Stumble / Cut Out / Hesitation at Specific RPM

  • Thread starter Thread starter laverlesmains
  • Start date Start date
Hmmm, in that case check all main earth points throughout the bike.
Quality of wiring from ignition triggers to the DMC?
Condition of R/R?
Mounting point for the DMC?
Replaced coils when DMC fitted? (too lazy/busy to read back through)
 
Hmmm, in that case check all main earth points throughout the bike.
Quality of wiring from ignition triggers to the DMC? - good
Condition of R/R? - brand new
Mounting point for the DMC? - solid
Replaced coils when DMC fitted? (too lazy/busy to read back through) - yep
 
Ok, one last try.
Have you checked if the fuel line filters (on each carb and in the fuel taps themselves) are free of silt?
Could be a restriction at higher revs. Long shot though.
Also, try opening the fuel cap when it does it in case it's an air lock.
Have you tried switching on the choke when it stumbles?

That's me done now.  :D
 
Hi Grant

I think the fuel flow is good as the stumble is only at a certain rpm - above and below that rpm the engine runs fine.  In other words, accelerate nicely up to 4500ish, stumble cuts in at 4500ish, twist throttle more and engine climbs nicely to redline.    The DMC failures I've had occur at about 80mph on the highway, which I think is about 4500, and at that RPM the stumble causes the DMC to barf out.  Which is why I've been focussed on electrical - I believe that there must be some short or voltage spike that's feeding into the DMC at that RPM.

Maybe I should just buy one of your 1200s...    ;-)

 
A couple of updates.    Thinking that this may be an electrical issue, I connected up a timing light and taped it to the front of the bike with the strobe light facing me.    I couldn't ascertain any interruption in the pulses as I rode along at 4500-5000 rpm (the stumble zone).

I then replaced the main jets (from 120 to 130) and installed new K1 needles.    Again, no change to the stumble zone.

I stuck my GoPro to the sidecover, removed one airboot and rode down the street.  A little too much windnoise to properly hear the stumble while riding, so here's a video clip of me revving the bike twice through the stumble zone.   

Anyone hazard a guess?  Short of the wiring loom itself, I think I've replaced just about everything in the ignition and fuel systems !!

Laverda Revving at Stoplight
 
The setup of the fuel line has made a large air lock. Perhaps this is causing the hesitation. If you rearrange the V fitting on the carb and the length of the hoses the airlock can be eliminated.
 
Any hose can have air trapped in a high point in a loop, it is not an "air lock" and the fluid will flow through it regardless. It will affect syphon systems but not a gravity feed like here.
 
  If the triple is going to go into a "rich shift" it will do it at 4.500 RPM and clear at around 6000RPM. On a standard unmolested machine this should not cause much of a noticeable effect whilst riding on the road. If cam timing, carbs or exhaust have been modified or incorrectly set up this can start to show up. If you start to pull the choke on at these rpm and motor gets immediately worse - then it could be this. I doubt this is the problem, but easy to find out - regards - Phil
 
If the triple is going to go into a "rich shift" it will do it at 4.500 RPM and clear at around 6000RPM. On a standard unmolested machine this should not cause much of a noticeable effect whilst riding on the road. If cam timing, carbs or exhaust have been modified or incorrectly set up this can start to show up. If you start to pull the choke on at these rpm and motor gets immediately worse - then it could be this. I doubt this is the problem, but easy to find out - regards - Phil
Click to expand...

Phil..  I have pulled the choke while riding in this zone and if I recall the engine bogged down...  I think my carbs are set up properly - 120 mains, K1 needle, 55 pilot.  1.5-1.75 turns out, sync'd with a Carbtune.  Plugs are a little sooty around the edge but brown at the electrode.    Exhaust are repro Jota's from Wolfgang.    I haven't touched the cams.  Not sure what range this clears up at as my tach needle gets a bit fluttery and wildly optimistic over 5000 rpm.  :D
 
per Wolfgang's (and others) advice, I put a little paint mark on the throttle grip, and then marked "0", "1/4", "1/2", and "Full" stops on the switch gear.

When accelerating from stop, it's about 1/4 throttle before reaching 5000rpm where it stumbles.

switching into second gear, give it about 3/8 throttle and it accelerates smoothly 'til the 5000rpm stumble

shift into third gear, crack the throttle to a little over 1/2 and it accelerates smartly 'til the 5000rpm stumble.

3 different throttle openings - 1 constant rpm..    I'm thinking it's electrical.

 
From my experience, a hesitancy in acceleration from 5000-6000 rpm under WOT is most often caused by fuel starvation due to too low float levels.  But this will only show under hard acceleration in the higher gears, ie, 4th or 5th.  Max torque is made at around this point, requiring a reliable and steady fuel supply.  Usually shows on hard-worked 120s with a bit of a maintenance deficiancy, the float valve wears at the piston stop and allows the piston to protrude further, effectively lengthening the valve and lowering the float height.  Reckon the harsher vibes of the 180s don't wear the valve as severely as the fine vibes of the 120s, but it does show up on 180s as well.  Once the hesitant zone is overcome, the bike accelerates normally... till a piston is holed!

What plugs are you using?  The DMC doesn't like suppressed caps and plugs.

piet
 
Well, you've got resistor caps and resistor-less plugs, should work OK.  Still a good idea to check if the resistance is the same in all caps.

Float level should be at 18mm, better to err on the high side... I know Dellorto specifies 18-18.5mm, I go for 17.5-18mm(the further the float is from the gasket surface, the lower the fuel level).

piet
 
Here's a strange scenario I ran into this summer...3C runs fine until a certain RPM. There were no boots on the HT leads at the coil and the spark was going around the wire to the frame...do you have boots on the coils?
 
Check for any arcing from the coils to the frame - I expected the scenario I spoke of would be rare but it may be the new latest thing....
 
Easiest way to check for arcing at the plugs, plug caps or plug leads is to do it at night. Take the fuel tank off the bike, turn any lights off and start the engine in the dark. Have a close look at the leads while it's running. Any arcing will be obvious.
 
From my experience, a hesitancy in acceleration from 5000-6000 rpm under WOT is most often caused by fuel starvation due to too low float levels.  But this will only show under hard acceleration in the higher gears, ie, 4th or 5th.  Max torque is made at around this point, requiring a reliable and steady fuel supply.  Usually shows on hard-worked 120s with a bit of a maintenance deficiancy, the float valve wears at the piston stop and allows the piston to protrude further, effectively lengthening the valve and lowering the float height.  Reckon the harsher vibes of the 180s don't wear the valve as severely as the fine vibes of the 120s, but it does show up on 180s as well.  Once the hesitant zone is overcome, the bike accelerates normally... till a piston is holed!
Click to expand...

I agree and undersized Main Jets will do the same especially if you are not running the original Exhaust Pipes. I experienced the same problem on my 71 750SF when I changed out the original Dual Exhaust System for 2 into 1 Headers and had to increase the size of the Main Jets from 125's to 130's in Delorto VHB 30A's with K2 Needles in 2nd notch.
 
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