Intermittent Miss

T

treoleileagh

Full member
Location
Australia
I need some help to diagnose an intermittent miss (Jota1979).
The miss comes in on a trailing throttle or at low to mid engine speed (usually under 3000 rpm). It can come in under load at higher revs.
It usually comes in after riding ten kilometers or so (from cold) or two or three kms (from warm).
The miss persists, sometimes for several kilometers.
It usually clears eventually at high revs (if I can coax it above around 4500 rpm).
After clearing, it returns intermittently if engine speed falls below 4000rpm
Feels like it is dropping one cylinder, sometimes two.
Sometimes accompanied by farting and backfiring.
Giving it a shot of wide open throttle doesn't cure the miss. Closing the throttle doesn't cure it.
Giving it choke makes no difference

Here was what I've done to track it down:
Changed the fuel
Cleaned carburetor jets
Checked battery condition and charge rate. (13.3v ign off, 13.8v running).
Checked continuity from switch to coils, coils to ign triggers.
Bench tested coils (both show around 5 amps)

Ideas?
 
New OEM alternator 200 miles ago, New wiring everywhere, New Boyer/Todd ign amplifiers. Starts easily every time, runs brilliantly before the miss kicks in.
 
I’ve always been a big fan of doing the easy stuff first. For this I’d always start by changing the plugs. If one plug is different then that narrows the search
 
My 3CL had an itermittet miss years that was hard to find. It would run fine until warm and then have problems. Turned out to be the resistor plug caps. Put on new NGK caps and the problem was fixed.
 
1200ts said:
My 3CL had an itermittet miss years that was hard to find. It would run fine until warm and then have problems. Turned out to be the resistor plug caps. Put on new NGK caps and the problem was fixed.
Click to expand...
Good idea. Thanks. Plugs are less than 100 miles old and all look the same, pipes too. Anyhow, I'll go there again and caps as well.
 
treoleileagh said:
Bench tested coils (both show around 5 amps)
Click to expand...
Do you actually mean Amps or Ohms? To bench test the current draw, you would have to connect it to a battery and measure the current. Seems like a lot of trouble to go to when a simple resistance test will tell you the same thing.

Anyway, whether its Ohms or Amps, it doesn't necessarily mean the coils are OK. A coil can be OK when cold but fail when warm. If the failure is in the secondary winding (which is most likely) it won't show up with any test of the primary winding.

As others have suggested, try new plugs first. They can fail in all sorts of strange ways that you wouldn't expect, and can be affected by engine temperature. I've had engine symptoms like you describe, and a new set of plugs fixed it. It used to happen on a regular basis when the bike had a DMC1 ignition with original pickups. That system must have been very sensitive to plug condition. It's never happened since I fitted the Ignitech ignition with Hall effect triggers.
Note: I'm not criticising DMC ignitions in general. Mine was a very old ignition system that was well past its prime. Electronic components can deteriorate with time.

If fitting new plugs doesn't sort the problem, it might help your diagnosis if you can figure out which cylinder is missing. Perhaps an exhaust header temperature measurement will tell you. Or you could try disconnecting the spark plugs one at a time and test riding it on 2 cylinders. If it doesn't miss, then you've disconnected the dodgy cylinder. I dunno whether Boyer ignitions are OK to run with a plug lead disconnected, but it's best to earth the disconnected lead to be safe anyway.

Once you've identified the faulty cylinder, try swapping components around to see if the fault moves to another cylinder. I know you can't swap coils on a 180° engine, but you can swap plug leads and resistor caps. From your description of the Buyer/Todd ignition (you used the plural "amplifiers") it sounds like it has separate ignition boxes for each coil. If so, you could also swap those around.
 
treoleileagh said:
New OEM alternator 200 miles ago, New wiring everywhere, New Boyer/Todd ign amplifiers. Starts easily every time, runs brilliantly before the miss kicks in.
Click to expand...
New Boyer/Todd? When did Phil stop selling them???

I suspect your problem is also jet related. What jets and fuel is recommended now for you guys down-under?

I have had bad plug fouling problems, largely cured by down jetting for modern fuels. iirc I am now on 55 pilots with 130 mains using standard UK 95 unleaded.
 
Dellortoman said:
Do you actually mean Amps or Ohms? To bench test the current draw, you would have to connect it to a battery and measure the current. Seems like a lot of trouble to go to when a simple resistance test will tell you the same thing.

Anyway, whether its Ohms or Amps, it doesn't necessarily mean the coils are OK. A coil can be OK when cold but fail when warm. If the failure is in the secondary winding (which is most likely) it won't show up with any test of the primary winding.

As others have suggested, try new plugs first. They can fail in all sorts of strange ways that you wouldn't expect, and can be affected by engine temperature. I've had engine symptoms like you describe, and a new set of plugs fixed it. It used to happen on a regular basis when the bike had a DMC1 ignition with original pickups. That system must have been very sensitive to plug condition. It's never happened since I fitted the Ignitech ignition with Hall effect triggers.
Note: I'm not criticising DMC ignitions in general. Mine was a very old ignition system that was well past its prime. Electronic components can deteriorate with time.

If fitting new plugs doesn't sort the problem, it might help your diagnosis if you can figure out which cylinder is missing. Perhaps an exhaust header temperature measurement will tell you. Or you could try disconnecting the spark plugs one at a time and test riding it on 2 cylinders. If it doesn't miss, then you've disconnected the dodgy cylinder. I dunno whether Boyer ignitions are OK to run with a plug lead disconnected, but it's best to earth the disconnected lead to be safe anyway.

Once you've identified the faulty cylinder, try swapping components around to see if the fault moves to another cylinder. I know you can't swap coils on a 180° engine, but you can swap plug leads and resistor caps. From your description of the Buyer/Todd ignition (you used the plural "amplifiers") it sounds like it has separate ignition boxes for each coil. If so, you could also swap those around.
Click to expand...

Yes, I'll change the plugs next. I just changed the resistor caps and the problem remains. At idle, all plugs spark so it's hard to identify the faulty cylinder(s).
Yes I did mean ohms. The double coil is the original and I guess age makes it suspect anyway. Since the pattern of the miss is to drop back to two and then one cylinder, I'm starting to think the double coil might be a candidate for replacement.
Yes, there are two Boyer boxes.
Thanks for all this. I'm learning all the time about these electrics and this was helpful.
 
OneThinBloke said:
New Boyer/Todd? When did Phil stop selling them???

I suspect your problem is also jet related. What jets and fuel is recommended now for you guys down-under?

I have had bad plug fouling problems, largely cured by down jetting for modern fuels. iirc I am now on 55 pilots with 130 mains using standard UK 95 unleaded.
Click to expand...
I bought them from Phil a couple of years ago, however the bike has only done a hundred miles or so on them. Yes, same issue with plug fouling, fixed by changing the jets as you have. I started out in the belief this was a carburetor issue, but I'm sure now it's an electrical one. Thanks for your thoughts
 
did she become the intermittent Mrs?
I made a misstake once, got Sued by her for broach of contact
CLEM
 
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