Stumble / Cut Out / Hesitation at Specific RPM

  • Thread starter Thread starter laverlesmains
  • Start date Start date
I'm going to assume that although I've got carb issues to sort out and tune, that's not the root cause of the problem I'm experiencing.

I think it has to be with the triggers for a couple of reasons.    First, when I set the DMC to "0" and direct connect the blue and yellow wires to the +ve terminal on the battery I get constant strong sparks across all three plugs when I'm grounding them against the engine.  This tells me that:

1) I've got decent coils
2) I've got good leads to the plugs (was worried that maybe I hadn't connected the plug caps to the wires properly)
3) The DMC is working (at least in test mode)
4) I've got decent ground to the engine.

If I hook everything back to normal, set the DMC on curve 1, and crank the engine over I'm not really getting consistently good spark on Cyl #3 when grounding against the engine.  This would lead me to believe that the trigger is not sending the right pulses to the DMC.    Further, it seems like a bitch to get a decent strobe off of Cyl 3 when trying to set timing.  Sometimes the light simply doesn't come on.    If I use the same light on my 1979 Suzuki it pulses very strong so I know the strobe light is ok..      This makes me think that I'm not getting a full pulse to the plugs, but I'm not sure how that relates to the trigger.  I would think that the DMC pulse to the coils is the same strength regardless of its input pulse strength from the triggers.




 
But cylinder 1 & 3 fire at the same time on a 180.

If there's a problem with 3 and not 1,then the fault is with something that is connect to 3 and not 1,like the HT lead to 3!?

Andy
 
Try it on number one (shared coil with #3). I assume it's a 180...
If you get a good spark on number one then it's not the coil or trigger.
Could be the HT lead on #3, or the plug cap, or the plug.
 
Yes - it's a 180 so I should check#1 as well as #3..  will do that tonight

With regards to wiring of the charging coils, they are wired within the alternator so I'll move that connection outside.    Where do you have yours connected Stefan?


 
3) The DMC is working (at least in test mode)
Click to expand...

Then that really points to the triggers....

I would think that the DMC pulse to the coils is the same strength regardless of its input pulse strength from the triggers.
Click to expand...

Up to a point. If the input is within the specified range, then yes.  If the input is distorted or does not reach the threshold, then the plug(s) will either fire incorrectly or not fire at all.  The shape of the trigger pulse is a factor, as well as its size ('strength')
 
holy crap Stefan...  that's quite the setup!    Where's the flux capacitor?  :D

nice job.
 
not sure if this is a symptom or a cause, but I put the DMC into test mode and was not getting spark on 1 & 3

I swapped the wires between the coils and still wasn't getting spark on 1 & 3 (so it's not the signal from the DMC)

I removed the coils.  The dual coil has 3.6 Ohm of primary resistance and is open in secondary - that's not good

The single coils has 4.5 in primary and 12K or so of secondary - that's good

I wonder if the problem's been the coil from the outset, or the coil's gone bad due to other issues in the system.

Will see if I can source another dual coil
 
Will see if I can source another dual coil
Click to expand...

Tim - plenty to choose from as has been discussed before.  Important thing is minimum 3 ohm to avoid cooking the DMC output driver.  In fact you've already done some work in this respect....
I did replace the coil with a 5 ohm Dyna from my Suzuki and wired it up.      The bike started much stronger and settled into an idle much better and the strobe light is much stronger.
Click to expand...
 

I was about to suggest you repeat the exercise and take the bike for a test run.

However, checking back through your earlier posts, the coil issue would seem to be a red herring....or another problem!
The following symptom occurred with the original Ignition system and with the new DMC2.    <snip>
When accelerating on moderate throttle, all is well until about 4000-5000 rpm at which point the motor cuts out and sputters.    If I keep giving it throttle it will smoothen out.
Click to expand...

So, I'd suggest refitting a known good coil - your Suzi Dyna - and leave it in place to continue your troubleshooting.  I still think you would benefit from swapping out the stator plate for a known good one....

 
Rob

totally agree on red herring potential.  I've sent a request to Wolfgang to see if he's got a stator plate.  I think I may source another coil 'cause I want to ride the Suzuki!

Cheers, Tim
 
laverlesmains said:
Rob

totally agree on red herring potential.   I've sent a request to Wolfgang to see if he's got a stator plate.  I think I may source another coil 'cause I want to ride the Suzuki!

Cheers, Tim
Click to expand...

Tim,
I have dozens of stator plates here, if you cannot find one let me know
send me a PM with a postal address

Red
 
well. some progress....

while waiting for electrical parts, I picked up a carbtune....  nice.. my GS750E is now perfectly balanced!  :)

With a replacement dual coil from Wolfgang, and a replacement stator from Red (thanks a bunch Red!) I set about sorting it out.

First I replaced the coil..  Started the bike and it settled into a lovely idle.  Amazing what having all three cylinders firing does for the idle.    Let the bike warm up for awhile - got the carbtune out and balanced the carbs.    Idling nicely.    Turned it off replaced the stator with the replacement from Red.    Started up - seemed to idle even better.  Put all the bits and pieces on, grabbed the helmet and went off down the road.

Accelerated beautifully until about 4500rpm cut out and then picked up again..  Damn !!  If anything, it seems like the "bad" RPM range is narrower than before...  i.e. the symptoms are getting better.  but the stumble is still there.

I think I'll replace the needles and choke plungers..  float height is good.    can't be fuel starvation
 
Tim - you said at the start of this thread that the same problem existed with the old ignition system.  Presumably it's been happening for quite some time?  Did it become apparent suddenly or gradually develop over time?  Had you worked on the bike just before the problem started?

A couple of thoughts on the electrical side: have you taken the DMC earth direct to the battery negative terminal?  What sort of spark plug caps are you using (they need to be 5kohm suppressed type) and were these replaced with the new DMC install?
 
Rob

I got the bike this spring with the intent of doing a mechanical restoration.    Rode it a couple of times, noticed the stumble but figured it was just a carb thing that I'd sort out with a carb rebuild.

After working on the bike for a couple of months - rebuilding carbs, brakes, exhaust, forks, electrical tidying etc (but not DMC yet) I had it back out on the road and again noticed the stumble.    I installed the DMC and again there was the stumble.  The DMC ground does go direct to the -ve terminal, and I'm using 5ohm NGK caps.  It's got a new (old) stator, new bullet connectors (not using the connector block under the tank) new coils, HT leads, and caps.  The coils are powered directly from battery via a relay.    I think by now I've eliminated anything electrical.  very odd.

I also bypassed the starter switch for a test run and still had the problem.

Cheers, Tim
 
Its times like these that really test you,been there,it will be some small stupid thing that you least expect.Maybe time to start looking at those oddball thing.Have you checked for leaks in the balance hoses.Anyone around you with another set of carbs you could try.How about the key switch,had a couple of them get intermittent faults.Don't get discouraged.Good luck
 
Have you fitted new plugs and caps, trimming 1 cm or so off the HT leads?
Have the carbs been totally disassembled?
If so were they untrasonically cleaned?
Did you use a proper seal/gasket kit on each carb?
Can you tell us exactly which jets/valves/slides/emulsifiers/needles etc are in there?

Still sounds like a carb jet transition stumble to me.
 
Have you fitted new plugs and caps, trimming 1 cm or so off the HT leads? - Yes
Have the carbs been totally disassembled? - Yes
If so were they untrasonically cleaned? - No - carb dip
Did you use a proper seal/gasket kit on each carb? - Yes
Can you tell us exactly which jets/valves/slides/emulsifiers/needles etc are in there? - will do

Still sounds like a carb jet transition stumble to me. - Agreed - except it caused the DMC to shut down twice, which is why I was thinking electrical
 
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