750 amal tuning tips ?

hobot said:

I found one can not instant snap open any carb Cdo unless over about 12-1300 rpm even my P!! dragster, but if set up right just a few 10ths second smoother opening till at that rpm can snap WOT and not die but if not set up right can still bog down to lugging chugging bucking detonation state, so must ease off a bit till nailing again. Both my 750s can semi lug just over 30 mph just above 2000 rpm oil flow and charging rpm and take increasing smooth opening a few 100 rpm more then can hit WOT to fast as I dare. If can start cold w/o tickle prime or choke then its considered too rich once fully hot. For a bit more off idle spunk I notched spray tubes like 850 and set float to richness so under one pilot screw turn out gives good idle, set to 900 for road life, yet eager to get going. Do note Amals may not be able to sustain WOT long if not slotting open the float bowl filling restriction.

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Four strokes are no different to two strokes when they are tuned correctly. For max performance the jetting is very lean on the needles and you still need to feed the throttle on with a four stroke. It's a habit you develop when you ride two strokes, where if you whack the throttle open they can stop running. Four strokes are usually a bit more forgiving, most are tuned to run a bit rich.

acotrel said:

The main jet size is pretty much irrelevant unless you are doing a blast down a long straight road with the throttle wide open. For most commuting you will be metering the mixture with the needle and needle jet with the throttle part closed. As long as the mains are rich enough you will stay out of trouble. The height of the needles is critical - too high and acceleration will be sluggish - too low and the motor will cough as you go up and down through the gears. If the choke circuit is not operating correctly, it is difficult to tune the needles and needle jets to make sense.

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Agree with all that except main jet being irrelevant unless at WOT down a long straight. One may not do this without a downshift but consider what happens at say 3000RPM in fourth... Wick the throttle all the way open and the gas is getting to the engine up through the main jets without influence from needle position or slide. These are not CV carbs that ultimately regulate flow through vacuum. Purely mechanical here.

Biscuit said:

ewgoforth said:

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Unless you're going up a big hill you can't hit WOT in first gear and probably not in second either. The engine will bog down if you try. That assumes a healthy engine, which his is not. Marking the throttle and keeping an eye on it while riding is easy to do and will show you what circuit you're actually using, not what you think you are using.

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Mine will. Granted things happen pretty fast in 1 and 2 but for the sake of argument, even in third if your wrist has wound it to the stops the slide and needle is out of the picture,,, you are on the mains regardless of RPM. If you are bogging at WOT in first and second it may be time to re-think your own main jet number.

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My 850 has a TZ350 Yamaha twist grip which is very quick acting. Most of the time you don't need that fast action, however it stops the situation where my wrist gets bent at an extreme angle at full throttle. It takes a bit of self control to avoid winding it on too fast as I come out of corners. It's easy to get the situation where the motor is running on the mains where the revs are too low to need that rich mixture. A nice progressive firm squirt is the answer.

The hardest part of the jetting to get right is at 3/4 throttle. If you lower the needles until you get the cough as you go up and down through the gears, the correct setting is by lifting the needles one notch higher. If you cannot induce the cough fit smaller needle jets. For max performance the taper on the needles becomes important. However if you are doing that you will also be measuring air temperatures. I use Mikuni needles in my 34mm MK2 Amals.

You can only sensibly roll a standard AMAL open, not crack it open, because they don't have accelerator pumps.
The jets pass fuel only due to the low pressure created by air speed across the spray nozzle. The low pressure is manifold vacuum dependent which drops toward zero if the throttle is cracked open. This results in very little fuel draw (the 'lean' flat spot) until air:fuel balance is restored. Tuning rich right across the jet range is more forgiving but wastes fuel, dilutes oil, kills plugs, etc.
Ta.

acotrel said:

The hardest part of the jetting to get right is at 3/4 throttle. If you lower the needles until you get the cough as you go up and down through the gears, the correct setting is by lifting the needles one notch higher. If you cannot induce the cough fit smaller needle jets. For max performance the taper on the needles becomes important. However if you are doing that you will also be measuring air temperatures. I use Mikuni needles in my 34mm MK2 Amals.

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Can we also point out that road bikes aren't tuned by getting them so lean they 'cough'.
Thats more an alcohol/racing/tuning thing.... (?).
Petrol engines will just get sluggish or 'hunt' or pause on the throttle well before that.

needing said:

You can only sensibly roll a standard AMAL open, not crack it open, because they don't have accelerator pumps.
The jets pass fuel only due to the low pressure created by air speed across the spray nozzle. The low pressure is manifold vacuum dependent which drops toward zero if the throttle is cracked open. This results in very little fuel draw (the 'lean' flat spot) until air:fuel balance is restored. Tuning rich right across the jet range is more forgiving but wastes fuel, dilutes oil, kills plugs, etc.
Ta.

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I'm hoping no one thought I meant snapping the throttle instantly wide open as if solenoid activated. I looked back at my own posts and could only find reference to full throttle at lower revs as being on the main jet and really nothing to suggest instantaneous throttle action was necessary for this.

stu, once you get the timing sorted, if no, or not much change, no harm will come to reducing main jets a step at a time. And of course check plugs. By the way is this something that has always been, or just started?

Huh seems only hobot and maybe acertel Alan tend to like to Snap WOT for the few 10ths sec difference ahead than just smoothly opening fast. Sorry if yoosse guys have bog on snap opens over 1500 rpm - as that is not normal nor expected in Commandos though maybe too common, so keep diddling > if immature thrill seeker. I KNOW how wide scope capable good ole Amals can be and know the hardest proof of good across the range tune is being able to give almost WOT throttle creerping up steep slope on poor traction and not bog out or spin out just lug as needed under 2000 rpm but making torque to plow through tall grass carrying a couple 50 lb feed sacks. My SVb50 is not capable of this as bogs out or errupts to spin out. Oh yeah besides lowering the spray tubes a bit plus notching them and raising bowl level, I also fitted #3 cut out anodized slides rather than 3.5 to both make up for some fluted bore wear and off idle snap response. I do not have video of it yet but until i get over 30 mph off tarmac I often power steer some by a WOT SNAP then release for the instant tire spin tail swign out then .5 sec later re hook in desired direction w/o worry of front tire not doing its job on low traction.

Moderns are wheelie limited compared to a Commando so they can not put down or hook up the power a Commando can if spun up though power band in 1st 2nd as modern pilot must chop throttle or wheelie control kicks in to keep modern from tumble over back wards. A really good Cdo can take moderns up to about 90 mph a normal power Cdo maybe to 75 mph.

My pure factory Combat had to out accelerate in 90-115 range the most elite type barely street legal current moderns Aprilla 1000 SRs, GRXs etc in order to keep up with their pace in opens d/t being last in line of half dozen superbikes -as leaves little passing room left after they pass a cage one by one before the the next blind closes in. I had to constantly snap to WOT to do so but I was pleased and they were pissed they could not operate - use their excess to any advantage in real world. To brag and get spit at Ms Peel would of gotten bored with them and passed in the opens to let hair out in the turns so abundant in Ozarks. I had half dozen all out work outs on Trixie a few years ago to find out what I could and could not keep up with, so know unless at least 70 hp/350 lb no way can keep up with serious modern joy riders, let along tease to excess when on Peel with - can I tag alone till bored. I had 2 Nortons i would not could not just snap WOT throttle w/o pre griping, tucking down to take the run out from under leap force or dealing with turning into smoking pavement flat tracker. P!! was pure bee line wonder while so so compared power Peel did not have to back off for turns, so combined should just kill me.

Alan was talking about getting em too lean while tuning them Steve, so you are not even on the same planet.

Besides, show us the footage/data plot to prove you really ride em like that.
The current MotoGP pilots can show footage of em being spat off if they try it, and the electronics aren't closely dialled in !!!

Carbs with accelerator pumps are a different world, I really loved my Delorto pumpers.
Alas, they won't fit between the frame rails in a Commando, I looked at that.
Suppose could make do with one, Hmmmmmm.

stu said:

Hi
looking for a few pointers for a novice mechanic
Ive a pair of amal mk1 932/26 carbs jetted with 230 mains feeding my 1973 750 motor. These are stock for the 1973/750 and combat but my bike is dated to the last few hundred of the mark 5 roadsters so not a combat. The bike starts easy, idles real low and steady and pulls fine up to about 2500 rpm - it struggles to make power above 3000 rpm (tho maybe a wee bit clutch slip too) and the plugs are sooty black. I replaced the needles and 106 jets, checked the float hieghts and put a set of iridium plugs in which seem to tolerate a rich mix better - now I'm about to start experimenting with reducing the mains to limit fuel flow - 210 then 200.
Bit of a novice so any advice appreciated - d'you reckon I'm on the right track here and what are warning indications of going too lean other than white plugs ? What sort of mains size could i safely go down too before i'm into engine damage territory ?
cheers
Stu

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Hi Stu.
Checked air cleaner? A clogged one makes for rich mixture - take it off and trial run again.
Note: Main jet only affects up near the wide open throttle range. 2500 - 3000 rpm is just coming off slide cutaway and onto needle jet (in straight needle range not even at taper).
Ta.

Having read LABs boyer info, back out to the shed and tried timing up to 5000rpm. Nosebleeds, dogs barking and commando dancing around on the centrestand and i reckon it was getting up to about 28 but def not 30 so in the ballpark ?? Checked the jets - all clean and nipped in firm (needlejet and mainjet), floats floating, bowls showing clean fuel and gauze baskets clean. Opened the tops, both needles/springs etc bedded in properly, but i raised them a tick to centre notch anyway. Air slides and throttle valves both sliding fine and stuck some fresh (N7YC) plugs in. Recent air filter in and fresh carb boots on so don't suspect air leaks. In line HT lead testers show both plugs sparking away.
Out to roadtest and fires up fine, steady idle and smooth at walking speed. Up to 2000rpm on the flat, cruising and with easy accelaration it gives a steady pulsing surge, more throttle and struggles to make power with the surge getting worse. I managed to get up the road okay, condition deteriorated and struggled back to the shed, esp over the wee hill, so i'm thinking got worse as engine heated up / both plugs very black. I've a few smaller mains jets ordered up so maybe I'll try drop the needle back down and step down the mains 230 - 220 see what happens.

Hi Stu.
Check the ignition connections.
I recently had a gauge that read ok at the lights but dithered at 2500 rpm - loose earth.
A friend's 850 started ok but stumbled with revs - ignition wires frayed at joint.
Ta.

Mark,

If you have a solid (good) side stand check the time using that instead of the main stand. It usually prevents the Norton shuffle.

Pete

OK stu, from your last post I'm getting the idea that your bike falls flat on it's face around 2500 RPM whether you are trying accelerate up through the gears or just add a bit more power/RPM.

Is this a recent problem or have you been plagued with this issue since your ownership?

If it has always been like this for you, and perhaps someone has been into it before your ownership, suspect the cam timing. Degrees of cam retardation will vary the usable RPM downward but still allow for easy starting and idle. (to a point)

Try checking the timing on the wheels with a buddy straddling it, briefly running it up to 5000 while you get an earful with the strobe. Gives a much more solid strobe mark. I know you say it's running rich (maybe at low settings) but check the fuel tap is capable of delivering enough fuel to the carbs at higher throttle settings. Both taps open for high speed runs.

hi biscuit - definitely a problem thats gotten worse over the last few weeks tho the top end power has always been lacking guessing from what i've read and plugs have been dark since i had the bike. The usable power definitely sits around 2000-3000 (till now) and I seldom roll on past 3500 but happy running with commuter traffic at 60mph. Can the cam timing be verified as correct (ie is it obviously marked) when you open up the timing side ? Do you have to dismantle/remove the boyer to open up the timing cover ? all new to me :shock:

You said you have a Boyer ignition? Symptoms sound a lot like what happened to my Boyer and about which volumes have been written.
It is VERY common for one of the two wires which attach to the Boyer sending unit under the points cover to fracture inside the insulation
due to the heavy vibration of the engine. Engine will still start and run at lower RPM but be unable to go past 2500 or so rpm . Search the forum for past discussions of Boyer failures. You would be lucky if this is the issue as the fix is pretty easy.

stu said:

definitely a problem thats gotten worse over the last few weeks

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In that case I would think it's less likely to be a jetting (or cam timing) problem.
However, something has obviously begun to deteriorate recently and it could be an ignition problem or that the engine is simply overdue for a rebuild?
A lack of power could perhaps indicate a cam lobe has worn away in which case one of the valves, either inlet or exhaust may have noticeably less lift when compared with its counterpart on the 'other' cylinder?

stu said:

Can the cam timing be verified as correct (ie is it obviously marked) when you open up the timing side ?

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Yes (see manual) http://www.classicbike.biz/Norton/Repai ... mmando.pdf

stu said:

Do you have to dismantle/remove the boyer to open up the timing cover ?

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Yes.

Checking the cam timing is not as hard as the manual makes it seem. In the manual that L.A.B. was kind enough to post go right to section C30 and illustration C37 for the procedure. Illustration C37 shows two fat black arrows each pointing directly at a small dimple on the cam gear and on the cam drive gear. Note the pin count between those dimples. There are 10. That's what you are counting to verify correct timing. Just be sure you are at T.D.C.

Note that if the two dimples happen to be at the bottom when you have turned the engine to T.D.C., just turn the engine through to T.D.C. again and you see that the dimples will be at the top.

Best way to get the Boyer rotor free of the camshaft, is after removing the center bolt, stick a suitably sized Phillips screw driver into the hole and give it a good sideways slap with the palm of your hand. This will break the taper fit loose. You can pre-mark everything in the Boyer with a paint dot and be pretty sure it'll be in the ball park if not perfect when you put it all together.

The cam lobe alert does twice what ya might think as besdies reducing valve open flow it also puts a whip lash slack in cam chain run that can oscillate the trigger signals to bonkers timing, usually in over adv so engine just can not work against the too soon combustion. Commandos can stump most rocket scientists and Cdo really can-do burn up or blow up too.