Help needed please

Did you check that the jet sizes and settings of your carb are what's recommended in the manual? That would take a couple of minutes to find out.

Tony. Did you buy it as having a rebuilt engine, complete with new carburettor, and if so, was it from a dealer? If it was, and you have just bought it, get him to sort it out.

By the sound of it, it is not a good idea for you to delve into the carburettor to change settings, as it would be easy to go the wrong way and possibly cause engine damage, as David said.

Can you explain the symptoms in more detail? At what speed, and in what circumstances, does this flat spot appear?

Tony. The needle height may not be the answer. It depends on what you call a flat spot. At what road speed, and in what circumstances does it appear, and does it pull through it, and accelerate again, and if so, at what speed?

Can you elaborate on moving the needle up by a clip?

Is the main needle and what is a clip?

Sorry if this is all basic too you but its hard going for me?

Thanks.

The carb is a new Monobloc carb if that helps.

If you have not seen the inside of a Monobloc, now is a good time to take it apart. Just unscrew the ring at the top of the carb and the slide and needle will come out together. Be careful lining things up when you put it back together. The slide needs a precise orientation (cutaway facing air intake). Needle must enter needle jet or it will jam. The carb top has a locating pip. And don't crossthread the carb top ring. I usually take the carb off and do stuff on the bench. It is a lot easier. But I urge you to look inside your carb. You don't need any gaskets or washers for the parts I have mentioned. If you look at the main jet, you may need to replace a fibre washer.

I'm sure others will comment soon. Anna will tell you to change your spark plugs (since it is well established that 90% of fuel problems are actually ignition problems - and vice versa!)The air screw is the pilot jet adjuster screw and is the horizontal one. Usually it's about 1.5 turns out to start with - but it mostly affects starting and tickover so it's not likely to be that if the problem is running in top gear. Turn it out for more air (weaker) and in for less air (leaner).Then the air slide - for small throttle openings including pulling away steadily. But they are expensive to swap in and out if you don't already have a spares box full of them. But again if the carb was supplied by a specialist then they should have put the right one in.Then the needle. Move it up (clip further down) to allow more fuel and make it richer. Move it down (clip further up) to close the aperture and reduce the fuel (leaner).Finally the main jet when throttle is fully open or nearly so. Probably not that unless you are going flat out. In which case stop the engine (at full throttle) coast to a halt and look at the plugs.The air slide lever partly blocks the throat of the carb when the wire is slack. You only need that when starting up. You have lifted it when it starts, haven't you? Anyway - if running improves a little when you lower it a bit, it then must be getting too much air so it is too lean. And vice versa.Must do something productive now!best wishes...

Hello Now thank you Dave How nice you are, ! Well Our Next Meeting is this Thursday from 7.30 pm on At the Bowman,s Hotel top Noch Place Food is the Tops, Drinks from anything you like even Tea or Coffee , I bring My Big Book on Amal Carburettors this about a 1.1/2 thick with lots of Information inside and you have us three engineers to hand too as well as the rest of our lovely members all good guys every last man jack of them yes the East yorkshire Branch is one of the best around, has it was me that Started it all off, and this weekend Saturday you find us all round at the Life Museum In Hull on Saturday 10am on , a nice walk round the Trawler Artic Corsair one of the last side winder Trawlers , you get too see the Engine room and the Big Murrlees Blackstone Main Engine all 6000hp Engines I used to rebuilding in my time 40 years ago

So anyone like to come they be made very welcome , By the EYB members , so there you are see you around , yours Anna J

Anthony, could be the main jet too big? I don't know much about mono blocks but if it were a concentric I would suggest raising the fuel level, this worked on my Commando with the same issue.

I use these carbs on all my bikes except the 16h, that has pre mono 276.

Oh and Champion plugs in em all !!!!!!!!

Previously anthony_neale wrote:

Previously ian_cordes wrote:

Tony. The needle height may not be the answer. It depends on what you call a flat spot. At what road speed, and in what circumstances does it appear, and does it pull through it, and accelerate again, and if so, at what speed?

Managed to get on the road today for a test run..

1st and 2nd gears no problems. 3rd gear accelerating around 3000rpm(50-60mph) flat spot. carry on gently accelerating 3500 rpm clears and no problems afer on full throttle. Same in 4th..stopped and gave air screw half turn in. didnt improve. returned air screw to 1 1/2 turns out. took plugs out and they are black with soot. the carb has no air filter but the wire mesh type and has a larger main jet than normal I have been told...

Keep this in mind

Jonathan. That is all very well, but that chart is for TT racing carburettors, for which the recommended needle jet, main jet etc will be of no assistance whatsoever in this case, because they refer to race-tuned engines with big valves and high lift cams, whose only throttle setting will be fully open, or shut! And that is for running on 80 octane petrol!! Even the standard pump rubbish we have to put up with now starts at 95 octane, with very different characteristics to 'pool' petrol.

Tony. Sooty plugs usually means too much fuel to air, i.e. a rich mixture. Ideally you need to do a 'plug chop', as previously mentioned, which involves starting with clean, preferably new plugs, and riding the bike flat out in top gear; preferably slightly up hill, or against the wind, to put it under load; then pulling in the clutch whilst simultaneously killing the engine. Coast to a halt, whilst holding the clutch lever in, and remove the (very hot) plugs. If they are black, you have too large a main jet. If they are white'ish, too small. However, it seems to be pulling at the higher end of the rev range, the problem being in the mid-range. This is controlled by slide cut-away, needle jet and needle height.

You need to go to basics, and see if your carb is set up to the book. If not, then do so. Standard settings are your datum point. It is a waste of time altering needle height if the incorrect needle jet for your application is fitted. Note what the settings are at present, what they should be as per the book; and that includes carburettor bore size; and that should give you an indication as to what is going on.

One caveat to this is that, if as you say the engine has just been rebuilt, then it will be tight, will not want to rev, and you shouldn't be thrashing it yet! So, check your standard settings first.

Quite a few people here in Cornish villages have 6 fingers......

Tony. Remove the top of the carb as previously described. You may have to remove, or raise the rear of the tank in order to get clearance. The carburettor model no. will be stamped on themounting flange.Ideally it should read 376/67. If it reads 389/##, then it is the wrong carb, and needs replacing; in which case, ignore what follows! If it reads 376/## (not 67), then you will need to check the bore size. The slide, which will be dangling from the cable, will have a number stamped on top, 3, or 3 1/2 etc. If not 3, replace it. The needle will be dangling from the slide, retained by a spring clip at the top, located in one of the 5 grooves in the needle. Check it is in the centre groove. If not, withdraw the spring clip and re- position it. Before doing so, check the number stamped on the top of the needle, very small! It must read 376/063. If not, change it. Disconnect the fuel line at the carb end. Clean the filter and housing. Note whether there is a washer under the housing. Undo the smaller of the two chrome nuts at the bottom of the carb body. That will reveal the main jet. Carefully unscrew the main jet, blow it through, and check it's size, clearly stamped on it. If incorrect, change it. Next, undo the larger chrome nut at the bottom, and remove the jet holder. The needle jet is on the top. Check the number of that; hopefully 106. If not, change it. Ensure it is clear, and has a cross-drilling. If those numbers are as they should be, they can be re-fitted. If not, replace them with the correct ones. Do not try to re-fit them with the carburettor top on. Check the pilot jet, by removing the small chrome domed nut on the side, ensuring it has an 'O' ring fitted. Carefully unscrew the pilot jet, and check it's number. If not 25, change it. Check that it is clear, blow it through, then before fitting it, blow through the orifice to which it fits, to ensure all drillings are clear.

Remove the float chamber cover. Carefully withdraw the float, noting where the spacer goes. Check the float needle is clean, and not worn. If worn, replace it. Check the float for damage or deterioration. If faulty, replace it.

I would suggest that, before re-fitting any of the components, blow through all drillings in jets, and carb body with an air line, making sure you are happy that all holes are clear before re-assembly. That includes removing the air screw c/w spring, ensure the end is clean and unworn. When re-fitting, screw it right in, then out 1 1/2 turns.

If you have removed the carb, check the bore size at the mounting flange. It should be 1 1/16". Ensure that the hole in the manifold is the same size, and that they mate nicely. Do not overtighten the nuts when re-fitting, it will distort the carburettor flange, causing air leaks, throttle jamming open as the slide sticks in the body etc; not good.

Carefully re-assemble in reverse order. The slide will only fit one way, but ensure the needle is located in the needle jet as you lower the slide. Don't force anything.

Ideally, as you requested in your first post, it would be good if someone nearby could pop round and do it with you, it would be so much clearer.

You should only need to buy a new carb if yours is the wrong body, or worn out. Otherwise, everything is replaceable.

Good luck!

You should only need to buy a new carb if yours is the wrong body, or worn out. Otherwise, everything is replaceable.

Good luck!

Tony, if you do have a new carb there is every chance that the numbers stamped on the flange are incorrect. But that does not mean it is the wrong body. Wrong body only means not 1-1/16" diameter where it connects to the manifold. Don't expect to be able to buy a carb with the right stampings on the flange. If it is 1-1/16" at the flange then it is the right body and you have to assemble it with the right jets, needle, slide, etc. It will then be 100% correct for your model (as it left the factory, at least). If your engine has modifications like a non-standard manifold then that changes everything. Also don't assume you can fix the bike by immediately replacing parts. You have to go through all the steps Ian mentions above. Then if all adjustments are correct and the bike doesn't run right, you may need a new part but it may not be a carburettor. You may have a sluggish or sticky auto-advance or any number of other problems.

Looks as though you have got to the bottom of it Tony. Sucking in air at the manifold, so someone has raised the needle as far as it will go in an attempt to compensate.....

Amal list a #3 slide for that application, but you could see how you get on with the 3 1/2. The larger cutaway will allow more air at 1/4-1/2 throttle (ish), so if it spits back or feels flat, you may have to get a #3 slide.

If the engine has just been rebuilt, it may be that the builder fitted a slightly larger main jet, which is good practice whilst running in, as it will keep the engine running cooler. Once it has loosened up nicely, you can fit the smaller jet, if the larger one is choking the engine slightly at the top end of the rev range.

The problem seems to be occurring at the transition between the mixture being supplied from the needle to main jet. A 270 main jet is larger than originally fitted, but, the handbook suggests going up one, or two, sizes for high speed use, so, it will do no harm. I have a 270 main jet, in my '54 Donny 88, and it still does 64 mpg ridden hard. The needle is probably too high.

The manifold air leak and incorrect needle height appear to be the root of the problem. With the air leak fixed, needle in the correct position,I would err on the side of caution and try the 270 first, and see how you get on. It is not massively too large. Anyway, the factory settings can only be used as a starting point, as the pump fuel now is very different from that which was available when your bike was new.

When you say 'cafe racer tweaks', you have got me worried now! What exactly are they.....?

I have been running my Dominator since 1969 without an air filter (shock, horror!) and haven't noticed abnormal bore wear. No need to panic.

A cafe racer tweak like a megaphone exhaust would justify that larger main jet.

That looks pretty, Tony! The O ring will have messed it all up, for sure.

Remember that, when it goes on a dyno, they thrash it flat out through the gears, and hold it against the throttle stop in top. It is an air-cooled engine, run statically. They will have fans, but are you sure your engine is run in enough to cope with that? I have known of old engines seize, or grenade, on dyno's. Do make sure it has loosened up enough to rev freely in top gear under load. It will tell you what is going on with the fueling, for sure.

So should I hold off on the Dyno test for a whilst or even not bother now I have hopefully found the problem. I can put the carb back together with the 270 jet slightly larger than standard but as it has no air filter and non standard exhausts does this sound a better idea?. Hopefully I will be able to tell if its right or not.

Tony

I watched some small race cars go through this procedure. They did a dyno run then made changes to their fuel-injection then did another run, etc. They had the lap-top in their laps all the while.

Do you plan to do a dyno run, change jets and do another run? You should have a procedure ready because it is time consuming and costly.

Tony. Sorry I did not reply to your post on Wednesday, but I cleared off to the Goodwood Festival of Speed for a few days,with no computers in sight! No reply needed, though, as you are sorted, but no, I wouldn't bother with a dyno. I only ever used one on a 4 cylinder, 4 carb motor which was being tuned for track work, but no need on your Norton. Glad it is ok, and, as is very often the case, something quite simple, and cheap to fix, in the end. Ian