750 amal tuning tips ?

stu · Jun 10, 2015

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

Nortiboy · Jun 10, 2015

stu said:
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 cheers
Stu

Hello Stu,

All the stock MKV 750 I have seen have a 30mm carb. Someone else can chip in here.

Cheers

Mark

Rohan · Jun 10, 2015

The 1973 750 Commandos also had 32 mm Amals, post the 1972 Combat saga.

stu said:
What sort of mains size could i safely go down too before i'm into engine damage territory ?

At 2500 or 3000 rpm, they are no where near running on the main jets.
They'd still be well down on the needlejets.

Think you should try raising or lowering the needles.
I'd lift them up a notch 1st, no chance of damage then, see if richer in the mid range improves or gets worse.
If it gets worse, its already too rich, lower them a notch, and see if that helps. (be gentle, in case it is lean. Watch for heat).

As they say, 90% of carb problems are electrical, and vice versa.
What ignition is this, and what advance have you given it.
A tooth out on the cam timing is also not impossible....

L.A.B. · Jun 10, 2015

Nortiboy said:
All the stock MKV 750 I have seen have a 30mm carb.

Post-Combat 750's with RH5 and RH6 heads would normally have had 32mm (932) carbs, and 30mm for RH1.

http://www.classicbike.biz/Norton/Parts ... _73-74.pdf

Heads, Group 2.
Carbs, Group 11.

stu said:
Ive a pair of amal mk1 932/26 carbs jetted with 230 mains feeding my 1973 750 motor.

Left should be a 932/27 (for 750 MkV, although 932/26 & 27 are often incorrectly listed for: "1973 Combat"-even by Amal :roll: ).

batrider · Jun 10, 2015

Are you operating the choke correctly? Choke off = cable pulled tight so choke slides are up. Some feel that the Norton choke control is "backwards".

stu · Jun 10, 2015

Hi

choke lever pulled tight so the slides are up for running, yes.

The ignition is a boyer but the timing chain slack is needing checked - I've been wary of opening up the timing side so far for breaking old wires etc (it is my daily transport). I've acquired a strobe light too so i'm hoping to learn how to use that first and check the disc on the primary cover for timing.

The needles are on their top clip ie lowered right down - just figured with the plugs so sooty it must be overrich - but i think maybe the engine does run hot - the old rubber plug boots have left a melted mark around the top of the plugs

Plugs were Champion N7YC and now NGK BPR7EIX (change in engine tone) but still very sooty.

cheers
Stu

Matt Spencer · Jun 10, 2015

Check the hidious coils for hideous spark before LEANING it .Unless you like it melted .

a HALF INCH spark is required on infernal combustion engines , includeing NORTONs .

6 V Lucas coils arnt often capeable of that . Or 12 V ones . unless ' SPORT ' .

also rich fuel is SOOT and OIL is OIL . if its OILY its Oil , if its SOOTY its Enid Blighton , or Fuel
or more likely SPARK .

Where do we send the Medal ? . " Everyday Transport " . 10 points for effort . But is not the 1970s,
though Id expect any half decent Commando capeable of it . If the jackles havnt been at it .

brxpb · Jun 10, 2015

Have you replaced the jets through the hole in the floatbowl?, I had a very similar problem to yours and found that on unscrewing the jet it had slightly unscrewed the jet holder. When I took the bowls off the bottom of the needle wasn't visible with the jets out, equivalent of raising the needle about another three notches. Tightened home the holder, refitted my jets and it was like a new one.

Nater_Potater · Jun 10, 2015

stu said:
...(it is my daily transport)...

Assuming it hasn't always been this way, how did it come on? Suddenly? Gradually? Any recent change in fuel sources? Some background history would help. You mention a timing light; get that on and verify the ignition's advancing properly as the rpms come up. Retarded timing can show sooty plugs just as if it was too rich.

Nathan

Biscuit · Jun 10, 2015

Rohan said:
The 1973 750 Commandos also had 32 mm Amals, post the 1972 Combat saga.

stu said:

What sort of mains size could i safely go down too before i'm into engine damage territory ?

Click to expand...

At 2500 or 3000 rpm, they are no where near running on the main jets.
They'd still be well down on the needlejets.

This is not necessarily so. It depends on throttle position, not RPM. Under moderate acceleration or if running at a steady 2500-3000 RPM, true, main jet has little influence. But consider this. WOT in each gear yanks the slides up as far as they go and the metering needles are consequently lifted out of the main jets as far as they can be. You are running on the mains no matter what the RPM, before, going through, and after 3000 revs.

stu · Jun 10, 2015

Matt - definitely sooty not oily plugs and both plugs do seem to be sparking away, i'm not getting any missed beats or backfires just a lack of power rolling up beyond 3000rpm so didn't suspect the coils. How would i check coils are producing big enough sparks ?
brxbp - i took off the bowls and nipped the jets up into the carbs pretty good but I'll have a look for looseness there. I reckon I'll try drop the clips too (raise the needles) 1 notch and see what that does and meantime read up on strobes and timing marks
Nater - uk premium petrol and no sudden changes but the electrical system is ancient

stu · Jun 10, 2015

Biscuit - i did wonder about this, the bike will purr along at 60mph on a steady throttle and only an issue when the engine is loaded - esp going up a hill. Considering the state of the sparkplugs i figured it was bogging down with a rich mix ? timing needs checked first tho

Hobot · Jun 10, 2015

If starts and idles and takes throttle to 3000 rpm then its not very much fuel related as should still pull beyond that even if stumbles and jerks and backfires and farts from bad mixture, so suspect spark timing is somehow not advancing enough, which can be from too low initial timing or cam chain too slack to stay in time. I have had similar happen with boyah trigger wires and other bad conduction that was good till some threshold of vibes blocked electrons. Time light check would be my next easy step before open TS cover, which should be looked into as part of our worship rituals anyway.

stu · Jun 10, 2015

well i hooked up the timing light and set the bike running. At a steady 900 rpm idle the wee mark is way off (would be sitting about 10 if the scale didn't stop at 20). At a healthy rev, (reading about 2800 on the tacho) the mark moves up to about 20. More revs and it doesn't gain a lot - i chickened out at about 3500 rpm with the bike roaring in the shed and the mark was getting up around 24 / 26 maybe.
So do i need to advance the ignition for the mark to be at 28 around 3000rpm (my usual running speed), is the primary side timing gauge reliable ? And if so is it simply a case of opening the contact breaker housing cover, loosening the boyer disc and rotating a snitch to get the mark sitting on 28 ? question questions !

L.A.B. · Jun 10, 2015

stu said:
So do i need to advance the ignition for the mark to be at 28 around 3000rpm (my usual running speed),

You need to rev it until it stops advancing, and for a Boyer this should be 31 degrees BTDC and Boyer say to rev up to 5,000 RPM.

Boyer (Micro MkIII & IV) instructions: http://www.boyerbransden.com/pdf/KIT00053.pdf

stu said:
is the primary side timing gauge reliable ?

No, not always. ignition-timing-questions-t17871.html

stu said:
And if so is it simply a case of opening the contact breaker housing cover, loosening the boyer disc and rotating a snitch to get the mark sitting on 28 ?

Yes (but 31 degrees).

Hobot · Jun 10, 2015

What a wuzzie - there is absolutely no risk to rev unloaded to 6000 rpm to get a steady look at full adv. If engine could be harmed by this then best experienced mechanics in the world would want to find out at home, sheeze. In practice just have to blip up over 5000 a few times to see where blurry marks line up on average so not as nerve wracking to endure holding a constant roaring 5+grand. hobot seasoned lazy route would just try adv tiny amounts till hints of backfire then slightly back off till no hints then maybe put time light on - IGNORING ANY NUMBER VALUES, just to mark where it runs right - ONLY THEN if wanting to be a proper online annal Nortoneer reporter can go through all the steps to verify TDC mark really is and factory dial actually aligned to trust. Put a couple+ layers of carpet under center stand or likes dances with the devil, but then miss out on one the funniest ancient Commando imitation rites.

ewgoforth · Jun 10, 2015

Biscuit said:
Rohan said:

The 1973 750 Commandos also had 32 mm Amals, post the 1972 Combat saga.

stu said:

What sort of mains size could i safely go down too before i'm into engine damage territory ?

Click to expand...

At 2500 or 3000 rpm, they are no where near running on the main jets.
They'd still be well down on the needlejets.

Click to expand...

This is not necessarily so. It depends on throttle position, not RPM. Under moderate acceleration or if running at a steady 2500-3000 RPM, true, main jet has little influence. But consider this. WOT in each gear yanks the slides up as far as they go and the metering needles are consequently lifted out of the main jets as far as they can be. You are running on the mains no matter what the RPM, before, going through, and after 3000 revs.

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.

Hobot · Jun 10, 2015

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.

acotrel · Jun 10, 2015

Biscuit said:
Rohan said:

The 1973 750 Commandos also had 32 mm Amals, post the 1972 Combat saga.

stu said:

What sort of mains size could i safely go down too before i'm into engine damage territory ?

Click to expand...

At 2500 or 3000 rpm, they are no where near running on the main jets.
They'd still be well down on the needlejets.

Click to expand...

This is not necessarily so. It depends on throttle position, not RPM. Under moderate acceleration or if running at a steady 2500-3000 RPM, true, main jet has little influence. But consider this. WOT in each gear yanks the slides up as far as they go and the metering needles are consequently lifted out of the main jets as far as they can be. You are running on the mains no matter what the RPM, before, going through, and after 3000 revs.

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.

Biscuit · Jun 10, 2015

ewgoforth said:
[

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.

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.

750 amal tuning tips ?

stu

Nortiboy

Rohan

L.A.B.

Moderator

batrider

stu

Matt Spencer

brxpb

Nater_Potater

Biscuit

stu

stu

Hobot

stu

L.A.B.

Moderator

Hobot

ewgoforth

Hobot

acotrel

Biscuit

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