second service/ new rear/ engine bogging. - KTM Duke 390 Forum
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post #1 of 26 Old 08-05-2016, 07:37 PM Thread Starter
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second service/ new rear/ engine bogging.

i got back from a motorcycle trip that was approx 600 miles and took my bike into the shop the next day. i needed to get my second service, but i also needed a new tire. between the the nasty roads, and long stretches of highway, i was riding over in the texas hill country, i had worn my tire down to the cords. i also noticed that my top end was beginning to suffer on my way back home. it was becoming difficult to reach speeds above 85mph.
my engine temp didnt seem to be any higher than usual, and my fan was working, but it still scared me enough to put it back in the shop. i dont know if this is because of the stop leak stuff i put in my radiator, which did stop the leak, at least... i told the shop guys to go ahead and flush it out, replacing the coolant with engine ice. i cant remember what tire i wound up getting, but the guy said it should be wider, which i am hoping will give me just a couple of extra inches, so that i'm not scraping my pegs.

also, for any other texas d390 riders, pedernales falls st park has a really fun entrance road.


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post #2 of 26 Old 08-05-2016, 07:48 PM Thread Starter
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my favorite pic from the ride. my d390 and my buddy's rc390 at our campsite.
we also had a friend in a pack mule kia soul.
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post #3 of 26 Old 08-06-2016, 12:53 AM
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NEVER use stop leak! It will stop up your leak all right as well as any other passages and can lead to a fatal failure of your motor.Ask me how I know, $3700 wiser. Hey, I was a kid what did I know. Glad to hear your bike is still saveable.
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post #4 of 26 Old 08-06-2016, 01:16 PM Thread Starter
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i dont typically resort to such rash actions, but in this case i wanted to do what i could on a very limited time and money budget so that i could go on this long planned trip. i got to get out, and knew i'd be getting the bike into the shop, at least for the 2nd service, so the tire and rad flush hopefully wont add up to too much... at least i get a little discount there for being on a local forum they support... :/
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post #5 of 26 Old 08-07-2016, 11:04 AM
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Matt, I thought about the "engine bogging"/perceived loss of top end speed that concerns you. A couple of thoughts:

I saw in the photo that you have a generously-sized tank bag on your bike. I surmise that on a camping trip you might also have had some additional luggage hanging off your Duke.

You say that you could not get the bike to run beyond 85 mph (as opposed to its normal top end of just over the ton). My thought is that you were retarded by aerodynamic drag rather than loss of engine performance.

To greatly simplify a complicated formula that accounts for several variables, let us just observe that aerodynamic drag increases as the SQUARE of velocity. For example, if you are riding at 60 mph, your aero drag would be 3,600 units above what it would be at zero speed. But if you increase speed to 85 mph, your aero drag soars to 7,225 units, about TWICE as much as at 60mph. Any extra obstacles to aero flow at this speed (ie, rider's body, external luggage) will create correspondingly large increases in drag.

Riders who like to do the ton on a Duke typically ride solo, with no external loads or attachments and they lie down on the tank to minimize drag. Clearly, with a large tank bag it would be impossible for you to assume a racing crouch. In addition, your external luggage would be creating heightened drag, magnified by riding at high speed. My supposition is that the combination of an upright/semi-upright riding position, along with additional drag from luggage, would be sufficient to create a level of aero drag that would impede you from reaching theoretical top speed. This is particularly true if you were riding into any degree of headwind, which would aggravate the bike's coefficient of drag.

I believe your engine is probably OK and normal. When you get your bike back from the shop, if you are worried I suggest you find an isolated road, then try a top speed run, with no luggage, no headwind and using a racing crouch, flat on the tank. I think you should be able to do the ton or thereabouts. Top speed is subject to variables such as rider size, weight, position and clothing, engine tune, optional equipment, fuel type, ambient temperature, wind direction, tire inflation, road surface, sprocket sizes, chain lubrication, tension and alignment, etc.

Cheers, Will

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post #6 of 26 Old 08-07-2016, 05:28 PM Thread Starter
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thanks, diplo, but i dont think that's the case here.
the only luggage i had on the bike was the tank bag. my friend in the kia was the designated pack mule.
even if i had my small amount of baggage on the bike, i dont think it would be enough to slow me down that much. everything i brought fit into my back pack and tank bag.
i was fairly spartan, while my compadres were packing dig folding chairs, full size pillows, several outfits, that big tent, and a bunch of other crap we didnt need.


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post #7 of 26 Old 08-08-2016, 07:40 AM
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Interesting that the shop would suggest a wider tire, as the 150 that is on there is kind of too wide to begin with, at least it terms of allowing the rider to access the edges of the tires when leaning. Most of the RC Cup riders have gone to a narrower 140 tire.

Good luck, hopefully the speed issue is just a tight valve and the shop will catch it with the valve adjustment. I agree the luggage was not the problem, as I had a large tank bag and tail bag when I rode from Texas to Florida and I was still able to crest 100 with ease.

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post #8 of 26 Old 08-08-2016, 08:00 AM
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I agree with BSH: If your shop recommended a wider rear tire than the already-too-wide OEM 150, it sounds as if they don't have the slightest clue about tire dynamics. Perhaps they have a 160 in stock that they want to sell.

Cheers, Will

"If you don't know where you're going, you might wind up somewhere else."
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post #9 of 26 Old 08-09-2016, 04:25 PM Thread Starter
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i got the bike back this morning and the **** thing tops out around 91mph and it's working hard to get there.
i cant call the shop yet, because my phone is in the middle of a lengthy restore. my phone got crippled somehow on last week's trip. battery life went to a few hours and it ran super hot. yeah, my luck... so while my phone is being activated, i'm hoping yall can provide me with some good material to talk to the shop when i call them.

they didnt even touch the radiator. if i'm off work tomorrow, like i am supposed to be, i intend to do another radiator flush on the bike. i want to put some clean engine ice in there, and get rid of the remaining stop leak crud.
i dont think that the stop leak is why my engine is running poorly. the bike doesnt appear to be running any hotter than it normally ran before all this trouble.


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post #10 of 26 Old 08-09-2016, 05:01 PM
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Running your rear to the cords means it has a greatly reduced circumference, since speed is taken from your front wheel this could very well be the "problem". Other than that I'd be looking at chain tension (know how to set it that it isn't too tight) or possibly a faulty thermostat.
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