Knocking / rattling inside primary drive case + backfire

Hi

I'm new to the forum, having just acquired my first Norton - a 1973 Commando 850 mk2a.

I have two issues, that are not related, but would really appreciate any advice that anyone is able to please give about them. I'm desperate to get the bike out onto the open road!

Issue 1
Knocking or chain slap in the primary chain case (see video here: ). I've checked the chain tension and it's about 3/8's (as recommended in the handbook). If anything it may be a little tight. I've not had the casing apart yet, but am willing to give that a try. The bike was restored in 2020 and has not been run since, so I'm wondering if there could be some kind of clutch issue, although the noise does seem to be more from the front of the casing.

Issue 2
Backfire over 2000rpm. When I received the bike I noticed that the right side exhaust header was loose. I thought this was causing the backfire, but after tightening it I'm still getting backfire. I could simply be using the choke incorrectly (or it may need adjustment) but the backfire seems to happen at any choke position. I could still have a leaky header of course.

Any ideas on either or both of these issue would be hugely appreciated. Also, if any Commando experts are in the North Norfolk area I'd be happy to pay for a bit of assistance!

Cheers, this looks like a great forum and I'm sure I'm going to find it a really useful resource. I'm also looking forward to the day when I know enough that I can post help for other members.

Take the case off and look before running that engine again. Something is loose or free that shouldn't be.

A loose chain typically causes a rattle near the front of the primary case. You state the chain tension is 3/8 inch .....is that total up/down movement? You might try a little tighter chain adjustment before following kommando's advice.

If the noise persists after a chain adjustment, immediately pull the cover and have a look.

Slick

+1 on opening up the primary, it is not much work and you can make a proper inspection. Check chain tension at several spots, it is possible that due to even slight out of roundness that of either sprocket it can vary considerably. The clutch nut could be loose, the stock tab washer is soft and gets thinner over time and that will make the nut loose. A fix I learned of here is to use a hardened Belleville washer and blue threadlocker. The diaphragm spring needs to be removed to inspect the nut (and clutch plates) but it is easy to make a DIY clutch spring tool. Finally if you do all this be aware that the manual spec of 70 ft-lbs for the clutch nut is excessive and can cause the circlip to break, something like 40 ft-lbs is better. Finally if you do adjust the primary tensIon (by pivoting the gearbox) be aware that this will affect the final drive tension too so check that before you ride your bike...

Thanks for the replies, fantastic. I think I'll take the case cover off as the next step. I'll keep the thread updated.

Any clues on the backfiring issue would also be appreciated. Cheers!

Agree with Bluto. Open up primary and have a look. My guess is loose clutch basket. You do need a special tool to rmove the diaphragm spring. DON't do it without it.

Thanks MichaelB - this tool? https://andover-norton.co.uk/en/shop-details/15499/clutch-compressor-tool-c-4805

M-Hawk said:

Any clues on the backfiring issue would also be appreciated.

Click to expand...

M-Hawk said:

Backfire over 2000rpm.

Click to expand...

Only when you close the throttle suddenly or at other times so perhaps you could give a bit more detail about when the backfiring occurs?
Which exhaust pipe set does it have, specifically the fittings into the cylinder head?
Are you certain that it's a Mk2A and not a Mk2?

M-Hawk said:

Thanks MichaelB - this tool? https://andover-norton.co.uk/en/shop-details/15499/clutch-compressor-tool-c-4805

Click to expand...

Exactly. You can easily make your own. See what others have done.....

Slick

L.A.B. said:

Only when you close the throttle suddenly or at other times so perhaps you could give a bit more detail about when the backfiring occurs?
Which exhaust pipe set does it have, specifically the fittings into the cylinder head?
Are you certain that it's a Mk2A and not a Mk2?

Click to expand...

Thanks for the reply. I think it's happening on the run up, rather than roll off.

I'm not sure what pipes it has. I'll pull the right side and get some photos tomorrow.

I was told that it's a 2a but have no idea if that's true or not. It was a USA import in 2018.

Thanks again for your help (everyone). I'm learning.

texasSlick said:

Exactly. You can easily make your own. See what others have done.....

Tools You Have Made to Maintain your Norton

In another thread Rob (robs ss) gave us a new tool ..... Show us what special tools you have made. Show me yours, and I'll show you mine. Slick

www.accessnorton.com

Slick

Click to expand...

Thanks Slick, great stuff.

L.A.B. said:

Only when you close the throttle suddenly or at other times so perhaps you could give a bit more detail about when the backfiring occurs?
Which exhaust pipe set does it have, specifically the fittings into the cylinder head?
Are you certain that it's a Mk2A and not a Mk2?

Click to expand...

Actually, I just remembered that the original advert is still up: https://www.carandclassic.com/auctions/1973-norton-commando-nMQPJn

Is it a 2a?

Remove the timing inspection cap (smaller forwardmost cap on primary cover) to see if the rotor nut has loosened leading rotor contacting/damaging the timing scale. Had that happen once due to a "pro" shop not using the shake proof washer under rotor nut.
Also with primary cover removed, check the three screws holding inner primary to crank case. These should have little tabbed washers to secure them but I have had one let go regardless, leading screw flying around primary, damaging chain rollers. Now I always use locktite on these in addition to tabbed washers.
Before pulling clutch apart, also check if chain might be rubbing against inner primary. Can happen if inner primary mounting stud is not properly shimmed, casing sitting askew slightly.

Backfire could a number of things, carb tuning/condition of jets/needles are a possibility. I had a main jet starting to unwind from carb body giving backfire that got progressively worse over a matter of days. What is condition of spark plug on side that backfires? Is this points of EI ign? Air leaks are carb to manifold or mani to head?

M-Hawk said:

I was told that it's a 2a but have no idea if that's true or not. It was a USA import in 2018.

Click to expand...

M-Hawk said:

Actually, I just remembered that the original advert is still up: https://www.carandclassic.com/auctions/1973-norton-commando-nMQPJn

Is it a 2a?

Click to expand...

It's a Mk2, (see following message) as the Mk2 was the US market version with the perforated metal airbox, peashooter silencers, (etc, etc,).

The Mk2A was the European market version with the black plastic airbox, black cap silencers (etc., etc.,), although some late Mk2As were sold on the US market yours, doesn't appear to be one. The tail light assembly also wouldn't be correct for an 850 of any type.

A minor detail but the side panel pinstripes would normally be facing the other way with the single stripe to the front of the panel (and double horizontals.
It has the 850 Mk3 alloy left-foot gearchange lever fitted with a non-standard rubber.
The petrol tank looks like an Indian-made copy.
Personally, I'd suggest removing the automatic anti-drain valve or at least fit a manual valve with an ignition cut-out.

Serial 303165 actually makes it an 850 Mk1 (Mk2/2A after 307311).
The rocker feed line normally goes from the timing cover to the right-hand side of the cylinder head.
The wiring for speedo and tacho illumination is missing.

Thanks for the info. Sounds like I've got a bit of work to do if I want to make it as original as possible.

It is a nice looking bike.

You could ride it a bit before deciding whether to take it back to very original. It has alloy rims which are nice but not original and the horn is obviously in the wrong place. Plus the things LAB mentioned.

Bit honestly unless you want to show it a few sensible upgrades are a good idea in my opinion if you want to ride it. For example the front brake may benefit from improvements. There are options from discrete up to full on modern. Lots of info on the site on this and in my opinion pretty much essential for modern traffic.

Thanks John, appreciate the comments.

I plan to ride it and use it well, so I'm happy for it to have a few oddities and imperfections about it. I'll look at the horn location, and some of the other things that L.A.B. mentioned though.

First job is to get it running sweetly!

The horn location is wrong but frankly more sensible than the original location under the bike at the back. The joke is that Norton built the bike around the horn.

I have a smaller modern horn mounted between the coils on my bike and it works well.

Fit new spark plugs and take it from there.