|
|
Post by oldchopperguy on Jun 8, 2016 21:52:45 GMT -5
I have a quick question about rev-limiters...
Yesterday, I ran errands on "Minnie Mouse" my old Kymco 250 Grandvista. On the highway, I was running with traffic, near WOT for a couple miles, 80-82 mph. On a downhill stretch, all traffic increased speed as it often does, and the old scoot did likewise, nudging indicated ninety-mph (actual, 87 mph).
After a mile or so, the engine simply "shut down". EEEWWW!! Surprise! I quickly made it over to the right lane to go onto the shoulder, and when my speed dropped to about 75 mph, the engine started right up and ran fine the rest of the day.
I have run downhill with a tail-wind, well over ninety-mph in the past with no engine cut-out. This time though, the tach was up to 9K+ rpm. In past fast runs, it stayed below that.
My question is: Did I exceed a max-rpm limit, and the rev-limiter shut the ignition off, and it returned function when the engine got back down around 8K rpm? Or, is it likely the CDI or coil crapped-out momentarily... Or, is it likely my fuel-pump could not keep up with the engine's fuel-consumption at that higher-than-spec speed?
This is the first time old Minnie has ever acted up. I consider that the clutch was engaged, and the engine was spinning at over 8Krpm for a good while, NOT firing... And when the speed dropped down below 80 again, the engine started up smoothly with no "exploding wet gas" in the muffler, no backfire, etc. It simply resumed running smoothly.
I don't fully understand "rev-limiters" in the CDI. I'm thinking it might have kicked in around 9K rpm and then released again at a more "proper" rpm... But I just am not sure.
I rode for a full hour after the event, and the motor ran perfectly, though I did not get back on the limited-access, or exceed 65 mph the rest of the day.
Whatever happened, I either lost spark, OR fuel delivery, or BOTH for around 20 seconds at WOT, and then the engine ran fine after slowing down.
Any advice is welcome! I do NOT want to get stranded on the highway... It might be best to replace the CDI and coil as a precaution but I'm not sure it was electrical. Could still be fuel.
Thanks in advance for any ideas!
Leo in Texas
PS: I asked the dealer about it, and he had never heard of a Grandvista being ABLE to exceed the rev-limit, so he was not sure what the limiter would do to the engine. With my final gears up-toothed a tooth or two, and a downhill stretch, it DOES look like exceeding max rpm IS possible.
|
|
|
|
Post by rockynv on Jun 9, 2016 4:34:59 GMT -5
Rev limiters usually cause the engine to sputter and resume in a very small rpm band going off and on again right away so I would be inclined to think fuel pump not being able to keep up or exhaust valve sticking. Bike has been out of operation for a while during the light upgrade so it may be time for a good fuel system cleaning and possibly for some Mystery Oil.
When was the last valve adjustment?
|
|
|
|
Post by oldchopperguy on Jun 9, 2016 10:19:17 GMT -5
Rev limiters usually cause the engine to sputter and resume in a very small rpm band going off and on again right away so I would be inclined to think fuel pump not being able to keep up or exhaust valve sticking. Bike has been out of operation for a while during the light upgrade so it may be time for a good fuel system cleaning and possibly for some Mystery Oil. When was the last valve adjustment?
Rockynv,
I tend to think you are right. The shutdown seemed more like "no-fuel" than "no-ignition". The downhill allowed me to get to a speed much higher than the 70 mph top-speed listed in the shop-manual. Even with a 1-tooth up in the final drive, top-speed should really be no more than 75 or so, but on level road with little wind, the scoot will hit 80 mph with little trouble... BUT... This is usually at around 8K rpm. This time, with no tail-wind, and only the down-hill run adding to the speed, I was running around 9K rpm. I'm thinking this may have caused the engine to be consuming gas faster than the pump could deliver.
I had similar experience with my old Chinese 150 when the vacuum-petcock would close at high rpm.It seems like if the IGNITION was off, there should have been some noticeable popping/sputtering and a blast of unburned gas in the muffler when spark returned after coasting for a good 10 seconds with the motor spinning 8K rpm.
It seemed much more like the fuel was shut off, then fuel-flow returned again.I hate to admit it but I'm not even sure if the old scoot has an electric, or mechanical fuel-pump... Got to check the manual! One thing I know is that I really should not be running the motor that fast anyway... Another good reason to swap out the worn original rollers for new Dr. Pulley sliders.
I had the valves adjusted and a full tune-up, with coolant flush/refill over the winter, and the engine normally runs absolutely perfectly at any reasonable speed... This IS the smoothest single-cylinder engine I've ever had. The throttle-response and overall performance of this scoot is truly a joy. I figured a few miles at WOT would be good for the scoot, and clear out some of the carbon from city-driving, but 9K rpm is probably a tad much on the old girl... The manual says she's made for 70 mph tops, and with the up-tooth final drive, 75 mph is likely all I should push her to. I will be watchful for any further "hiccups" on the freeway... and, I'll be watching the TACH as much as the SPEEDO!
Thanks for the advice!
Leo (going to slow down a little...) in Texas |
|
|
|
Post by oldchopperguy on Jun 9, 2016 20:08:15 GMT -5
Well, duh... I think I may have answered my own question.
When in doubt, check the MANUAL! My coveted, printed in Taiwan, in broken English "Grand Dink" shop-manual explains Minnie Mouse's FUEL PUMP is indeed mechanical, and, VACUUM-operated... Vacuum and crank-case pressure "blow and suck" (bad wording I suppose) diaphragm-operated somehow functioning off both intake vacuum, AND crankcase pulses. And... "extended full-throttle operation can cause less-than-optimum fuel-delivery"... and is "not recommended".
No kidding...  Well, I've never had a problem so long as the old Mouse stays under 9K rpm, so, READ the manual, and "live and learn".Even with the original, worn rollers, the old Mouse will run 70-75 mph at 8K rpm. Good enough for now for sure... I really do want to get some Dr. Pulley SLIDERS installed when I can though. I'm betting they will give me 80 mph at 8-grand, while increasing acceleration, gas-mileage AND top-speed at the same time. I've read good things about those sliders!
Once again, if you are able to get a shop manual, it's prudent to actually READ the thing!  Ride safe! ...and READ that MANUAL...Leo in Texas
PS: I do realize many guyz and galz will ask "Why DO you want to go 80 mph on a 250". Honestly, simply to keep up with the "slow lane" on local freeways. That's the ONLY reason.
 % of my riding is on local 40-45 mph limit 4-lane surface-streets, where traffic runs 55-65 mph and those are a JOY on the old 250...
I don't ride the limited-access often, but when I do, I feel INFINITELY better if traffic is NOT passing me on the shoulder... And THAT is what happens even at 70... The old scoot's handling, braking, suspension and all are easily up to 80 mph speeds, so if it's feasible, I'll go for it... LOL!
|
|
|
|
Post by rockynv on Jun 10, 2016 4:13:35 GMT -5
The problem is not actually the pump most times but that at high rpm the vacuum pulses get too short to cause the pump diaphragm to flutter as substantially as when idling which is really the opposite of what you want to have happen. A few tanks of a good fuel system cleaner might help get it back to where it was before. Fuel deposits on the pump diaphragm from when it dried out while sitting idle can greatly reduce its efficiency. They do like to be kept clean and free of any residue that makes the diaphragm less pliable or the directional valves too stiff to move freely. On some the valves are just tabs cut into the excess parts of the diaphragm and when they get stiff then they may take longer to close so that there is not enough time for that to occur during high speed operation when they are dirty and the pulses are short and weaker. A well sorted 250 should be able to safely cruise the Interstate and its not unreasonable to want to reduce the risk becoming part of a Texas Hood Ornament on some Big Ole Boat attached to the Long Horns so popular in your neck of the woods.  |
|
|
|
Post by SylvreKat on Jun 10, 2016 8:19:50 GMT -5
A well sorted 250 should be able to safely cruise the Interstate and its not unreasonable to want to reduce the risk becoming part of a Texas Hood Ornament on some Big Ole Boat attached to the Long Horns so popular in your neck of the woods.  I thought you were giving Leo an idea for Minnie's next nose-ornament, in case the swan gets swiped....  >'Kat |
|
|
|
Post by rockynv on Jun 10, 2016 11:12:54 GMT -5
This will give you some perspective:
|
|
|
|
Post by oldchopperguy on Jun 10, 2016 15:05:32 GMT -5
Rocky,
You definitely hit it. Have a bone! At 9K rpm the diaphragm can't keep up, AND the vacuum portion loses its function too. Double-whammy! I think the fuel system is pretty clean, but I mostly need to keep the revs under 9K. Over 9K is only possible downhill anyway... Not an everyday thing... I can find no factory-suggested "redline" but I'm sure Kymco never meant the old GV to run faster than 8K. True, any 250 should be able to run the highway (even a 60 mph Chinese one) and I can cruise 70 all day long. THAT should be enough, but often it just simply is not. Rocky and Kat...
I love those longhorns... I worked with a rodeo performer who had a trained longhorn and it was amazing! Those horns spanned over 7 feet across! And that steer could whip his head around an pick off a fly on his butt! THAT is ACCURACY... but you did NOT want to be anywhere near the "arc" of those horns around fly season!!!
It's really hilarious, but even the young bikers seem to love my swan! Surprise, SURPRISE... Several (along with a couple of rat-rod jockeys) have stopped me to ask where they could get one... One REALLY young guy even wants one on his otherwise super-clean, no-frills crotch-rocket! I guess it's the light-up wings that get to 'em... They'd really like the "nekkid-lady" version! THOSE are nearly impossible to find!
Go figger...  At least I haven't run into any smart-guy remarks... AMAZING!Ride safe,Leo
|
|
% of my riding is on local 40-45 mph limit 4-lane surface-streets, where traffic runs 55-65 mph and those are a JOY on the old 250...
