it does.
We have pretty good fuel in this country, and very consistent between brands too, so I get away with using 95 RON (98 RON getting hard to find) and over 10:1 CR on my Trident. I think 9:1 would work in a 750 Commando with a standard cam.
This particular Combat has led a hard life, or we would probably had gone for a full Combat spec, but I think caution is in order here.
Besides there's nothing wrong with a standard 750 Commando's performance anyway.
IMHO of course.
 
tri-man said:
Hi David, and thanks for your input.
My recommendations to the bike's owner, after considering all information given me on here, is to stay with the Combat head, install a standard cam and lower the compression ratio to 8.5-9:1
The Combat head has fine ports IMHO, tapering down from 32 to 28.5mm as it does.
We have pretty good fuel in this country, and very consistent between brands too, so I get away with using 95 RON (98 RON getting hard to find) and over 10:1 CR on my Trident. I think 9:1 would work in a 750 Commando with a standard cam.
This particular Combat has led a hard life, or we would probably had gone for a full Combat spec, but I think caution is in order here.
Besides there's nothing wrong with a standard 750 Commando's performance anyway.
IMHO of course.
Click to expand...

Yes this works. I don't like the idea of a thick cylinder base spacer as it moves the tappets from the normal position. There are thick aluminum head gaskets/spacers that I have hear leak. I have one of these, but never tried it. Makes more sense to put the spacer where the material was originally removed. Also would put the rocker geometry back to normal when using standard pushrods.
 
I went through the detuning of my Combat along the lines of what you have in mind for the same reason you are.
Replaced 2S cam with standard one.
New JCC Pistons to replace original ones with oil slot prone to failing.
Spacer under barrel to slightly higher compression than Standard. (I believe I got it from Old Britts)
Replaced carbs with 34 MM VM Mikuni
Replaced 19 tooth gearbox sprocket with 21 tooth one.
Pazon Sure Fire Electronic Ignition
Podtronics rectifier

Starts easy.
Runs very strong.
High 40's to mid 50s per US gallon
Almost 12,000 miles with no mechanical issues
 
Status
Not open for further replies.
Back
Top