The impossible question.

Intermitent ignition sounds the most likely. Check the earth brush in the mag ,check that the champion caps are NON resistor. If the mag is a bit weak it wont like them. That popping and banging is fuel being ignited in the exhaust. Try closing the points gap on the mag to 10 thou.Check you have the leads the right way round.

That sounds very similar to how my lightweight was running after I bought a new concentric (I was sold one that was too large) fitting a new monoblock sorted it for me! Do you have another carb you could try to rule out carb problems? Have you tried altering the needle clip position to see if that has any effect?

Dan

Hi Vinny.....When you first get your bike running, it will only be using the pilot jet circuit. As soon as you open the throttle the second stage of the carb begins to take over........this will be the needle jet section. Which I would say from your description of the engine response is set far too weak.

928 carbs are a little too big for a 99. So the engine is alreay getting too much air to start with. A 626 would have been much closer in size to the original 376 Monobloc used on these engines. The main jet of the Concentric sounds too small as well. I would suggest using at least 220 with a cartridge filter and 240 with an open gauze K&N cone type.

I would suggest also checking the fuel flow to and through the carb.

had a similar problem when unknown to me the block the main jet screws into had come loose and dropped down into the float bowl. i only had tick over and the only way the bike would pull off was if i mauled the engine and pulled off in second gear using full throttle

If the leads were crossed it would not idle,so its not that. I still wonder about a weak spark, close up the plug gaps to about 10 thou,If that helps it will be an indication. Mag condensor?,

Looking at the video clip, it seems that when you open the throttle you are getting a weak cut (to use piston aero engine terminology). Check you are getting a decent fuel flow from the petrol tank. First thing is to up the main jet a few sizes. Next, raise the needle a notch. Either or both should help improve things. If it still spits when opening the throttle, try a smaller cutaway.

If none of the above help, look to the ignition side. What's your plug gap?

Were it mine, I would fit a more appropriate carb, a 1 1/16" monobloc preferably.

That's not a fibreglass tank is it?

I've had similar problems with Commandos. One thing that I didn't think to check until the end of a similar experience was the fuel taps and filters. If the petrol tank has been sitting around with ethanol fuel and general crud in it then it's worth removing the taps and cleaning the filters and bores. They can seem to be delivering fuel but the actual flow rate is not enough to keep the engine running (float bowl filled) above tickover speed. Are the petrol taps ethanol proof? If not, the rubber seals will swell up and restrict fuel flow.

Take the carb off and check the by pass drilling is clear, this is the supply that takes over after the pilot and before the needle jet. Your pipes are showing a weak mixture.

My 88ss has a pair of 928 for historical reasons. They might not be the optimum size but it runs fine. So I dont believe over size to be the issue.

And if you had the same issue with the previous carb it sounds like something else.

Are both float bowls full and clean? If there is fuel blockage in tap or tank it should at least run fine fot a minute or two. So if the carbs are about right then that pretty much leaves ignition.

Good luck!

It's the slide cutaway that takes over from the pilot not the needle jet (although they all overlap to some extent). My ES2 was spitting like mad till I changed the number 4 slide (theoretically correct) for a 3.5. It transformed the carburation.

Does it spit back through the carb (which would indicate weakness in that range) or fire in the exhaust, which may point to intermittent sparks as said.

I find that it's always worth fitting new HT leads and plug caps (non-resistor as said) which are cheap enough from the likes of the Green Spark Plug Co and eliminate at least one potential fault.

You can check flow rate through the tap(s) by directing the pipe into a large jug or similar and seeing how much flows in say 30 seconds. If it's more than half a pint or so it should be fine.

Not quite correct Ian. The bypass drilling takes over before the slide. Its the hole behind the pilot and the engine side of the slide. As the slide is lifted it supplies fuel for the transition.

True, but it's not the needle jet. And we folk with monoblocs don't have bypass drilling (as far as I know...)

Note that the standard carburettor for a 99 is a 1 1/16" Monobloc, but the cylinder head is the same as a Model 88, with 15/16" inlet tracts, so the larger carb won't make much difference.

I had a problem with my Model 88 when I first got it running, it would stumble on opening the throttle from idle.. A richer slide in the Monobloc reduced the issue, but did not entirely eliminate it. The problem turned out to be the idle by-pass compensator hole between the spray tube and head. Clearing it cured the problem.

But the carburettor here is new, so that is unlikely to be the issue Bear it in mind that the needle jet on the old carburettor is likely to be worn, so setting a new one to the same spec may mean a lean needle mixture. If the problem is carburetion, it will be the transition from idle jet to cutaway. If the idle mixture is good, and the compensator hole is not the issue, try a richer slide

I use NGK BP7ES (they are low mileage but late '80s), which is a little harder, but this small difference is not likely to be causing this problem. Have you checked the retarded timing of the ignition? If the ATD is worn, it may be too retarded. If the ATD springs are worn, it will advance too soon and give problems

Be patient and keep checking

Paul

The attachment, with this posting, might be of some help to owners tuning a concentric carb.

Of interest.....the same article also points out that not all Concentric carbs had needle jets with acceleration (bypass) bleed holes.

I just watched the video. The yellow flame from the exhaust accompanying the off-idle snatch, and discoloured pipes at the bend indicate lean mixture. So check the compensator hole, and if that is not the issue, try a richer slide

There's still flame by the time the exhaust gases exit the 'silencers', so I think the exhaust pipes and silencers are too short, but I suppose that's a style decision.

Paul

The monoblock does have a by pass, It draws fuel from the same well as the pilot but at a later point in the action, I have found it to be blocked . I have also had to drill it a bit bigger on ocasion. Thats a bit scary as you can't reverse the process. The Concentric does have a secondary by pass port ,in the groove below the slide. Seems to be back to front compared to a monoblock. If the secondary is blocked it will idle fine, but not pick up.

hello now have watched your video clip the things that are not right with this bike one weak mixture bluing on the exhaust pipes tell me there is weak mixture try putting the choke on a then rev the the engine you need bigger jet if you have a filter fitted as phil say a 180 main is more for a model 88, try a 200 first if no better try a 220 main if to rich drop the needle jet a few notches and try champion N9BCY there better igniting ethanol fuels yours anna j>

When I changed comp ratio it had no noticeable effect on settings. I have a 928 on s 500. I played with slides. 2.5 was all.wrong, 3 was much better and 3.5 further improved. But even with 2.5 it did not misbehave like yours.

My bet is fuel starvation or ignition.

Anything resin based is suspect under the attack of ethanol contaminated fuel. The resins dissolve and are burnt in the engine coating the plug insulator with a hard irremovable shell that carbons up and shorts out the spark ,symptons are difficult starting and violent popping and banging. Tank liners are a problem. Almost impossible to save the plugs,unless you find a powerfull solvent pure ethanol??,or an old sandblasting plug cleaning machine. New carbs have been found blocked with swarf,anything is possible with old bikes

If you have a sound steel tank and ride the bike regularly replacing old fuel with new ,have extra good fuel pipes and don't spill the fuel over that expensive paint job.then you would think we are all doom mongers talking rubbish. Later,rather than sooner you will wish you had listened.

The present ethanol-enriched petrol will quietly corrode carb jets. However, GRP resin dissolves quite readily and can (will) coat carb internals with gunge.

Previously robert_tuck wrote:

Intermitent ignition sounds the most likely. Check the earth brush in the mag ,check that the champion caps are NON resistor. If the mag is a bit weak it wont like them. That popping and banging is fuel being ignited in the exhaust. Try closing the points gap on the mag to 10 thou.Check you have the leads the right way round.

I used to cross over leads when I parked my bike in the street as a means of preventing the engine from being started. Left the bike (Triumph 5T ) outside a hotel in London once. In the morning it as gone. Found it a few hundred yards away parked by the kerb with a small patch of oil under it. Think the T-leaf must have kicked is heart out before giving up thinking the bike wasn't worth stealing!

Probably too tired from kicking the bike to push it away.

My bike has a pull-knob for a permanent kill on the magneto, but if someone disconnects the lead, that stops working. Your idea is a very good one.

But one thing, surely a 5T left more than a small patch of oil?

Paul

Previously peter_maple wrote:

Previously robert_tuck wrote:

Intermitent ignition sounds the most likely. Check the earth brush in the mag ,check that the champion caps are NON resistor. If the mag is a bit weak it wont like them. That popping and banging is fuel being ignited in the exhaust. Try closing the points gap on the mag to 10 thou.Check you have the leads the right way round.

I used to cross over leads when I parked my bike in the street as a means of preventing the engine from being started. Left the bike (Triumph 5T ) outside a hotel in London once. In the morning it as gone. Found it a few hundred yards away parked by the kerb with a small patch of oil under it. Think the T-leaf must have kicked is heart out before giving up thinking the bike wasn't worth stealing!

hello well i have a well hided switch on my bikes and it wired in to handle bar kill button but in this norton model 99 case its not the ignition problem ,but a fuel problem with wrong jetting and maybe the pilot jet too small witch should be a 25 pilot jet and a 220 main jet 106 needle jet for 928 mk1 concentric carburettors or obtain a mono bloc 376 1.1/16th with a 250 main jet 25 pilot 106 needle jet with a D needle and 3.1/2 cut away throttle side and champion N9BYC Or N6Y both have the same heat rating as N5 but with better shielding yours anna j

Previously vinny_tray wrote:

Good evening everyone.

Thankyou again to everyone who has posted advice on this thread.

As promised I am posting an update and can report we have had success today on the Norton.

As the majority of you all advised it was a feeling issue and were correct it was running very weak.

Even though I had checked the fuel flow from the fibre glass tank through the petrol tap I thought it to be sufficient it turns out that I was incorrect.

After removing the tank and rigging up a remote tank I can confirm that we now have a free revving engine.

All be it the jetting is not right but at least it starts and revs at will.

I have changed the plug caps and checked the earth brush on the mag but it was definitely the tank and fuel tap.

So it looks like a new tank and petrol tap is on the horizon.

Can anyone recommend one of the Indian suppliers or is that just a no go area?

Once again thanks to all that took the trouble to advise me.

Ride safe.

Vinny.

Hello be very wary of Indian made Norton Tanks some are a real nightmare and do NOT fit Right, try to find an Original one is my add vice and have it pro cleaned and repainted and Enthrenol treated with Harley Davidson Tank Sealer 100% ethanol proof, yours Anna J Dixon

Previously robert_tuck wrote:

If the leads were crossed it would not idle,so its not that. I still wonder about a weak spark, close up the plug gaps to about 10 thou,If that helps it will be an indication. Mag condenser?

Robert, I realise it's a bit late to answer this post, but if the leads were crossed

the engine wouldn't even run!!

Ian Hay.

Who is this Ghost? We have a Paul Wolf who repairs Magnetos. We learn from the above, this problem is not this or that but is this and that.