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Jubilee running badly

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Jubilee running badly

Posted by Dan Field at July 12. 2017

Ok I'm coming to the end of my options! ... and tether! I've got the jubilee running, it ticks over nicely and accelerates well if I open the throttle slowly, however it completely dies if I open the throttle quickly and this is at any revs. This is made better if I put the choke on which I thought meant it was running lean. The bike has a concentric fitted with jets the same as the monoblock. I've tried raising the needle and increasing the size of the main jet to 140 to no avail, the problem continues, I've also fitted navi monoblock with jubilee jets but that did exactly the same thing leading me to think it must be something else.... maybe the old rule of 90% of carburation problems are electrical is holding true!

ive checked the timing with a strobe (Boyer) changed plugs and made sure the coil earths properly. I've checked for air leaks as well.

Its not helped by the fact that I don't have much space to test ride .... does anyone have any bright ideas or is this cutting out when you wack open the throttle normal?!

Dan

Re: Jubilee running badly

Posted by andrew_wibmer at July 12. 2017

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.

Re: Jubilee running badly

Posted by George Phillips at July 12. 2017
I seem to recall, dan, that Triumphs did that in the late fifties. It was a way of stopping the engine. Could it be that the engine is flooded owing to too swift a taper on the needle? Or possibly a worn needle? George

Re: Jubilee running badly

Posted by George Phillips at July 13. 2017
Have you tried lowering the needle, Dan? It might reduce the sudden flooding you get when the throttle is flicked open.

Re: Jubilee running badly

Posted by Dan Field at July 13. 2017

Yep needle has been up and down all the way, the carb is new. It has new Boyer ignition and new plugs, leads and resistor caps. Engine dies when throttle is opened wide which is on the main jet and it really dies, no misfire or anything, it's like you've hit the kill Switch! If I run it with the choke on the plugs are really sooty, if I don't then they have little colour (not really running it for long enough to colour up).

Re: Jubilee running badly

Posted by robert_tuck at July 13. 2017

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!!.

Re: Jubilee running badly

Posted by Dan Field at July 13. 2017

Tank vent is clear and no tank liner present, I've blown the carb through too many times to count! Plus it ticks over beautifully and the air screw works. Riding it around the field it accelerates nicely if you open the throttle slowly -  it's a quick opening that results in a dead engine, it does the same on the stand. Engine isn't running hot and exhausts not glowing or bluing! I would have expected it to bog slightly when you open the throttle but not die so that it stops dead. However it will pick up if I put the choke on, which suggests a lean mixture but everything I have done to riches it up has made no difference. I thought it might be the concentric or the slide cutaway, but fitting the monoblock from the navigator with jubilee jetting (130 main) made no difference.

It is running with a coil with 2 outputs. Ive got a spare set of coils I could try but it runs so well on slow throttle openings I doubt this is the prob.

Ive got a 150 main somewhere, I might try that and recheck the timing but why on earth does it want so much fuel? I should add that it's just had a rebore and new pistons and the valves were holding compression well.

Re: Jubilee running badly

Posted by robert_tuck at July 13. 2017

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 .

Re: Jubilee running badly

Posted by Dan Field at July 13. 2017

Thanks Robert!

Re: Jubilee running badly

Posted by les_howard at July 13. 2017

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

Re: Jubilee running badly

Posted by peter_holland1 at July 13. 2017

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

Re: Jubilee running badly

Posted by les_howard at July 13. 2017

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.....Wink....Les

Re: Jubilee running badly

Posted by patrick_mullen at July 13. 2017

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

 

Re: Jubilee running badly

Posted by john_holmes at July 13. 2017

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.

Re: Jubilee running badly

Posted by Dan Field at July 13. 2017

It has a single 12v coil with two outputs (prob chinese!)but I have just noticed that it has 2 champion plug caps with 10k ohm resistance. I've re read the Boyer instructions, it says use 5k ohm resistor caps. Could that be the issue?  I do have two single 12v coils ready to go on the navigator that I could try.

I too don't think it's carburation, the fact that it did the same when I fitted a monoblock would seem to indicate that it's electrical. I'm going to get a new strobe and check the timing, but am pretty sure it's ok. Will swap over coil leads and caps and see what that does, but it's off for its mot tomorrow, so may be at the weekend. Might give Al a ring too.

Re: Jubilee running badly

Posted by john_holmes at July 13. 2017

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).

Re: Jubilee running badly

Posted by Ian Easton at July 13. 2017

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.

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Re: Jubilee running badly

Posted by Dan Field at July 13. 2017

Ian that's interesting, I'm running a new concentric rather than a monoblock, but it's worth checking.  I've changed the plug caps this evening and that's made no difference. I've also changed the main jet from 130 to 140 then to 120 and that made no difference either so I'm certain it's not a jetting issue. I've booked it in for an MOT tomorrow so if it passes I'll be able to give it a better run.

Re: Jubilee running badly

Posted by Dan Field at July 14. 2017

Well it's now mot'd and taxed as historic, :) so I can top everything up with oil and take it for a proper test run!

Re: Jubilee running badly

Posted by patrick_mullen at July 14. 2017

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

Re: Jubilee running badly

Posted by George Phillips at July 14. 2017
Do let is know what the problem/solution was when you find it, Dan. As I noted earlier I came across this years ago but just assumed it was a quirk of the machine/carb.

Re: Jubilee running badly

Posted by Dan Field at July 14. 2017

Previously patrick_mullen wrote:

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

 

 

And the oil leaks!

Re: Jubilee running badly

Posted by jim_royce at July 15. 2017

Dan,

 

The symptoms you describe are the ignition coil 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

Re: Jubilee running badly

Posted by michael_sullivan at July 15. 2017

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?

Re: Jubilee running badly

Posted by Dan Field at July 16. 2017

Yep tried the old monoblock (well one off a navi with jubilee jets) and the result was the same. Job today is to take it for a good run and see how it performs on the road, then float height and timing check.

School reunion yesterday 40 years since we left .... am a bit jaded today!

Re: Jubilee running badly

Posted by Dan Field at July 16. 2017

Took it out for a run today, was actually much better than I remember but even after a good warm up it wouldn't rev out and the engine still died completely if you opened the throttle quickly. Putting the choke on just alleviated the symptoms it didn't cure it. I was running a 120 main in the concentric so upped it to a 130, but no difference. Except I'd found many oil leaks. I flattened the rocker covers and made new gaskets which has stopped most of them but it's dripping oil from the back of the chain cases expect that seal will need doing, I don't think it's gearbox oil that is dripping. Anyway next job will be to change the coil. The plug looks interesting hot in the middle and rich on the outside!

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Re: Jubilee running badly

Posted by Dan Field at Sunday 17:08

Ok, I spent a few hours today in the garage, I changed the coil (no difference) checked the timing, this was retarded a bit max advance was 25 degrees. Took it for a test run, it was better but still struggled to get to 50 mph, checked the valve clearances, these were a few tho out and upped the main to 140 .... plug was still showing lean, by now it was running better so I took it for a longer run, sadly the hesitation is still there and the bike still spits, but I'm not sure that this isn't a blown headgasket rather than spitting from the carb. To be honest other than valve timing there's not much else to try!

Dan

Re: Jubilee running badly

Posted by robert_tuck at Sunday 18:49

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.

Re: Jubilee running badly

Posted by Dan Field at Sunday 20:05

That pic was from the previous test run, the last one looked a bit better!  Needle is as high as it can go, but it should run reasonably well with stock settings,  it's got a concentric fitted at the moment but am going to try the stock monoblock but not until I've checked the heads. The cutaway only affects a small part of the throttle action, I think something else is amiss!

Re: Jubilee running badly

Posted by patrick_mullen at Monday 07:45

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

 

 

 

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