Spray tube question

Hello,
I'm new to this site. Have owned a few Nortons in the past and now have another. The latest is a rebuilt 74 but I cant get the tuning right. I've just noticed that it is using the MK 3 carb setup ( for the different pipes and air box). Im wondering if someone here has insight into what I will need to change to bring it closer to 74 specs. Will the cut-away spray tube have a detrimental affect? Should I just get new carbs?
Cheers, Dan

Welcome to the forum, Dan.

dbdan said:

I've just noticed that it is using the MK 3 carb setup ( for the different pipes and air box).

Will the cut-away spray tube have a detrimental affect?

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All 850 models had carbs fitted with the cut-away (stepped) spray tubes, (and 4 ID ring needles) so it shouldn't be a problem.

Best wishes on Commando curve balls. Of all the things on an Amal that can affect anything the very least of them is the spray tube height and whether a cut out or not. Its only to help the 850's not bog on off idle throttle but also helps 750's a bit off idle fuel draw response too. There is a whole book of check lists so need more data points beyond telling ya sniffing up wrong tree with spray tubes affecting state of tune. Start with ignition dial in first, ie: advance till slightly backfires then retard til it just don't, ugh by nth degrees.

Thanks for tithe replies. The bike is running too rich so I will change out the main jet and needle. Can I just use the 4 band needle at the lowest position? Also, is the slide different? Dan

Dan it's true the spray tube is a non-factor here. Your main jet should be fine. It's the needle and needle jet that progressively wear out from all the violent shaking going on. Amal sell them in matched kits so you will need 2 kits totalling the 4 items to fit. The 2 needle clips would then be set at factory manual positions. Fitting new float needles and stay- up floats a good move too. Use the best most expensive premium fuel. Non-ethanol. Reach in with thumb and forefinger to grab a slide and see if it wiggles around in bore. If so it's shot like those needles then contemplate new Premiers or other choice$.

Torontonian said:

Dan it's true the spray tube is a non-factor here. Your main jet should be fine. Reach in with thumb and forefinger to grab a slide and see if it wiggles around in bore. If so it's shot like those needles then contemplate new Premiers or other choice$.

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Thanks. The slide is solid in the carb but behaves like is worn. When the bike is warming up the idle is good to start with then begins to race. As the bike warms up further, the idle falls away. I've checked the timing and its bang on (electronic);
The idle circuit is clean and the carbs are well balanced. Some other symptoms: the bike misfires off idle with A roll-on, has a flat spot in mid revs; really hunts under light loads (2nd gear, 2500rpm on flat ground) and shows really sooty on a plug chop. Note that the the timing does not appear to wander under strobe. Dan

The hunting & the just off idle missfire sounds like poor transition from idle circut to throttle cut-away and needle position. I believe I would make doubley sure the idle passages were clear and adjust the idle mix so it would idle when hot and for the time being not worry about dying when cold. Are air slides (chokes) fitted and used? When are the plugs inspected? After a lot of just low speed running while trying to tune carbs it's likely they'd be sooty. Worn metering needles and needle jets can sneak up on you but if an 'all of a sudden' thing probably a tune issue. Make sure you have the 4 ring, long, 850 needles and try the weakest (top) groove. Pretty sure slides should be 3 1/2 in the '74.

After the ignition is set right, then next step is get float level about right, which is indicated by the engine actually responding to pilot-air screw idle turns, ideal as designed is 1.5 turns out but as long as idle can be set stable 800-900 then its quite good enough to get the other items dialed in. Hunting could be slide wear air leaks. Put some grease on em and if helps you got more a clue. My grease lasts about 100 miles before the idle got too high and won't go back. Anodized slides with one size less cut out number worked a treat for my '72 on Amals. Can't go by plugs much on lean burn gas so response is main way. When right all Commandos should need a bit of a tickle or choke or enricher action to fire, then one kick no or low throttle starts rest of the day, if not way below freezen. Bores can get our of round but soft enough can be beaten back on a dowl or just the ole slides.

Yeah, and don't overtighten anything in the carbs or manifolds as they are unforgiving ,meaning you can't go back...

Yeah sure Matt I've had bowls and manifold gaskets hanging down on rides because I wasn't brave enough or small enough to nip em up enough. I know symptoms of air leaks now and also water in bowls. I use thick heat blocking gasket at head so its crush resistance keep nut/bolts snug and a cushy thick one on carb flange so don't have to bust a nut tightening yet seal and stay snug. Then the carb body screws that a little wipe of RTV will hold yet breaks loose in couple turns so not a swearing straining slot screwing task to remove again if loctite used. So 2 places I approve myself to use RTV, rockers rims the other in a pinch. 3rd the primary till fixed.

dbdan said:

Some other symptoms: the bike misfires off idle with A roll-on, has a flat spot in mid revs; really hunts under light loads (2nd gear, 2500rpm on flat ground) and shows really sooty on a plug chop. Note that the the timing does not appear to wander under strobe. Dan

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Dan, you probably already know this but it's more a question of throttle position rather than RPM's. Fred at Old Britts has a good section on Amals. The different circuits in the carb come into and out of play at different throttle positions. You can foul the plugs at one setting, like idle, and not be able to burn them clean to get a reading at higher openings so make sure to start there first. Adjusting float height is a project with the earlier setup, needles and their jets wear. Consider buying a stay up kit, jets, including a few mains, and maybe new slides then it can become an easy choice just to get a set of Premiers. I'm going to go back to Amals at some time after a single Mikuni and I can't see the resurrection of my old Amals over a pair new Premiers, considering the improvements.

http://www.oldbritts.com/amal_tun.html