Main jet starting point?

A quick question to the gurus out there. Can anybody suggest a good starting point for the main jet size for a 74 850, standard cam, standard compression, 32mm carbs with velocity stacks and peashooters?

The standard settings for the MK2 seem to be 260 jets, and the Mk2A with bean cans went down to 230. It would be nice to know roughly where to start - it gets a touch expensive buying a whole range of jets, and it's also expensive buying several times, why with postage etc.

Cheers,
Steve in Copenhagen.

These are Amal Concentrics Mk1 - you don't actually say. ?

Taking off the air cleaner usually makes them run a couple of ranges leaner (?),
I think is how it works ?,
so you'd be looking at ~ 280 mains. (??).

Unless you run it hard, the main jet doesn't usually come into play.
In fact, you can take them right out, and rarely even find the difference !

Have you actually tried it with the stock jets.
Unless its hunting or bogging at full throttle,
and thats FULL THROTTLE, it may not need much change.
And you can always try adding a little choke, if your choke system is still fitted and working....

If the jets are too small, you can always just open them out with a drill.
Saves buying new ones.
There are certain sizes for drill jets though, and it needs to be drilled carefully.

With real care, you can also silver solder them up and redrill.
Silver soldering needs to be VERY tidy though, an extra blob of metal can undo all your good work.

All in the tuners repertoire ....

I would go with the standard setting for your equipment. 260's seem fair. That what Old Britts says. Yet Amal show states 220 for the MKIIa and 260 for the MKII. Remember, It won't effect your riding much unless you are running the throttle wide open often..

My new Amal Premiers came with 220 main jets, specified for sea level 850 peashooters, stock motor

Start with the stock main jets. Depending on what air filter (none ?!), cam, pistons, head work, mufflers, etc. done in the past 40 years you will need to run the bike at WOT for half a mile or so ( up hill if you can get it ) and see how a new set of plugs look. Don't let the bike idle at the end of the full throttle run. Change the main jets until you are satisfied with the color of the plugs. As noted before, they don't make a huge difference until you are in the upper 1/3 of the throttle range unless you have restrictive (i.e. quiet) mufflers.

BTW, the cost of main jets in a Norton tends to be very minor expense. Buy a range all at once to save postage or find someone nearby who has a collection of carb parts.

Greg

Thanks for the replies. Yes, it has Concentrics, Mk1. I very much hope to be taking the bike down to Spa in Belgium in early July to run in the Bikers Classic. Spa is a really fast track, with lots of time spent flat on the tank at full throttle, hence the question.

I'll probably order 3 or 4 sizes and see how it goes. If all else fails, I do have access to a friend with a set of jet reamers, and there's usually people selling Amal jets at the track.

Cheers,
steve.

Rohan said:

If the jets are too small, you can always just open them out with a drill.
Saves buying new ones.
There are certain sizes for drill jets though, and it needs to be drilled carefully.

With real care, you can also silver solder them up and redrill.
Silver soldering needs to be VERY tidy though, an extra blob of metal can undo all your good work.

All in the tuners repertoire ....

Click to expand...

That's cheating,but I've done it too.If I ever had to make a jet smaller,I just used soft solder.I drilled the entry and exit angles to the same as previous.There are no moving parts that wear out a main jet,and soft solder seems to work OK.You can get away with this if your jet drills go up in approximately 0.001" steps.Mine would go 1.40mm,1.42mm,1.45mm,1.48mm,1.50 mm etc.
I take the easy way on selecting main jet size.Fit a jet that's big enough to cause 8-stroking at full throttle.It only takes a few seconds at full throttle and high rpm to figure that out.Reduce main jet size until 8-stroking stops at full throttle.Select a main jet that's 20cc smaller than the smallest jet that causes 8-stroking at full throttle.You'll be safe at that.You could make slightly more power with a smaller jet,but you do it at the risk of detonation.

My new Amal Premiers came with 220 main jets, specified for sea level 850 peashooters, stock motor

Click to expand...

That seems small, my 850 came with peashooters and 230 jets (its was a MK2A originally with Black Cap silencers and 230 is factory spec), at full throttle it was sluggish so dropped some 260's in and it runs fine.

Like what some others guys posted, I would start out with a 260 main jet. At full throttle you can apply a little bit of choke and if the bike runs better, you need to go up a size in the main jets. I found the choke to be a handy tool for this type of tuning other than just using it for cold starting.

I believe the rule of thumb is start rich and adjust towards the lean.