Cutting out at sustained high RPM

Got a chance to run my smoker-tard at a longer track yesterday. It ran OK as long as I was off pace. When pushed and holding the throttle open longer it started to cut out on top. 5th gear, approaching topped out and it'd just cut off like I hit the kill switch. Quickly close the throttle and open it again and nothing. Close and wait a moment and it'd be back, but may cut again. Close for a little longer and we're good to go. Only at the top of the rev range, in what is effectively over-rev territory, but AFAIK I don't have a limiter in my ancient CDI. So I'm thinking the bowl isn't filling fast enough?

Two other symptoms that bothered me: Only in 4th and 5th, again if I was pushing would I get detonation some times, oddly never on the first lap. I've been running the bike on 93 on a smaller track, no issues. I swapped over for 260GT (100 octane), fattened the jetting by raising the needle and switched my ratio from 32:1 to 40:1 to try and get more fuel in the equation, didn't have a richer needle for my lectron style carb and no flywheel puller to allow me to pull timing. This helped, but I was still able to hear it ping every now and then in my last session. I also need to see why I'm getting what looks like exhaust carbon mess along one side of the base gasket, hoping it's not leaking, if it was I'd expect to see water there?

 

Check for air leaks if you have never had this problem.



Sent from my SM-G930P using Tapatalk

 

Same technique as a four stroke, propane / brake and parts cleaner around joints and listen for a change in RPM?

The other thing I'm contemplating is fuel foaming in the bowl from vibration given it only cuts out at sustained overrev? This is the first time I've been able to really wring the bike's neck like this so I dunno if this is a new issue or not. Prior to this the suspension was so off that even at Loudon I was lugging around turning 1:30s so never really nailed it WFO sustained there.

 

You're sucking air at the gasket. Explains the carbon mess and the "fix" of going fatter. Next symptom is no compression, due to hole in piston dome. Might as well do the crank's ignition side dry seal, too.
I assume the bike had been sitting a long while before you brought it back to life?

 

Gah, not the diag I was hoping for, but it makes sense. Not going to be able to fix that in time for next weekend, bah. Seals are about 6 years old, top end was replaced this spring, the bike sits over the winter but sees use every year when there isn't snow on the ground. It sat for about 4 weeks from the last race (short track) to this weekend.

Is there a test I can do to verify a seal leak other than replace and test (Won't have another opportunity at a big track this year)? I'll be putting a bore scope into the cylinder today to see how the piston and cylinder look too.

 

Yes. There is an available 2-stroke leak down tester.
Can't give a recommendation but check Leakdown Test Kit

 

Follow up question - If I want to have a pro rebuild the motor, any suggestions? I love the way the bike handles, I like the motor when it's working, but I'm not exactly experienced with the intricacies of smokers as you may have guessed by now. I've rebuilt this unit a couple times myself but at this point I'm willing to buy some expertise to have it really setup right.

 

Now I think I'm cursed, just jumped on my 09 KTM 300 XC-W, it's doing the exact same thing, only worse. Admittedly, it does sit far longer than the motard, so I guess I shouldn't be surprised. Bah.

 

My Beta seems to be intolerant of ethanol fuel when sitting. Could be a eurobike thing.

Service...ask these guys?
About Us or Complete Rebuilding and Custom Fabrication Services

 

My first hope was fuel, but I drained and replaced with fresh, no improvement.

 

Visual inspection of the dry seal shows it looking good, only crank movement is slight in and out which I'd expect with ball bearings. Firing it up and hitting the seal with brake and parts cleaner had no impact on idle. I was going to try propane in the oil fill to test the wet side next but the clutch slings oil out the opening making a nice mess.

Plug looks good, rich which I'd expect given the jetting changes I made.

Piston and cylinder look ok, detonation apparently never got bad enough to start cratering the piston. Inspection vid:

https://www.facebook.com/video.php?v=1452710581448767

 

6psi pressure test goes a long way in a two stroke

 

Yup, just gotta find the tools now.

 

i made it using a pvc cap, a rubber freeze plug, a pressure gauge, and a motorcycle tire valve..

my dad did the same thing but used a stethoscope...

pressure tests are the best and the worst. they're the best because once you fix all your leaks, the bike runs and carburetes much more predictably

theyre the worst because usually it involves splitting the cases to reseal with threebond or replace a crank seal that you can't easily get to

 

What are you using for a borescope?

 

It's a cheapie WiFi one off Amazon I'm borrowing from a friend. Damn handy, I'm going to snag one of my own soon.

https://www.amazon.com/Depstech-Wat...id=1503929486&sr=8-17&keywords=wifi+borescope

My friend's kit has the 8.5mm camera and a smaller one, otherwise same package. There are cheaper units out there now that use the same cameras but have a smaller base pack.

 

Initial inspections for this condition would be:

- Compression check (as previously mentioned) - use a brand new schrader valve as 2-strokes tend to stress them and I have seen countless valves fall apart during the test and enter the engine
- Engine pressure check @ 4-6psi (as previously mentioned) with soapy water or simple green sprayed at every seam to verify no leaks - simple fab of exhaust plug and carb block off plate with a hose barb attached to a pressure regulator from a compressed air system
- Fuel flow verification, which requires a general calculation using BSFC of the engine at max power - use a graduated cylinder and stop watch to confirm flow through tank/petcock is adequate
- Ensure your carburetor float level is set correctly
- Timing verification across entire operating range (including expected over-rev), as I have seen rudimentary CDI systems go off the deep end with respect to advance once you get outside the "expected operating range"
- Ensure proper spark plug gap for ignition system
- Ensure proper coil operation - failing coils tend to manifest in reduced output at high temp/power conditions and revert back to normal operation once cooled down
- Follow on tests are warranted after these results are determined

Do you experience a slight "surge" when the engine cuts out? If the engine is tuned correctly the sudden reduction in fuel or addition of air will produce a lean condition that tends to momentarily increase power prior to the complete loss of or fluctuation in power based on fueling condition. How does the engine transition when the throttle is rapidly opened from idle to 100% - any lag/stumble or indicating improper fuel/air mixture.

Are you sure that you are over-revving under WOT, as I would expect that to only occur on down-shift if the gearing is set correctly.

 

One thing I forgot to mention is confirm your fuel is still capable, as I have a strict requirement on highly tuned engines that any fuel stored more than 90 days in HDPE containers is no longer usable.

You mentioned you checked operation using new fuel/oil mix, but I just wanted to confirm

 

New fuel / mix was from a fresh, unopened can of Sunoco 260GT, prior fuel was pump gas put into the jug that day.

On overrevving, I'm well past peak HP / peak torque, I've normally shifted before this point when not ragged out in 5th, bike is still pulling but not with authority so this is what I consider overrev. Considering +6mm bore on there, if I overrev too badly I'll likely find out when the piston lifts the cylinder off the base?

No surge, if anything a warning stumble/detonation before hand with pump gas in which case I'd back off but most of the time it just cut out like you had hit the kill switch.

- I haven't done a compression test yet, but my kicker says I'm still doing pretty good there.
- Pressure test is on the list of things to do to see if I can spot anything then the plan is a full rebuild over the winter, bearings, seals, errything and then a pressure test when done.
- Fuel flow - out of the petcock is solid, will be testing out of the carb next. Float level was good at the start of this season but will recheck.
- Timing - at this point short of buying a whole new ignition I'm semi in the dark here. I can't find a service manual for a 92 WR250ZD, and Yamaha was changing the curves and initial pickup plate timing year to year so I dunno what to set to other than using the base setting marks on the cases/plate. To make things more fun, the CDIs aren't marked either so unless I find a NOS one still in a bag it's still a guess. That said, I've had the bike spun to 9k on the dyno as currently timed, will put a light on it to see if I can watch it go AWOL next.
- Gaps set to the book with each plug, just got a fresh plug this evening.
- Coil is antique, may just replace if I can get NOS...

 

Moving the needle does not affect wide open throttle but your change in oil ratio would some. A two stroke when rich will top out at a certain RPM and not rev higher. Your plug says rich but you said the motor was cutting out not just ceasing to rev so it may not be due to richness. If it is actually pinging, that needs worked out first before the accurate jetting can be done. If your bike has a tach, is it just reving too high and needs a sprocket change?

Two strokes not reving when rich may seem counterintuitive but it is true.

Weak spark for any reason can make a motor sigh off early. I have seen it happen when the gap is wrong.

The higher the RPM the more important the fuel is. Most real race fuels are formulated to burn fast. I'm sorry if missed it but what RPM do you consider top and what is it turning when the problem occurs?