1999 C40 TLRX, 2-stroke: Miss or RPM Loss at WOT, Video

[a387673]

Here is a link showing the RPM loss I'm experiencing at WOT. At :18 in the video, the RPMs drop by 300 or 400 (from about 5,300 to about 4,900). I ran out of room and backed off two seconds later, so there's not a lot to hear, and there is some wind noise at that point, but it is typical that if I massage the throttle it will pick back up, and fall off again.

I have done basic checks of compression and spark, cleaned the carbs and numerous other items, like fuel pressure/vacuum, ignition coil resistance, thermostat, inductive diagnostic tachometers, etc., etc., with no real clues yet as to what the issue might be. Thanks for any insight.

https://youtu.be/_Ho-kSFqLBQ

[99yam40]

I had a similar problem with my C40TLRX.
It took me a very long time to find out a previous owner drilled out the main jets.
I had to buy one new jet just to measure the hole and compare to the ones in the carbs.
running with the top cowling off and the silencer off, when I ran into the wind it ran fine, but when I ran with the wind there was a miss or RPM drop .
Just like when running with everything in place.
that led to me thinking about too rich.

not saying that is your problem, but it might be.

have you changed the fuel pump?
if it leaks into #3 crankcase that will cause a miss.

if the rubber plugs fall out of the pilot jet opening it will cause too rich condition also.

if a jet/passage gets partially plugged it could cause too lean condition.
Timing fluctuations or missing spark can be seen with a good timing light

[a387673]

When I tested spark at 7/16”, it showed what I thought was a very thin blue line, across all cylinders, with an audible snap. I thought it might should have been bolder and brighter. Is the quality of this spark good enough when blue, or should I expect more?

If this was an automobile, I most certainly would have replaced plug wires by now, but the cost of the plug wires/ignition coils causes hesitation. I had a resistance test on a coil, secondary lead, with a null value because one of the pinned connectors was not making contact. I re-did that connector and got an acceptable value, but it's safe to say I don't trust these components. Still hate to just throw parts at it.

[99yam40]

test what you can and make sure it meets minimum specs given in service manual.
need to have proper test equipment for peak voltages
KV tester would be nice, but not many have those

as I said before a good inductive timing light will show if a plug is missing spark even intermittingly ,
ones with RPM will show one cylinder different than the others.
and will show if the timing on each cylinder is bouncing around

if all tests show good then you are left with too rich or too lean in a cylinder to fire properly.
you can spray some fuel mix into each carb while the problem is there to see if it is leaning out, but testing for too rich is hard to do.

lean sneezing can damage seals in the crank case. top and bottom crank seals can be changed, but those between the cylinders on the crank will need to have the crank pressed apart.

I have found the old rubber plugs I mentioned earlier come out and be floating around the fuel bowls.
new ones solved that problem for me

does the motor drop a cylinder and then come back, or does it stay down?

[a387673]

The RPMs drop down and typically come back up for a period, then drop again. Sometimes it takes a minute or 30 seconds for a cycle,sometimes it cycles more quickly (down and back up), but I haven't let it run for extended periods when it wasn't revving, for fear of leaning out a cylinder.

[bedwar44]

I had a similar problem once on a c40tlrv. It tool a very long time to finally cure it. Ended up it was the lower crank shaft seal(s), which are actually quite easy to change out. My best guess is the case slowly lost vacuum, affecting the scavenging system or whatnot inside (I am no expert on the internals), then the vacuum slowly built back up as you describe and if accelerated slowly would get back to full throttle. Only happened when running higher rpm range. When i changed mine out, the problem was fixed immediately and the motor ran better than ever. If you are gonna start replacing stuff, seals are cheap and it takes 10 minutes to remove the power head, assuming you have a hoist (or my go to, a come along and sturdy ladder or tree) and your bolts are in good enough shape to remove. The seals are in a little cone shaped housing on bottom of case. 1 bolt, change seals, and back together. Will need a new gasket between case and housing as well. Good thing about doing this, if you are maintenance-inclined, is you should probably do it anyway if it has never been done.

Eventually, this issue on mine was accompanied by what some call a lean sneeze or similar sound when idling and not warmed up, etc. But, a little different from an actual lean sneeze, there was a distinct metallic sound at the end. Not like metal moving, but like air being forced through or over a metal piece that is just the right thickness to vibrate and make a sound (drive shaft tube, crank seal housing, something in there...). It is hard to describe, but instead of a woomph noise of normal lean condition it was a bit more drawn out, increased in volume as it happened, and ended more sharply, like woooomphht, with the metal sounding noise in the hht. There was a youtube video floating around some years ago with the same issue and good example of the noise.

[a387673]

Quick question..... I was thinking a little deformation on the pilot jet in one of the carbs should not cause this problem with the higher RPM miss (or whatever it is), but now wonder...... One of the pilot jets was stubborn when removing at one point, and the screwdriver slipped, causing slight deformation to the slotted end of the jet. It appeared that the opening was neither obstructed or widened,but the basic question is, could what appears to be a minor issue, if an issue at all, with the pilot jet on one of the carbs, cause this type of problem with higher RPM performance? Thanks for any feedback.

[99yam40]

I agree, it should not affect higher RPM and it is just the slot and not the jet orfice