Off Idle Sputter On My Velocette

Tornado

Tornado

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Slowly working my way through various issues with my newly acquired 54 MSS Velocette, 500 cc all alloy engine.

Bike has a 928 Amal Concentric Premier with appropriate jetting as per workshop manuals i have reviewed. Needle at middle position. Also has a newer BTH mag with electronic ignition setup, auto advance etc.
Bike starts very easily, pretty much every first kick fires ups for past 20 or 30 tries.

Problem is at slight throttle opening just off idle, engine under load while driving along, so 1/8-1/4 open zone, engine sputters quite harshly. Makes low speeds around neighbourhood side roads very unpleasant. Higher opening throttle problem goes away entirely. Also no sign of sputterring if on CS in Nuetral, slowing opening throttle.

Assumed a lean condition off idle. Closing down air screw, even fully closed, made no improvments. Tried swapping the #3-1/2 t-slide for a #3, as per Amal booklet guidance to similar issue, but again no improvement.

Now thinking perhaps an air leak at carb to head gasket. Also, could spark timing be a possibility here?

Any other things i should consider? Needle position?
 
Tornado said:
Slowly working my way through various issues with my newly acquired 54 MSS Velocette, 500 cc all alloy engine.

Bike has a 928 Amal Concentric Premier with appropriate jetting as per workshop manuals i have reviewed. Needle at middle position. Also has a newer BTH mag with electronic ignition setup, auto advance etc.
Bike starts very easily, pretty much every first kick fires ups for past 20 or 30 tries.

Problem is at slight throttle opening just off idle, engine under load while driving along, so 1/8-1/4 open zone, engine sputters quite harshly. Makes low speeds around neighbourhood side roads very unpleasant. Higher opening throttle problem goes away entirely. Also no sign of sputterring if on CS in Nuetral, slowing opening throttle.

Assumed a lean condition off idle. Closing down air screw, even fully closed, made no improvments. Tried swapping the #3-1/2 t-slide for a #3, as per Amal booklet guidance to similar issue, but again no improvement.

Now thinking perhaps an air leak at carb to head gasket. Also, could spark timing be a possibility here?

Any other things i should consider? Needle position?
Click to expand...
Pilot jet size too small.

Which do you have fitted ?
 
Air leak into inlet can be tested for by having the bike idling and spraying the joints that might be leaking with WD40 or similar. The engine note will change if it finds a leak.
 
Triton Thrasher said:
Air leak into inlet can be tested for by having the bike idling and spraying the joints that might be leaking with WD40 or similar. The engine note will change if it finds a leak.
Click to expand...
Yes will be trying that method soon. Feeling it unlikely a leak as other throttle positions, idling and starting all ok, a leak should affect many throttle areas id think.
 
Triton Thrasher said:
An inlet air leak has most effect at closed throttle and small throttle openings, when vacuum is high.
Click to expand...
OK, hitting the carb/head flange gaskets with WD40 had no impact on idle rpm. Also hit the carb air mixture & tickover screws in case the rubber o-rings were passing air....again, no difference in idle rpm.
Had the bike out for a longer ride at higher speeds/rpms, covered about 45 miles. The sputter is most apparent at smaller throttle openings, but I did have occasional misfires at higher rpms as well. So thinking its just an off-idle thing may be misleading.

Next I'm thinking to check valve rocker clearances and ignition timing (once I figure out how on this bike).
 
Hmmm, something interesting on an old thread in a Vincent forum...a BTH electronic magneto having issues, bkke sputtering under load. Reported to be from having the "kill" wire coming out the mag bundled with or too close to the HT wires, getting interference and affecting the spark triggering.

On my bike this black kill wire runs out of mag together with two output wires in a single plastic conduit to the external coil/capacitor thingie that then outputs to the HT lead.

Reminds me of the Trispark ign issue from noisey podtronic reg/rec output. My velo also has a modern looking reg/rec (podtronic?).
 
Have you thought about stripping the carb? A known issue with Premiers has been machining swarf left in the pilot circuit.
 
FWIW…

I’ve got a mate who’s had a Venom for about 100 years, give or take.

He’s run it on MK1 (not Premier), MK2, and Monobloc carbs.

He says it starts better and runs better all round on the Monobloc.
 
Tornado said:
Reported to be from having the "kill" wire coming out the mag bundled with or too close to the HT wires,
Click to expand...
Some fixes are cheap to try.

Also- new spark plug and using a suppressor plug cap or plug.

Even trying another ignition coil doesn’t have to cost much. I think the BT-H “magneto” is a CDI type, so needs a CDI coil, of low impedance.
 
Triton Thrasher said:
Some fixes are cheap to try.

Also- new spark plug and using a suppressor plug cap or plug.

Even trying another ignition coil doesn’t have to cost much. I think the BT-H “magneto” is a CDI type, so needs a CDI coil, of low impedance.
Click to expand...
Bike came with a 10kOhm cap on the recommended N4C plug. Bike did fire up with that but it was early days dealing with very grabby clutch so was unrideable and if sputter exisited was not noted then. It seems to become obvious only when under load, not idling to reving slowly up in neutral.

BTH coil pack runs around 20 or 30 GBP i think. So that is an option. Trying to hold off on the parts/money cannon for a bit.
 
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Triton Thrasher said:
Some fixes are cheap to try.

Also- new spark plug and using a suppressor plug cap or plug.

Even trying another ignition coil doesn’t have to cost much. I think the BT-H “magneto” is a CDI type, so needs a CDI coil, of low impedance.
Click to expand...
Oh, i have since fit non resistor cap to it after reading on BTH site and other places no more than 5k or no resistor at all is best for mag setups.
Was somewhat problematic to start for a spell, might habe been my technique, but now a New N4C plug fitted and shes starting easier than my commando, basically first kick and it goes fine.
 
Tornado said:
Oh, i have since fit non resistor cap to it after reading on BTH site and other places no more than 5k or no resistor at all is best for mag setups.
Was somewhat problematic to start for a spell, might habe been my technique, but now a New N4C plug fitted and shes starting easier than my commando, basically first kick and it goes fine.
Click to expand...
Yes but your BTH ‘mag’ isn’t actually a magneto, it’s a self generating electric ignition…
 
Thinking more on possible electrical interference disrupting mag trigger events, the 12v dynamo output wires run along top frame rail to a modern reg/rec on rear mudguard. The low tension output and kill wire from mag are running along same frame rail all wrapped up to the other power lines, before reachingcthe coil/cdi unit mounted top frame rail forward of engine.
Might try a dyno disconnected, not being driven off belt and batt disconnected for a test. And will unwrap the mag wires, seperate from others to test for cross talk.
 
Fast Eddie said:
Yes but your BTH ‘mag’ isn’t actually a magneto, it’s a self generating electric ignition…
Click to expand...
FAQ on BTH site states not to run more than 5k resistor as it could lead to CDI damage. But in another section mention no resister should be used at all. I tried to contact them about this apparent contradiction but no response to two emails from me.
 
5 kohm resistance would be [edited] a 5 kohm resistor cap and no other resistances in the HT.

It’s never impossible that they sold you a bad spark plug. One of the last pair of Champions I bought only lasted 200 miles before it started missing under load.
 
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