Jubilee running badly

Hello Dan,

have you fitted an air filter?, are the plugs showing lean or rich. Also are you sure advance / retard is ok, I would be inclined to try advancing a touch.

My 250 single used to cut dead after a few miles. The petrol cap vent was blocked. My vintage 500 single ran very hot and weak till I drilled out the tank liner that was blocking the flow in 3 places, tank link pipe, tap internals and tank filter. The Atlas has been acting up and I have just found the pilot jet partly blocked with ----yes detaching tank liner. Plugs that look ok will totally die with a deposit of --------tank liner. Whats really annoying is I have never used tank liner or ethanol fuel!, just have to put right the actions of others who just will not listen and think i'm just making a fuss!!.

Don't lose heart, you WILL find the problem ,its only you don't know WHEN!. Coils should not be earthed?, mine are mounted in rubber .

Hi Dan As you have triple checked everything on the fuel side, I would expect it to be a very weak spark. What is happening is that at tickover the effective cylinder pressure is very low but the instant you open the throttle the engine sucks in lots more air which raises the cylinder pressure. Now even a weak spark can jump the low pressure easily but when the pressure rises it requires a higher energy spark which, if you have a problem, will simply snuff the spark out and the engine stops. I suspect you have a duff capacitor (condenser) or possibly (less likely) a coil that is not too good. Check the items in this order by substitution.

Les

Dan,

I've noticed that some electronic ignitions are very versatile.

Are you sure the Boyer is set up right? For a positive earth and a rotor running in the same direction as the crankshaft?

I'm thinking weak spark.

I'm thinking your Boyer is swinging towards 22 degrees of retard rather than 22 degrees of advance.

If this is madness, I'm sure the replies will come soon.

Peter

OH! OK. I misread your first message Dan....didn't notice the fact that it has electronic ignition,...it reads as if you have a "Boyer" strobe..... but regardless of that, I reckon as said it is caused by a weak spark....You could always refit the standard points and coil set up,.... far less to go wrong, far easier to sort out, far easier to time, far more reliable and the engine will run and start EXACTLY the same...or in your case BETTER.........Les

Hi Dan,

From your description of plugs sooting up when you put on the choke and the engine running nicely at idle off the choke it sounds that the carburation is pretty close and unlikely to be the problem.

I agree with that Les has just said. It's has to be a spark or timing problem and as you have electronic ignition it isn't a condenser problem so possibly the coil is weak or the plug leads/caps are the cause. You could try advancing the timing by a few degrees but be careful and don't overdo this.

Patrick

Is your bike 12V, if so is the boyer feeding two 6V coils in series or two 12V coils in parallel. The 6V in series is the best option, the 12V one is marginal.

Boyer analogue does not need any resistance eg MK3 and MK4 (black box), only the Boyer Digital versions (red and blue box)need a resistor plug or plug cap of 5K, the less resistance the better as long as you do not go below 5K for the digital, I use NGK resistor plugs (5K ohm) as the resistor caps can go bad plus solid copper HT wire for the digital boxes on my Commando and B44 (twin plugged).

We had a similar issue. We put a perspex side on the carburetor to see what the fuel was doing (see photo) and that's when we found out that the little hole in to top of the tickler is actually the vent for the float bowl. Fuel was coming in but couldn't displace the air so once it had run off the choke it would die. We cleaned out the tickler and it ran perfectly. Fuel should come up to the little pip below "Amal " on the cover. you can see our before and after photos here.

Well done. That's a major step forward- up and running and road legal. Only the fine tuning to sort out.

Dan,

The symptoms you describe are the ignitioncoil breaking down under load. Dad had a Sunbeam S8 which did the exact same thing. He changed the carb, faffed around with the jetting but it made zero difference. Changing the coil turned it into a rocket machine.

We also had a car with the same issue even after correcting the timing it still wouldn't take quick throttle openings. New coil and the problem vanished.

Got to be worth a try.

Jim

Dan: Have you checked the fuel flow from the petcock? The symptoms sound to me as if the fuel mixture is going massively lean when you whack the throttle open. Could also be caused by an incorrect float setting, i.e. fuel in the float bowl too low.

Keep working on it, you'll get it.

Also, haveyou tried running it with the old monoblock?

The ring of carbon on the plug body is pretty normal however the insulator and earth electrode are showing very weak so you need to try a smaller cutaway ,raising the needle as these indicators are from the range where you have been riding.

Hi Dan,

As you are suspicious of the head gasket have you tried a compression test ? This is always the first thing to establish with car engines before altering any other standard settings. The Jubilee is 8.75/1 so is similar to a standard MGB -so I would expect a minimum of 150PSI and both cylinders within a few pounds of each other. I have never carried out a compression test on a kick start engine so it might not be as simple as it sounds. As a guide a good MGB normally gives more like 190 PSI but you might get a lower reading as you can't spin it over properly on the starter.

Patrick

Hello Dan!

Shold be quicker than removing a Commando head with those pesky bolts from underneath...

Spray some brake cleaner around the head so you can localize the leak.

My tappet adjuster lock bolts will make a huge difference, haha!

Previously john_holmes wrote:

Is your bike 12V, if so is the boyer feeding two 6V coils in series or two 12V coils in parallel. The 6V in series is the best option, the 12V one is marginal.

You can't run 2X12V coils on a Boyer (or RITA) ignition in parallel unless you use two steering diodes! 2X6V is the best option. A twin output coil has one good output and one mediocre output. So again it is 2X6V and the original Wipac is often the best option.

Dan

not sure where I saw this info but I recall reading that mono block carbs run very different settings to later concentric??

might be worth enquiringly with your supplier.

Hi Dan Any chance it could be contamination in the fuel tank?. My fuel tap filter was clogged ,the bike ticked over ok but the partially blocked filter starved the carb cause the fuel couldnt keep up with demand when on a run. When I got round to sealing the tank all was revealed. I know its a long shot,but it`s a possibility.

GRAHAM

Dan:

Have you checked the jet needle for the proper taper?

It sounds to me that the mixture is still going lean when the throttle is opened.

Mike

Hi Dan The manifold on the left (your original) looks like a Navigator one, the one on the right looks standard Jubilee. The difference is pretty small though-the Jubilee carb is officially 25/32" and the Navigator is 7/8th. That is a difference of only 3/32" or a couple of mm. I was told many times that the traditional way to 'tune' the Jubilee was to use the Navigator manifold and 7/8 carb. This should improve the top end at the expense of idling - the opposite of what you were getting.

I think you should clean up all the joints on the old monoblock - particularly the mounting flange-, get it to stop leaking, then do a proper trial run just to confirm that it is now noticeably improved. I have seen quite a few reports from people saying that the new carbs often contain manufacturing swarf so it doesn't say much for their standard of quality control. Maybe just a rebuild on the old monoblock will do the job.

Patrick

A friend of mine a few years ago bought a pair of brand new standard HS4 SUs for his MGB. (SU is the same company as AMAL I believe). These idled perfectly -better then the old carbs- but were no improvement at all in ordinary running despite all our attention. So I am a bit dubious as to the benefits of buying a new carb. It seemed a lot of money for little purpose (unless your engine idled most of the time).

Don't know if this will help but when I rebuilt my 375 Monobloc it peed petrol everywhere, until I discovered that the seating washer under the banjo that is shown in all the drawings should be left out as it prevents the needle from closing off the fuel supply.

Terry

Previously Terry Grimwood wrote:

Don't know if this will help but when I rebuilt my 375 Monobloc it peed petrol everywhere, until I discovered that the seating washer under the banjo that is shown in all the drawings should be left out as it prevents the needle from closing off the fuel supply.

Terry

The overhaul kit from Burlen includes the washer and shows it in the drawing, which is what caught me out - naturally I fitted it. It was only when I looked at photos of my carb taken prior to dismantling that I realised there hadn't been one in the first place.

Of course this might not be the problem with your Monobloc but if it is it could save you discarding a perfectly satisfactory carb.

Terry

Dan,

Those slide fixing rings seem to vary in size by design so if you have any choice in the matter try swopping over for another fixing ring. I was told by an 'expert' at an autojumble that Amal actually made the fixing rings in undersizes to compensate for wear in the thread of the carb body.

I can vouch that when I tried different fixings from autojumbles there were noticeable differences i.e some were very tight on the thread, others could be so loose as to be useless. Maybe easier said than done unless you have a box full of assorted carb bits. You used to get these 375 bits at the autojumbles for a few pence

Patrick

Getting back to your plugs, I believe the insulator colour will indicate the heat range and the outer ring colour the mixture. It you have a porcelain white insulator and a black outer ring then the plug may be too hot.

While it won't help your situation Dan, I have had two branch members replace Commando carbs with new ones (not premiers) and never get them running right to the extent they gave up and went down the mikuni line. The source of the carbs was probably the issue. ie not the genuine article. They looked ok though and I couldn't see anything wrong with them.