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DRZ400S Fuel/Carb/Idle issue

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My 2007 DRZ400s with  Mikukni BRS36, 3x3 mod and JD jets has been running like a champ for all these yeas, until several weeks ago, for no obvious reason (except maybe one mid pressure washing), it started dying with no particular pattern. Sometimes it would die cruising on a highway, sometimes going slow in a lower gear, really - no pattern. However, it looked like it was not dying if the petcock fuel selector was set on PRIme. So, I decided to rebuild the petcock with new O-rings and a new diaphragm. After doing that and having the same problems, it turned out that my petcock vacuum hose was too porous - vacuum was there, but not enough of it. Changing the hose brought the vacuum back and the petcock operated as expected. However, the engine was still dying. One way to make it dye for sure was to bring RPM really high and than to just let it idle while disengaging the clutch (squeezing the clutch lever), so it goes all the way to idle. If clutch was left engaged, the engine wouldn't die. I tried to change idle setting and the fuel screw setting, but nothing really helped. So, my first question is what happens with the fuel inside BRS36 when throttle is not applied? This is not really idle as engine still creates vacuum, so which fuel circuit is active? 

Maybe I should mention that I changed oil and found no traces of gas.

Not really knowing what to do, yesterday I removed the carb (with JD jets) to see if there is something obvious and found nothing. I didn't remove and clean all the jets. Put everything together and what I have today is that the engine not only dies like before, but it idles really really strange - at times it would be "normal" thumper sound, but sometimes it would be at the 2000+ RPM range, One really weird thing is that once it is in this very high idle mode, I can screw the fuel screw all the way in, and nothing changes. So it sounds like fuel is still coming from the pilot or the main jet. 

I'll open the carburetor again, but I wanted to ask you experts if you can give me some directions - what to look for. Everything looks really clean, JD jets are maybe 200 miles old, float height was also set 2000 miles ago, so I don't know what to do once I remove the  carburetor.

 

Thanks a lot.

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bumtarder, thanks a lot.

No I haven't. I removed the carburetor again and this time I'll check the air intake as well. I thing I forgot to mention is that before this happened I cleaned and oiled my No Toil foam filter. I have no idea whether this can mess up with air intake somehow.

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Check the engine compression and do a cyl. leakdown test. If it has leaking valves your idle will be poor. Check valve clearances. 

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Thanks a lot. Compression test is on my ToDo list, but I don't have the tool for the leakdown test. I think that the problem with the variable idle is something I  introduced. The initial problem was that the engine started dying down, mostly at highway speeds. I fixed it by hiding it behind a bigger problem.

BTW, Bucket-list congratulations on your impressive trip to Alaska and back.

 

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So far I removed the carb several times, cleaned and re-cleaned, compression checked, petcock rebuilt, but no progress.

However, I found a pattern of behavior, so I just wanted to check whether it sounds familiar to anybody.

Throttling up creates no problems. However, throttling down has two distinct behaviors. If I just close the throttle at any engine speed, while bike is moving and clutch is engaged, the engine will slow the bike down, and carburetor will slowly come to 'normal' idle. Of course, at that point I have to disengage the clutch, otherwise engine dies. Scenario 2 happens if I close the throttle and disengage clutch while engine is at higher RPM. Rather than slowing down immediately, the engine stays at the high RPM. Over the next several seconds it will slow down somewhat, but not necessarily all the way to idle.  

It sound like there is some issue with the pilot circuit, but I couldn't find anything wrong when inspecting.

Idle behaves better, almost normal, when engine is cold, so I have a related question - what can be different depending on engine temperature? Carb doesn't 'see' the engine temperature, only the vacuum engine creates.

Thanks.

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Make sure your gas cap is venting properly. Don't know what the clutch lever would have to do with anything unless you were in gear. If you still have the kickstand switch wired up, make sure it's functioning correctly. If it's disconnected, make sure it's not shorting out. That's all I have.

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1 hour ago, ndjur said:

So far I removed the carb several times, cleaned and re-cleaned, compression checked, petcock rebuilt, but no progress.

However, I found a pattern of behavior, so I just wanted to check whether it sounds familiar to anybody.

Throttling up creates no problems. However, throttling down has two distinct behaviors. If I just close the throttle at any engine speed, while bike is moving and clutch is engaged, the engine will slow the bike down, and carburetor will slowly come to 'normal' idle. Of course, at that point I have to disengage the clutch, otherwise engine dies. Scenario 2 happens if I close the throttle and disengage clutch while engine is at higher RPM. Rather than slowing down immediately, the engine stays at the high RPM. Over the next several seconds it will slow down somewhat, but not necessarily all the way to idle.  

It sound like there is some issue with the pilot circuit, but I couldn't find anything wrong when inspecting.

Idle behaves better, almost normal, when engine is cold, so I have a related question - what can be different depending on engine temperature? Carb doesn't 'see' the engine temperature, only the vacuum engine creates.

Thanks.

You have a slight hanging idle. You probably just need to fine tune the idle adjuster and fuel screw.

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Thanks a lot for your suggestions.

Two days ago I was riding with the gas cap unlocked and completely loose to find out if I can believe it. Nothing changed. 75xtdrvr, you are right - bike is in gear all the time. I cac produce this idle problem reliably by standing in one place - bike in gear, clutch depressed, blip of throttle - RPM stays up high. Slowly release clutch - before bike moves RPMs go down to 'normal' range. I know this is called "hanging idle", but so far I wasn't able to find any explanation why is this happening. Lean mixture, apparently? But why? I think it has something to do with the transfer (progression) ports, but I was just hoping somebody will recognize the pattern and have an explanation.

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Just to report that this issue is now solved. So far I haven't found this exact problem being mentioned here so I though I should add it to a "data base" of issues. It was all about the faulty petcock. I still use the original one - the one that requires vacuum to let the fuel though. I did check the usual modes of failure but it behaved the same regardless whether it was in ON or in PRI position. So I decided the petcock was OK and kept torturing the carburetor. After removing it and cleaning it several times, I finally realized that there was absolutely nothing wrong with it, and decided to go back to other things, starting again from the petcock.  I replaced the vacuum line with a transparent one and that's how it became obvious that the petcock has a very, very slow leak of fuel into the vacuum line. A drop or two of raw fuel would be sucked into the engine, usually when engine vacuum is the strongest - high RPM + throttle closed. Of course, it was doing this regardless whether it was in ON or in PRI position. I'm now running with the valve in PRI position and the vacuum line pinched until I get a replacement valve. It runs as good as it always was.

Just one more word about the "hanging idle". During this ordeal I was changing everything - different jets, pilot screw setting, idle screw settings, float level, everything I could think of. Several times during these experiments I had very noticeable hanging idle. What is usually mentioned as a reason for this condition is a lean mixture. However, I now think that the main reason is the idle screw setting. It is easy to create the hanging idle by having the throttle plate right above the first pilot transfer port, uncovering just a very, very small part of this hole. I think that in that case the fuel coming out of that port may go either way - towards the engine (high idle), or behind the plate (low idle). Actually it is enough to very, very slowly roll the gas handle to see the big jump in RPM as the first transfer port gets open.

Thanks to all of you who tried to help with your suggestions.

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Thanks for the update and explanation. I run my idle circuit very lean and Don't have hanging problems, as long as I keep my idle speed below 1,700 rpm, over that it will hang.

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48 minutes ago, bucket list said:

Thanks for the update and explanation. I run my idle circuit very lean and Don't have hanging problems, as long as I keep my idle speed below 1,700 rpm, over that it will hang.

Yeah, I don't think it is caused by lean pilot circuit. I just think these bikes are sensitive to idle rpm and fuel screw setting. If I set idle too high it'll hang. My bike does best with low idle, but it needs to be thoroughly warmed up to keep from stalling.

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