Eight Stroking/Rich Mixture/Black Smoke


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Dear All,
Some of you will probably have read this on jtan but please bare with me.
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
1 Disable the pumps by removing the foot valves. No improvement.
2 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.
3 I then secured restrictor wires in the progression holes of 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.
4 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.
> 5 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'.
6 Adjusting the fuel mixture screws doesn't fix the problem either.
7 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.
> 8 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.
9 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.
10 The tapered fuel metering needles and needle jets had seen a lot of work so working on the possibility that they may be worn, I replaced them and then substituted leaner jets. No improvement.
> 11 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.
> 12 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.
> 13 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.
> 14 I run a measured 8.2:1 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'.
> 15 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.
16 My next avenue of attack is to remove the carburettors totally and substitute some Amals and see what that does.
17 As stated previously 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. 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.
18 Last week I removed the 36mm PHF Dellortos' and replaced then with a pair of 32mm PHF Dellortos' The problem is still there.
Now make sure you read it all and see if I have missed something. For someone that rides this bike everyday this is a most frustrating problem. I would welcome any suggestions that may cure this problem.
> Ken Phelps, in Traralgon Victoria, Australia


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8 Stroking


Hmm, you seem to have done it all. The final act of substituting for a another pair of carbs - with the same result leads me to think beyond carburation - and beyond ignition too from what you've said. Is there any chance that you've had a small slippage of a cam in its pinion thus screwing-up the valve timing? If not, my final act would be to change to Amals and if it still happens, then swap mags (and HT leads/caps) out of desperation.

I am with you entirely on the non-use of NGKs - particlarly with magnetos. I have very few ignition problems with my bikes - but all those I have had in the last 15 years have been when I've relented to peer pressure and purchased NGKs. One wouldn't spark from the day I bought it, another lasted 3 miles, another lasted 200 - it got me to Dover but then gave me heartahe trying to get on the car ferry on one cylinder... Refit any old 2nd-hand plug of another make and it starts and runs like a dream hot or cold. I overheard a couple BSA/Norton guys at an Autojumble last week having the same discussion - both had similar experiences with NGKs.


Peter Barker
Bristol, Uk


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G'day Peter,
George Charleton drew my attention to ignition timing and cam timing. Thats when I recalled mistiming at 44 degrees and it not making any difference. How ever the cam timing is making me think. Although again, why both cylinders are effected?. I do not believe that the cams have slipped in the wheels as it would be unheard of for both to slip the same amount at the same time and besides I pushed the cams together many years ago and they were quite tight. That moves the problem down lower to the half time pinion and mainshaft. Again what are the odds of a slippage in this area only occurring a small amount and then locking up again and staying locked up for so many years.
However I will look into it and see what transpires. Although being a Norvin I won't be able to put a dial guage on the front exhaust or rear inlet valves but I should be able to get a fair idea of the figures.



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Muffler Effects

The muffler hasn't changed. It has always been a straight through absorbtion type megaphone that I repack with fibre glass when necessary.


Eight stroking is common on megaphone motors, as I am sure you know, but it could be worth trying something else to see if it affects running. Air cleaners?


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Exhaust Effects

It is megaphone in external shape only. It is a perforated straight tube through the middle.
I did think of the aircleaners but two things:- if they are restricting the flow at just over idle then there is no way that they would pass enough air at over 120mph. Second I put some ram tubes on the other day to show my son and it made no difference.


Ian Savage

VOC Vice President
VOC Member
Are you able confirm this is happening on both cylinders?
The thing that occurs to me is that during idle not the all the fuel is being drawn into the cylinder and it is ponding in the inlet track. On opening the throttle the increase in velocity draws this in, in one slug.
I’ve no explanation as to way it should have started to do this after not doing it for so long.
Ian S


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Fuel pooling

G'day Ian,
Thanks for replying. I have thought along these lines. I do have long manifolds but they have been there since day one so don't believe that they are causing the fuel to pool. If I could spot this "pool" of petrol I could then investigate its source but I don't see an actual "pool" of petrol anywhere.
Another thought is that unburnt fuel is accumulating in the combustion chamber but again it is a long shot.



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Eight Stroking/Black Smoke/Rich Mixture

I forgot to mention Ian that yes, it is occurring on both cylinders.