MISFIRING AFTER 8 -10 MILES

Strange problem on my mk3 850 with boyer, new coils and leads,new amals with 230 main jets.Starts ok and runs ok after choke has been turned off (after 2-3 miles) but at about 8 - 10 miles it starts to miss on acceleration at low to half throttle.Putting the choke back on made no difference.I whipped out the air filter(plastic black box type) and tried it again, but it was worse.Needles are set at the richest setting.Anyone any ideas?

Some people report that the low speed pilot jet on new Amals can be buggered right from the factory. Search here for "pilot jet" and you'll see numerous posts on how to clear them.

Also check your float levels. One of the new Amals I bought was way off.

Not 100% sure about the MKIII but my MKII uses 160 mains. But you won't notice that until about 3/4 throttle.

I had a similar problem with my Mark III last year. I searched and experimented with everything till I finally found the culprit.

The last thing I checked was the simplest (and cheapest) to fix. I had opened the points cover a couple of times for a visual inspection and tracing of the leads from the Boyer pick up plate up to the black box, but everything looked kosher.

This time I tapped the electrical tape over the connection on the leads under the points cover, and voila..... the wires came unconnected right before my eyes. It was a loose connection between the wire ends all the time. If only I had known that before buying a new battery, a Sparx alternator, etc.

My symptoms were the same - started to miss and spit on low to med revs after about 10 miles of riding.

Check your connections all along the Boyer wire routing. (Then again it may be the carbs).

Good luck

Keith

If you have fitted the peashooter silencers then 260 mains work better, doubt this is your issue though.

The symptoms are also similar to the coil/s being faulty.

I know that you say that you have new coils fitted, but if you are still in possession of the old coils, if they were servicable, you could substitute them? This would eliminate the coils....or not, but it is best to do one thing at a time in my experience.

Check that youre air vent in the petrol tank is clear. When the symptoms start, you could release the filler cap and see if there appears to be a vacuum.

A problem that I had on mine when I was first running it after building was at 3500 to 4000 revs whilst cruising in top gear, or accelerating, it would missfire and hesitate and I was confident that it was an electrical problem such as bad H.T. lead, but the cure was to drop the needles down one notch to weaken the mixture, much to my suprise (and I have dropped them another notch also and it runs fine now). This was running with early type air filter, peashooters 260 main jets and 3.5 slide if my memory serves me correctly.

Your main jet doesn't sound too big, but what size air slide are you using, and do you have black caps or peashooters on?

COMMANDO1954 said:

Starts ok and runs ok after choke has been turned off (after 2-3 miles) but at about 8 - 10 miles it starts to miss on acceleration at low to half throttle.

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I had a similar problem with my Triumph Bonneville recently, shortly after I fitted a new set of spark plugs, - one of the new plugs was a dud.

Balance pipe leak? check this very simple thing.

Yes I am on peashooters and do have some 260 main jets so I will try them.

Turned out the problem was spark plugs.They had fouled up at low revs with the old carbs on that were running too rich.I had just wiped them clean and re-fitted them buy obviously they needed a better clean.Now on 260 main jets and Iridium plugs and it is running perfectly.T hanks for your help guys.

I had the same problem. Make sure your grounds are good at the coils. Run a ground wire back to the (+) terminal on the battery. I also installed a braided strap from the engine to the frame. Problem went away!!