New Norton knocking help please

Hello all,

Im new to the Norton thing and have a question....please bear with me.

I bought the 73 commando a few weeks a ago and after cleaning and fixing oil leaks have ridden around the area ...country side...Nebraska a few times to check things out. The bike as I see, is pretty much stock but has SS pipes, Boyer ignition, 34MM Amal MKll carb. The plugs are ngk br8es and have .035 gap.

when I’m in 4th gear and pull up a hill I’m getting KNOCK!...that’s my concern! I let off gas and took it easy under the load but when you go through the gears it pulls hard! I have not checked jetting but it could go down on the needle clip one notch. The plugs are coffee brown...maybe a touch darker. I have not set timing since I would like advice since I’m new to these engines.

from what I can see, the engine has been gone through and the compression is 175 in both cylinders and I was told the valves were adjusted too. The bikes starts fine and cruises ok just want to get rid of the knocking. Most of the gas is 91 shit or 93with ethanol but race gas in many blends is available but I’m going to be limited on riding since I live in the country side.
any help would be appreciated and if anyone is around the greater Omaha Nebraska area I could use help.

thanks. much.

Plugs are normally 7's rather than 8's.. in England we call it 'pinking' ... check your ignition timing at high RPM.

You need to check the timing with a strobe. 29° BTDC @ 4000 rpm or 31° @ 5000.

Actually the original spec spark plug is BP7ES, an extended tip plug. Likely a previous owner went to a colder plug due to the pinging (pinking - although I’ve never seen that color (colour) in the combustion chamber) . As mentioned retarding the timing helps. Is this a ‘73 750 or ‘73 850? The 750 will have more tendency for knocking due to higher compression than 850s.

750...sorry. I will check ignition Timing.

Should I try draining gas and dropping some 100 octane to see if that helps? I will still set timing.

thanks for the reply’s very much.

Nate.

Just so we know..... From Longman Dictionary Of The English Language.... (wot! no 'American!'):
Ping: N Am ignition knock.
Pink: To make a series of sharp popping noises because of faulty combustion of the fuel-air mixture; knock.
So.... K turns to G Mid Atlantic !!

Do you know if any work was done to the motor that would have raised the compression ratio? A stock 73 750 should not ping on pump gas, but a 10:1 motor will.

Education is great.

Ooooofff. That I’d not know. I will try and find out.

We talking engine noises or exhaust noises?

Engine

Best not to do ANY prolonged riding if the engine is pinking, the end result will be a damaged piston (expensive)

It’s only been a few miles and not hard miles.

EstuaryBoy said:

Just so we know..... From Longman Dictionary Of The English Language.... (wot! no 'American!'):
Ping: N Am ignition knock.
Pink: To make a series of sharp popping noises because of faulty combustion of the fuel-air mixture; knock.
So.... K turns to G Mid Atlantic !!

Click to expand...

The ad campaigns ingrained some things in our mind too.

Definitely check your timing and even then it won't hurt to back it off a tad as you don't know if the engine is standard compression
Is there any kickback when you start it?
If you still have problems after this you will need to know what throttle opening you are at when the pinking occurs
Marking the throttle drum will help with this
I once had high compression pistons and a combat head on my 750 I never did stop it pinking until I changed to std pistons

yes. double check timing run highest octane gas you can get nonethanol if possible don't 'lug' engine

Some basic stuff to look at. Is the 1973 year registered or year of manufacture? Check the red tag on the headstock and see if it shows manufacture date. It is in theory possible that it is a Combat motor (1972) that was not registered until 1973.

I'm not an expert on the '73 750s but apparently there was a high compression head and a low compression head. Look above the right hand (timing side) exhaust rocker and see if there is a stamped number.

If it has a Combat head, there would be a "C" stamped under the headsteady, removal of which is necessary to find it. It's always possible somebody built a Combat from their '73 bike.

Run a high octane fuel and put some lead replacement additive in your fuel, just put the recommended amount in per fuel tank and see how it goes, todays fuel is not the same as yesteryears fuel, also check your timing, as well change your plugs, try a set of Champion N7YC and see if that helps, my Norton runs best with the Champion plugs, have tried NGKs, Bosh, Desmo plugs, but the Champions run best and out last the rest, well in my Norton anyway.

Ashley

rvich said:

I'm not an expert on the '73 750s but apparently there was a high compression head and a low compression head. Look above the right hand (timing side) exhaust rocker and see if there is a stamped number.

Click to expand...

The (RH5 standard comp. or RH6 high comp.) number is also likely to be found under the head steady.

The recommended octane numbers quoted in British bike handbooks, manuals etc. are for RON octane.

Octane rating - Wikipedia

en.wikipedia.org

"Anti-Knock Index (AKI) or (R+M)/2
In most countries in Europe (also in Australia, Pakistan and New Zealand) the "headline" octane rating shown on the pump is the RON, but in Canada, the United States, Brazil, and some other countries, the headline number is the simple mean or average of the RON and the MON, called the Anti-Knock Index (AKI), and often written on pumps as (R+M)/2.

Difference between RON, MON, and AKI
Because of the 8 to 12 octane number difference between RON and MON noted above, the AKI shown in Canada and the United States is 4 to 6 octane numbers lower than elsewhere in the world for the same fuel. This difference between RON and MON is known as the fuel's sensitivity, and is not typically published for those countries that use the Anti-Knock Index labelling system."