DECEL BACKFIRE

[ALEX2158]

I might have missed the thread but I have a 2017 DRZ400 that has a FCR40MM, MRD exhaust, 3x3 airbox, heads ported, cams, big bore, and stroker crank. My issue is on hard decel it backfires like crazy where can I start to solve this issue. Should I follow FCR39 jetting or FCR41 any help would be great. 

[xxdabroxx]

bigger on the pilot jet.  Sounds lean.

[William1]

Nolt a jetting issue. You have an open pipe that does not muffle and a carb with  no decel valve system, so on throttle chop, it will pop/backfire/. Use the brakes harder.

If you try top 'jet it away', the bike will run terrible.

[ALEX2158]

How much bigger should I go?

[ohiodrz400sm]

Jetting should be slightly richer than the commonly recommended 39mm settings. The larger engine pulls more fuel and requires leaner jetting than a stock displacement engine though.
Some popping on decel is normal with the MRD, or any open exhaust. As William said, if you jet it away you are WAY TOO RICH.

[ALEX2158]

Well I dropped my main jet down to a 160 is that to small? 

[William1]

1 hour ago, ALEX2158 said:

Well I dropped my main jet down to a 160 is that to small? 

Not at all. My DRZ SM has a 155.

Your pilot should be a #45

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.
Gently turn the fuel screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle knob to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly).  Once warmed, slow the idle speed (knob) to the lowest possible speed.
*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***
Turn the fuel screw in until the idle becomes rough or the bike stalls.
if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.
If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.
Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.
If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.
If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.
If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,850 rpm, but go by the spec in your manual.

[dg400sm]

is it a decel back fire or popping,they are not the same,a backfire is a rich condition,sounds like a gunshot

[ALEX2158]

I guess more of a popping issue

 

[Flash1034]

Newbie question...does somebody have a picture of the stock Mikuni which shows the locations of the various adjustment screws?

 

Thanks

[ohiodrz400sm]

Newbie question...does somebody have a picture of the stock Mikuni which shows the locations of the various adjustment screws?
 
Thanks

The idle is the white knob.
The fuel screw is circled. It is blocked by a brass plug from the factory.

[Flash1034]

Thank you, sir!   That fuel screw looks hard to get to without removing the carb.  Is there a special tool?

[ohiodrz400sm]

You'll have to remove the carb to pull the brass plug out.

https://thumpertalk.com/shop/ThumperTalk-DRZ400S-SM-Extended-Fuel-Mixture-Screw-p2006766350.html

After that, install this screw to make adjustment simple.

[Flash1034]

You are a saint.  Thanks for taking the time to help me out!  

[ohiodrz400sm]

Np

[EvanNaculum]

On 3/30/2018 at 6:57 PM, William1 said:

Not at all. My DRZ SM has a 155.

Your pilot should be a #45

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.
Gently turn the fuel screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle knob to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly).  Once warmed, slow the idle speed (knob) to the lowest possible speed.
*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***
Turn the fuel screw in until the idle becomes rough or the bike stalls.
if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.
If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.
Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.
If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.
If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.
If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,850 rpm, but go by the spec in your manual.

Idiot question, but when using the nomenclature “turns” does that mean a full 360 degree rotation? Where a quarter turn is a 90 degree rotation? Also, turn the screw “out” is counter clockwise, and “in” is clockwise? Lastly, turning the screw “out” richens or leans with pilot circuit? Oh and my r&d flex jet is confusing because on the sticker it has an “L” with an arrow and a “R” with an arrow, but turning the screw in clockwise is in the direction of the L arrow...does the L stand for lean and the R stand for rich as opposed to left/right?

 

[William1]

2 hours ago, EvanNaculum said:

Idiot question, but when using the nomenclature “turns” does that mean a full 360 degree rotation? Where a quarter turn is a 90 degree rotation? Also, turn the screw “out” is counter clockwise, and “in” is clockwise? Lastly, turning the screw “out” richens or leans with pilot circuit? Oh and my r&d flex jet is confusing because on the sticker it has an “L” with an arrow and a “R” with an arrow, but turning the screw in clockwise is in the direction of the L arrow...does the L stand for lean and the R stand for rich as opposed to left/right?

 

Righty-tighty, Lefty-loosey.. 360 is a full turn. The R is more open (R-Richer) The L is more closed (L - leaner)

To determine where it is, GENTLY tighten (turn right) until it stops, seeing exactly how much it takes. Typical setting is around 1.75 turns (or 630 degrees).