combat engine pinging

I pulled the head off last weekend after stripping a spark plug hole. While in there, measure the lift on the cams.
What I found was more like a standard 750 camshaft. Definitely not a .390 intake lift, closer to .330. Explains a few things. There never was that surge of power at 6500rpm as I remember from my first combat and the 268 duration of the standard cam with 9.0:1 compression ratio at sea level equates to 182psi compression according to a website "not2fast Engine Cranking Compression" which is what I measure with my Sun guage. With a double S cam duration of 274 the cranking compression would be 171psi. The high cranking compression could be the reason I can't find any current fuel the engine likes.

Now the choice and I'm asking opinions. Should I put in a double S cam or is there a better choice for a combat engine now?
Would like to here only from the combat owners with experience, please.

illf8ed said:

Would like to here only from the combat owners with experience, please.

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Suggest you send a PM to Windy ("bill" on the Forum).

Psi numbers can be deceiving. Longer duration may show less psi even though the compression ratio is higher. If you want performance, dependability and just a great running street rod, go with the JS stage 1 with the radius lifter setup. Throw on the JS flatsides and be wow'ed.

Now I'm not trying to push Jims stuff, but i am saying that it worked for me. My cases are reinforced and re bearing ed all around.

Not that i need to push it to 8000 rpm and beyond, but with this stuff I could. You should go with at least new srpings if you wan to take it there. Otherwise 7000 is a piece of cake on stock valve springs. Still has grunt at low end. May require some cam timing. I have mine advanced 10 degree from standard. I cannot tell you if this is from the grinding or that is just want i needed to get it right. Not uncommon.

Again, the main point with this stuff is making sure the rest of the drive train is up to snuff. No weak links, including brakes because you will be incline to see the ton mark. Tightening up the iso's a tad will make it steady and smooth at greater speeds. A higher drive sprocket won't bring you down either. Cruise at 80 all day long. I cannot emphasize how smooth the stuff runs.

PM sent.

pvisseriii said:

Psi numbers can be deceiving. Longer duration may show less psi even though the compression ratio is higher. If you want performance, dependability and just a great running street rod, go with the JS stage 1 with the radius lifter setup. Throw on the JS flatsides and be wow'ed.

Now I'm not trying to push Jims stuff, but i am saying that it worked for me. My cases are reinforced and re bearing ed all around.

Not that i need to push it to 8000 rpm and beyond, but with this stuff I could. You should go with at least new srpings if you wan to take it there. Otherwise 7000 is a piece of cake on stock valve springs. Still has grunt at low end. May require some cam timing. I have mine advanced 10 degree from standard. I cannot tell you if this is from the grinding or that is just want i needed to get it right. Not uncommon.

Again, the main point with this stuff is making sure the rest of the drive train is up to snuff. No weak links, including brakes because you will be incline to see the ton mark. Tightening up the iso's a tad will make it steady and smooth at greater speeds. A higher drive sprocket won't bring you down either. Cruise at 80 all day long. I cannot emphasize how smooth the stuff runs.

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Wow, just looking for camshaft.

Its old news that trying to detune a Combat with standard make them detonation prone. I vote 2S and return to 10:1 CR. The rpm limits is not the Combat head but the common cast iron flywheel fragmentation bomb and the crank shaft jump rope.
If you want to live near or past red line long a steel flywheel is where to start and I'd higly recommend the whole crank be cyrogenic tempered too. If you decide on a another head or expensive mods, I'd sure think about a standard small port head with skinny ligher valves or even better the $2500 Fullauto/Combstock hi velocity head. By accident I discovered the 6500+ 2S coming on cam is the big head ports delaying the ram effect. Small ports come on cam a couple 1000 rpm earlier and don't quit till something else gives up.

illf8ed said:

pvisseriii said:

Psi numbers can be deceiving. Longer duration may show less psi even though the compression ratio is higher. If you want performance, dependability and just a great running street rod, go with the JS stage 1 with the radius lifter setup. Throw on the JS flatsides and be wow'ed.

Now I'm not trying to push Jims stuff, but i am saying that it worked for me. My cases are reinforced and re bearing ed all around.

Not that i need to push it to 8000 rpm and beyond, but with this stuff I could. You should go with at least new srpings if you wan to take it there. Otherwise 7000 is a piece of cake on stock valve springs. Still has grunt at low end. May require some cam timing. I have mine advanced 10 degree from standard. I cannot tell you if this is from the grinding or that is just want i needed to get it right. Not uncommon.

Again, the main point with this stuff is making sure the rest of the drive train is up to snuff. No weak links, including brakes because you will be incline to see the ton mark. Tightening up the iso's a tad will make it steady and smooth at greater speeds. A higher drive sprocket won't bring you down either. Cruise at 80 all day long. I cannot emphasize how smooth the stuff runs.

Click to expand...

Wow, just looking for camshaft.

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You asked if there was a better choice.
Well there is. I may have gone over the word limit justifying it.
Sorry. At least it wasn't cryptic.

By accident I discovered the 6500+ 2S coming on cam is the big head ports delaying the ram effect. Small ports come on cam a couple 1000 rpm earlier and don't quit till something else gives up.

Standard camshaft doesn't have any "on cam" thrust. I remember my first combat, stock with only 300miles when I got it, had the "launch point" at around 6500rpm.....I like it!

My 1st Combat had early factory hogged out ports CHO head. Norton later backed off port size in later casting/milling, so mine didn't really kick in till 6800, then felt like a 3rd piston added. Front would lift lain over finishing up 1st and then 2nd, whoohoo couldn't believe it at first. British Toy Chief Michael Starkey took casts of Ms Peel's ports and said someone over did the opening up porting, which backs up the history I've read on Combat bombs. Alas the rest of the engine ain't made to stand much over red line w/o mo money.

May have to try the 10' cam advance mentioned as better all 2S around behavior.
I didn't notice or suffer any lack of low end grunt on either of my 2 Combats, only it got better and better till scary even. Cream of the crop to me and more of em made and sold than any other model, thank goodness.

I'm going into the engine to change to a long duration camshaft. Haven't decided yet which one. With the cylinders off I can see the camshaft in there doesn't have "SS" stamped on it...it's a std cam.

I rest my case along with decades of back up reports that the worse thing to do to a Combat is put in the standard valve bumper stick. Standard nil over lap cams give higher compression test results, Until the RPMS hit air velocity momentum to pack it in. Combats came with early hogged out head and later less hogged out factory heads. My hogged out Combat got 3rd piston kicking in at 6800, others a few 100 rpms sooner. BY far the best thing I did accidentally for POwer was to put on a standard low compression small port head. Then stick on a bigger carb, whoohoo.

At the suggestion of a mechanic who has worked on Nortons since the sixties, I stuck a stock Roadster cam in my Combat when I rebuilt the engine last year. The Combat cam was knackered. His comment was "do you want improved reliability or performance?" I have not noticed any appreciable difference in performance.... a little top end. The bike is still quicker than my 850 Commando and my Triumph T160. Quicker but not faster. It gets almost twice the gas mileage of the T160 as well. The engine does not ping on normal 91 octane pump gas (no ethanol) and the timing is set to spec (Boyer). Mick

Duh, Boyers are made to fit all the Brit twins, which most are more detonation prone than Norton so the advance curve is sluggish compared to what a Commando likes most so enjoy your sluggish non-Combat response on tame cam a long time on premium gas. Jim Schimdt in his Norton Race Manual goes into detail how/why to alter analog Boyer to pull better out of apexes. Cam profile, compression ratio and ignition timing are all interrelated to make power w/o detonation. Fudge one of these factors off optimal then one or more of the others must be fudged down too. My totally factory Trixie Combat gets 87 octane very often as only gas available and have not been able to induce ping or knock all summer long loaded down for out of state trip or just me on it enjoying the wake up call of a Combat pulling harder after 6000 rpm instead of dropping off power curve like non-Combats.

Can't ignore coke or rough chamber surfaces glowing to pre-ignite or heat range of spark plugs which is a big heat flow path to cool chamber, not just keep plug clean to fire or glowing white hot. To de-coke the hillbilly way I spray water into carbs till almost choking the fire, until the greyish crap stops blowing out the tail pipes. It you over do it it just stalls out no damage.

Duh? Always with the negative waves Moriarty. Not everyone's an expert like some folks.... merely meant to point out that octane levels and timing play a large role in detonation. Maybe the original poster knew that, maybe he didn't. Didn't want to open up the Boyer advance curve again.... its been beaten to death on numerous forums. Yup, I guess my Combat is sluggish. After all I was passed yesterday on a twisty back road by Suzuki GSX-R. But then again I never said I was faster than a modern Jap sport bike. Mick

Hehe, what do ya expect with a subject line like a Combat pinging issue as they don't do that w/o good reason PLUS a *mis behaving* pilot beyond what a police would consider legal or lugging it up hill loaded in 4th gear accelerating from 30 mph. IN other words for a 'Combat' to detonate even with too much CR and too fast or advanced spark on low octane, the pilot is causing it, period end of tale. So either configure it back to stock Combat with enough octane or tame your throttle heat and gear loads to get around quite nicely to just fall in line with even freeway traffic. At low-ish rpm and easy throttle use a Combat operates under less compression pressure than a standard 750. Don't expect a standard cam Boyer Combat to lift its front w/o damage racing it over 6000 rpm on hi throttle. So your remark reveals either a lack of understanding or just upset that you ain't experienced hobot's wild flings on Ms Peel 2S Combat with the small ports, single 34 carb and sluggish analog Boyer curve. I had 2 factory set up Combats, neither of them needed much pilot attention to stay in saddle no matter how manic I got with them but boy howdy on Ms Peel's accidental set up it only took me one ride to develop reflex to grip white knuckled and plant butt BEFORE i snapped throttle or it'd flat run right out from under me. With the rump rod handling - sports bikes no matter how powerful didn't stand a change up to 90 mph and then couldn't shake her in opens until well over 120. 91 octane was the highest I could buy here. I did not press hard with only 87 octane in tank though as it would begin to ping climbing in 4th behind trucks and such so I down shifted or eased off throttle.

So my message to put in your chamber is detonation is mainly d't pilot error/behavior not mechanical or fuel issues even if they make an engine more prone to it.

"Hehe, what do ya expect with a subject line like a Combat pinging issue as they don't do that w/o good reason PLUS a *mis behaving* pilot beyond what a police would consider legal or lugging it up hill loaded in 4th gear accelerating from 30 mph. IN other words for a 'Combat' to detonate even with too much CR and too fast or advanced spark on low octane, the pilot is causing it, period end of tale."

Nah, it wasn't the pilot. Didn't suspect the previous owner had violated the engine with a camshaft not in the spirit of "combat". Combination of poor choice in cam plus the every deteriorating quality of fuel.

Nah, it wasn't the pilot. Didn't suspect the previous owner had violated the engine with a camshaft not in the spirit of "combat". Combination of poor choice in cam plus the every deteriorating quality of fuel.

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Ok sure that could surprise ping on anyone till they realized they had to baby it till corrected. Thanks for the reality check. Too bad the cam is so deep in, ugh. Its times and findings like that I reflect on Ken Canaga telling a year ago he had a cycle all put back but rocker covers or such and dropped a nut down the cases, so nothing for it but tear it down to open cases and do it all over again, ugh. I really like the plain jane Combat performance where it always feels a bit more eager than I am the faster it goes. I often forget I've put in 87 octane going to WOT but I don't snap on throttle in lugging conditions but do lug 30-35mph in 4th for miles at at time up/dn hill and dell but on light throttle just enough not to stumble. I don't like to spend much time in lower gears as the bushes don't get lubed then.

Everyone of us has their own special problem child to raise up, best wishes on yours.

To get back to the original question about what cam to choose, I had an Axtel 5S in my race bike ( until I replaced it with a PW3)

I was very happy with the Axtel, it had ( from memory) .350 lift and gave good all round power. It was strongest from 5000 to 7000 but still had good driveability below that.
It also was happy on standard valve springs so no extra expense there.

There are a number of options that are almost identical to the Axtel cam
Everything you want to know about cams is here:

http://atlanticgreen.com/camsurvey.htm

Vintage builder racers from 40ish year ago know that Williams copied the Axtel cam to come up with the PW3. Most the S cam range are slight variation of the 2S and need rather more lash. Hotter the cam the higher the CR needs to be to make it work right down low as well as up high. If no external power adder I'd sure be saving up for a Comstock/Fullaouto head while spiffing up the rest of the engine and drive train to take good advantage of it.

G'day David,
I had to rebiuld my Combat after an internal malfunction saw the cam and bores get ruined.
I replaced my 2s (SS) Cam with the 4s after much deliberation.
When I asked the question, re which cam, I recieved PMs from several forum members
who have used them (4S) for many years and still fit them if they require added performance.
Locally I spoke to guys at the Races, and they too said good things for both on the track
and on the road.
I also had a local cam biulder radius the bottom of the lifters.
Standard valve springs.
Note... to fit the 4s cam I had to machine the cam tunnel a little as the lobes are slightly higher than the 2s.
With the added compression of the .040" oversize rebore I decided to remove .020" from the tops of the pistons,
half that which is removed from the head for the Combat, just for peace of mind.
At this stage I still have the 32mm Amals, sleeved.
It is a very smooth, torquey combination.
I run a 22 tooth front sprocket, and the bike is happy with two people on board.
Anything from 2,500 revs is smooth right through to 7,000.
Our speed limit of 110 kph (70 mph) arrives at about 3,750 and the cam is working beautifully.
There is a graph avaliable here somewhere that shows a comparison of the most talked about cams,
they are very similar.
As I bought all my rebiuld stuff from RGM in England it was an easy choice to go for the 4s as that's what they had.
I spoke to Peter Williams, but as his totol price for all the parts needed was slightly more than RGM, I decided to go their way,
and If I had bought all my parts from the States I would have probably gone with the JS. kit.
That's my story.
AC.