Lubrication When Engine Braking?

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jc7622
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Lubrication When Engine Braking?

Post by jc7622 »

I was reading the "Oil Injection" post an he said something about lubrication when engine braking. Not wanting to hijack his post I started a new one.

What do you mean by that? The reason I ask is because my oldest kid recently burned up the crank bearings in his KTM85. I took the top end off and there was no oil on the crank. Usually when I pull a top end off (on any bike) the crank has a fair bit of oil on it. This time it didn't, which puzzled me because we all use the same fuel jug at 36:1 (amsoil).

The day it fried he and his friends were riding at our farm and made a loop that had a 1/4 mile straight. They would be WFO for most of it. It was at the end of the straight when his bike shut down. Could this engine braking/lack of oil thing be partly to blame for the lack of oil found on the crank? Can someone explain it to me?
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Post by kawagumby »

You can fry a two-stroke engine if you run hot and shut down hard or decend a long downhill with the throttle completely closed, as it doesn't get enough oil to the moving parts. The trick is to pull the clutch and blip the throttle at regular intervals - you might want to check and make sure you have an idle setting that affords a decent amount of fuel/air flow also. Check your jetting and plug color. Basically, you never should run a closed throttle for any amount of time with a hot 2-smoke engine. When I used to race hare-scrambles there'd be times when you'd be hard the gas, engine starting to ping from heat, then drop down a long downhill - you learn to keep that jug cool with the throttle or join the other guys standing with their bikes alongside the course.

Although it's been a while since I owned an oil-injection bike (the last one was a yamaha 80 rotary-valved street bike back in 1967 or so, which had some amount of delivery adjustment, I think), the injection system on that bugger was to a large degree modulated by a cable connected to the throttle, so I could see that such a design would only complicate matters if adjustments aren't perfect and you don't blip that baby on the downside anyway, OR if the system malfunctions. Bottom line, you have a full control with premix, jetting and technique - something you don't have with an oil-injection system. It's okay for lubing a chain I guess.... :mrgreen:
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Post by canyncarvr »

The reason an injected system is better in such an instance over pre-mix is that the oil pump is generally modulated by RPM. My R5s/RDs had pumps that were regulated by both throttle position AND RPM. So...with the throttle completely closed there was still some oil being injected via the RPM factor. The number of pump strokes was set by RPM, the length of pump stroke was set by the throttle.

More throttle, more oil. More RPM, more oil.

There are a number of oil outfits that will specify a considerably richer pre-mix (more oil) for a small-bore screaming meemie (KTM85) than a considerably more mellow and RPM reduced woods bike (like a KDX). 36:1 seems on the low side for an 85cc engine.

Re: 'They would be WFO for most of it. It was at the end of the straight when his bike shut down. Could this engine braking/lack of oil thing be partly to blame for the lack of oil found on the crank?


Yup. And arguably more than 'partly'. :cry:


Re: 'Bottom line, you have a full control with premix, jetting and technique.

Yup. :wink:

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Post by jc7622 »

>|<>QBB<
canyncarvr wrote:
There are a number of oil outfits that will specify a considerably richer pre-mix (more oil) for a small-bore screaming meemie (KTM85) than a considerably more mellow and RPM reduced woods bike (like a KDX). 36:1 seems on the low side for an 85cc engine.

Re: 'They would be WFO for most of it. It was at the end of the straight when his bike shut down. Could this engine braking/lack of oil thing be partly to blame for the lack of oil found on the crank?


Yup. And arguably more than 'partly'. :cry:


Re: 'Bottom line, you have a full control with premix, jetting and technique.

Yup. :wink:
Assuming we are using Amsoil, what is a good ratio for 65's and 85's? 32:1? Any negatives to running my KDX at the same ratio? It's so much easier with everyone pouring out of the same jug.

I have control of premix, jetting and technique on my bike. On his bike...two out of the three.
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Post by fuzzy »

As much oil as you can get away with and stay reliable plug-wise is pretty much always my answer to this question. High-rpm road race bike? 12-16:1. That being said 32:1 should be just fine....The 36:1 is probably fine too, but for insurance you can keep it a little rich off the bottom if the riding is going to be fast paced. Just turn the air screw in 'til she's a little burbly. Teach the technique.
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Post by KarlP »

Back in the day I had a Kawy 350 2st with the carb down in the right side engine case, forward of the clutch, aspirating through a rotary valve on the crank shaft. Nifty arrangement I always thought, unless you went through a puddle.
It had a oil injection sytem. My understanding was that oil was injected into the mains based on RPM and into the throat of the carb based on throttle position. A lot of people disconnected the throttle cable link and let it inject only to the main bearings and ran premix at ~50:1 or so.
My bike, as a whole, never ran right. One of my buddies had one and so did my brother. They both worked great, real tourqey and responsive motors.

The above has nothing to do with the original poster, I guess.

"Could this engine braking/lack of oil thing be partly to blame for the lack of oil found on the crank? Can someone explain it to me?"

I would say that the last few seconds of operation of that screaming 85 explain the lack of visible oil on the crank. It does not explain why the bearings let loose, though, IMO. Oil you can see there ain't lubing any bearings, anyway. It seems more likely age related, including the age of the rider.

I bought my wife a KX100. She goofed around on it a bit and I realized I should have bought her a TTR or an XR. Whatever. I added a FWW, tuned it for her riding style, and did a few other things to make it sing in the 3/4 throttle range where she liked it. One of her friends from church came to visit. This friend had a 16 year old boy along and when my wife showed off her bike to him he, of course, asked if he could ride it. Twice around the yard and WFO up the street on a cold motor. Stuck the piston of course.

There is now a sticker on her bike that says "YES, it's fast. NO, you can't ride it." :lol:

Anyway, in good condition and decent tune that motor ought to be able to take anything you can dish out, within reason. A 1/4 mile burn might not be within reason.


Just musing........
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Post by canyncarvr »

You won't likely get an agreed upon answer.

Amsoil (Interceptor or Dominator) says they're good at 50:1. While I'm in no position to argue with that statement, it does seem odd that the same ratio is recommended for, say, an 85cc and a 200cc engine.

All purely conjecture...I have zip.point.nada experience with pre-mix mixes in bikes that size....but while you could be richer pre-mix-wise, I would think 36:1 would work in the 85, but maybe NOT at the razor's edge of jetting. What's going to happen if you richen (air/fuel-wise) things up a bit? It will still run satisfactorily?

Considering the oil quantity difference between 32/36:1, I wouldn't think it would be the difference between a bike that seized and one that was OK.

The thing is water cooled? Any other points that matter to the situation? Maybe coolant was low? Is there a habit of idling the bike too much soas to be overheating the bike to start with?


Probably the best result overall is to teach the technique. Get him used to either pulling the clutch at least some of the time on decels, or blipping the throttle on decel.


Mr. Gumby's first sentence sums it up perfectly: 'You can fry a two-stroke engine if you run hot and shut down hard or decend a long downhill with the throttle completely closed...'

Continuing to do that (run hot and shut down hard)..and trying to PREVENT the end result with a jetting change or pre-mix ratio change may well be counterproductive, frustrating, and replete with repetitive engine destruction.

Something to keep in mind..you ARE running leaner air/fuel-wise at 36:1 than you are at 50:1.

Take some plug readings on the 85. Not a chop or throttle range read, but after a lap or two. See if 'overall' the engine isn't running too lean?


Besides...maybe it was an air leak from a bad crank seal! Now THERE'S a happy thought!!

Good luck!!

**I'm a slow typer..saw only your earlier response when I started...
Last edited by canyncarvr on 07:40 pm Nov 20 2008, edited 2 times in total.

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Post by jc7622 »

Thanks for the good info.
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Post by meterpig »

Someone who has been around far longer than many, told me that in he "old" days, two stroke oil was not a product on the shelves. At that time, he simply mixed regular oil into the gas at 16-20:1.

Today I use fancy, schmancy pink $25/gallon oil at or around the recommended dosage.


From all that I have read, it seems the KDX motor is tougher than most. Today I ran it at 70MPH on the road keeping up with a DR. Crazy I know, but the motor ran great.

If only I could say as much for the rear bearing.

Personally, I think sometimes folks get jumpy around the KDX motor expecting it to blow at any moment. Don't worry, it's not a KTM.
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Post by Julien D »

I was definitely "jumpy" the first time I had my KDX200 running. Sounded as if it was going to fly all to pieces. Enough reading has reassured me that the motor is fine. It quiets down when you rev it, and it pulls strong all through the RPM range. Power wheelies through 3rd are no problem.

I have a 73 kawi f-11 as well. It has oil injection, which I think is a very nice feature. It's comforting to know the amount of oil in the mix is varied dependent on throttle position and engine RPM. Nice to know that sucker is pumping oil directly into the intake and crank bearings at idle too.

I've always wondered why manufacturers moved away from oil injection. It seems more precise to me, and just makes sense to apply the necessary amount of oil for different RPM ranges and riding styles rather than just a constant mix. I can putter around on the F-11 all day long without fouling a plug, or I can beat it up WOF for hours with no problems either. I'm in the process of freshening the old gal up right now. New crank bearings and top end. It's a very fun old bike, I doubt I'll ever get rid of it. The KDX is now queen of the garage though!


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Post by meterpig »

They probably dropped it due to cost, service and support. Oil mixing is far simpler. Not to mention, for the US market, it is unusual for people to convert their 2T dirt bike to street. Thus, while it would be nice, it is more or less not wanted by the market.

Heck, as of now, 2T's are disappearing from the lineups. So in the end, nobody really wants it.

Now on snow machines with the long distances they travel, it makes sense to go oil injection.
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Post by Kurt Franz »

>|<>QBB<
meterpig wrote:They probably dropped it due to cost, service and support. Oil mixing is far simpler. Not to mention, for the US market, it is unusual for people to convert their 2T dirt bike to street. Thus, while it would be nice, it is more or less not wanted by the market.

Heck, as of now, 2T's are disappearing from the lineups. So in the end, nobody really wants it.

Now on snow machines with the long distances they travel, it makes sense to go oil injection.
Been trying to find a complete engine off of a KDX200SR. This is the oil injected model of our KDX that is not available in the states. Too bad about 2 strokes here in the US - the lower maintenance cost and lighter weight are good and the emissions could also be controlled.
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