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07 DRZ400sm FCR MX - Fuel screw all the way in still idles fine? Studder at 1/8?

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Hey guys,

Just looking for some information. I purchased this bike with the mods already on it. FCR, Yosh pipe, K&N Filter. The basic problem I am trying to solve is a stumble at about 1/8 throttle. I have tried the fuel screw out 2.5 turns and in this position the bike runs decent (barely noticeable stumble) when the outside temps are around 8-12 degree celcius. When warmer (17-25 degrees C) at the 2.5 turns out setting the bike seems to be running quite rich, stumble and not pulling hard.

Turn the fuel screw in about 1 turn and it gets better (1.5 turns out, but similar the "miss" or "studder" is always present when at a constant throttle position for a period of time.)

I also tried turning the screw completely in yesterday and the bike idled good the whole way in. Turning the screw out the bike only stalled around 3 turns or so.

I am a newbie to the carb tuning scene as my previous rides have been fairly stock.

Thanks in advance.

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Pilot is for idle only. (99% anyhow) If you have dropped the idle speed from normal until it just runs and you can turn the fuel screw in without the motor stalling, then you need a smaller pilot. Check the condition of the screw ('O' ring intact), search for proper procedure for setting the idle circuit (try a post from William1) and go from there.

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As always, DG is right on the money. Pilot/fuel screw is for idle only, set it and leave it alone. From just off of idle to just under about 1/4 throttle is the straight diameter of the needle. What needle is in the carb and what clip position are you are?

Fuel Screw/Pilot Jet

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 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 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 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 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 1,800 rpm, but go by the spec in your manual.

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Thanks for all the input. To be honest I am not sure of the jetting. Haven't opened anything up. I did however try something a littl different yesterday. From what I could tell the previous owner installed a K&N filter a couple of weeks before he put the bike up for sale. He did however include the foam filter with the sale. Yesterday out of curiousity I installed the foam filter and went for a ride. All I can say is WOW! The bike pulled harder then it has ever before. I was not able to confirm if the miss or stumble was still present because I couldn't get over how much more power I had.

This would mean the bike was running lean with the K&N correct? Anyways I am going to test this for a week or so and then see what I can do from there.

Hypothetically speaking if the foam filter is a stocker what would be the "typical" jetting/needle change when going to a K&N?

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Very possibily it is running a little lean.

Until you know what needle, pilot, main and if the slow air jet is installed and what size, guessing is pointless.

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Alright. I guess guessing is no good. Will have to wait until I have some time and tear it down. Similar conclusion to my last post. Thanks for all the input. Cheers!

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Alright found a moment today to check the needle. I know I still have to check the jets, etc. The bike is running the EMN needle set at 3 rings from the top.

I also noticed today that Kitchener, Ontario, Canada is listed as being 1070 ft above sea level. Does this mean that the EMP needle is more appropriate?

Jetting values to come, just need my daughter to have a few more naps while I am at home.

Any additional information would be helpful.

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Hmmmm EMN should be fine. Check to see if it has a slow air jet installed (in the carb bell, left side) if so, what size? Do you know the source of the FCR?

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I will have a peak when I get a chance. My assumption is that it is a TT sourced carb, but I have no way to be sure. My reason for believing this, it is a TT carb, is that TT case covers are also on the bike. Also the individual I bought it from said the previous owner told him all the TT recommended mods were done to the bike.

I will keep going along my investigative route and get you guys all the necessary information. I appreciate all the help so far.

Thanks again.

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Ok the ride home tonight was the last straw. On the way to work this morning the bike pulled hard and had little to no stumble. On the way home completely different story.

So I figured I would take everyones advice and stop guessing. So I ripped the carb off and:

2007 DRZ400sm

14000 KM's

Mainly commuting and agressive street

Yosh Stainless Comp Series Pipe (RS-2 I believe)

Spark Arrestor In Place

3x3 Hole in the airbox

K&N Air Filter

T-Handle Fuel Screw (source unknown; I think TT store)

FCR-MX Carb

Symptom:

Studder at small throttle openings, constant throttle. <1/4, maybe <1/8???

Worse when motor is hot and weather is warmer. Less noticeable in the am ride over the pm ride.

Current Jetting:

EMN Needle 4th clip from top (just made this change yesterday, made the morning ride great thought all was almost fixed. Ride home problem returned.)

SLOW AIR JET REMOVED

PILOT JET 45

MAIN AIR JET 200

MAIN JET 158

Thanks in advance.

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Problem still exists in that setup(3rd clip) as that is the way I found it.

I was trying to determine if the bike needed a bit more fuel, it did run a bit better in the morning but went back to it's same old tricks in the afternoon.

One item I noticed when having a look at the bike was that the throttle wheel was making slight contact with the frame when the throttle was closed. Can this cause a problem if vabration is being transferred to the carb?

Any other ideas as to what could be the problem?

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Sorry Eddie can't take a picture the carb is back on the bike. Here are the items I found during the tear down clean and reassembly.

1. Throttle wheel on the carb was making contact with the frame.

2. O-ring mod previously done had bit the dust.

3. Intake boot on output of the carb was in incorrectly. I think basically there is a UP indicator on the boot. It was facing the engine head when I took the carb off and up was sideways. This seemed incorrect to me so I reversed the boot had it facing the carb with the UP actually UP.

4. I cleaned the whole carb with carb cleaner. Everything looked good but I figured I had it off anyways why not.

So the result today is, rode the bike to and from work. No stumbling, surgine, etc. Up to now I have never been able to do this. That being said the weather was quite moist and cool, so I am hoping that this did not disquise the issue.

I will report back after a few warm rides. I would however like to thank everyone for the responses thus far, you all have been of a great help. I am also impressed at your ability at getting someone to take apart a portion of a motor that they never have before to investigate. I will now be unafraid of working on a carb in the future. This is probably a good thing as who knows what will happen when the normal warm weather returns.

I will keep you all posted.

Thanks again.

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It's back, but slightly different.

So the warm weather is back, and I went for a ride yesterday. Essentially the problem has returned at a much reduced level. Similar throttle position (around 1/8th) I am noticing a slight stumble/surging. I am hypersensitive to the problem now so that might be part of it.

So what's left, any ideas? Should I move to a larger pilot jet? Bike starts nicer than before and comes off the choke easily (it would typically stall before).

Maybe the TPS sensor is a issue, previous owner did not unplug but rather covered and zip tied? Just wondering if it can send the wrong message back to the bike if it is present and not attached.

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