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vectradam

detonation above half throttle

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my 2018 280 rr detonates above approx 1/2 throttle. needle clip is on mid position but i'm wondering whether this is more main jet, i have a 125 main jet at present. does anyone have any suggestions as to which main jet i may be best to go to or any other ideas 

 

many thanks

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The main jet really has no effect until more than 3/4 throttle.

Enriching the clip position may help somewhat, but the needle taper itself has more influence at 1/2+ throttle.

Cleaning carbon from the head and piston would help if there is an accumulation.  

Have you tried a higher octane fuel?

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 This could go either way with jetting. I assume you are in the Uk since you had a bike stolen. Summertime jetting can be different on a two stroke. Have you asked Steve?

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6 hours ago, konrad said:

The main jet really has no effect until more than 3/4 throttle.

Konrad your statement above is incorrect.

On most all carbs the fuel has to flow past / through the orifices in the main jet to get to the needle jet.

therefore the size of the main jet will ultimately control of much fuel gets up to the needle jet regardless of what you do with the clip.

The taper on the needle just controls when it gets the fuel not how much fuel as that is untimely determined by the size of the main jet.

The position of the clip on the needle can't make any more fuel flow through the hole in the main jet... period.....  

When you turn the throttle it lifts the slide lifting the needle and allowing gas to flow through the main jet and up through the needle jet.

At 1/4 to half throttle the pilot jet, main jet and needle jet are all contributing to the amount of gas

It is only when the throttle is wide open that only the pilot jet and main jet have the main contribution as the needle will be as high as it will go 

making the needle jet at a fixed orifice at that point.

The pilot jet and main jet is always contributing it is only the needle jet that s the variable in the equation.

 

 

 

 

 

 

Edited by billyt

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10 hours ago, billyt said:

Konrad your statement above is incorrect.

Tell that to the carburetor manufacturers. Here's just one example: http://www.sudco.com/jetting_assistance.html All are similar. 

I do agree with your statement in that if you install an extremely small main jet, the flow will be limited at part throttle.  But for any reasonable size main jet for the application, that does not happen. 

Edited by konrad

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 Here is one of the best graphs for understanding jetting. Everyone always forgets the pilot circuit is on 100% of the time. Across the pond we tend to jet the pilot jet richer, seems in the UK they go right to replacing the slide. Must be the difference in fuel.  

 

effects3.gif

Edited by lineaway
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25 minutes ago, lineaway said:

 Here is one of the best graphs for understanding jetting.

What is the original source of that chart?

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Konrad.   I worked for Mikuni (still do) as a carb tech on trials bikes so no need for me to tell them anything.

 

 

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27 minutes ago, billyt said:

Konrad.   I worked for Mikuni (still do) as a carb tech on trials bikes so no need for me to tell them anything.

Then I will defer to your experience in the matter, sir.  Trials bikes are weird.

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2 hours ago, billyt said:

Konrad.   I worked for Mikuni (still do) as a carb tech on trials bikes so no need for me to tell them anything.

 

 

So what would you suggest he does billy to eliminate the lean area he describes?

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Vectradam You Wrote:

my 2018 280 rr detonates above approx 1/2 throttle. needle clip is on mid position but i'm wondering whether this is more main jet, i have a 125 main jet at present. does anyone have any suggestions as to which main jet i may be best to go to or any other ideas 

 

We need some more information you:

Is this a stock bike?

Carb is stock?

Jetting stock?

What altitude are you at?

What is the ambiant temperature you are running at?

What is your gas/oil ratio mix?

What do you mean it detonates above 1/2 throttle?

Is that you holding it wide open for a period of time or just revving it up?

Can you hold the throttle open and steady at 1/8 throttle (pilot jet and main jet) with the revs just holding steady and not running away?

Can you hold the throttle open and steady at 1/4 throttle (pilot jet & low needle jet & main jet) with the revs just holding steady and not running away?

Can you hold the throttle open and steady at 1/2 throttle (pilot jet, mid needle & main jet)  with the revs just holding steady and not running away?

Can you hold the throttle open and steady at 3/4 throttle (pilot jet & High needle jet & main jet) with the revs just holding steady and not running away?

Does the bike ping/detonate when you let go of the throttle from say 1/2 throttle as the revs drop?

When it makes this detonating sound have you tried pulling the choke on to see if the sound goes away?

Edited by billyt
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I've been studying the effect the main jet has on part-throttle fueling and learning a lot. 

Have found an equation for two flow restrictions in series (in this case the main jet and the needle system) and have written a small spreadsheet based on the Dellorto PHBL 26.

Is anyone interested in the results?  I ask because presenting/explaining them would be a fair amount of effort.  I'm also uncertain what the best way to actually present them would be (I know not everyone is as enamored with numbers as I am). 

Would anyone be interested in working with the spreadsheet?  It's in OpenOffice Calc, but I could convert to MS Excel.

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I would be interested in hearing from the carb technician billy as to what is the best way to eliminate that lean spot in the mid range you allude to in your original post.  I know this very same scenerio afflicts bikes with keihin pwk carbs gas gas and TRS to name two.

 

Edited by the dabster

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