Firing on one cylinder

paulspice

paulspice

Club Member
I recently fitted a new top motor mount and this meant I had to remove my coils and coil mounting bracket. I also had to have a tank mounting bracket welded to the frame. When I put everything back together and tried to start the bike it would only fire on 1 cylinder. I removed the ht lead from the rear cylinder to see what one was not firing and as it still ran on one cylinder it had to be the rear cylinder not firing. When I held the plug cap near the spark plug, it started firing on both cylinders but when I put the cap onto the plug it stopped firing again. It does this all the time now. Does anyone have any idea what could cause this to happen. I’ve tried different plugs and swapped the front and back leads around. I’m really baffled by this.
 
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Many years ago Meriden Triumphs came with a outer metal body over porcelain inner and the inner used to fracture and so plug would short out on metal cover. I can see your caps have no similarity to the Triumph one but not knowing how they are constructed could you see if easing the cap up and down will make a reaction? just a thought for a possible clue...good luck.
 
Thanks for your reply and suggestion Paul. I swapped the leads around and it still only happened on the rear cylinder so I can discount it being a lead problem and I’ve tried different plugs so I can discount it being a plug problem. The only thing I can think of doing is swapping the coils around. This should tell me if it’s a coil problem or not. I guess it’s going to be a process of elimination.
 
Oh yeah, sounds like you are chasing a bad earth maybe - usually the sort of thing that is only solved once X amount off effort is expended!
 
I disconnected the battery and put protection around the cables in the area of the welding. The strange thing is that it runs OK when the cap is held near the spark plug. I’m wondering if I have a short somewhere and it’s shorting out when connected to the plug. I’m not sure if that’s even possible.
 
This is so weird it must be identifiable by that alone? Is there anything to do with the bracket that could be interfering?
I remember that when HD went to capacitive discharge (ie; no points) ignition the spark gap had to be enlarged to be a tiny bit more difficult to fire so the magnetic "wave" that triggered the spark did not anticipate, what could affect yours that way? sorry. I'm rambling out loud, that doesn't help - do you have an old plug lead and cap to try..?
 
Hi Paul I think the problem may be where the lead plugs into the coil the metal female insert inside the coil may be moving as the plug lead is moved toward the plug allowing to fire and when lead is on the plug it moves to break circuit and cut spark seen a similar problem on a drag bike at the bulldog the dyna coil was the culprit the insert had come away inside the coil we found it by inserting a screw driver and gently wiggling it about the insert should not move at all :unsure:
 
paulspice said:
I disconnected the battery and put protection around the cables in the area of the welding. The strange thing is that it runs OK when the cap is held near the spark plug. I’m wondering if I have a short somewhere and it’s shorting out when connected to the plug. I’m not sure if that’s even possible.
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Further to what watchman said, waggle the HT Lead at the Coil end to see if is indeed the Coil connection.
 
paulspice said:
I disconnected the battery and put protection around the cables in the area of the welding. The strange thing is that it runs OK when the cap is held near the spark plug. I’m wondering if I have a short somewhere and it’s shorting out when connected to the plug. I’m not sure if that’s even possible.
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in my experience, when something goes wrong it's usually always related to whatever it it you've faffed on with last. hope you find the root of the problem. it'l be a bad connection somewhere or other.
 
watchman said:
Hi Paul I think the problem may be where the lead plugs into the coil the metal female insert inside the coil may be moving as the plug lead is moved toward the plug allowing to fire and when lead is on the plug it moves to break circuit and cut spark seen a similar problem on a drag bike at the bulldog the dyna coil was the culprit the insert had come away inside the coil we found it by inserting a screw driver and gently wiggling it about the insert should not move at all :unsure:
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Thanks Barry, I think the thing to do is to swap the coils around, that way I’ll know for sure if it’s a coil problem or not. The plug leads are a real tight fit both on the coil and the spark plug.
 
chrissy said:
in my experience, when something goes wrong it's usually always related to whatever it it you've faffed on with last. hope you find the root of the problem. it'l be a bad connection somewhere or other.
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Completely agree with you, it was working before I had the welding done and before I removed the coils to fit the new motor mount.
 
Paul P. said:
This is so weird it must be identifiable by that alone? Is there anything to do with the bracket that could be interfering?
I remember that when HD went to capacitive discharge (ie; no points) ignition the spark gap had to be enlarged to be a tiny bit more difficult to fire so the magnetic "wave" that triggered the spark did not anticipate, what could affect yours that way? sorry. I'm rambling out loud, that doesn't help - do you have an old plug lead and cap to try..?
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I’ve tried swapping the plug leads around and that made no difference so I’m 99% sure it’s not an HT lead issue. I’m also 100% sure I haven’t got a clue what the problem is.
 
Putting the extra gap into the plug lead can make the spark at the plug stronger. If the coil is giving a weak spark the extra gap will make it stronger.
There are/were gadgets around which you could fit into the HT leads to do this or you could make you own with a couple of screws and a bit of clear pipe to hold the two ends in line.
Check for this (weak spark failing under compression) by swapping coils around.
 
I’ve just swapped the coils around and it’s still the same. So far I’ve ruled out plugs, leads and coils. I made up a new link cable between the 2 coils. I guess my next move is going to have to be checking the wire that goes from the rear coil to the TwinTec ignition module.
 
paulspice said:
I’ve just swapped the coils around and it’s still the same. So far I’ve ruled out plugs, leads and coils. I made up a new link cable between the 2 coils. I guess my next move is going to have to be checking the wire that goes from the rear coil to the TwinTec ignition module.
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It seems like you have checked all possibilities on the output side of the ignition so I agree you should now check the feeds going to the Coil. Maybe a loose connection or fraying wire. Good luck.
 
paulspice said:
Completely agree with you, it was working before I had the welding done and before I removed the coils to fit the new motor mount.
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i thought it was a new tank mount not a motor mount? are you using this new mount as an earthing point?
hope you don't mind or thinking i'm telling you how to suck eggs and i'm just thinking aloud but i've seen folk having issues when they've painted frames and not properly cleaned earthing points.
 
I had to have a tank mount welded back as the previous weld had broken down. I fitted the new billet motor mount at the same time. Neither of these are earthing points.
No, definitely not thinking you’re teaching me to suck eggs, all suggestions are really appreciated.
 
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