Stumble / Cut Out / Hesitation at Specific RPM

  • Thread starter Thread starter laverlesmains
  • Start date Start date
Did you buy new petcocks from Wolfgang? The reason I ask is that the ones I bought have "on/off" in the same position - meaning that one tap would need to face forward and the other to the rear...
Have you checked can timing? I don't imagine you could ride through "a zone" but it wouldn't run perfectly anywhere but it would run...you may be a tooth off....
 
Did you buy new petcocks from Wolfgang? The reason I ask is that the ones I bought have "on/off" in the same position - meaning that one tap would need to face forward and the other to the rear...
Have you checked can timing? I don't imagine you could ride through "a zone" but it wouldn't run perfectly anywhere but it would run...you may be a tooth off....
Click to expand...

Yes on the petcocks from Wolfgang..  and I haven't checked cam timing yet.  I've got two other tests to do first, and then I'll have to figure out how to check cam timing !  ;-)
 
I bought an inductive Tachometer online for about $15.00..  As seen below, I taped it to the instrument panel and with the wire wrapped around Lead #3, took off down the road

IMG_1200.JPG



At about 5000ish RPM, I hit the stumble zone and the revs on the Inductive Tach started to drop..    Eureka !  Ahah !  Thar she blows !    I pulled over and switched the inductive wire to Lead #1 (same coil of course) and ran the bike back down the road and sure enough when I hit the stumble zone the RPMs on the Inductive Tach started to drop.    In both cases btw, I could accelerate through the zone and the RPMs on the Inductive Tach would pick back up.

Finally, I switched the wire to Lead #2 and repeated the experiment with the same results...

So - what say the jury now?    Is this confirmation of electrical ?    In all cases the mechanical Tach stays (fluttering madly) about 4500RPM which says to me that the engine is still revolving at about the same RPM due to the momentum of the bike etc, but the drop in the Inductive Tach RPM reading says that the coils are not emitting their required/expected number of sparkys...    either due to some arcing near the coils, or some other elec-trickery further upstream towards Mr Kurtz.

 
Specifically the HT leads...?
Click to expand...

Nope - no boots.  The HT leads enter the coils with what appears to be a bit of clear silicon sealant.  Got these coils new from Wolfgang about a year ago..

I dunno what your problem might be (dodgy coils, leads, pickups ... ?) but your diagnostic method is innovative.
Click to expand...

Ha !  thanks Dellortoman..  it's half the fun.    I've changed the pickups and the coils and the leads..  I'll see what I can do about checking for arcing tonight once the sun goes down and I can really piss off my neighbor by revving the bike..
 
don't know how I have missed this thread until now - so, stumbling at approx. 4,500 rpm regardless of throttle position, coils have been replaced, and using a WITT ignition. Was doing the same thing prior to replacing the coils...

so, for starters, which WITT box/version, and which switch setting on the WITT, meaning, which curve?

have you put a timing light on the bike and revved it up past the stumble point to see what the timing is doing?

Paul LeClair
 
so, for starters, which WITT box/version, and which switch setting on the WITT, meaning, which curve?

have you put a timing light on the bike and revved it up past the stumble point to see what the timing is doing?
Click to expand...

Bronze/silver colored DMC unit on Curve 10..  Have also used Curve 1 - no noticeable difference vis a vis the stumble.

Just ran it again with the timing light, and sure enough the strobe is irregular during the stumble zone.  The timing itself is fully advanced before the stumble and after.

Now I have to figure out how to track if the signal to the coils is irregular during the stumble
 
Somewhere back a few pages it was suggested it might be a poor ground [earth]. It still sounds like it is harmonic/vibration induced. I would go over all the ground connections and add star washers between the terminals and the frame. That goes for the ground leads for the DMC and I would make absolutely sure the engine case has a good connection to the frame, maybe add an extra cable for insurance. If nothing else it will make life easier for the starter.
On thinking further about vibes you might try isolating the Witt box from its mounting. Possibly a poor internal connection?
 
sort of - but I'm waiting for a remote fuel tank that I ordered to arrive so that I can do some static tests in the garage.

For absolutely no reason at all I was able to break through the stumble and hit 85-90mph on Saturday, which was odd 'cause the bike was idling poorly that day.    While prepping it for a run on Sunday I noticed that the airbox boot to carb #3 was askew - perhaps that caused the poor idling.  Yet more red herrings.

Since owning the bike I've changed or fiddled with just about every carb/electrical component on the bike.  This makes me think that the electrical fault is simply a ground or connection somewhere in the harness.  To that end I removed the fuse panel, cleaned up all the contacts, and redid the connections.  Of course it made no difference, but at least it's one more thing off the list.

Once I get the fuel tank, I'll start tracing the circuit back from the coils.    If my little inductive Tach was correct and showed sparks dropping off in the stumble zone then I'll see if I get similar interruption in the feed to the coils, then in the feed to the DMC etc..etc.

I'll also try see if I can see arcing from the coils/leads to the frame.

Thanks, Tim
 
Tim - I know you've covered off most of the many suggestions made...but reading back through all the posts I see that the coils are fed via a relay.  Does this feed also supply the DMC box?  Also, has the relay always been in circuit when you've been suffering the misfire?  I'm a bit suspicious about relays and the effect of vibration on them. 

My strong preference is to minimise the number of connections in ignition circuits to reduce the risk of any one failing.  Which begs another question: have you tried running the bike with a (positive) feed direct from the battery to the DMC and coils?  That will remove all supply side wiring, switches & relays from the equation. (I know you have bypassed the 'starter switch' and still had the problem...not sure if this is the ignition switch or the kill switch.  I'd like to see a direct feed from battery to DMC & coils which bypasses swiches, connectors and wiring - the DMC turns the current off after a few secs if the engine doesn't run, so nothing will burn out).

Whilst checking earths, the two that are key are the coil/frame mountings which need to be clean and shiny.  Also it might be worth strapping something from engine case to battery negative...again, just to rule that out.

Finally, for running in the garage, it's usually possible to sit the fuel tank the wrong way round (front of tank facing the back of the bike) on the seat and hook the fuel lines up to the taps.

Good luck!
 
Also, has the relay always been in circuit when you've been suffering the misfire?
Click to expand...

No - the relay was out of circuit and I still suffered the misfire


Which begs another question: have you tried running the bike with a (positive) feed direct from the battery to the DMC and coils?  That will remove all supply side wiring, switches & relays from the equation. (I know you have bypassed the 'starter switch' and still had the problem...not sure if this is the ignition switch or the kill switch.  I'd like to see a direct feed from battery to DMC & coils which bypasses swiches, connectors and wiring - the DMC turns the current off after a few secs if the engine doesn't run, so nothing will burn out).
Click to expand...

If memory serves, I isolated both the Ignition and the Kill switch.    I think I'll redo this though.

Whilst checking earths, the two that are key are the coil/frame mountings which need to be clean and shiny.  Also it might be worth strapping something from engine case to battery negative...again, just to rule that out.
Click to expand...

Re: the coils.  I read a separate post where someone discussed the pros/cons of running a ground wire from the coil bracket to the battery.    Are you suggesting that I check the bracket mount to the frame to ensure good clean connection there?    I did run a ground from the bracket to the -ve terminal on the battery.    Good suggestion on the earth strap to the engine - that's a quick thing to do.

Finally, for running in the garage, it's usually possible to sit the fuel tank the wrong way round (front of tank facing the back of the bike) on the seat and hook the fuel lines up to the taps
Click to expand...

you're being too practical!  remember, the other joy of owning a vintage bike is accumulating new tools and hardware...  I mean how else would I have been able to justify buying an oscilloscope !!    But seriously, I did try this method but it just felt insecure with all the vibrations...

thanks for all the suggestions Rob.

Cheers, Tim
 
No - the relay was out of circuit and I still suffered the misfire
Click to expand...
Damn - so much for that idea! 

If memory serves, I isolated both the Ignition and the Kill switch.    I think I'll redo this though.
Click to expand...
OK - would be worthwhile.  Just another thought....have you always used the same battery when the misfire occurs?  Just wonder if a plate might be falling apart or shorting and the volts collapse.

Re: the coils.  I read a separate post where someone discussed the pros/cons of running a ground wire from the coil bracket to the battery.    Are you suggesting that I check the bracket mount to the frame to ensure good clean connection there?    I did run a ground from the bracket to the -ve terminal on the battery.
Click to expand...
If you've already run a strap from the coil bracket, that will discount the bracket/frame connection.  A good connection is required there.

One other thing - you mentioned that the DMC box is solidly mounted.  Did you really mean that, or did you use the 4 little silentbloc rubber mounts that it comes with?
 
Since it sounds like you're taking the tank off go ahead and remove the valve cover to check cam timing. With the #1 at TDC the marks on the cam should align with marks on the towers. It may take a couple revolutions to get on the correct stroke but the marks should both line up
 
One other thing - you mentioned that the DMC box is solidly mounted.  Did you really mean that, or did you use the 4 little silentbloc rubber mounts that it comes with?
Click to expand...

On the silentbloc rubbers..

remove the valve cover to check cam timing. With the #1 at TDC the marks on the cam should align with marks on the towers. It may take a couple revolutions to get on the correct stroke but the marks should both line up
Click to expand...

thanks - will take a look at that as well.

Cheers, Tim
 
Hi Tim - what about my battery question? 
Just another thought....have you always used the same battery when the misfire occurs?  Just wonder if a plate might be falling apart or shorting and the volts collapse.
Click to expand...
 
Just another thought....have you always used the same battery when the misfire occurs?  Just wonder if a plate might be falling apart or shorting and the volts collapse.
Click to expand...

Hi Rob - nope - this is the second battery - and odyssey AGM.

Cheers, Tim
 
this is the second battery
Click to expand...
Damn - again! 
Just leaves a direct feed from battery +ve to the coils and DMC box to try...oh, and an earth strap between motor and frame.

If it's any consolation, it took me 7 years to track down a duff soldered connection to an indicator spade connector that sometimes worked and sometimes didn't.  Outwardly it looked fine.  (By way of mitigation, I wasn't working on it continuously  :-[ , only when I remembered the problem after coming back from a long ride when the indicator did and didn't work.)  Give me a decent crimp over a soldered joint every time!  :)
 
You must log in or register to reply here.
Back
Top