Mk3 850 build won't idle

Hello all,
New too the forum and have a problem that's just a little beyond my scope of mechanical knowledge. I have recently built my Mk3 from the frame up after a wreck took it out of commission. Specs are as follows:
- cylinders bored +.060"
- pistons, rods, and crank balance by HCV Motorsports
- head ported and polished by Leo Goff (intake manifolds matched)
- Megacycle 560-00 cam with radiused lifters
- new Black Diamond valves
- R/D valve spring kit with Titanium retainers
- Tri-spark ignition system
- Dual Amal Mk2 34mm carbs (#25 pilot, 106 needle jet, #3 slide, needle set to middle position) with K&N in place of non existent factory air box
- Podtronics 200w voltage regulator and 200w stator

I finished the build and started up the bike with minimal trouble. Started the bike one day to warm it up and take it for a break in ride and the idle started climbing without me touching the throttle, has a intermittent back fire through the carbs, and if the rpms settle down it won't idle without taking the slack out of the throttle cables. I re-checked the carb settings, idle screws 1.5 turns out, sync still good, and float level around .100". The slides aren't sticking in their bores. Couldn't find any apparent air leaks between the carbs and the head. Checked timing and it's still set at 28 degrees BTDC. Valve lash was still correct. (I set lash to Combat specs for reference). Everything else about the bike is working perfectly outside of a few oil weeps/leaks and hesitation when snapping the throttle open that tells me I need a richer slide (already ordered).

I'm open to any suggestions and hoping I've just missed something obvious. Thanks!

Did you spray WD-40/carb cleaner around the carbs while it was running?
Vacuum leaks are where I'd start.

Also check the cable routing- that has caught me a few times.

Try turning the idle stop screws in until it does idle.
If taking the slack out of the cables gets an idle then the idle stop screws must be set too low to do their job.
It also seems there may be a cable routing issue ( causing idle to climb) as mentioned above.

Glen

The backfiring would indicate running lean, so the leak check is a good place to start, as well as pilot circuit tuning.
Idle screws should be set to whatever gives the best idle rather than a pre-set value.
As Glen says, you need to bring them in until you get an idle. Then you can get the pilot mixture set.
Until these are sorted anything else is a guess.
Pilot screw settings are typically 1 - 1,1/2 turns out, but can vary quite a bit. My first 850 was less than one turn.

Changing the throttle valve cutaway is best done (1) when you've got the above sorted, and (2) after you've ridden the bike to establish if the lower mid-range fuelling is still out.

A good study of the Amal carb manuals available online would be very beneficial.

As already mentioned, sounds like cable routing to me too.

I’ve just been through a steep learning curve with mk11 carbs! Are they new or old? What needles do they have? Do they have the small alloy Bush, called and ‘air jet’ fitted into the small hole directly below the intake throat (you have to remove the velocity stack stubs to see and to remove)?

Do not jump to conclusions, stick to the basic stuff first, whilst I was struggling with my carbs recently I was serious torpedoed by a faulty brand new spark plug !!

@ Andy, I learned recently that the pilot screw tells you a bit more than that... yes their precise settings can vary, but if less than one turn is best, you need a richer pilot jet. If more than two turns, you need a weaker one. The air screw is designed to work correctly when 1 1/2 turns out. Well, this is according to John Healey at least.

Fast Eddie said:

@ Andy, I learned recently that the pilot screw tells you a bit more than that... yes their precise settings can vary, but if less than one turn is best, you need a richer pilot jet. If more than two turns, you need a weaker one. The air screw is designed to work correctly when 1 1/2 turns out. Well, this is according to John Healey at least.

Click to expand...

I wouldn't argue with that Eddie, but these were original Mk1 932s with pressed-in pilots, so pretty limited options! Nevertheless, good to bear in mind.
The bike ran great and tickover at 800 rpm, and we're only talking a bug's pube less than a full turn out

B+Bogus said:

I wouldn't argue with that Eddie, but these were original Mk1 932s with pressed-in pilots, so pretty limited options! Nevertheless, good to bear in mind.
The bike ran great and tickover at 800 rpm, and we're only talking a bug's pube less than a full turn out

Click to expand...

Indeed, fair point, you’re kinda limited with the fixed pilot Bush!

Another point for the OP... mk11 carbs are very sensitive to cable routing of the cold start plungers too. Unless there is a good amount of free play in the cable outer ‘lock to lock’ the plungers will be lifted, and the cold start circuit opened, and all your carb settings are then completely up in the air.

And talking of the cold start, don’t forget it goes the opposite way on mk11s, ie slack cable = choke OFF. That’s caught out many a fine fellow before now !!

Even more simple as above start with the basics, go over them again, to create a bang you need fuel, air and a spark. You have sparks, the airway is open and thus it has to be the fuel. Unless you have forgot to do something up on new build you can discount air leaks elsewhere. Another trick is to raise the throttle stops to fast idle remove one plug if it dies the metering is not correct, when it is, it will usually start on one cylinder with the elevated throttle stop. You can then work back from this position for each side.

I did spray WD40 around all the intake joints (carb to flange, flange to manifold, and manifold to head) with no change in the motor. Cable routing is something I will look into. I've read that between the head steady brackets is preferred over anything else and I'll see if my cables will allow for that routing. I've read the amal carb tuning manuals many times already but going over them again never hurt anyone. Keep a copy next to my bed for night time reading anyway. The carbs are brand new from England as of last year with factory jetting. I don't have cables run for the cold-start levers for the carbs so I don't see that being a problem. I just set up the carbs to pre-sets (found here http://www.jba.bc.ca/Bushmans Carb Tuning.html) to give me a starting point. I'll be rolling the bike out today to kick it over after checking the cables and see what happens.

What do you mean by ‘factory settings’?

The Norton factory never fitted mk11s. So they can’t be Norton factory settings.

So I assume you mean Amal / Burlen factory settings?

If so, unfortunately, they are most certainly NOT a reliable starting point. I’d urge you to canvass opinion from people with the same carbs, on the same bike, that are known to run 100% sweet and use their settings as a start point.

Why do I say all this? Cos I’ve just been through a proper headache myself, with brand new mk11s set up by Amal to suit the bike, which is nothing special, just a box stock T140!

For inspiration / depression (delete as appropriate) read this:

https://www.accessnorton.com/NortonCommando/struggling-with-mk11-carbs.26419/

I meant factory amal jetting. Apologies for the confusion.

I've looked around on this forum and other place for suggested settings and I found factory Amal Mk11 setup wasn't a bad place to start. Although my judgement may not be the best being this is the first time I've tried to set up a bike from scratch.