Fast idle and a few other problems

Hi all,
well, my very stock 74 850 Mk2 (Boyer ignition) is registered and back on the road after very long lay up. It’s just delightful, exceeding my memories of it, yesterday on a perfect cool evening on an isolated mountain road, to visit a mate for a few beers and home again. How good is this!
I think that perhaps owning a Trident as my primary bike tended to jaundice my initial experience of the Commando by not allowing me to apprec it’s lower power band and letting the bike just ‘lope along’. Rather, I found myself wanting to change down and rev it beyond it’s comfortable rpm band. I’ve decided that it’s relaxed gait is deceptive and actually disguises it’s performance after the busier T160. Just leave it in fourth and let the motor do the work.
The handling, different to the Triumph, I don’t quite feel as much a part of the bike but more in partnership with it. Unless pushed too hard, it’s light and effortless.

A FEW PROBLEMS
The bike is a perfect starter, almost always first prod with an appropriate tickle when cold. It instantly settles into a perfect idle. However, once warm, the idle stubbornly hangs up around 2000rpm. From reading the forum I’m led to believe it’s slide wear.
# Does over-tightening the carburettor to the manifold lead to distortion of the body contributing to the problem?
# Does simply fitting new slides help with this problem?
# Should I have the bodies rebored and sleeved? Who would perform this work in Australia? I really don’t want to fit a single Mikuni.

Wow, is that front brake useless. I always thought I had large strong hands but at 62 and with a condition called Dupuytrens Contracture I can barely slow the bike down. I Have a reduced master cylinder kit ordered so I hope that helps, otherwise I will have to rethink the front brakes
# I have heard that Ferodo Platinum pads gives improved stops, is this correct?
# Does anyone have any other suggestions.

I find the clutch quite difficult to use trying to get a smooth take off. I find I’m tending to stall the bike off the mark, something I would never do on other bikes. In fact I’m using a fair amount of throttle to get her underway which can be quite interesting on wet grass or a loose gravel road. It has metal plates.
# Would disassembling and washing the clutch plates help this problem?
# Is ATF the preferred oil for the primary?

The otherwise oil-tight bike has an annoying weep at the head gasket. Not quite bad enough to make me remove the head and attempt to fix it but enough to make the bike look untidy after a good run, particularly on dusty roads.
# Do most home mechanics remove the engine from the frame or take the head off in situation?
# Does simply changing the head gasket solve this problem?

Any advise or suggestions on any of the listed problems would be greatly appreciate.

regards
Al

Apart from the above problems

The oil leak is likely from either the front rocker covers or the oil feed into the head. The oil runs down and looks like the head gasket is leaking. I'd check those first before I would pull the head off.

Alan L said:

A FEW PROBLEMS
The bike is a perfect starter, almost always first prod with an appropriate tickle when cold. It instantly settles into a perfect idle. However, once warm, the idle stubbornly hangs up around 2000rpm. From reading the forum I’m led to believe it’s slide wear.
# Does over-tightening the carburettor to the manifold lead to distortion of the body contributing to the problem?
# Does simply fitting new slides help with this problem?
# Should I have the bodies rebored and sleeved? Who would perform this work in Australia? I really don’t want to fit a single Mikuni.

Click to expand...

Yes.
Maybe.
If it comes to it, buy new Amal Premiers. Annodized slides, new bodies, other minor improvements.

Have you had them apart and cleaned them thoroughly post layup?

Does it hang off idle at 2k when cold?

Check to see if you can make it stumble when warm by spraying some carb cleaner at the carb/manifold and manifold/head joins to see any air leaks.

What happens if you drop slide height with the idle screws? They really should be set for idle at warm. Keep at 2k when cold to keep oil to the head at a good rate.

Alan L said:

I find the clutch quite difficult to use trying to get a smooth take off. I find I’m tending to stall the bike off the mark, something I would never do on other bikes. In fact I’m using a fair amount of throttle to get her underway which can be quite interesting on wet grass or a loose gravel road. It has metal plates.
# Would disassembling and washing the clutch plates help this problem?
# Is ATF the preferred oil for the primary?

Click to expand...

Cleaning the bronze clutch plates helps keep the from sticking.

Yes, use Type F ATF.

Read this.

Sounds to me like you’ve simply got the throttle stop screws wound in too far as Gortnipper says.

The fact it idles so well immediately from cold is partly what makes me think that. A cold bike shouldn’t idle perfectly, and if it does, it’s probably gonna idle too fast when up to temp.

Great summary of the Commando in comparison to the Trident by the way !

Hi Fast Eddie (and all)
I wish it was as simple as incorrectly adjusted throttle screws.
it really does feel like the slides are not dropping all the way down when it is hot.
In view of it’s excellent idle before a run is there any way of ensuring the slides do drop down when hot, a light hone for example?
al

Check your cables and how the speing is seated in the slide. I have had weird in behavior and sticky slides with a cockeyed cable and/or a cable that was poorly routed or spring/needle slightly dislocated.from.its seat.

If the slides have sticktion with playing with no tops on the carb, get Premieres.

It will be a case of a lot of fiddly with the carbs until you get it to run right and by then they will be worn out. So just bite ye olde bullet and procure the Premiers
and save yourself the bother. New carbs means that everything is new.

How do you know the throttle stop screws aren’t at fault? Have you tried them? You haven’t said so... Just try winding out your throttle stop screws a tad when it’s ticking over at 2,000rpm, it will either lower the revs, or it won’t. Simple as that, and then you’ll know. Honestly, there is zero point in lengthy hypothesising until you’ve confirmed this.

If it’s not that, then as Gortnipper says, def check your cables condition and routing. Also your twist grip assembly. Always check the easy and cheap stuff first !

If it’s not that, and in answer to your earlier question, YES over tightening can cause warped bodies, which causes slide sticking, very easily actually. But personally, I’ve never had warped bodies that only stick when warm. And they have tended to stick higher up in the bodies, not in the almost closed position.

If it is warped bodies, they can be fixed and / or botched back into functionality, but as others have posted, there really is little point in expending time and effort on the bodies IF they are already worn out. So a full strip and inspection would be wise before deciding on the next steps at that point.

Alan L said:

Wow, is that front brake useless. I always thought I had large strong hands but at 62 and with a condition called Dupuytrens Contracture I can barely slow the bike down. I Have a reduced master cylinder kit ordered so I hope that helps, otherwise I will have to rethink the front brakes
# I have heard that Ferodo Platinum pads gives improved stops, is this correct?
# Does anyone have any other suggestions.

Click to expand...

Getting new pads and a 13mm mc goes far. You likely need a new brake line, as the old one is probably acting like a balloon. A SS line.

I have Dupuytrens as well, and I went to a 13mm Brembo MC (with the full CNW Brembo kit) and used shorty after market levers for a Duc with an adjustable cam to change the reach and that made all the difference to cover the brake with my crooked fingers.

A good option is Don Penders brake kit as well.

gortnipper said:

Getting new pads and a 13mm mc goes far. You likely need a new brake line, as the old one is probably acting like a balloon. A SS line.

I have Dupuytrens as well, and I went to a 13mm Brembo MC (with the full CNW Brembo kit) and used shorty after market levers for a Duc with an adjustable cam to change the reach and that made all the difference to cover the brake with my crooked fingers.

A good option is Don Penders brake kit as well.

Click to expand...

I am very pleased with the brake conversion from Don.

Alan L said:

# Do most home mechanics remove the engine from the frame or take the head off in situation?
# Does simply changing the head gasket solve this problem?

Click to expand...

You don't have to remove the engine to remove the head. What kind of head gasket do you have? The copper gaskets tend to leak much more than the fiber flame ring type. Look at the other sources of leaks as already mentioned, but if you do conclude that it's the head gasket there are threads here about the front cylinder studs leaking because they sometimes protrude into the pushrod tunnels.

I am currently finishing a rebuild on a Mk3 that I purchased in pieces which is my 4th 850 and have owned Norton's for 45+ years, on this rebuild I have noticed that the quality of "named" replacement parts is not what it used to be in particular to reply to this thread I purchased new throttle cables from a reputable supplier under the Norton p/ns and the inner cable from the throttle to junction box was to short so had to deepen the plunger grove in the junction box to get the carbs to close correctly and this appears to be a common problem. Has Alan L fitted new cables?

Fast Eddie said:

How do you know the throttle stop screws aren’t at fault? Have you tried them? You haven’t said so... Just try winding out your throttle stop screws a tad when it’s ticking over at 2,000rpm, it will either lower the revs, or it won’t. Simple as that, and then you’ll know. Honestly, there is zero point in lengthy hypothesising until you’ve confirmed this.

If it’s not that, then as Gortnipper says, def check your cables condition and routing. Also your twist grip assembly. Always check the easy and cheap stuff first !

If it’s not that, and in answer to your earlier question, YES over tightening can cause warped bodies, which causes slide sticking, very easily actually. But personally, I’ve never had warped bodies that only stick when warm. And they have tended to stick higher up in the bodies, not in the almost closed position.

If it is warped bodies, they can be fixed and / or botched back into functionality, but as others have posted, there really is little point in expending time and effort on the bodies IF they are already worn out. So a full strip and inspection would be wise before deciding on the next steps at that point.

Click to expand...

Hi Eddie,
Thank you again for your reply.
It is hard to include all relevant information in my initial post and you are quite correct in asking the basic questions.
Yes, I have played with the idle adjustment screws and ensured that there is slack in the throttle cables.
In fact I am concerned that the idle adjustment screws are set so low they may not even be engaging the slide.
When I say the bike idles perfectly after start, I do of course mean once the bike has warmed up sufficiently to reasonably provide a good idle, not the second the bike fires up. The ‘holding up’ at high rpm (approaching 2000) is profound, not just a higher than normal idle.
The issue only makes itself known at the end of a long run. My gut feeling is some kind of warping or distortion of the body of the carb.
I will remove the carbs (again) and see if I can see any evidence of binding and check if the screws mounting them are too tight.
Thank you for your interest
Al

You don't have to pull the carbs, just pull the tops. Then you can check the spring seating or if you have the tops on back to front (which I did once and it bound up when opening up ) as well as the tolerances.

Alan L said:

Any advise or suggestions on any of the listed problems would be greatly appreciate.

Click to expand...

Available at most bicycle shops.
The black bottle is a dry lube with carrier liquid.

Hi Alan,
Thanks for the info, I really don’t want to sound all curmudgeonly and pedantic, honest... but when you say ‘idle adjustment screw’ we are really talking about the right screw? This certainly wouldn’t be the first thread to get it wrong! There is a pilot air adjuster screw (controls the air to the pilot circuit and is horizontal) and a throttle stop screw (controls the seating position of the slide and is pointed up at an angle).

If winding the throttle stop screws out does not lower the tickover speed, then they can not be holding the slides up, as you suggest.

However, it’s interesting that you say you doubt they are touching the slides anyway. If that were the case then the slides would close all the way and kill the engine (hot or cold there’d be zero tick over). So, if your throttle stop screws are not touching the slides, and the bike is ticking over, it can only be because something else is holding them up. Take one out completely, put your finger over the hole, see what that does.

A warped body is a possibility, but I’d still want to rule out cables 100% first.

How did you check for slack in the cables? If you have a 2:1 cable junction you cannot just check at the twist grip (as you would on the Trident for example), you have to check at the cable outlet at the carb, or more ideally, by checking the movement of the slides (which you have to do to balance the slides anyway).

I would really urge you to take the tank off and have a good, thorough, inspection of the cables, and routing, and the junction box, etc and rule out all possibilities there, it really is very easy to get something here wrong / twisted / tight / etc when rushing.

As a Trident owner I know I’m on the verge of insulting your intelligence with this next question, but it’s necessary to ‘rule it out of our enquires’... are you using the choke correctly ie slack cable = choke slide closed = choke ON / tight cable = choke slide open = choke OFF ?

If there is nothing found wrong with the cables and chokes etc, then it’s carb off time as you really need the bodies off and clean to be able to gently feel for possible tight spots, and to check the mounting flange for distortion, etc.

Fast Eddie said:

I really don’t want to sound all curmudgeonly and pedantic, honest...

Click to expand...

Uhh...err....umm ...

;-)

Hi Eddie,
Thank you so much for the interest you are showing regarding my little problem, I do really appreciate it. You are correct to check that we are both singing off the same hymn sheet regarding the various adjustment.
Now, ‘curmudgeonly‘ is not a word in common usage but no, I didn’t take your enquiry as that at all, it’s all about the lowest common denominator. I’m a huge believer in William of Occam’s philosophy that the simplest answer is the most likely be the correct one.
Unfortunately in this case I do know the difference between the idle mixture screw and the idle adjustment screw (pity really, it would have been an easy fix) and I do know that counterintuitively that the choke slides on our carbs pull upwards to the off position.
I have checked free play in the cable at the actual carbs not at the twist grip because I do know that they divide into two under the tank
No drama, I’ll take the tank off (again) and remove the slides and see whats going on.
again, thank you for your assistance and interest.
regards
Al

First, do the throttle slides really closes. Though the stop screws don't seem to affect idling it could be that the slides for some reason don't bottom.
If they do, how does the engine get air enough to idle at 2000+ Rpm? Though not much air needed for that. Noticed when hand choking my Manx for really cold starting. Air could either come in via leaking flanges (tested with starting fluid or brake cleaner) or leaking between throttle and body. Do you have choke cables or blanking screws fitted? It can also be that the float chambers top surface could be distorted by excessive tourque on the screws. That can get an air leak into the pilot circuit.
I presume you are familiar with http://amalcarb.co.uk/rebuilding-mark-1-concentric-carburetter