Jump to content
Sign in to follow this  

Rough Idle, Hesitation/sputtering at low throttle

Recommended Posts

I just bought an '04 DR-Z400S, it runs like a bat out of hell and I have been having a blast ripping around fire trails where I live in the Panhandle of Florida.  There are a few issues that I think are all just carb setting/jetting issues.  Any help or input is welcome. 

 

Problems:

 

1. The bike idles rough.  Pretty sure it actually misses combustion every now and then at idle.  If left alone, it will sometimes stall.

 

2.  When I'm cruising at the low end of the rpm range, maybe 1/4 throttle, the bike will hesitate and surge.  It's been hot and humid here, as usual.  

 

3. When I let off the throttle and clutch when I am coming to a stop, occasionally the engine will cut out.  

 

4.  Sometimes when I clutch and let off the throttle, or even put it in neutral the engine won't come all the way down to idle.

 

Setup:

 

Stock exhaust

3x3 mod

Keihin FCR (pretty sure it's the 39mm)

If there are other mods, I'm not yet aware of them.  I don't know how it's jetted. 

 

Thanks!

Share this post


Link to post
Share on other sites

When you say clean the carb, do you mean rebuild and thoroughly clean, or use something like b-12?

 

Probably best to disassemble the carb, then clean out all of the jets and passages with carb cleaner (or similar product) and some compressed air.

 

Might not be a bad idea to check and record jet sizes and needle position while your in there.

  • Like 1

Share this post


Link to post
Share on other sites

Thanks!  I'll give it a go.  I haven't even taken the tank off of this thing yet.  I guess it's a good thing I haven't made any plans for Labor Day Weekend!

Share this post


Link to post
Share on other sites

Probably best to disassemble the carb, then clean out all of the jets and passages with carb cleaner (or similar product) and some compressed air.

 

Might not be a bad idea to check and record jet sizes and needle position while your in there.

 

Do I need a rebuild kit, or will I be able to put it all back together without new parts (assuming nothing is broken/in need of replacement).

Share this post


Link to post
Share on other sites

Having a stock exhaust and a/b mod with a FCR on it is going to be problematic. To much carb and not enough outflow. It is very wise to look into the carbs jetting and know where it is at. Suggest you go back to a stock carb if you choose to stay with the stock exhaust. It will be a better balance to the engine. The best way would be to buy a freer flowing exhaust. Best is the MRDssw for the price. It can support even big bores well.

Do not take apart the part of the carb that has security screws ! The rest you can clean as you will. Just remember everything and take notes for all jetting and for reassembly.

Share this post


Link to post
Share on other sites

Do I need a rebuild kit, or will I be able to put it all back together without new parts (assuming nothing is broken/in need of replacement).

 

No need for a rebuild kit, just clean it thoroughly and replace anything if need be. Also like burncycle said take your time disassembling the carb and take notes and/or pictures. Have a clean workspace to keep your parts/hardware in order, because it is easy to lose some little parts.

 

Taking the tank off is easy, getting the carb out/in is the hard part. Removing the top subframe bolts and loosening the bottom subframe bolts and moving the subframe a little will give you an extra 1/4" or so and makes it easier.

Share this post


Link to post
Share on other sites

Having a stock exhaust and a/b mod with a FCR on it is going to be problematic. To much carb and not enough outflow. It is very wise to look into the carbs jetting and know where it is at. Suggest you go back to a stock carb if you choose to stay with the stock exhaust. It will be a better balance to the engine. The best way would be to buy a freer flowing exhaust. Best is the MRDssw for the price. It can support even big bores well.

 

I'm running the FCR-MX with the air box mod and stock exhaust.  I noticed much better throttle response and a little bit more up top, it works very well when jetted correctly.  Though I do agree with you, the SSW/MRD you will see more gains up top than I have with the stock exhaust. 

  • Like 1

Share this post


Link to post
Share on other sites

Thanks y'all!  Glad I don't need to shell out and then wait for a rebuild kit to ship.

 

It's my intention to get at least a slip on exhaust eventually, but like most of us, cash flow is the main thing keeping me from doing ALL THE MODS.  Honestly, right now, this thing feels a lot happier when I'm ripping on it than when I'm trying to baby it down the quiet 15mph street I live on or cruise at 35 mph.  

 

Burncycle, I wish I knew where the stock carb is.  The guy I bought the bike from never mentioned it had any mods(I'm not even sure he knew it was modded at all), and it's my first dr-z, so i didn't really know where to start looking.  

 

I'm probably gonna start tearing this thing apart tomorrow morning.  I'm sure I'll have loads of questions then.  Thanks for all the advice!

Share this post


Link to post
Share on other sites

Ok, last weekend I checked the valve clearance(were within spec), changed the oil and filter, cleaned and oiled the air filter, and cleaned and reinstalled the carb. Been pretty busy all week, but just had a chance to mess with the bike some.  Now, when I start the bike, it runs fine in choke, but as soon as I take it off choke, it dies.  Fuel screw is set to 2 turns out.  Any ideas?

Share this post


Link to post
Share on other sites

Welp.  Seems like my issue yesterday may have just been that I had the idle speed adjuster set too low.  This afternoon I took the carb out again, cleaned the bowl and jets, with special attention to the pilot.  I put it back in, and had the same problem.  When I tried turning the idle speed adjuster up, the bike idled like normal.  It's still a little rough, but I think that's because my mix is off.  I ordered a zip ty fuel screw yesterday, so I'll see if getting that dialed in will smooth out the idle/hesitation issue.

Share this post


Link to post
Share on other sites

I just bought an '04 DR-Z400S, it runs like a bat out of hell and I have been having a blast ripping around fire trails where I live in the Panhandle of Florida.  There are a few issues that I think are all just carb setting/jetting issues.  Any help or input is welcome. 

 

Problems:

 

1. The bike idles rough.  Pretty sure it actually misses combustion every now and then at idle.  If left alone, it will sometimes stall.

 

2.  When I'm cruising at the low end of the rpm range, maybe 1/4 throttle, the bike will hesitate and surge.  It's been hot and humid here, as usual.  

 

3. When I let off the throttle and clutch when I am coming to a stop, occasionally the engine will cut out.  

 

4.  Sometimes when I clutch and let off the throttle, or even put it in neutral the engine won't come all the way down to idle.

 

Setup:

 

Stock exhaust

3x3 mod

Keihin FCR (pretty sure it's the 39mm)

If there are other mods, I'm not yet aware of them.  I don't know how it's jetted. 

 

Thanks!

 

List your current jetting

Share this post


Link to post
Share on other sites

I'd suggest removing the spring on the coast enricher - i had exactly the same problems you describe and it all went away. It's so easy to do anyway, you can put it back if nothing happens.

Share this post


Link to post
Share on other sites

I'd suggest removing the spring on the coast enricher - i had exactly the same problems you describe and it all went away. It's so easy to do anyway, you can put it back if nothing happens.

 

If it is the E model Fcr it will have a CE, ....Coast Enrichener removal is the way to go....

Share this post


Link to post
Share on other sites

List your current jetting

 

Pilot Jet: 45

Pilot Air Jet: 65

Main Jet: 152

 

Where is the coast enrichment spring, and how do I know if I have an E model FCR?  I know I have an E model intake boot, because it doesn't have a fitting for the crankcase breather hose.

Share this post


Link to post
Share on other sites

Mine is an S , has a Mikuni and it also has an enricher.

 

Right under the sensor plug, remove the screws while holding the cover down or it will fly off, because the spring is strong.

 

There are many suggested things you can do as to plugging vacuum intakes, etc, but as all i wanted to do was a quick test,

 

i simply removed the spring and left the diaphragm there as a seal (closing it back can be tricky because you no longer

have the spring to hold the diaphragm in place, watch out for those small guide holes). I am so happy with the result that i'm letting sleeping dogs lie for the moment, i'll look into all that plugging and o ring stuff later. One really has to wonder what was Mikuni thinking. Funny thing is, apart from all the problems it created, it also didn't do anything for the one thing it was meant for, namely the decel popping, which also went away after this mod. Go figure.

 

Could be the spring, it seems to be much too strong for that small diaphragm. But i checked the one on an Keihin FCR i also have and it's just as strong as the one on the Mikuni.

 

v78u.jpg

Share this post


Link to post
Share on other sites

Pilot Jet: 45

Pilot Air Jet: 65

Main Jet: 152

 

Where is the coast enrichment spring, and how do I know if I have an E model FCR?  I know I have an E model intake boot, because it doesn't have a fitting for the crankcase breather hose.

 

 

Post a pic of the carb...both sides please

what needle setting ?

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×