E: Engine Bafflement

[moto8500]

and I am not talking about silencers. I have an issue which has totally baffled me and I hope members could assist with. I am running a Black Shadow on a pair of Amal carbs with a BTH magneto and it takes 10/15 minutes of cranking the engine before it bursts into life - so there is no sign of the engine trying to fire up before all of a sudden starts. Plugs are wet with a strong spark. Conways have rejetted and set up the carbs so the machine achieves a strong tickover from cold. Once started the engine run fine. I trickle the carbs with the choke at half way. Electric start. New plugs. Checked chokes are working. The fact that both cylinders fire at the same time suggests the issue lies with one component. Any ideas gratefully received.

 

[greg brillus]

What kind of Amal carb's are we talking about.......it sounds over rich to me.......once the plugs dry out, then it fires. Generally an engine that starts and idles well from cold is running on the rich side.

 

[Speedtwin]

Yeah sounds flooded and too rich.
Take it you have tried with no choke and no tickle.
Lean it off a little and try and find the sweet spot.

 

[nigsey]

Sounds to me like a fuel issue if the plugs are wet. Apologies if you have already tried these things but it’s important not to overdo the carb tickling (it’s easily done) and using choke exasperates the problem. If it doesn’t start try turning off the fuel taps off and try again after a couple of minutes, sometimes I also hold the throttle half open if I’ve flooded mine (when kickstarting in my case) which can also work if I’ve flooded mine. Check your plug gaps too, should 18-20 tho for a BTH mag I believe.

 

[Chris Launders]

Do you have a mag cut out wire still, this could be a problem, 2nd you should not need the choke and tickling it unless it's around freezing.

I have a BTH and 32mm mk1s and have removed the chokes as it normally starts first kick after tickling.

 

[greg brillus]

On absolutely no account "DO NOT" open the throttle past not even a 1/4 whilst using the electric starter unless you wish to destroy it.......Even with the kick start lever, any major backfire is disastrous for the starter and all the transfer gears and spragg clutch........Most twins with the side bowl Amal carb's you don't need to do anything except hit the starter button and give it a tiny amount of throttle........Once you open the throttle by half or more, the volume of air/fuel in the cylinders goes up big time, thus the starter motor will struggle to crank the engine.......These starter kits are a 100% compromise, not a factory installation and need to be treated with much respect.......As suggested try starting the engine using no extra fuel enrichment and see how you go.

 

[oexing]

Greg, same idea I was having, too much throttle opened while kicking/starting. This is a typical habit with many riders and for fighting this bad habit Amal offers these throttle stops in my link. You can set this thingy for perfect idle speed with the adjustable lever and for starting you turn it in onto its stop and don´t have to care for best twist grip position. After the engine has fired and some time went by for minimal warmup you return the throttle stop back to the preset idle position. On Vincents this device may not have the space or access for fingers - just saying.
But certainly you use either choke or tickling the carb a tiny bit- without overflowing - but never both ways.

Vic
throttle stop

 

[vibrac]

The above reason is why Velos have a swing stop thingy on their pilot jet
Incidentally my TR5 52 Triumph Trophy was always a bad starter until I realised that opening the throttle wide was the answer when kicking it over...
funny things these old bikes

 

[MichaelOrgzey]

My Twin starts with full choke and no tickle first kick every time. No throttle. As soon as I tickle she's flooded and won't start till I have kicked over a dozen times with throttle wide open. Shadow carbs. Tickling seems to overdo it very easily.

 

[teunvandriel]

My Rapide starts 9 times out of 10 cold (summer or winter) first kick with the throttle wide open. During a drive after a (coffee) break slightly harder, 4 or 5 kicks. I always overrun both original 289 carbs before starting. Of course putting the pistons in the right position first. A few years back I had the carbs ultrasonically cleaned and put in new 3 1/2 throttle slides. The original magneto was rewound 25 years ago using Kapton insulation, which was also used at my work to make the engines for the TGV / Eurostar (High Speed Train, after the magneto overhaul I have now driven 30000 miles with this bike. As a spark plug, I use an NGK B7EVX at the back and an NGK B6ES at the front (no EVX type available in 6). In Holland we use 98 octane. Mostly I ride duo with wife. Top speed 100 miles, I think even faster, but yes, thing is 75 years old, and so am I. The speedlimit in Holland is 130 km/h and the fines are very expensive. By the way, most (99%) of 75-year-olds in Holland ride an Ebike. (I have no idea if those things exist in America or Australia as well). I myself still use an ordinary sports bike WITHOUT E(lectrish) in addition to the motorbikes, and certainly ride 3000 miles a year on that too, plus a couple of Laverda's (2 and 3 cylinders) that are also often used.
I sometimes have the idea that in the Vincent world, many people think they can solve a non-existent problem with extremely expensive modern parts like carbs and ignitions