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DRZ 400s backfiring

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Hello all this is my first post on here if i have put it in the wrong place please move.

I live in the uk i have just got a drz s with a full scorpion exhaust on with down pipe having some problems with it backfiring popping realy bad when you let the revs of it then when you open it up for a split second seems like its starved of petrol then it will take of again you can feel it missing when going threw gears.

I had the carb of last night give it a good clean and checked  jets give them a good blast out put it all back on today  still no better today.

Mixture is set at. 2.3/4 turns starts on the button with choke on anything less than this cuts out soon as i rev it or put choke back in.

Been told that you should have a jd jetting kit with this exhaust scorpion say different that you dont so i realy dont no what to do now thanks.

Forgot to mention it has a clarkson over sized tank on so there is no vacuum on it now just on off petrol tap thanks.

 

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You don't require the JD kit but it is the most simple way of getting all the bits.  Barely running at idle and popping on decel are both indicating too lean on idle so unless the jet is plugged it is too small.

 

The mixture screw is to fine tune the setting of the idle jet only.  Since the jet is so small rather than making 22.5, 23, 23.5, sizes they are provided in increments of 2.5 and the mixture screw provides the fine tuning between those sizes.  The main jet doesn't have a screw because it doesn't need to be as precise at the larger air and fuel flows.

 

The idle jet flows at all throttle settings but as the revs go up the influence becomes less.  When you close the throttle it is back to the idle jet.

 

You should purchase the next size larger idle jet and try again.  You could get the kit which will give a few sizes of jets, screws, and also the needle.  The needle is an improvement as well as being adjustable and is only available in the kits.  If you would have to wait for the kit to be ordered and don't wish to delay then for a few pounds you can obtain the larger idle jet now and have a spare if you get the kit.

 

Note:  The mixture screw adds to the flow through the jet in a separate circuit.  Closing the screw will not restrict what flows through the jet but opening it will add additonal fuel up to the limited amount the circuit can provide.  The intent is to provide fuel up to what the next sized idle jet does.

Edited by slowriding

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Increasing the exhaust gas flow means you need to balance the equation by increasing the intake flow....

 

I would be installing a jet kit..and doing the . DRZ400 3X3 mod

 

When your riding , your mostly on the needle and the stock needle just don't cut it with an open pipe...

 

I suggest the JD Jetting Jet Kit  and fitting an Extended Fuel Mixture Screw to make the idle circuit fine tuning much easier....

Edited by Craigo 485sm
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Thanks for your replies i will i try with some larger jetts first if that dont work i will go with the JD jetting kit. It already has an extended fuel mixture screw on the carb is there any where in the UK that you can buy the JD Jet kit from thank you again. 

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You said "jets" but I hope you meant "jet" as in just the idle jet, unless you also have problems at full throttle.

 

The idle jet and fuel screw have effect from idle to about 1/4 throttle.  Problems in this throttle range (not rpm) change these.

 

The needle affects from about 1/4 to 3/4 throttle.

 

The main jet affects 3/4 to WOT (wide open throttle)

 

There is some overlap but avoid a common mistake to make changes to something that was set just fine.

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Another common mistake is to correct one fuel circuit without correcting the overall set up.....

Increasing the exhaust gas flow effects all fuel circuits and can not be corrected by sorting out one....

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As Craig said you should go through the procedure to set up the jetting in all ranges, including the JD kit because only the included needle will clean up the 1/4 to 3/4 throttle range.

 

Be sure to follow the procedure though.  Don't just increase the size of everything because that seems right.  If you get the kit it will have suggested sizes and needle position.  You can do the door on/door off tests to get an idea about the mid and top end before you dismantle to change the idle jet but ultimately each range needs to be addressed separately.  Once you get your idle mixture good leave that alone.

 

You said you have just purchased this bike and it already had the pipe on it, correct?  If it was set up properly before it may still just have some junk in the idle jet that needs cleaning or there is a vacuum leak.  How is the vacuum port that previously was for the petcock capped?  Confirm the size numbers on the existing jets when you take it apart again.   It may already have been jetted and perhaps even has the kit needle in it.  Is the 3x3 mod done to the airbox yet?  What kind of air filter?  If it is foam, is it oiled with proper foam filter oil?  If it is K&N style cotton, is it oiled with the proper oil?  All of these should be determined before spending the time to jet it.  Different air flows will change the pressure differential between the carb throat and atmospheric and that affects fuel flow.

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meant jet as for the air box that is standard not beet cut out all jets are standard as is needle..

Just sent for new air filter oil filter and 2 ltrs oil all this will be done this week will be ordering jd kit next week thanks again.

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A pre-oiled air filter?  Unless the air filter comes pre-oiled you need to oil it.  Use ONLY the appropriate oil for the filter.  Not engine oil.

 

Do not remove the flame arrester screen between the air filter and the carb.  It does not restrict air flow and it could save your bike from catching fire.

 

You will want to do the 3x3 before jetting as it changes it.

 

The larger jets with the 3x3 are not because of more air flow (possibly at higher rpms) but changed air flow.  The change in airbox pressure means there is less difference between ambient pressure outside the carb, also inside the float bowl, and inside the throat of the carb.  Less pressure differential needs a larger jet to flow the same amount of fuel.

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