Misfire

It sounds like it could be an electrical fault to me, I had exactly these symptoms  once and it proved to be the dreaded kill switch. 

You are running rich. What throttle opening/speed 25,35, 45, or ?? As obviously you will  be on a different part of the carb setting. If its ok when cold-it likes a rich mixture when cold. When you accelerate it wants to be rich. You don't say what carbs?

 I have also blanked off the balance pipe between the inlet manifolds  don't know why it has one to be honest .

It helps more if the carbs are not well balanced but little effect if they are well balanced.

A misfire only at a certain throttle opening and not at particular revs points to a carb issue, the opposite points to ignition issues. One that happens at small throttle openings and goes away at large openings suggests the slides are not lifting in unison possibly coupled with pilots not fully tuned. So do a full carb tune including a carb balance (see page 5 for carb syncing).

Bushmans%20Carb%20Tuning%20Secrets.pdf

Won't be slides out as you have already identified richness. At this position on the carbs it is the cutaways you need one size leaner. But suggest you replace the needle jets and try dropping the needles first-cheaper.

The air screws in to slow it is the problem. Adjust air screws for fastest then slow with slide screws. Then see if miss-fire goes.

Check the battery.

Sounds like an issue I had (And have again) with my Mk 3. 

A new battery solved the miss fire.

I'm going to try a yellow Motobat next Spring. 

The starting point is usually 1.5 - 2 turns out. This is probably where your richness is coming from. Follow Al Oz's advice.

With 260 mains, the needle should be on the weakest setting. if the carbs are good, then it must be electrical. As a MK3 owner myself the plugs under the tank need checking and cleaning, also make sure nothing in the headlamp is vibrating as the revs rise to cause the problem. Most of my misfires in 27years have been electrical, the carbs just get to point where they will accept adjustment in which case they are worn out. 

Reset the tick-over first. What is the ignition system?

So how many volts are in the battery, John?   

Good question, Alan, if low V in Bat and especially Mk 3 Boyer it will misfire. Been there.

When I first got my Fastback it showed similar symptoms; irregular running on light throttle, but perfectly happy if one opened the taps. Investigation showed that the needles were shagged — replaced these and the needle jets and the problems went away.

Needle Jets wear out 3 to 1 compared to the needle. But we haven't heard from John yet as to what happens when he sets the tick-over correctly, and what ignition system he has?

I had a terrible misfire develop on my Commando MK3 on a recent ride-out and thought it was most likely a fuelling problem. However, I decided to check the electrics first for grounding issues as I came across this on another bike some years ago. Turned out to be a faulty fuse holder which was failing intermittently under vibration. Replaced and problem solved. 

I had a terrible misfire develop on my Commando MK3 on a recent ride-out and thought it was most likely a fuelling problem. However, I decided to check the electrics first for grounding issues as I came across this on another bike some years ago. Turned out to be a faulty fuse holder which was failing intermittently under vibration. Replaced and problem solved. 

Hi John

The Pilot Air Circuit on Amal 930 carb's is very fine; thickness of a human hair, goes in

3 directions with abrupt corners. Ideal for muck to lodge!

Recently on my bike had same problem as you. After trying many things including

cleaning carb's and checking all base settings, in desperation [on cold engine!] I removed the pilot air screw, sprayed carb' cleaner in and re-set the screw to 1 1/2 turns out, base setting.

Started it and immediately it tried to tickover! On the road, misfire at low revs and the overrun has gone. Worth a try.

This original misfire is self inflicted, The air screws are fully in if they are opened the tick over goes up and when you slow it down with the slide screws  the revs take a long time to return to tick over. 

As advised if air-screws/tick-over is correctly adjusted then we feel that this will sort itself.

How long does it take to check the battery voltage?

Yes you can start  the motor using Pazon ign on low volts BUT if the battery has seen better days, it might not take the charge, Hence, misfire (Been there)

Wonder when the capacitor was last changed, but battery V first. 

...cause all sorts of problems.

Have you replaced the wiring from ignition unit to pick-up ?

The symptoms sound exactly like those caused by a break in the wiring an inch or two inside the insulation where it exits the rear of the timing cover.

As the motor shakes, at maximum iso movement, it makes and breaks and then clears as the engine smooths. I use twin electrical flex. It seems more able to cope with the movement.

John, does the Pazon come with new wiring to run down the frame from the ignition unit to the pick-up ? I'd suspect the wiring or the connectors with any RPM-related misfire on a Commando as everything shakes and moves about so much.

Thanks John,

This eliminates the voltage issue. (A new duff battery is not unknown) 

Battery good, charging good.  To me I'd next look at air leaks but I'm sure you will get to this.

Good luck.

Misfire at low throttle openings. The last part of setting the carbs is most probably the most important part, but if you follow the guidance you will have problems as it effectively leads to leaning each side out way too much. When each cylinder is set in turn and then with both caps refitted you then lower the slides to achieve a tickover. You have now set the carbs to maintain tickover and not much else, so when you open the throttle from the shut position the pilot circuit will struggle to keep up as the slide rises and the cutaway comes into play. 

Well done John and all.  Almost there.

Perhaps the tick over will settle quickly once everything is bedded in?

Looks like you have checked and corrected everything. I know that some of these new slides can be a very tight fit. 

IF the ignition system was points with mechanical advance retard then a worn/sticking ATD would give this effect. Can you run the bike without air filters and see if the slides are actually dropping  onto their stops? Other wise report back.

I agree with Alan, what I had in mind in not so many words.

The other thing might be the throttle cables not running smooth.

We look fwd to your next report.

Hi John.

I had a similar problem with my 650SS a few days ago where the tickover was too high though it had been OK before. When I released the throttle quickly I couldn't hear the slide contacting the throttle stop as I should. I checked out the carb and it was all OK and then realised that when I'd fitted the new handlebars just before that last ride I'd been so careful to ensure I had the twistgrip located so it didn't interfere with the front brake lever that I hadn't noticed that the angle of the throttle cable coming out of the twistgrip was rotated too far round and that was what was preventing the slide completely dropping down. A quick adjustment of the twistgrip positioning solved my problem.

HTH.

Hi John

This reluctance for the revs to drop when hot was due to worn carb bodies on one of my Commandos. But maybe yours are not particularly old/worn? 

Whilst setting up you mention adjusting air screws which increases engine speed then dropping the slide to bring the speed back.  But if you go too far you are running on bypassed air.  Have you tried closing the air screws right down to see if she still runs?  Do it on alternate carbs.

Only other point to look at is the balance tube on the manifolds, split, cracked or stretched.

cheers

Jon

It could be your pilot bushes are still partially blocked. They are notoriously hard to clean on concentrics. To quote an excellent booklet "Super-tuning Amal Mk 1s"  "If when bike is revved, tick-over is slow to settle to set-point. Pilot jet is too small. If, however it settles quickly to BELOW set point then rises to it-- pilot jet is too big.

hello,

would the chokes play a part in whats happening.

just to add. i had a faulty battery once it was an internal connection

fault. where the terminal fits to the plates inside.

it would charge, show good power then it just failed when it felt like it.

move the terminal, all ok until the next stop.

causing misfire ect. bike was running on a pazon ign. not commando.

barry

Jonathan is making a valid point.  If you have the slides set too high then you can be running on the bypass system and any adjustments you make to the pilot mixture screws has minimal effect.

In my experience a hanging idle is usually caused by a partially blocked pilot circuit, either the jets or the carb body, ultrasonic cleaning doesn’t always get the blockages out. That assumes the cables and slides are operating correctly.

dan