Amal jetting woes

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Apr 15, 2004
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I can't seem to find the right jetting. Everything I try is too rich or too lean. The local brit bike shop, Isles Motorcycles, recommended 105 needle jets and 200 mains for our mile-high altitude. So I put those in. Nowhere to run the bike wide open around here, so no idea if the mains are right. I guess they'll have to do.

The 105 NJ's with needles in the middle position gave bone white plugs and pinging. So I raised the needles a notch (one notch is all there is). That was way too rich - black plugs, 8-stroking. I put the 106's back in with the needles on the top notch. Better but still too rich. Of course those came with the bike and might be worn out. I could try new ones.

Seems like what I need is to use the 105's and raise the needle 1/2 notch or maybe less. I'd need some shim washers like Dynojet supplies with their jet kits. I don't suppose anyone has tried that with Amals???

Or maybe I'll just have to give up and put a Mikuni on it. Higher quality and I think more adjustable. Would really rather keep the amals though...


It appears that you have replaced both the needle as well as the needle jet? If not, you should.

Also, what size is your throttle slide? A #3 slide is richer than a #3-1/2 slide. I've used #3-1/2 slides on both 750 and 850 Commandos with good success.

Is there excessive clearance between the throttle slides and the carburetor body? If so, you will have problems, cheif among them is difficulty adjusting the idle.

Are your pilot air screws adjusted about 1-1/2 turns out?

Let us know your progress.

I think the next step will be new 106 needle jets and needles. The ones in now came with the bike and quite likely are worn out.

The slides and carb bodies seem to be ok. Bike has a nice smooth steady idle. I've got it idling at about 1200 rpm when warm and it's pretty consistent. So I don't think it's going to need resleeving just yet. The problems are in the needle jet/needles' domain.

Shimming the needles to work with the 105's would probably work but fabricating things like that is beyond my abilities. And it should run with (new) 106's. Most bikes run fine up here with standard jetting...

reading the plugs

Was looking at some of those websites norbsa posted on another thread about reading plugs. Sounds like I may have been confused on how to read them. If the porcelain is white but the base ring is tan or gray that's ok? If so those 105's may be correct. I thought the *porcelain* was supposed to have the nice tan color all the way down...

Yes Debby, The center needs to be white with no speckles they would indicate pre- ing. I have been getting hotter and hotter plugs and as I move up the scale slowly I find the bike better running. Most of the plugs we run in Nortons are too cold but don't take anyones word on witch one to run learn to read them yourself. You may find as I have found that you have long trip x-way plugs and a hotter set for the 55-70 mph we all like to do. Once you find the hottest plug your bike can stand you may want to try retarding the timming ever so slightly for better 1/4 mile times. Just learning this stuff as I go. norbsa
Here in Reno I use stock needles and needle jets, with a one size main reduction. I have a Combat that runs on regular...

Hi Eric,

I used to live in the Reno area about ten years ago. I'm a UNR grad as a matter of fact. Still miss the area. Great riding there. A couple of my faves were Monitor pass and Ebbetts pass. No back roads here in Colo - all roads lead to a subdivision and all are clogged with SUVs :(

When I lived there I was Norton-less but did have a Suz GS750. It ran fine with stock jetting. I'd think the Norton should run ok with stock jetting here (our altitude is about the same as Northern Nevada's) but I think the 106 needle jets that came with the bike are probably worn out. I have a new set of 105's so those are going back in.

The local brit bike shop recommends 200 mains and 105 NJ's. Mine is not a combat BTW.

Woebegone! I put the 105's back in with the needles in the middle clip. Was running well except it was pinging on accelleration. So I went back, dug out my timing light, and checked the timing. Sure enough - both cyls were off, at 32 deg instead of 28.

So I had to do something I haven't done in a long long time - adjust the timing (I have points). I remember I used to get pretty frustrated in the old days whenever I had to do it to the old bike. Now I remember why. Wow it's hard to get the adjustment right. Every time I moved the backing plate it was too much or not enough. I suppose that's why Boyers are so popular huh? I finally got them to 25 deg and decided that was good enough.

I test rode again and the pinging was gone. The bike seemed to be running smoother and better too. And the plugs look good. Maybe being retarded a few degrees isn't so bad anyway considering our low octane modern gas. So I think I'll leave it like this. I'm pretty happy with how it's running now :)

71 750 Roadster
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