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Off Throttle Meltdown?

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I've been reading up on tuning 2 strokes and have seen this mentioned a number of times. While riding the other day I noticed that I do a lot of off throttle coasting down hills. Where I ride is mostly very mountainous terrain so if I'm not going up, I'm going downhill.

Assuming the bike is set to stock carb settings, what is the better way to coast the bike? Would pulling the clutch level eliminate the decel meltdown?

I try to give the trottle a blip or a little gas if possible, especially when riding forrest service roads at pretty steady speeds. I don't think I am at risk of meltdown but I'd like to keep risk at a minimum.

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I always hear people talking about it on this forum, but have never actually seen or even heard of it actually happening to anyone. In my opinion it is not something that you really need to worry about. If you are using the correct oil ratio in your gas then you will have a reservoir of oil built up in your bottom end that will be more than enough to lubricate the engine under engine braking.

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Are you talking about coasting with the clutch out & no throtte, because yes that would cause the bike to run lean but I don't think it'd make the bike melt

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I always hear people talking about it on this forum, but have never actually seen or even heard of it actually happening to anyone. In my opinion it is not something that you really need to worry about. If you are using the correct oil ratio in your gas then you will have a reservoir of oil built up in your bottom end that will be more than enough to lubricate the engine under engine braking.

 

 

+1

 

You always here this story, but I've yet to see it transpire. And like you here in BC I coast down some long hills, can be minutes long.

As KJ said there is a build up of oil in your crank. I've read in a study before that it takes 1/2 an hour for the oil to do it's complete passage through the crank and out the exhaust.

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If you mean pulling in the clutch on a downhill and coasting the engine, there is no difference between this and idling the motor in your driveway.  The water pump pushes water through the radiator proportionately to the RPMs, so if you are coasting downhill at 8 MPH vs sitting still in your driveway, your bike will be better off coasting downhill at 8 MPH to get airflow through the radiators.

 

If you DON'T pull in the clutch while descending down a mountain, and the engine is going naanananaananana and almost stuttering as it slows down, then you are running a little lean.  Richen your needle 1 or 2 clips and you'll be fine.  

 

What kind of bike do you ride??

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Not a problem when jetted correctly, and most modern 2Ts should be OK. 

I had a plated 70s vintage air cooled Bultaco 370 Frontera that would run down the hiway with cars.

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It happens on things like shifter carts and less frequently some desert raced bikes.  But after running the engine at WOT for an extended period of time and then maintaining high rpms while coasting can cause a piston seizure while coasting.  Again this is after running wide open for half a mile then shutting the throttle but not slowing down while still in high gear with the engine still holding high rpm.  It will never be a problem coasting down the side of a mountain, in fact after coasting for long periods the engine will be so loaded with fuel and lube that you may foul the plug when you roll on the throttle again at the bottom of the hill.

Edited by 1987CR250R

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For reference I have a 95 wr250, I am getting the jetting dialed in currently so I just want to take everything into account. I've read about it online and in tuning books, but nothing regarding dirt bikes specifically.

I know that with efi that no gas gets injected on decel (throttle closed, rpm raised above idle), is this also the case with carbs?

I had my pilot air screw fall out and on decel it would do what the poster above described. But I also had my needle set too rich and had the air screw backed out way too far to compensate. Once that was sorted it no longer sounded lean on decel.

Thanks for all the help so far! I ride in huge elevatikn swings so I am working on logging all my changes and getting some jets to have a good tuning range. There is a local winter harescramble series i am hoping to get into and I am just working out all the bugs before i enter my first event

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In the Mojave Desert we would tear across Johnson Valley pinned for sometimes 4 or 5 minutes at a time on CR500s.  My uncle is an expert desert racer so he showed me the ropes.  After riding one day, we took out the spark plug on my bike and the tip was white.  So we richened it up on the main jet because of all the fast riding we were doing that day.

 

One thing he said will always stick with me: "If you ever chop the throttle at high speed when your bike is running a little too lean, you can easily seize the motor because the cutoff of fuel flow will heat up the combustion chamber instantly."  Recently on my KTM 300 I rejetted it for cooler weather, but I didn't richen it enough.  When I cut the throttle, the motor would "hang up" rather than smoothly idle down.  This is the result of running too lean.  So I was careful when riding WFO because rolling off the throttle too fast could damage the motor (hit the killswitch before cutting the throttle, that pours cool fuel into the motor).

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Downhill, in gear,closed throttle on a 2 stroke is an accident waiting to happen, especially if your a gear too low to slow your desent. There was a member on here not too long ago who suffered this fate............seized at the bottom of the hill. Clutch it occasionally and give it a rev.

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Downhill, in gear,closed throttle on a 2 stroke is an accident waiting to happen, especially if your a gear too low to slow your desent. There was a member on here not too long ago who suffered this fate............seized at the bottom of the hill. Clutch it occasionally and give it a rev.

That is exactly what I do. If I don't blip the throttle occasionally on the way down it gets loaded up with fuel and will break up a bit at the bottom of the hill when I get back on the gas.

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