Amal needles FYI (2010)

maylar

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Posting this as a FYI for the archives. Not a question, just a report on my recent experieces with Amal needles.

Background: '74 850 MKII, stock intake (paper filter), 750 head pipes, Wassell silencers. Amal 932 carbs, 260 mains, 3-1/2 slides, flat top spray tubes. Carbs were sleeved about 7,000 miles ago.

I did some work to the bike last season and somehow ended up with the long 4 band needles (928/104) in these carbs. With the needle in the top groove (needle all the way down) the engine ran much too lean and hot. Raised the needle to the middle groove and it was much better, although a bit lame in the lower revs and mid range. I also got an unrealistic 60 mpg cruising at 50 mph. This indicates it's still lean, though the plugs looked OK. I ran it that way for the last 3000 miles.

So recently with the discussion of needles here, I decided to put the correct shorter, 2 band needles (622/124) in there to see what would happen. With the needle all the way down, my midrange throttle response was much improved. But the motor would surge a bit just off idle, and sometimes backfire on throttle - off. These symptoms are very similar to what happens with a partially clogged pilot jet.

Raised the needles to the mid position yesterday and the surging went away. Still had an occasional pop on decel, but cured that with 1/8 turn more rich on the idle mix. Throtle response is now excellent across the band, like an 850 should have.

So, be aware that needle taper and position have a profound effect just off idle and through the mid range. Just FYI.

Will report back on the change in MPG after a tankful.
 
Alrighty Dave, Now you are at a point to do my experiments with float level vs pilot jet optimal setting. May I leave you restless until you try bumping up bowl fuel level till 1.5 turns out gives most pleasant behavior.
Next experiment is file a nip out of back side of spray tubes.
 
I went through all this balony years ago after I tuned my motor.
Stuck flat top spray tubes with 2 band needles, different settings. Tried different slides etc.
Never ran really right, so I put the cutaway spray tubes back in with 4 band needles & set it to the book. Hey Presto! It ran like it should.
 
Posting this as a FYI for the archives. Not a question, just a report on my recent experieces with Amal needles.

Background: '74 850 MKII, stock intake (paper filter), 750 head pipes, Wassell silencers. Amal 932 carbs, 260 mains, 3-1/2 slides, flat top spray tubes. Carbs were sleeved about 7,000 miles ago.

I did some work to the bike last season and somehow ended up with the long 4 band needles (928/104) in these carbs. With the needle in the top groove (needle all the way down) the engine ran much too lean and hot. Raised the needle to the middle groove and it was much better, although a bit lame in the lower revs and mid range. I also got an unrealistic 60 mpg cruising at 50 mph. This indicates it's still lean, though the plugs looked OK. I ran it that way for the last 3000 miles.

So recently with the discussion of needles here, I decided to put the correct shorter, 2 band needles (622/124) in there to see what would happen. With the needle all the way down, my midrange throttle response was much improved. But the motor would surge a bit just off idle, and sometimes backfire on throttle - off. These symptoms are very similar to what happens with a partially clogged pilot jet.

Raised the needles to the mid position yesterday and the surging went away. Still had an occasional pop on decel, but cured that with 1/8 turn more rich on the idle mix. Throtle response is now excellent across the band, like an 850 should have.

So, be aware that needle taper and position have a profound effect just off idle and through the mid range. Just FYI.

Will report back on the change in MPG after a tankful.
Seems like you are using the carbs for a 750 Combat with the flat top tubes, with the change of needle you are at Combat specs except for the main jet. Why didn't you fit carbs with 850 style 1/2 cut down fuel nozzles (tubes)?
 
Seems like you are using the carbs for a 750 Combat with the flat top tubes, with the change of needle you are at Combat specs except for the main jet. Why didn't you fit carbs with 850 style 1/2 cut down fuel nozzles (tubes)?
Short answer:
I didn't have any.

Long answer:
I bought the bike new. Being young and clueless, I decided to "improve" the power by installing a pair of 34 mm Mikuni's. Sold the original Amals to a neighborhood kid for his BSA. I had no clue about tuning carbs, other than changing main jets. That adventure was an epic failure, and I went to my dealer and bought a new set of 32 mm Amals. They had flat top spray tubes, and they worked fine for years. At some point I decided to replace the needles and bought a pair that was correct for an 850, from Old Britts. That's how I ended up with long needles and flat top spray tubes.
 
I went through all this balony years ago after I tuned my motor.
Stuck flat top spray tubes with 2 band needles, different settings. Tried different slides etc.
Never ran really right, so I put the cutaway spray tubes back in with 4 band needles & set it to the book. Hey Presto! It ran like it should.
Stock settings are best on a stock 850, the engineers knew what they were doing, why do people think they can fix something that's not broken?
 
Stock settings are best on a stock 850, the engineers knew what they were doing, why do people think they can fix something that's not broken?

The engineers certainly knew what they were doing, however, the 928/104 (4 ID ring) needles and 928/107 stepped spray tubes were only required so 850s would pass a US rolling acceleration test where the throttle had to be snapped fully open at low road speed in top gear when noise reduction muffler mutes required in certain US states were fitted and under conditions unlikely to be encountered during normal riding and I doubt any riders now (or for very long then) have exhausts with mutes.

Amal needles FYI (2010)

Amal needles FYI (2010)
 
If you fit the largest specified main jet when you are tuning your motor, the metering of mid-throttle positions should only be done by the needles and slides. Most slides these days have a number 3 cutaway, and the tuning is done by the needles and needle jets. The needles in both Mikuni and Amal carbs are the same diameter above the shoulder where the taper begins. The differences are in the length of the shoulder and the taper of the needles. If you have the situation where you can induce a cough in the motor when you ride the bike, by lowering the needles, raise them one notch so the tuning becomes richer. and you will usually have the needles in their best positions. However, then there is the issue of the taper on the needles. The taper is there to provide more fuel if you lose vacuum when you wind the throttle on too fast. If you have a very slow taper, your tuning will stay lean as you wind the throttle on, but if you wind the throttle on too fast, your motor will gasp. A slow taper needle is better if you want more acceleration, but if you cannot control your throttle hand, you need faster taper needles.
I have 34mm Mk2 Amals on my Seeley,850, but I use 6D Mikuni needles which are very lean all the way down. My inlet ports are the standard 30mm for most of their length. I ride the bike as though it is a two stroke. It is fast enough to win races. If you have bigger ports, you need a faster taper.
I only use plug readings when I am setting the main jets, but usually I leave them a bit too rich. If you are going to burn valves, it will usually happen when the motor is on full throttle.
 
The engineers certainly knew what they were doing, however, the 928/104 (4 ID ring) needles and 928/107 stepped spray tubes were only required so 850s would pass a US rolling acceleration test where the throttle had to be snapped fully open at low road speed in top gear when noise reduction muffler mutes required in certain US states were fitted and under conditions unlikely to be encountered during normal riding and I doubt any riders now (or for very long then) have exhausts with mutes.

Amal needles FYI (2010)

Amal needles FYI (2010)
Whatever exhaust system you have on your bike or what mods you have done, the tuning sequence is always the same. You cannot just fit jets , slides and needles on somebody elses' recommendation and expect them to be correct. Every motor is different. The exhaust system can change the valve timing needed to get the best performance. If you fit megaphones to an already well tuned standard bike, you will probably burn a valve almost immediately
 
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