MK111 Idle

Hey folks,

Changed my big plastic airbox to the earlier type and installed Denso IW 22 iridium plugs.

Pulled out the Boyer and have gone back to points as I have replaced the sprag and associated parts
twice due to kickback.

Also now have a four brush starter, bigger cables, new solenoid and a Yuasa YTX 20 CH BS battery installed.

Ran okay for about a week, but now when I'm sitting at a traffic light at idle (900rpm), the idle jumps up to
2000rpm with hand off the throttle.

I let the clutch out slowly, bike in first, brake on and the idle dies down to 900rpm.

Pull clutch in again and 30 seconds later off it goes again up to 2000rpm!

I was on an incline front wheel pointing downhill at a light when this happened first time but it happens
all the time now, flat, hill, whatever.

Don't know if the incline thing was the cause or it was going to happen anyway.

All advice appreciated, Cheers.

I think it is in the carb balancing if you have dual Amals. Also, check and adjust the float level befor you start carb adjusting.
By the way, have you seen the MK3 airboxes on ebay recently? $300 - $400. Unbelievable. I've thrown away 2 or 3 of them in the past.

Check the AAU, if its sticking advanced the idle will be too high, hence why the EI is best on non Electric start bikes.

agreed AAU.
Not necessarily only caused by sticking though.
At cold start it will idle fine then creep up as the day lengthens.
The return springs are not fully returning the flyweights, the advance increases the idle, as the idle increases the flyweights move farther out further increasing the idle.
A low voltage tolerant EI is a decent fix for this.
If there was the opportunity to match the springs with the flyweights and monitor the advance curve it would be better than just putting newly available springs in place and hoping all will be well.
Points ignition is easier on the starting mechanism, leg included, when set up properly. Points don't need the revolving wake up signal that some EI does.
Points are easily fixed on the side of the road if need be.
All the Best.

I've got an Altair in now and my idle and off-idle is better than with either the points or an early analogue Boyer.

I'm curious as to why you removed the airbox?

scotnort75 said:

Changed my big plastic airbox to the earlier type and installed Denso IW 22 iridium plugs.


Ran okay for about a week, but now when I'm sitting at a traffic light at idle (900rpm), the idle jumps up to
2000rpm with hand off the throttle.

I let the clutch out slowly, bike in first, brake on and the idle dies down to 900rpm.

Click to expand...

A runaway idle can also be caused by an intake air leak. You must have had to remove the carbs in order to extract the plastic airbox (unless you cut the thing out) so, it's possible that has resulted in an air leak at one of the intake manifold joints?
Also, check the carb balance tube is tight and there are no holes or splits.

After a ride, when the engine is at normal operating temperature, pull your air cleaner, then operate the twist grip (engine off) making sure that the carb sides are consistently returning. Amals are sensitive to mounting fastener torque; it doesn't take much over-torque or differential torque to distort the carb body.

A sticking AAU, as mentioned earlier, is another good real possibility, you can check this by (temporally) putting a dished washer under the AAU fixing bolt which will freeze the AAU (prevent it from articulating).

Additionally, if there is an air leak, as mentioned earlier, or if the carbs are out of sync, either of these can produce a "stepped" idle, although an air leak tends to be consistent. You can check for an air leak by spraying carb cleaner, some use WD-40, all over the intake tracts, if the idle changes or white smoke issues from the silencers, you have an (external) air leak, leaks that originate from under the valve covers are usually accompanied by noticeable oil consumption, which you didn't mention.

Likely AAU. Pull the points plate and lightly grease up the pins and tracks and fit new springs. Those pin tracks can wear out if not lubed every 3000 miles I think the Manual states , so inspect. :shock:

I'm also voting for the AAU being wonky. At 10,000 miles, my idle was getting more erratic, even with a single Mikuni. I since went to a Pazon Altair with the idle stabilization feature built into it. Now, it'll idle all day long with nary a waver. Granted, it's a kick start, but it hasn't shown any propensity to kick back with the Altair, even with a low battery.

Nathan

Dump the points and the junk AAU and reinstall the Boyer. Set the timing to 28 degrees BTDC instead of the 31 that Boyer says to use. You won't have the kick backs when starting. On MK111's I never set it over 28 total degrees and have the original sprag in mine after close to 60K miles.

milfordite said:

Dump the points and the junk AAU and reinstall the Boyer. Set the timing to 28 degrees BTDC instead of the 31 that Boyer says to use. You won't have the kick backs when starting. On MK111's I never set it over 28 total degrees and have the original sprag in mine after close to 60K miles.

Click to expand...

If an analogue (early) Boyer, see Dyno Dave's website or search this forum for erratic firing with low voltage. Of course if you have a great charging system or do nothing but long drives it may never be a problem. The kickback and sprag destruction I've experienced were night rides with the original single phase alternator. Had I changed that first I may still have the Boyer.

Having said that, I definitely notice a better idle with the Altai but more than that, better off-idle.

Thanks everyone, I'll give all of these
suggestions a go.
BTW, I removed the black air box
as I like the earlier look.
I have also installed a single phase power
box from Norvil.
A local Norton owner is suggesting I also
change to NGK BP 5 ES plugs instead
of the DENSO IW22 I am running.
Cheers.

scotnort75 said:

A local Norton owner is suggesting I also
change to NGK BP 5 ES plugs instead
of the DENSO IW22 I am running.

Click to expand...

The standard NGK plug for your Commando is BP7ES.
http://www.sparkplug-crossreference.com ... SO_PN/IW22

Replacement spark plugs for DENSO IW22
.
.
NGK BP7ES

Click to expand...

I wouldn't recommend you fit BP5ES (two heat ranges hotter than BP7ES).
Even BP6ES isn't recommended. spark-plug-question-t25110.html#p331251

If the engine starts and runs ok apart from the high idle then there doesn't seem to be any need to change the plugs.

The one thing it is unlikely to be is the plugs. Stick with the Densos.

I have 220 main jets installed.

Also acquired a Gunson Colortune that I'm
going to try.

They have a few vids on Youtube.

Warm the bike up with a run then try the
Colortune to set the mixture and try again
and see what happens.

Cheers.

As well as the other suggestions of course.
Cheers.

scotnort75 said:

I have 220 main jets installed.

Also acquired a Gunson Colortune that I'm
going to try.

They have a few vids on Youtube.

Warm the bike up with a run then try the
Colortune to set the mixture and try again
and see what happens.

Click to expand...

The Gunson 'Colortune' is really for setting idle mixture if that was your intention as it's no good for checking main jets.