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DRZ FCR carb input?

matty

matty

Well-known member
Joined
May 19, 2004
Location
Oakland
Moto(s)
Doctor650 and lil buddy
Name
see above
I picked up a 06 DRZsm, about a month ago and it came with some mild carb issues. Specifically, there is a slight bog/lag after I’ve chopped the throttle and then get back on the gas. My guess is that its running lean on the pilot. So I’m wondering what jets, pilot and main, local riders are using?

Some facts: the carb is a 39mm fcr-mx w/choke, it is not the famed thumpertalk carb and kit, that much I can tell just comparing mine to photos online. The airbox has been swiss cheesed on the right side, but not the 3x3 mod. I actually taped over a bunch of the holes and that helped, somewhat, with the bog. It has a yosh slip on and there is a slight leak where it mates to the header.

I’ve checked thumpertalk a fair bit, though my searching skills are a bit lacking, I’ve seen the ‘o ring mod’, but again my carb is slightly different than the carbs pictured on tt. Other suggestions were to remove the pilot air jet.

Admittedly, I have the uncanny ability to take a 3 hour job and make it 30, so I’m just looking for any input or thoughts from folks who have a similar set up, before I dig in and start mucking with the pilot and needle circuit, leak jet and ap timing. In all likelihood, I’ll probably take her into the shop this week, but I thought I’d post up here first.

Thanks in advance.
 
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If you suspect the pilot, there's an easy way to check that:

The pilot is too lean if you can turn the fuel screw out and the rpms keep going up, once you've turned the fuel screw more than 2.5-3 turns, if it's still getting better you need to go up on the pilot.
The reverse is true as well, if you can turn the fuel screw all the way in such that it's completely closed and the rpms keep getting higher then you're too rich on the pilot.

http://www.thumpertalk.com/forum/showthread.php?t=327405
 
If you have a FCR, then it has a pumper circuit to give a shot of fuel into the engine when you go from closed throttle to opened throttle.

You should take the carb out to check if the pumper is working properly, i.e. it squirts out fuel when you push the pumper rod and the fuel squirt duration isn't too long or too short. This info is available on TT.
 
^ True.. could definitely be that. If you're talking about it bogging when you whack the throttle, that's usually the culprit. If you take the carb off (without draining the float bowl) you can twist the throttle wheel and you should see a squirt of fuel come out the front. If no fuel comes out, that's a big problem and you'll need to clean the AP passages to get it working again. Too long of a squirt duration is usually a bigger problem with dirt bikes than street bikes, but I try to keep mine in the 1.5 second range.
 
Thanks, that's where I'll start. quick question, is the actuation of the pumper circuit visible externally? what i mean is i can see a brass rod, slowly depress when I wack the throttle. Is that pumper as seen from outside? thanks again.
 
While tuning these carbs.. turn OFF the pumper.. dial in the carb lean and then add the pumper shot..
 
Ironbutt, this is a flatslide carb, not a pumper. We're reffering to the accelerator pump, which adds additional fuel to handle the situation for when you turn the throttle and let in a bunch of air but the engine hasn't spun up yet to create enough vaccuum to pull the right amount of fuel.

Are you still recommending turning the AP off and tuning that way? If that's the case, what part of the carb would you start with? The pilot/needle/ or main?
 
Um, what makes it not a pumper if it has an AP? I had thought pumper just meant it had an AP, and didn't refer to anything else...
 
My bad, I've only heard the mikuni's called pumpers - but if having an AP is all that means then ok.

Still, shutting off the AP (I assume by removing the diaphragm?) seems counter intuitive. If you dial in the carb lean with the AP off, then when you are riding say on the freeway where there's a constant throttle position - isn't it going to stay lean because the AP is only active on that first application of the throttle?
 
jrbruin said:
Ironbutt, this is a flatslide carb, not a pumper. We're reffering to the accelerator pump, which adds additional fuel to handle the situation for when you turn the throttle and let in a bunch of air but the engine hasn't spun up yet to create enough vaccuum to pull the right amount of fuel.

Are you still recommending turning the AP off and tuning that way? If that's the case, what part of the carb would you start with? The pilot/needle/ or main?
Click to expand...

Yeah.. turn off the accelerator pump.. get the main set.. then the needle.. then the pilot in that order.. then add in accelerator pump for the final adjustment.
 
jrbruin said:
My bad, I've only heard the mikuni's called pumpers - but if having an AP is all that means then ok.

Still, shutting off the AP (I assume by removing the diaphragm?) seems counter intuitive. If you dial in the carb lean with the AP off, then when you are riding say on the freeway where there's a constant throttle position - isn't it going to stay lean because the AP is only active on that first application of the throttle?
Click to expand...

there should be a screw that increases or decreases the throw of the pumper arm.. make it so that arm doesn't pump fuel.. the main jet is first.. then needle.. then pilot. in that order.. you can do it the other way.. but it will take you forever and it still probably won't be correct.

Lean is better at idle... when your putting along it's using the transition into the needle/pilot.. it's the needle at that point that should be richer. this is why I say turn off the pumper until you get the main and needle squared away..

Then as you roll on the throttle, the slide cutaway/needle and main come into play. I'd leave the pilot alone unless it's super lean or rich.. concentrate on the main jet and the needle taper and if need be.. the slide cutaway.
 
A hearty thanks to all. Y'all have painted a perfect picture of what i should look for, where to start and how. I'll probably dig in on Saturday morning, throw in the towel out of frustration on Sunday evening and take her to subterranean on Tuesday.
 
Fix that air box first..that's probably fucking the works up. then... see how it rides.. tuning takes time when you don't have a base line. Plug the holes.. put the stock air cleaner back in.. or oil the new one. little subtle things matter more than you think.

The main is probably fine... the pilot is probably fine.. the needle is lean most likely... but everywhere is lean with a chopped up air box.. so don't bother going after the carb first until that's sealed up and working like it was designed.

I'll either leave the air box in.. or take it out.. not this 3x3 bullshit.. your tuning it.. not jerking it off.. cutting holes in the air box is called molesting and it's a waste of time.

What I would do? 1. fix air box after cursing the asshole who cut it up.. 2. raise the needle... 3. add accelerator pump 4. test ride... 5. drink beer.




matty said:
A hearty thanks to all. Y'all have painted a perfect picture of what i should look for, where to start and how. I'll probably dig in on Saturday morning, throw in the towel out of frustration on Sunday evening and take her to subterranean on Tuesday.
Click to expand...
 
i've already cursed the asshole that cut it up. one more hole to plug and it will be back to stock. that was actually the first thing i did (taping over the holes) and it made a world of difference, it went from a serious stumble to slight bog. and thanks for the words of support. i find it a bit challanging when someone else has done the mods and not me. i'm fine when i muck things up, i know what i've done and i can work backwards to fix. when its someone else's work, its hard to know where to start. but again, you've given me a good baseline and its appreciated.
 
matty said:
i've already cursed the asshole that cut it up. one more hole to plug and it will be back to stock. that was actually the first thing i did (taping over the holes) and it made a world of difference, it went from a serious stumble to slight bog. and thanks for the words of support. i find it a bit challanging when someone else has done the mods and not me. i'm fine when i muck things up, i know what i've done and i can work backwards to fix. when its someone else's work, its hard to know where to start. but again, you've given me a good baseline and its appreciated.
Click to expand...

I had/have the same problem, Previous owner of my bike had installed the airbox sidepanel with holes cut in it. The bike would just sputter and lag, putting the stock sidepanel fixed it somewhat.

Try holding the throttle open just a bit like 1/8th to 1/4 turn and if it sputters or feels like it isn't running smooth then your needle position is off.

My bike does not like the throttle to be maintained less than a 1/4 because it just fumbles almost as if it were misfiring but from the airbox (lean mixture?). But once I open it further than a 1/4 everything runs smoother
 
209manny said:
I had/have the same problem, Previous owner of my bike had installed the airbox sidepanel with holes cut in it. The bike would just sputter and lag, putting the stock sidepanel fixed it somewhat.

Try holding the throttle open just a bit like 1/8th to 1/4 turn and if it sputters or feels like it isn't running smooth then your needle position is off.

My bike does not like the throttle to be maintained less than a 1/4 because it just fumbles almost as if it were misfiring but from the airbox (lean mixture?). But once I open it further than a 1/4 everything runs smoother
Click to expand...

There is such a thing as a rich misfire.. it might be falling all over itself with too much fuel.. Everyone wants to fatten up the pilot circuit because of the known idle drop method.. but that was back when carbs had an adjustable main jet.. like back in 1941.... These days.. with them new fangled needles and slide cutaways... it's best to leave the pilot alone if it idles at all..
 
Ironbutt said:
There is such a thing as a rich misfire.. it might be falling all over itself with too much fuel.. Everyone wants to fatten up the pilot circuit because of the known idle drop method.. but that was back when carbs had an adjustable main jet.. like back in 1941.... These days.. with them new fangled needles and slide cutaways... it's best to leave the pilot alone if it idles at all..
Click to expand...

for my bike (keihin carb) the uncorking process recommends a 168 or 168s pilot stock is 165s. I tried the richer pilot and backing it out more than 1 turn would cause the idle too drop too low. It did feel like I had more snap neck throttle response but once I let off the gas it would not idle properly or just turn off due to there being too much fuel. Stock pilot works for me, but I did lose some throttle response. I'd rather have proper running idle than poor idle and having the bike die.

so to the OP. try tweaking one thing at a time. I made the mistake of changing main jet and pilot at same time, and also messed with the airbox. I got everything all messed up that I did not know where to begin pinpointing the problem
 
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