carb settings

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I'm new to Nortons and Amals so perhaps someone can offer some advice.

My stock 70 roadster runs perfect when warmed up except it wants to stall off-idle when I roll on the throttle.

As well, it is exceptionally cold blooded, needing at least 10 minutes before it's rideable with the choke only helping a bit when fully engaged.
When cold it wants to stall when rolling on the throttle as well, backfires, and lurches at low throttle settings.

Plugs indicate a slightly lean condition so would that along with the other issues indicate a lean condition that could be rectified with the pilot air screw? I haven't messed with the carb setting as yet with everything the way I received the bike after a rebuild. Is there a "standard" setting for these screws I can use as a guideline. I can't see anything in the manuals. Thanks for any help.
 

MichaelB

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I adjust with half turn increments.The standard setting is 1 1/2 full turns or 3 half turns.
You can easily check your settings by counting the 1/2 turns as you screw them in. You just want it to bottom out, not excessively tight.
Myself, I would screw them them in, noting the number, then back 1 1/2 full turns each side. Then tune from there.
My expereience at sea level with stock 850, is 1 1/4 turn. Can't remember 750 setting.
 
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Thanks Mike, just to confirm, in is richer and out leaner right.

After I posted I went and checked the settings and both screws are 1-1/8 trns out. I'm going to try running them at one turn out. Thanks.
 

MichaelB

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Yes, in rich, out lean. Turning them in will sometimes cure an off idle stumble.
Try to balance the carbs also.
 
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feliz,

Follow the excellent advice given by Michael.

However, if you still have an off idle lean spot, it may be the result of lean throttle slide. Make sure you have #3 slide installed in that 750 Commando.

Jason
 
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Just in case anyone else is having similar issues and reads this thread, I tried the bike today with one turn out on the pilot air screws and it ran considerably better so I figured if some is good more is better so I went in to 7/8 turn and it ran great at all speeds with no off idle problems, lurching, or backfiring. Oh, and starting improved as well.

I suspect that having to turn the screw in so far is really just masking another problem, probably the wrong number slides as suggested above or maybe the pilot jets. If I can stop riding the bike I'll dismantle the carbs and check things out.

Regards, feliz.
 

MichaelB

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Feliz, Reading your thread has jogged my memory. I have had only one 750 with 30mm carbs and I had it a short time. IIRC, the setting on them was between 3/4 and 1 turn. It may have been 3/4 on one and 1 on the other. I don't mess too much with 1/8 turns. These Amals were so worn the slides rattled like B B's in a tin can so I assumed they were heavily worn and leaking. Your settings may be closer than you realize, someone else with a similar bike will have to confirm.

Or maybe not. You need to screw it in till it stumbles, then back off. If you can screw it all the way in, and it still runs, that's bad. Usually a clogged pilot jet. Amals are famous for this. Or maybe a wrong slide or needle.
Regardless if it goes all the way in, it's wrong.
 
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Thanks for all the good advice.

Michael, I'll try turning the scews in as far as they'll go and see what happens. I'll let you know for interest sake. Thanks.
 
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Well I took the top caps and bowls off my carbs today to check the slides and so on and they are # 3-1/2 with the needles in the bottom position. Do you think I should switch and go with the #3s or would there be much of a difference? I would just as soon have every thing to specs and take it from there, easier to sort things out.

Taking the slides apart was like a jigsaw puzzle and I'm not sure how I did it really. Can anyone tell me if there's an easy or proper way to assemble the needle, clip, spring, slide and cable? I honestly can't figure out how. Help!!
 

Ron L

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feliz,
Slide the cable through the cap, thread on the spring and compress it into the cap so the cable end protrudes as much as possible. Slide the cable through the large side of the "keyhole" in the slide and pull it over to the small side. With the needle clip attached to the proper ring on the needle, drop it into the large side of the "keyhole" in the slide. You can now release the spring into the slide. If you have chokes, they are assembled by compressing the spring on the cable and hooking it into the slot on the choke slide. Then fit the choke slide into the throttle slide. Be careful when fitting the assembled slide into the carb body. The slide has a tab which slides into a slot in the body to align it. When first feeding the slide into the body use you finger through the mouth of the carb to align the needle with the jet. Don't let the needle hang up on the side of the jet and force the slide. You will lose the needle clip or worse, bend the needle. Go slow and it will fall easily in place.
 

MichaelB

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Ron is on the money. You'll just have to work through it.

3 1/2 in 30mm 750??? Interesting. IIRC you bought this bike from a 'British, but not necessarily Norton, shop.' Are you or this shop at high altitude?? #3 slide is standard for all 750's, 30 or 32mm.

On the side of the carb it is either 930 or 932.

Also, check the interference between the carbs. Norton carbs sit very close and unless relieved at the inside boss, they interfere, causing vacuum leaks. Original Norton carbs are machined here, replacements have to be filed. Most 'British, but not necessarily Norton, shops' do not understand this.

Check your spray tube. That is the brass item in the bottom of the carb. There are two. Straight and cutaway. Full circular is the straight one,
The cutaway wil be obvious. The needle needs to be matched to the tube.
The two ring needle goes with the straight tube, the 4 ring needle goes with the cutaway tube. These rings are located above the 3 ring clip grooves.

Let us know what you have and don't fret. Anything can be made to work with proper tuning.

Also, if I were buying new slides, I would probably buy chrome ones.
 
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Thanks again for all the great advice. Just waiting for my chrome slides, new gaskets and o-rings, new main jets to get here then I'll get it all together. The hard chromed slides were good advice and only cost a couple dollars more than the pot metal ones. Thanks again, great forum.
feliz
 
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Well I put everything together with the help of you folks and went for a short ride, it ran great. The off idle and backfiring problems I was having seem to have gone completely. I installed chrome #3 slides and 220 main jets with the clip in the middle position, all what the specs call for.

I tried tuning by ear and it turns out to be near the 1-1/2 turns the factory specified. I had trouble getting the air box in position and it's still not fitting tight like it should so I'll work with that. The chrome plate is about 1/4 inch out from the filter and won't seem to go in snug.
I never realised there was a snorkel in there either!!.

Thanks again.
 
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