Post by scootandboot on Mar 23, 2020 12:37:11 GMT -5
I get 4000 rpm maybe 5000 rpm tops and still somehow manage to get 35 cruising speeds and 45 downhills so idk but I've always assumed running like that's not a terrible thing please chime in if I'm wrong.
Yamaha Cranks have always had a voodoo limit 10 k but plenty of us have pipes that run over that The only time any but really had problems if the jetting was wrong or the ran closer to 12k rpms than 10k rpms
I have run at 10 k rpms alot but cruise 8500-9000 rpms on all 3 of my 2 stroke scooters . A temp gauge will help keep you out of jetting trouble .
If the Crank Blows put in a race crank and then go crazy .
I can see there is alot of mixed opinions on this subject, maybe it would be better to nailed down what "safe" rpms really means for a 139qmb engine.
Like what would be a good cylinder head temperature for just riding around/ cruising. And what the max temperature is, before it melts like a metal candle.
Don't know what temps I'm running but I'll throw a thermocouple at it after my next cruise, but I'm under the assumption 4k max 35 cruising speeds are 139qmb safe. I run something like a 90 jet but I'm pretty sure I've been running super rich but exhaust leaks have caused me not to care. Bought a new exhaust and carb so I'll give a update on what the difference is when the parts come in. Just did a leak down test and I'm good to go on that front other than the shitty exhaust flange.
What I am assuming...regarding zino and magoconnor's comments about "max rpm", is worthy of discussion. If you have a forged crank, the limit is way higher. We ran a TA Yamaha twin at 14,500. Not forged but quality.
The limits as I see it are the bearings. Not a set made that exceed the strength of a forged stainless piece. Never. They are supposed to break first. People who wipe cranks miss the boat, if they look at simply rpm. You need high grade crank bearings and end bearings. Same with tranny's.
The biggest TZ250 and NSR250 crank failures were mostly related to bad loads caused from too much timing advance or even retard too slow, in the critical rpm ranges. In other words, before max rpm...around when the power hits the pipes sweet spot. There is a dynamic FU that happens in the crankcase and the chamber that do not match. That is a high rpm, not max rpm aww shit of great vibration that your sack may not feel.
If you have not seen "Two Stroke Stuffing"...check it out! A solo Norweigian smoking a 50cc to the max...and he blows up! He vibrated off his chamber stinger....Tells me much! His engine can do 18,000 rpm.
NOTE: Since I mentioned "Critical RPM"...every rotating engine or motor has a critical rpm range. The bigger the shaft and it's outboard loads...the wider the CRPM. In the oils business, high pressure gas compressors took extreme loads upon start up. 18 or more stages taking 2500 pound gas and pumping it up to 3500. Everything along that shaft had to be perfect. (I think that was 168 machined surfaces, on a 25,000 pound piece of stainless.) Regardless...the bearing lubrication and getting through "Critical RPM" is important. On a two stroke? Just as bad as my huge ass compressor.
That is why we use tachs...and head temperatures. If you can find a "vibra-tach", you can see that on a test bench or dyno. I used one everyday on electric motors and pumps. I was not just joking about tingling nuts sacks.
I am struggling with this POS Samsung worthless pile of Microsoft diahrea...so having a tough time with clouds and pictures and shit. On my Mac two keystrokes...picture! Look at vibra-tach tech today. I think there is a probe that plugs into a smart phone.
Basic reading should show a vibration "needle" that is similar to your electronic tach...but a "vibra-tach" will show a group of needles that are being disturbed. More equals wider...wider equals issues...mis-align, wear, something. Thus, when I say ignition? change it while the engine is running! Better? Good. Worse? Bad. No changes? Bad bearing or ring or piston....make sense?
We have done a dyno pull just to discover "critical rpm". WE have done dyno pulls to find max alternator output. You do not need a dyno for that shit. Just a tach and a good multi-meter. When lucky enough to get dyno time...test everything!! You will learn the engine in a nutshell.
Last Edit: Mar 28, 2020 6:19:19 GMT -5 by ThaiGyro
Senna1Rossi: Thanks renagade281!
May 23, 2016 14:59:12 GMT -5
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