1999 KDX Knock

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May 31, 2001
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#1
My 16 yaer old son went riding thursday and filled the bike with non mixed gas (premix was in wrong can). He put on 15 miles in about 30-40 mins. The bike bogged down and would run but not idle. I drained the gas and added pre-mix and it runs OK but appears to have a knock in the bottom end. Is this an automatic rebuild engine situation or what. I have not torn the top end down yet, but I thought I would and check for big end bearing play. The bike has very low hours overall and we are just sick about the possibilty of needing a complete overhaul.
 
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#2
Sorry to be the bearer of bad news, but you will be rebuilding it. I am surprised it even survived in running condition, I guess that's a testimonial to the reliability of the KDX. The knock is certainly the rod bearing, and you will probably find a lot of scoring on the cylinder and piston. The refusal to idle was the extreme friction, and probably the beginning of a complete seizure. You are probably looking at $300-400 to rebuild the crank, replate the cylinder, replace the crank bearings, and replace the piston and ring, assuming you dissassemble it yourself. Add another $200-300 if you have a shop do it all.
 
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#3
I would definitely pull it apart and check everything before riding it anymore. You probably have toasted the crank bearings and caused some piston to cylinder seizure.:(
 

dirt bike dave

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#4
Just out of curiousity, approx. how much premix was in the tank before he topped it up with plain fuel? Just trying to get an idea what the fuel/oil ratio was.
 
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#5
The tank apparently had 1/3 of a tank when he topped it off. We pulled the top end tonite and found severe scoring at the intake side of the bore. I pulled up on the rod to check big end clearance and could not detect any play, but saw heavy grey oily sludge on top of the cases. Is there a possibility the big end is not damaged? Hate to split the cases, but I hate to take a chance and do more harm than good. Any recomendations on a shop to do the rechrome and piston?
 

Canadian Dave

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#6
If the top end needs to be replated you'll want to talk to Eric Gorr at forward motion racing. U.S. Chrome is who he recommends and uses. If you need to do cylinder work you might consider having Eric do some head and cylinder modifications while you're at it. If seem a shame to spend that much money and not improve the power deliver while you are at it. At least you'll be making some improvements . . . . I know it eases the pain when I need to make repairs.

Before you write off the cylinder head to the local hardware store and pick up some merradic (sp?) acid. It will remove the aluminum smeared on the cylinder walls so you can get a look at the damage, if any. Clean with merratic (sp?) acid, rinse with soap and water, then carb cleaner and finally spray it down with WD-40.

David
 

WoodsRider

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Damn Yankees
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#7
FWIW - I was talking to Fredette once about pre-mix ratios. Does anyone remember the SNAFU at the '99 Portugal ISDE, where straight gasoline was accidentally filled into the tanks of some US riders on two-strokes? Well Jeff rolled into that gas stop with just under a half-tank of pre-mix. The US support (pit) crew started filling his tank. When they had just about topped it off someone discovered the mistake. The pit crew wanted to drain his tank, but Jeff refused and kept riding. If I'm not mistaken this problem happened half-way through day two, with about four days of riding left. Jeff finished on silver that year.

The knocking you heard was probably piston-slap. Follow CDave's advice on cleaning the cylinder. The Nikasil plating can stand up to a fair amount of abuse. Use a glaze-breaker (dingle-berry) hone on the cylinder and install a new piston and rings. Make sure to check the side clearance on the main bearing to the connecting rod.
 
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#8
That is encouraging advise. I have some muriatic acid around and will give that a try. I was assuming the worst and thought the thing would definatly
need rechroming. (At the very least).

Joe
 

fishhead

die you sycophant !
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#9
I wouldn't be using muiratic acid on aluminum it will eat it up faster than you pull out your credit card and the green cloud is an explosive gas called hydrogen. I suggest using a scotch bright pad green or purple for cleaning the bore for inspection and you might send it or take it to a reputable tuner for an opinion. I thought I might have a bad bore and a local tuner who has a good reputation said I could probably get another season or two out of it and as it turns out he was right. a new wiesco piston and rings with a pin and rod bearing and away I went. Saved a lot of money.