Stumble / Cut Out / Hesitation at Specific RPM

  • Thread starter Thread starter laverlesmains
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Rob said:
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!  :)
Click to expand...

Isn't that what an indicators does

                          Dave
 
A couple of more efforts:

1) Directly wired a +ve lead from the battery to the coils and to the DMC.  Coils grounded directly back to -ve, DMC grounded directly back to -ve.  Disconnected leads from the Alternator to the R/R so that the bike would run only on the battery.  Problem at 5000 rpm still there.

2) Richard K from this forum is in the middle of his rebuild and lent me his set of Delortos..  Swapped over my choke cables, mounted the carbs to the manifold, added the fuel lines etc and she fired right up...  Ran it down the road and the problem still persisted at exactly the same RPM.

I think I might remove all wiring connections, and leave only direct wiring from the battery for coils and DMC, and maybe a toggle switch for the starter. 

With the same problem with different carbs it HAS to be electrical and I'm assuming a nasty ground short somewhere in the loom.

Thanks to Richard K for the loan of the carbs!


Cheers, Tim
 
bummer...

completely removed the wiring harness.    Put a +ve battery feed to the coils and DMC, and grounded via -ve leads back to the battery.  Apart from a wire to touch the +ve and spin the starter motor there's no other wires or grounds in the circuit.

And the stumble/cut out is exactly the same.  No change.
 
looks like its down to the motor...maybe your cams aren't timed right...sorry I'm not going back and re-reading from the start to see what's been done
 
Hi Tim

You've narrowed the problem down to one of ignition failure. As far as I recall, the only things you haven't swapped out are the Witt ignition box and the pickups. So it's gotta be one of those. I strongly suspect the Witt box because if it was a pickup, it would only happen on one ignition channel. You've confirmed that it's happening on both channels with your inductive tacho.

I have a new (unused) Witt DMC 2 box and a 180? trigger plate (known to be working). You're welcome to try them if you cover the postage cost from Australia.

Cam
 
Dellortoman - thanks for the very kind offer.

Murgatroid on this board swung by a couple of weeks back with his DMC and we swapped it out - and the problem persisted.    Last year Red was kind enough to send me a used pickups which I replaced, and of course the problem persisted.

Grant - I've tried a couple of different curves.

I will check the cam timing, but my measured drop in voltage coming FROM the DMC to the coils makes me focus on electrical.  I really have no way of checking the voltage/pulse from the pickups..  Does anyone know the expected voltage?  And I'm assuming it's AC?    I do have an oscilloscope that I picked up off of Craigslist - need to figure out if I can use that to determine if the pickups go "wonky" at 5000...    It's just odd that it would happen at a certain rev, and then be ok above that rpm.

Maybe I should just get the optical set up from Red.....
 
Isn't 4500 to 5000rpm around where the main jet kicks in on fuel flow?

Tell us more about the nature of the stumble.
Can you power through it?
Did you try pulling open the choke a bit when it stumbles to see if it is fuel flow related?
What plugs are you running? (tried new ones?)
Have you replaced plug caps too, after cutting off 1cm from the HT leads?
Are the chokes seating ok?
When it stumbles can you stop and pull the plugs to see if they are wet or a strange colour?

Just trying to think laterally as well as straightforwardly.
 
Grant

re: the nature of the stumble..  If I'm in gears 1-4, I can accelerate through the stumble and it just feels like a bit of a power cut out, as opposed to the engine bogging down.  In 5th gear on the highway it's like someone shut off the engine, and twisting the throttle back and forth MAY get it to power through (that's about 80mph).

Same behaviour when I used Richard K's carbs the other day.

I'm sure it's electrical because when I used to try and power through it would just shut down the DMC.  Not sure if there's an error protection circuit in the DMC, but it would require removing the connector and letting the DMC calm down before I could start it again.

Cheers, Tim
 
Can I throw my 2p in for what its worth.
Last year I had to replace my air filter and fuel taps.
Got everything changed, took it for a ride and experienced exactly the symptoms as described at the beginning of this thread.
to cut a very long story short one of the new fuel taps was faulty but took a long time to discover why.
Inside the gauze tube there is another tube that is shorter than the outside gauze tube.  This was to long and was touching the inside top of the outer gauze filter tube so no petrol flowed through it when the tap was in the open position. only when the tap was put into reserve did it actually flow any fuel.
So, with both taps in the open position only one tap was flowing fuel to the three carbs so when the revs rose the float chambers ran dry, fuel starvation.
Have a careful look at the insides of your fuel taps!
 
Good point.
Also make sure the fuel cap is venting.
Pop it open as it starts to stumble.
I still think it's fuel related.
Can you borrow another fuel tank?
 
By heck, this one's been running for some time.

Set my mind at rest and do a compression test please.

Then I'll shut up.

H.
 
Harry - http://www.laverdaforum.com/forum/index.php/topic,87375.msg244713.html#msg244713

#1 - 140psi
#2 - 130psi
#3 - 140psi

Grant/RCGBOB44.. thanks for the thoughts..  I will check fuel - but if it was starvation wouldn't it stumble at 5000 AND above as opposed to only at 5000 ?  Easy for me to pop the fuel cap (which was also replaced btw) and see if helps.

Cheers, Tim
 
This might help
At a race meeting recently a chap riding a steely framed Honda,had a problem where the bike miss fired and wasn't very smooth.
The 'experts' reckoned it was electrical,but even running on a dyno at the circuit couldn't identify the cause.A lot of electrical parts ended up being swop with known working replacement.

The cause  ----- a main jet ----- had undone its self and was sitting in the float bowel.

Could you problem be say a needle jet not in the same position as the other 2 - just thinking aloud.
Good hunting,
Andy
 
Hi Andy

Thanks for the suggestion.    I would suspect something like that if it weren't for the fact that the exact problem persisted when I put a whole new set of (used) Dellortos on it this weekend.  The odds of it being something as specific as blocked passage or needles etc would be very remote on two sets of carbs.

However - fuel starvation is outside of the carb (venting, fuel taps) so I'll give that a look see in addition to continuing to trace electrical

Hypothesis - could a slightly bent crankshaft end cause wobble on the thingymabob that rotates past the pickups?  Could that wobble introduce erratic pulses being sent to the DMC and confusing the heck out of it?  And no, I'm not going to swap out crankshafts to test the theory.....

 
shot gunning...

  • replace plug caps / snip off the last .25 of wire
    what do the plugs look like?
    chokes not closing
    carbs not balanced...properly
    run with and without air filter?
    weak valve springs
    vacuum leak
    any fuel filters?
    alternator failing
    and I hope this is not the case; nut loose behind the handlebars >:D
 
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