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Eight Stroking, Black Smoke, Rich Mixture


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VOC Member
Dear All,
At the recent Australian National Rally I had some discussions with some members who have been quietly watching this topic. Yesterday a long time friend and Vincent Rider/Mechanic dropped in and the subject came up for discussion again. As result of this I decided to carry out a test. Returning from a ride this morning I let the Norvin continue idling and parked it in the drive whilst I went inside. I let it continue idling for a timed ten minutes, then went out and turned it off and immediately slipped the carbys off the spigots. Inside on the bottom of each manifold (I have long manifolds) was about 1/2 to 1 teaspoon of petrol.
For those who have forotten or those who haven't read what I have done, heres a brief run down on what has happened:-

I have what I believe is a carburation problem on my bike. The mechanical device concerned is a Vincent HRD 1000 with a pair of 36mm Dellorto PHF pumper carburettors attached.
> The bike idles fine but after idling for a short time, eg. at a set of lights, upon opening up the throttle to move off the engine eight strokes badly (sometimes 16 strokes and blows black smoke. The longer the idle time the worse the problem gets. A long set of lights or a double wait and upon opening the throttle the engine will cut out all together . The way I get around it its to give it a rev a couple of times whilst waiting or keep the engine at a very fast idle. The engine will idle perfectly for quite a long time, the problem only occurring on opening the throttle. If I smartly snap open the throttle on moving off I can get around it as well.
> I have been riding this bike daily for 29 years and this problem started approximately five years ago. Previously it was running very well. I'm no novice at mechanical engineering having built this bike as well as another successful racer. I repair and tune all my bikes and cars.
> Now eight stroking and black smoke are a sure sign of a rich mixture. Why this should occur out of the blue with out any changes to the carburettor settings has me mystified.
> To try to cure the problem the first thing I did was to disable the pumps by removing the foot valves. No improvement. Various Italian Motorcycle mechanics I have questioned have suggested that the chokes are not closing off. I have checked this closing by making sure that there is slack in the cable, putting a smear of grease on the plunger pads to check they seat on the choke jets and even machining up new jets without holes in them!!. No improvement. I then secured restrictor wires in the idle circuit fuel entry point in the throat of the carburettor. This only served to cause spitting back whilst the bike was idling. A sign of weak mixture. I then tried an opposite approach and made up a variable restrictor to the idle circuit air inlet in the mouth of the carburettor. This served to only make the idle very rich.
> I have rejetted the idle jets from the 65s' that I have alway used, step by step down to 45s. This causes the engine to spit back whilst idling, getting progressively worse the further I go down, upsetting the mixture and stalling the engine. Replaced the 65s'. Adjusting the fuel mixture screws doesn't fix the problem either. I then changed throttle slides, going from the 60 cutaways I had been using, up to 80 cutaways. Again this served to weaken the mixture on opening and causing spitting back and upsetting carburation.
> I have set the float heights very very accurately mid may at 18mm as per the instruction sheets. I have lowered the float heights to 20mm with no improvement. Even though the fuel can remain turned on for over a week whilst I am on holidays, proving that the float needles and seats are sealing, I replaced them. No improvement. The tapered fuel metering needles and needle jets had seen a lot of work so I replaced them and substituted leaner jets. No improvement.
> Discussions with Moto One in Melbourne suggested that there was a lighter float made available at the time of change from super to unleaded fuel. This lighter float I deduced would sit higher on the fuel in the bowl thereby lowering the fuel level. As mentioned above, the lowering of the fuel level had no effect. In hind sight I do believe that this problem started at about the time super petrol became unavailable.
> Working on the theory that perhaps the fuel level was rising whilst the bike was idling, I machined up fuel bowl securing cap nuts with a spigot that I attached a clear plastic stand pipe to. With this I was able to monitor the fuel level whilst the engine was idling. The fuel level didn't change.
> Another train of thought was that as the crappy unleaded fuel was forced upon us, I had to progressively retard the full advance ignition point from 38 degrees BTDC down though 36 to 34 degrees BTDC, not taking into account that the retarded or starting and idling ignition point was also being retarded. As it turned out idling ignition point was occurring at about 4 degrees ATDC. Thinking that at last I had discovered the souce of my problem, I reworked the mechanical advance/retard mechanism to restrict its movement, returning the idling ignition point to 4 degrees BTDC. No improvement.
> I run 8:1 compression ratio, premium unleaded and Champion N4c spark plugs. I have tried up to N6c plugs with no change. Don't tell me to use NGKs' as I have used them and had them "switch off", causing me much grief working out why I can't start the bike. I have used NGKs' in my Ferrari as well and had cylinders drop off line. Not impressed with NGKs'.
> On the bike I run a magneto ignition that starts very well and idles reliably. Having dealt with magnetos for many years, a sign of a weak mag is hard starting so I don't think that the problem is there.
> I'm at my wits end with this one blokes and I am running out of knowledgeable people that I can consult with. I know that you guys have been in the Ducati/Dellorto game for many years so I am really hoping you can shed some light on this most perplexing of problems. My next avenue of attack is to remove the carburettors totally and substitute some Amals and see what that does.
I have been steadily bringing the advance back from 38 degrees down to 34 degrees to cut out the pinging from the bad petrol we have here. The time I thought I had found the cause I realised that the ignition point at idle and starting was also being bought back so I reworked the ATD to only come back to about 4 degrees BTDC. Conversely a few months ago I miss timed it at 44 degrees BTDC full advance but that didn't solve my rich mixture problem, only made it kick back all the time. Yes the magneto is still giving sterling service, starting first kick and idling very reliably. I have run the magneto on a test rig and it produces a fat blue spark in a pressure chamber at revolutions below idle.

Now before anybody says the long manifolds are the problem, The long manifolds have been on the bike since day one, 30 years and 178,000 miles. For the first 25 years it was fine, only in the last five years or so has this problem cropped up.
The question now is, what is causing the petrol to pool or puddle in the manifold and how do I solve the problem?.
> Ken Phelps, in Traralgon Victoria, Australia


Well Known and Active Forum Website User
VOC Member
Carby Change

G'day Greg,
Yes, I was just reiterating that I had changed the carbys. I ran the 36mm Dellortos' up until some months back and as a finally act of desperation I changed them to a pair of 34s' that I had sitting on the shelf. Made no difference and just today seeing as how I had to take them off when I checked and found the pool of fuel in the manifolds, I have now put the 36s' back on.


Comet Rider

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VOC Member
Is it Fuel or Carb's

Hi Ken,

Just to drop my two pennies worth in.

I have been having similar problems here in the UK on my 580cc Comet which has coil ignition and a 34mm Mikuni. The only thing I can positivly say is that the recent (6 months ago) introduction of Bio Ethanol in the petrol mix seems to be the main culprit. I have now stopped using so called cheap supermarket fuel and gone over to only using fuel from known main suppliers such as Shell BP etc. My bike is now running much better, but still requires some fine tuning at the bottom end, which is my next project.

Best of luck

Tnecniv Edipar

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VOC Member
You seem to have covered all bases in the process described above , except the one inferred by comet rider in post 4. Fuel characteristics could be the issue. Obviously the intake flow is lowest at idle , so maybe fuel droplets are dropping out of atomisation due to the fuel formulation and puddling on the floor of the manifold ??
Have you tried different fuels ?


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VOC Member
Less Volatile Fuel

Yes, having found the puddle of fuel that is what I believe is happening. I will try some avgas in it this week and see what effect it has. Also I may try some other additives. If this does correct the problem then I'm at a loss to what to do to solve the problem long term. As I ride the bike daily it will be nigh impossible to run avgas or to keep putting additives in.


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VOC Member
Intake Manifolds

I would if I could but it would take extensive modifications to the bottom of the fuel tank and a complete refabrication of the oil tank just to be able to site the carbys then remaking the manifolds.

Tnecniv Edipar

Well Known and Active Forum Website User
VOC Member
Well , I suppose the 1st thing is to determine if a change of fuel makes any difference. Simplest things 1st and all that !!
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