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Misc: Ignition Mysterious Meteor misfire - what am I missing?

CoreyL

Well Known and Active Website User
VOC Member
Several weeks ago I headed out for a ride on my '50 Meteor. The tank was full, the sun was shining and all was right with the world. The bike had been running fine since a rebuild about a year/1000 miles ago.

I was riding at 50-60 mph and, after about 45 miles of riding, the bike started to misfire. At first, just every now and then (at constant throttle). It got progressively worse and I eventually turned around and went home. The bike starts up fine and idles without problem.

It never got so bad that the bike wouldn't make progress - it just surged and bucked. It didn't seem to matter whether I was accelerating or under constant throttle or if I were going 45, 50 or 60 mph (or slower). I opened the filler cap to make sure there was no vacuum and opened both taps. It made no difference. I put in fresh plugs (both NGK B7EVX and Champion N4C) - no difference. When I got home, I replaced the small coil and HT lead for the new type BTH "magneto". I also verified that the ignition timing had not slipped. Also checked valve clearances. All checked out fine. I opened up the kill switch on the handlebar and made sure the connection was good and not shorting to ground.

The carb is a new Premier Amal Mk 1 32mm unit: 0.106 needle jet, 250 main jet, needle clip in the middle groove.

I have not drained and refilled the fuel tank yet since, if bad fuel were the problem, why would it begin to start misfiring after riding for 45 miles on the tank. I've only used non-ethanol fuel in the bike.

I'm running out of ideas for things to check. What am I missing? Anything else the group can suggest? I'm flummoxed...

Thanks in advance for any help you can offer!
 

Martyn Goodwin

Well Known and Active Website User
VOC Member
Kill Switch! This is a common problem when the kill switch wire acts as an antenna picking up high voltage EMF and mucking up the electronics inside the BTH. It is also a issue with Pazon

Fix is to ensure that the kill switch wire is kept as far away as possible form the high tension (spark plug) leads AND the ignition coil.
 

CoreyL

Well Known and Active Website User
VOC Member
Kill Switch! This is a common problem when the kill switch wire acts as an antenna picking up high voltage EMF and mucking up the electronics inside the BTH. It is also a issue with Pazon

Fix is to ensure that the kill switch wire is kept as far away as possible form the high tension (spark plug) leads AND the ignition coil.
I'm aware of that issue and have been careful to keep the HT wire and the kill switch wire well separated. The kill switch button is mounted on the handlebar. I have a bullet connection in the kill wire to facilitate removal of the front end of the bike (which I've has to do several times). Do you think that may have something to do with it? If so, why would it be fine for so many miles/months and then start acting up?
 

greg brillus

Well Known and Active Website User
VOC Member
The newer BTH mag's are not infallible.........Has the bike been in damp conditions at all.........The ends of the coil poles are prone to light corrosion, this will make them play up...........I have had a couple suffer complete failures.......A spare mag would be good to try.
 

Chris Launders

Well Known and Active Website User
VOC Member
I had something similar and it was fuel starvation, I found the float was fouling the float bowl before lifting the float needle, but it was taking pressure off the needle so you would expect fuel to flow, it did, but not enough so I phoned AMAL.
They explained to me the brass viton tipped needle was too heavy to be lifted properly with fuel pressure alone and now supply an alloy needle for this problem, just something to check.

Strangely it came on suddenly, the carb had been on over 1500 miles before this started, I had other problems with it around this time and haven't used the bike for 2 years so can't remember if I used it much after or not.
 

greg brillus

Well Known and Active Website User
VOC Member
I've had similar problems with the newer Amal Premier concentric's on a few different bikes.......Yes the needle fails to lift via the float.........had one bike were you could hold the tickler down and nothing would happen. Had to bend one of the float tangs to actually lift the needle off its seat when the float lowers........Never been a fan of concentics, they have always been a very poorly made carb in my opinion.
 

CoreyL

Well Known and Active Website User
VOC Member
I had something similar and it was fuel starvation, I found the float was fouling the float bowl before lifting the float needle, but it was taking pressure off the needle so you would expect fuel to flow, it did, but not enough so I phoned AMAL.
They explained to me the brass viton tipped needle was too heavy to be lifted properly with fuel pressure alone and now supply an alloy needle for this problem, just something to check.

Strangely it came on suddenly, the carb had been on over 1500 miles before this started, I had other problems with it around this time and haven't used the bike for 2 years so can't remember if I used it much after or not.
I have the rubber tipped alloy float needle in there already.
 

LoneStar

Well Known and Active Website User
VOC Member
Sediment in the float bowl, intermittently blocking main jet. Or bad gas - as you say, why would it take so long to manifest? But it's easy to replace the fuel and see for sure.
 

vibrac

Well Known and Active Website User
VOC Member
There is a lot of 'folk law' on many threads here about the concentric carb I am not alone in disliking them beg or borrow another carb and just see what happens
 

CoreyL

Well Known and Active Website User
VOC Member
Sediment in the float bowl, intermittently blocking main jet. Or bad gas - as you say, why would it take so long to manifest? But it's easy to replace the fuel and see for sure.
I drained and refilled the tank yesterday with fresh fuel - no improvement.
 

oexing

Well Known and Active Website User
VOC Member
Your last check ride was allright for half an hour ? Seems time is the factor so my idea is to check sparks on the plug wire AFTER half an hour ride for max length . Electronics and coils are notorious for defects in hot state so no luck when checking at cold .

Vic
 
Last edited:

LoneStar

Well Known and Active Website User
VOC Member
Does the misfire occur with a cold engine, or only after everything is hot? It idles well; does the misfire begin at a certain rpm, or with a certain throttle opening? Only under load, or revving in neutral?
 

CoreyL

Well Known and Active Website User
VOC Member
It seems to worsen as the engine warms up. It doesn't seem to be RPM-related. I haven't tried revving in neutral. It doesn't seem to matter if the engine is under load or not.

My attention is focused on the BTH unit now. Maybe a bad earth connection or something like that? I'm going to remove the mag cover and poke around and see what I can see...
Does the misfire occur with a cold engine, or only after everything is hot? It idles well; does the misfire begin at a certain rpm, or with a certain throttle opening? Only under load, or revving in neutral
 

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