Help Please - Low throttle opening misfire

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Mar 11, 2006
Hi Guys. Been on the forum looking at the posts for about 12 months or so but this is my first post so hello everyone :D

Can anyone shed any light on this one as I can't seem to get anywhere on it?

My 1974 850 Mk 2A has developed a hesitation and misfire when riding at low throttle openings.

The bike has Boyer electronics fitted. I've checked the timing is OK (32 deg btdc with a strobe >5000 revs). The Amal Concentric carbs are only 12 months old from new with chrome plated slides so nothing should be worn out. The bike goes like a train from 1/3 throttle up to full power with no problems at all and happily pulls up to 100. I've also fitted new plugs and plugleads and the bike is sparking fine.

In desperation to sort it i've reset the tickover and pilot air screws from scratch using a Colortune on each plug. The bike now ticks over fine and revs up sounding fine in the garage but when I take it out it will not run smoothly on the road on low throttle openings. Nothing seems to sort it!!!

Has anyone any ideas cos its doing my head in :evil: :evil: :( :?
Welcome Mike

Does this problem affect both cylinders?

Which version of Boyer ignition is fitted?
How many miles on the boyer? Do your new carbs have those shinny white floats? On the floats you look for leaking and the level of the float staying at .080 top of float to top of bowl. Boyer's on Norton's can break the wires after some miles.
Hello Mike,
Another couple of questions;
Does the misfire go away when you open the throttle, or put a little choke on?

Thanks for the replies.

I'll answer the questions in the order they came through:

The missfire goes away when you open the throttle and the choke is working fine.
The Boyer is a a Mk1 and was on the bike when I bought it two years ago. The bike did not have much service history and I have no idea how old it is.
The carbs are only 12 months old, the floats have no leaks and I presume the float level is Ok as it has been running fine with them on for the last 12 months. The floats are shiny white ones.
I have checked the continuitity and cleaned all the wiring on the ignition side. Except the the Boyer wiring which I have only cleaned.

I believe Boyer will test your box for £10.00 to see if it has any faults. I would do this but I am at Mallory Park race circuit next weekend at a vintage bike festival so dont have time to get it tested. The old Norton is booked in for a track session and I want it running 100% for then if possible.

Its always the way something will go wrong just when you want it right
mike916sp said:
The Boyer is a a Mk1

Are you sure it's not a Mk3? (Micro Mk3)

First thing I'd do as the Boyer's age is unknown (a Mk3 could be 20+ years old?) would be to renew the wires between the Boyer box and the pickup plate, as these wires can fatigue and break where they enter the engine casing without showing any outward signs of damage. I would also inspect the soldered wire joints on the back of the pickup plate as these are also known to be a failure point, and if in any doubt then solder in some new pieces of wire, and ensure they are cable-tied to the plate itself.
New pick up wires might fix it. I have kitted a fix for this problem it's so common.
The floats and low speed circuits on new Amals are kind of famous for having incomplete passages,float tangs that are bent and slowly sink. Old Amals re-sleeved might fix it also. Water drawn by alcohol in the fuel has been causing strange symptoms in some old bikes.
I think I may have fixed the problem. Ive removed the pilot screws and blown some compressed air through the air passages. Reset the carbs and it seems to be running ok. Only done about 10 miles on a test ride so far but all seems well :) Think some crud must have got in somewhere but all the fuel filters seem clean.

Out of interest LAB the Boyer is marked with NT1A on the box which is presume is Norton Triumph 1A.

Thanks for all your ideas. I'll be back on this forum as its seems to be the best one around I've found so far. Thanks all
I have found that a .016 drill bit glued into the end of a plastic tube from a brake clean can indispensable. Get the pilot jet back to it's designed size than re adjust it. It may surprise you how tight the bit is from the coating inside there. It's a #78 bit.
mike916sp said:
Out of interest LAB the Boyer is marked with NT1A on the box which is presume is Norton Triumph 1A.

That would appear to be quite an old unit then, as the Mk3 version has been around since about 1980, so a 1A could be getting on for 30 years old! I don't think I've ever seen one.

Here's some pilot jet info which you may find useful: ... html#PILOT
The oldest of the Boyer's had #4 screws and eyelet wire connectors on the plate. Now if they had only used coaxial braided wire connectors there would be no epoxy on the plate at all and it never would have broke.
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