Britbike forum Forums British Motorcycle specific Brands Norton bulletin board 75 Mk3 Running Poorly

As discussed in an earlier post, my 1975 Mk3 was smoking slightly during acceleration. One of the carb slides was also slightly hanging up when I had the throttle about 50% open, so I decided to replace both carbs with new AMAL Premiums. After installing the carbs, heavy black smoke was prolific from both exhausts, both at idle and higher RPM. The needles were set on the middle groove, so I changed them to the upper groove. After putting everything back together and starting the engine, at idle there was no longer any visible black smoke. The engine was idling somewhat rough, but not bad. I took it for a ~5-mile ride. The further I went, the rougher the engine ran. After about two miles, the engine wouldn't run over 3000 RPM without the engine losing almost all power. The further I went, the worse it ran. It seemed to be cutting out on both cylinders. I almost didn't make it back home. The current situation is that the engine will start but idle very rough and cuts out on both cylinders erratically at higher RPMS. At idle, there is a good hot blue spark on both cylinders. I also have very good compression on both cylinders, ~150. Any ideas?

battery condition?

The weakest needle notch can be a bit weak.

Moving the needle position shouldn't alter the idle mixture to any significan extent.

Are you sure the Premier (not Premium) pilot jets (not the air screws) were both fully screwed in? Are the pilot jets #19s (3 ID-rings)?



That sounds typical of what happens when the chokes are left on.



Which Premier kit did you buy? Was it the PACK 113 kit (mentioned in the previous thread because the 220 main jets supplied would very likely be too weak for peashooters)?
https://andover-norton.co.uk/en/sho...emier-32mm-850-mk2a-mk3-220-jet-pack-113

Check the fuel (not float) levels.
https://amalcarb.co.uk/optimising-mark-1-concentric-fuel-levels

We previously asked if this was black smoke or blue smoke. Don't think we got a solid answer ?

If its blue smoke, then rings or valve guide seals are likely culprits, and changing carbs may just be a red herring.
Plug condition ? If they are sooting up from burning oil, you may be chasing your tail ??


Indeed. This needs to be explored, and ruled in/out ?


Indeed. Can't recall what ignition your are running, but if the battery voltage is low - and not charging ? - then it may not like this, at all.
Could (also) explain a lot ...

Ain't old bikes fun !

One of the mechanics at the shop bought a new '75 Commando. Found the cam had not been hardened properly and one lobe wore away.

A not uncommon story with some Mk3, when you read about.
But, it wouldn't cause smoking, and may not give good compression on both cyls ?
(not that 150 psi is great, should be higher than that in tip top (new) condition.)
And the performance would be as poor at the start of the ride as at the end .... !

And you could spot that one rocker wasn't moving much, if you cared to observe ?

150 psi is pretty good for an 850 Commando:
https://www.accessnorton.com/NortonCommando/cylinder-compression.35549/

The carburetors were purchased from Andover Norton. According to the label on the box, the carbs received are labeled as follows.
GRN No. 390902
Type 930/38PREM
Main 240
N/Jet 106
Pilot RJ17
Valve 3.5
The chokes are definitely off. The main jets are #240. The smoke was definitely black, very heavy black. The ignition system is original points / condenser ignition. The battery is almost new and spins the engine over very fast. The actual compression I measured was 151 for the left and 152 for the right. Everything I have read indicates that this is good compression, especially since the two sides were so close together and there was very minimal leak-down for both cylinders. I find it interesting that hitting 3,000 RPM has such a dramatic adverse impact on running and power. While idling if I pull one spark plug off at a time, both sides continue to run very rough, to me indicating that whatever is happening is impacting both cylinders equally.

"930/38PREM" is BSA B44.
https://amalcarb.co.uk/premier-carburettor-for-a-bsa-b44-victor-special.html


The correct Andover Norton AMAL kit for the 850 Mk3 would be PACK113 932/33 & 932/34 (but with 260 main jets for peashooters).
https://andover-norton.co.uk/en/shop-drawing/919/amal-premier-carburettor-packs-

If #17 pilot jets instead of #19s then I can't see how it could have been rich at idle.
Also without the flats on the unused inner throttle stop bosses and mounting flanges they wouldn't fit side by side on the manifolds without clashing if the carbs are not machined for a Commando.

You apparently own a B44 Victor Special so perhaps you were looking at the wrong AMAL box?

[quote
You apparently own a B44 Victor Special so perhaps you were looking at the wrong AMAL box?[/quote]

I am embarrassed! I was handed the box and didn't look closely at what I was reading. You are absolutely correct. The correct information on the new carbs is:
06.7334
CARB PAIR - AMAL Premier 32MM (850 MK2A/Mk3) 220 JET PACK 113

The compression measurements I gave were from a while back. I just re-measured the compression for both cylinders and the results were almost identical with the previously reported values, 150 for the left and 151 for the right.

Is that tested HOT, with throttle wide open ?

My 850 Mk1 with brand new AE pistons and rebore consistently gave 180 psi on both cylinders.
I've seen other rebuilt 850s give similar numbers.

I didn't get to test it HOT before the rebore, but I'd say 150 would have rather large ring gaps ?
If you want confirmation of this, tip a spoonful of oil down each plughole, turn it over a few times,
and retest. If the numbers jump upwards, then the ring seal isn't the best ...
A leakdown test may show the same thing, but they are a lot of messing about for very little ...?

Have you tried tweaking the idle mixture, and indeed the basic idle settings ?

If the carbs will not respond to setting the idle, the jets may be blocked and its idling off the needle jets.
The 1st Commando I ever met was like this.
It ran - roughly - but cleaned up above 3000 rpm, and then pulled like a train.

Don't assume new Amals are perfectly clean inside - you see lots of reports of the earlier new ones needing lots of cleaning ?

And, as previously, 220 main jets are too small for peashooters ?
Shouldn't make that much difference, but it won't like revving out. ?
Especially if you have DROPPED the needles.

I'd say 150 is nothing to worry about. Interesting you are running points and with them full advance comes in around 3000 rpm. Is the mechanical advance working? Have you strobed both sides?

Hi, not a Commando person but have you checked auto advance is working.Sorry if this has been covered. Eric

Nethertheless, if he reported it was 'smoking' (blue or black smoke ??),
with blue smoke I'd say worn rings was quite a possibility.
While it shouldn't give such bad running - unless it was fouling up spark plugs ?

Nortons are quite a long stroke, and those rings get quite worn after a while ...

Enquiring minds are wondering how it would run on both cylinders, if taking off a plug cap

I wasn't clear. If I pull one spark plug wire off, the other cylinder continues to run. This applies to both cylinders. I'm going to try to do some work on the bike tomorrow, but I am scheduled for surgery the day after and will likely be unable to work on the bike for a time.

Good luck with that then.
Mere mechanical stuff takes a back seat to the important stuff...