New Norton Owner; tuning questions

Hi everyone, my name is Andrew. I'm 32 and have just purchased my first Norton! It is a very nice 73' MK1 850 in Blue. I've always loved Nortons, but they have been out of my price range for a long time. Came across this one at a steal and had to have it.



I'm not unfamiliar with old Brit Bikes as I've had several BSA's and have worked on a early 70's Triumph, but I'm having some tuning issues with my Commando and could use some help. Apologies to anyone that is also on Facebook or Britbike.com... I'm trying to use all resources to get on the road ASAP.

Anyway, I'll try to keep this short. Bike needed new AMAL Premiere carbs, so on they went. 106 needle jet, 19 pilot, 260 main, 3.5 cutaway slide, float level even with the top of the bowl. Bike was running VERY rich; black, sooty plugs. It would start easily but not rev past 3,500 or so. Dropped the needle from the richest to the leanest position, installed 3.0 slide and revving improved; to about 4,500.

Assuming it needed to be leaned out a bit, I installed 17 pilot jets and lowered the float height to .080" below the bowl surface. Conditions improved again, to about 5,000 RPM. Hoping to finally sort it, I installed a new air filter and 240 main jets. Sitting still, the bike will rev past 5,500-6k now, but on a ride it will still cut out and bog down around 5,000 RPM.

I've been using NGK BP7ES plugs the entire time. Several sets; changed when any other major change was done. Boyer Electronic Ignition installed. Thinking perhaps the advance curve was off, I just checked and here are the results:

Idle: 18 degrees BTDC on the RIGHT side of the boss that the mark is centered on.
2,500 RPM = 22-23 BTDC
3,500 RPM = 26-27 BTDC
5,000 RPM = 30-31 BTDC

Now I'm at a loss for what to do next. I do think there might be a small exhaust leak at the head, so new gaskets are on the way.

Sorry for being so long winded! I just thought all of the information could help diagnose the problem. Despite the tuning annoyances (and carb removal rite of passage) I'm over the moon to have a Commando! I can tell it will be a wonderful ride. Thanks for any help you all can give, I really appreciate it!

Good looking bike! Check out https://www.accessnorton.com/NortonCommando/amal-premiers.27334/ for a whole expose' on the Premiers.

Nater_Potater said:

Good looking bike! Check out https://www.accessnorton.com/NortonCommando/amal-premiers.27334/ for a whole expose' on the Premiers.

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Thanks! Just got to the end of that thread; lots of good info there! These aren't my first Premieres; probably 4 fitted previously, but they are being more picky than the ones I've fitted on BSA's. They've been a very nice upgrade in the past. I'm just confused because my plugs are showing sooty and dark indicating rich, but all of my settings are so different (in a lean way) than what seems to work for others with MK1 850's...

adunham1 said:

Anyway, I'll try to keep this short. Bike needed new AMAL Premiere carbs, so on they went. 106 needle jet, 19 pilot, 260 main, 3.5 cutaway slide, float level even with the top of the bowl. Bike was running VERY rich; black, sooty plugs. It would start easily but not rev past 3,500 or so. Dropped the needle from the richest to the leanest position, installed 3.0 slide and revving improved; to about 4,500.

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If you mean the gas is at the top of the bowl, then that's your problem - not supposed to be. Read this: http://amalcarb.co.uk/optimising-mark-1-concentric-fuel-levels or this: https://www.gregmarsh.com/MC/afh.pdf

marshg246 said:

If you mean the gas is at the top of the bowl, then that's your problem - not supposed to be. Read this: http://amalcarb.co.uk/optimising-mark-1-concentric-fuel-levels or this: https://www.gregmarsh.com/MC/afh.pdf

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Thanks for the links. Dang it, I knew I must have done something wrong. I measured .080” from the bowl mating surface to the top of the float... NOT the fuel level. From the second link, it sounds as if I need to measure to the liquid instead. Is that correct? Also sounds like .040” instead of .080”.

Yes, it's how far down from the bowl edge to the fuel and it's .21" plus or minus .040" (0.17" to 0.24"). Look at the last picture in the second link. You'll find that when the level is correct the float will look like that with it shutting off the fuel.

Also, you mentioned going to a 3 cutaway. That's richer. Assuming you have the right carbs and a standard bike, it should be very close as supplied by Amal. Do you know what "PACK" number you bought?

adunham1 said:

Cool, I can make those changes tomorrow no problem. Swapping the slide back is easy. Thank you for the specs; sometimes I get lost with someone saying one is good, but another person says it’s bad, etc. typical internet things, I suppose.

I don’t know; is there any way I can tell? I still have the boxes the carbs came in. I bought straight from AMAL.

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The PACK number is on the side of the box if both carbs came in the same box. If they came in separate boxes then it will say type: 932/xx and the left and right will be one number apart. Also, what are the first three numbers of the engine number. Titles often have the year wrong and Nortons are notorious for that.

interesting. I will swap the slide tomorrow. That’s easy. I thought I had measured the fuel level correctly using a hose from the drain plug. It fell within that spec; 0.17-0.24”. I guess this means I am significantly wrong as currently the level would be .080” + the height of the float from the surface...

I don’t know; is there any way I can tell? I still have the boxes the carbs came in. I bought straight from AMAL.[/QUOTE]

I will check what the box says tomorrow and get back to you. The title is indeed incorrect; it claims it is a 1974 model. I believe the first 3 numbers are “306.”

adunham1 said:

I will check what the box says tomorrow and get back to you. The title is indeed incorrect; it claims it is a 1974 model. I believe the first 3 numbers are “306.”

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In theory, 307311 was the first 74. Yours is probably a late calendar year 73 build, but was imported and titled by the US dealer in 1974. Norton is not like BSA and Triumph. Norton didn't have a well-defined model year.

Wrong title years even happened with Triumph that did have a defined model year. I have three Tridents, a 73 that is titled as 74, a 74 that is titled as 73, and a 74 that is titled as 74.

Nice looking bike. I had new Blue one in back 73.
Timing and Boyer sound good.

I would...
1. Verify strong blue spark by grounding a plug.
2. Confirm spray tubes have cutaways.
3. Confirm Needles have 4 ID rings at the top.
4. Set float height.
5. Install 17 pilot, 3.5 slide, needle in center groove, 260 main.
6. Sync slides, fresh plugs.
7. Test and tune without air cleaner.

Are those mufflers the ones with the restriction at the output end or the "see-through" muffs? 260's will be much too rich with the restricted muffs.

I haven't found float level on Amals to be that big of a deal as far as operation as long as it's in some reasonable range and I've had it all over the place.

My current bike was running rich with 240's and I was just fooling around a while back and installed a pair of long velocity stacks with no screen/gauze on them. Immediately went way too lean; Installed 260's and they were still a bit lean with the stacks. Not suggesting you should install stacks/no filters but definitely try tuning as mikeB suggested, without the air filter and see if there is any difference. FWIW, I liked the bike's response/performance so much with the stacks that I left them on (JB Welded them on to the new premiers) and that's the way the bike is currently configured.

Thanks for all of the advice, MichaelB. Impressive list of bikes you have there! I have been out of town for work since my last post, so I will begin testing again this week. I do wish there was a more definitive way to identify the needles and pilot jets. The rings work, but misinformation is abundant on the internet! I'm very curious to see how the bike will run sans filter.

MexicoMike, I am not sure to be honest. I will have to identify what each version looks like. They appear to be pretty open, without any obvious restrictions. I agree; I think testing without the air filter will tell me a whole lot about what is happening or needs to happen.

Thanks again guys. I'm happy to have some energy to work on it again. It always gets a little defeating when it doesn't work after so much fiddling.

That is a very nice looking bike. Get it sorted. It's worth it. How many miles on it? Are you sure it needed new carbs? Have you tried running it on the originals and comparing jets and needle settings?

rvich, Thank you I appreciate it. Yes it definitely needed new carbs. The old ones were well worn. I checked the needle and jet settings upon installing the new carbs and all the specs were the same.

So I removed the stock air box today and the bike ran just a little better; nothing obvious. Still some backfiring on deceleration, but I think thats down to an exhaust leak at the head on the RH side. I have new seals on the way and I'm going to order the nut tool today. I wish I knew someone with the tool I could borrow for a day!

well, it may help the revving past 5000rpm thing with advancing the ignition a degree or two as I feel you have some room there

you say you have a Boyer electronic ignition, an older analog or newer digital that probably require resister plugs or wires
like NGK BP"R" 7ES?

adunham1 said:

Hi everyone, my name is Andrew. I'm 32 and have just purchased my first Norton! It is a very nice 73' MK1 850 in Blue. I've always loved Nortons, but they have been out of my price range for a long time. Came across this one at a steal and had to have it.

I'm not unfamiliar with old Brit Bikes as I've had several BSA's and have worked on a early 70's Triumph, but I'm having some tuning issues with my Commando and could use some help. Apologies to anyone that is also on Facebook or Britbike.com... I'm trying to use all resources to get on the road ASAP.

Anyway, I'll try to keep this short. Bike needed new AMAL Premiere carbs, so on they went. 106 needle jet, 19 pilot, 260 main, 3.5 cutaway slide, float level even with the top of the bowl. Bike was running VERY rich; black, sooty plugs. It would start easily but not rev past 3,500 or so. Dropped the needle from the richest to the leanest position, installed 3.0 slide and revving improved; to about 4,500.

Assuming it needed to be leaned out a bit, I installed 17 pilot jets and lowered the float height to .080" below the bowl surface. Conditions improved again, to about 5,000 RPM. Hoping to finally sort it, I installed a new air filter and 240 main jets. Sitting still, the bike will rev past 5,500-6k now, but on a ride it will still cut out and bog down around 5,000 RPM.

I've been using NGK BP7ES plugs the entire time. Several sets; changed when any other major change was done. Boyer Electronic Ignition installed. Thinking perhaps the advance curve was off, I just checked and here are the results:

Idle: 18 degrees BTDC on the RIGHT side of the boss that the mark is centered on.
2,500 RPM = 22-23 BTDC
3,500 RPM = 26-27 BTDC
5,000 RPM = 30-31 BTDC

Now I'm at a loss for what to do next. I do think there might be a small exhaust leak at the head, so new gaskets are on the way.

Sorry for being so long winded! I just thought all of the information could help diagnose the problem. Despite the tuning annoyances (and carb removal rite of passage) I'm over the moon to have a Commando! I can tell it will be a wonderful ride. Thanks for any help you all can give, I really appreciate it!

Click to expand...

i hope you have the straight through peashooters ! a friend had the same bother with baffled silencers
changed them and it was cured

it would help if you would fix your profile to say just where you are at.

adunham1 said:

I wish I knew someone with the tool I could borrow for a day!

Click to expand...

bill said:

it would help if you would fix your profile to say just where you are at.

Click to expand...

Sorry about that. Forgot to edit it after creating the account. I am in Columbus, Ohio. I think I updated it.