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Blew up kx500 on highway need tuning help

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Blew up kx500 on highway need tuning help

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Old 08-31-2018, 03:47 AM
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Default Blew up kx500 on highway need tuning help

Hey guys I just bought and blew up a kx500 super Moto. It currently has an FMF Gnarly woods pipe with a pro circuit 304 factory sound, vforce reeds and what was a pro lite piston. Unsure of gearing and can check. After further inspection I ended up overheating the exhaust port and destroyed the exhaust side of the piston. Total engine salad of the bore and piston. I have a 20 mile one way all highway to work ride and would like to be able to run it to work and not have to worry Iím gonna toss a piston. Was doing about 80 when it lost all compression. I want to be able to run this bike at short burst once or twice a week at 100mph and cruise at 65 to 75. I know the current fmf is for down low and not a top end pipe. I want to switch to pro Circuit and need a mid to high end pipe. 30 seconds after all heíll broke loose the engine read at 152 degrees on the radiator and 155 on the hour meter style temp gauge. Bike was filled at the gas station about 20 mins before the top end let out and was running on 93 gulf with a basic mix of 32:1 but had a bit more oil added for the highway use. Bike was smoking like a big bore should and was not caused from lack of lube as I mixed in more than needed. So gearing recommendation and pipe would be nice. Engine is a 2000 kx5 with an 06 kx250f frame
 
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Old 08-31-2018, 12:18 PM
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I'd recommend you go to the Supermoto Junkies forum for the most help.

I will tell you trying to use any MX bike for commuting on roads is not the best thing to do. They aren't going to be very dependable. I'd see if you can de-tune the 500 if you want to make it live longer.

From the water temps you give, that is way cold. Most LC engines are running thermostats to try to keep them at about 180-195 degrees, same with cars. If your piston damage was from seizing up it sounds more like you may have had a cold seizure with temps like that. But I could be wrong.

Do some research on engine temperatures for a two stroke, like the RZ350LC to find out what Yamaha had them running for temps. If they had a thermostat in the engine that would give the hint. If you need to add a thermostat you can do one in-line using a product called the Thermo Bob. The product is used by a lot of KLX and KLR riders to get even cooling through out their cylinders at lower temperatures. I have one on my KLX250. It runs a 180 degree thermostat. You could also do some research on the old Suzuki GT750 water buffalo two stroke triple to see what they ran. IC engines need a certain amount of heat to keep things right. Two stroke or four stroke, car, truck, or motorcycle, they need to maintain a certain amount of heat in the cylinder to deal with the kind of heat the piston is going to deal with. If the thermal expansion of the piston is too great for the cylinder thermal expansion you're looking at sticking a piston or damaging it.

The other possibility is if you holed a piston. If that's the case you're dealing with excessively lean jetting at your cruising speed. You need to look at the plug if it's still able to be checked to see if it ran lean. A product that might help after you get the jetting close, would be a Dial-A-Jet fuel adder. It allows fuel mix bleed in when the engine draws a vacuum running lean.

Regardless, two stroke motocrossers aren't going to do the job you want for a bike to commute to work. It would be a riot to play, but something like a street legal from the factory KTM dual sport/supermoto set up would do the job you ask far better than any two stroke, plus the mpg most likely would be far better.
 
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Old 08-31-2018, 02:38 PM
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Thanks for the tips, I have pictures of the damage but canít seem to get them to load. Iím not looking for an everyday commuter just a Friday / weekend play toy. I have the car for most days. It also has a 14 oz fly wheel weight I believe.
 
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Old 08-31-2018, 02:44 PM
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Also the bike did not seize I let off the gas and it lost power I pulled the clutch in and it heat seized after I let it back out and left a skid about 20 ft long, I let the bike cool down and it unlocked but then felt like a pw80 trying to kick over
 
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Old 08-31-2018, 06:15 PM
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As I said though, that engine temp is low. I had missed putting in a piece for my oil pump on my old TM125, so it was getting insufficient oil through the pump. The bike ran, but after a bit at higher throttle it would drag down on power, which was the piston starting to stick due to lack of lubricant. I could start it and run it, but that would happen. Like throwing out an anchor. Same kind of thing could happen if the engine cold seizes. Doesn't have to actually stick in the cylinder, just the heat and friction could cause the piston to squeeze the rings in their grooves, then when you try to start there is no compression because the rings are stuck.

Of course I'm not there so I can't see anything, I'm just putting the thought out there for what could be the issue. It might just be running too cool for an extended run with the piston getting heat in the crown from the slightly leaner partial throttle cruising, while the cylinder is getting too much cooling if there is no thermostat, which I find there isn't.


Here is a short thread on the heat situation. Remember odds are some of these guys saying don't worry are likely off roaders who don't ride the bike like a street bike is ridden. Personally after seeing that I'd take a serious look at the Thermo Bob. Inconsistent cylinder temperatures are a serious problem. Also most off road riding is around 25-35 mph, it may seem faster on some of those trails, but even Pro Motocross speeds seldom exceed 45 mph. So your 80 mph is jamming a whole lot of air through that radiator set up and likely overcooling. Another road race trick is to put duct tape over some of the radiator core to cut cooling to the point where it is at the preferred temp range. They do it because it is easy, bullet proof, and easy to undo. Want a bit cooler, pull a strip off, a bit hotter, put a strip on. That is one way you could deal with it at first.

Edit: Apparently with a bit more looking, it seems KTM is using a thermostat on the 300EXC. There have been mentions of thermostats in several Google links, including for the CR
 

Last edited by klx678; 08-31-2018 at 06:23 PM.
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Old 09-03-2018, 04:10 PM
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Now to real question I have is would could changing gearing keep my rpms lower? The pipe thatís on it itís a low to mid range pipe and I when I blew it up it was on the top end for a few miles. I ended up finding a way to work thatís about 15% highway and the rest city.
 
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Old 09-03-2018, 04:11 PM
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I also have a fan on the bike that kicks on around 175ish deg
 
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Old 09-03-2018, 06:49 PM
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I did some quick research on temperature. The average temp for fans on four stokes is around 210 degrees. I'm reading anywhere from 140-200 degrees on the two strokes.

If it was me, I'd see if I could get information on specifically coolant temperature, not head or cylinder, from Kawasaki. It can be gotten, try their USA site and/or a friendly dealer. Get it "from the horse's mouth" if you know what I mean. I just don't trust the general internet with something like this. Who would know better than Kawasaki.

Next best would be to try to get a hold of someone like Destry Abbott. He raced the KX in the desert and certainly would have a handle on what should be going on. Do a search on his name. He has a facebook page and twitter account, so you should be able to contact him somehow.

I just read a bit about him. He found he had Leukemia back in 2016 but is in remission if I read right so be aware of this in your approach. Most riders want to help each other, so my bet is he would be receptive and if he had no answer he might know who would.
 
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Old 09-03-2018, 07:16 PM
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Originally Posted by Kx500af View Post
Now to real question I have is would could changing gearing keep my rpms lower? The pipe thatís on it itís a low to mid range pipe and I when I blew it up it was on the top end for a few miles. I ended up finding a way to work thatís about 15% highway and the rest city.

Yes, gearing can change your rpm.
 
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