Any tuning gurus out there? I've got a drz400sm with full yosh, stage 2 hot cams and a fcr39mx carb that's giving hell. All installed by the PO. Its definately lacking power for the upgrades it has. When I got the bike, it was leak fuel through the vacuum line on the petcock into the motor. It had amazing power ... I replaced the petcock and now it feels like a dud. I would throw the old petcock back on if it didn't make my oil reek of fuel. Bog/die from idle to wot... unless I set the idle above 2200. I'm a couple weeks away from just buying a power bowl 2 if I can tune the problem out by then.
Sounds lean, if leaking fuel into the manifold was helping. Does the choke make it better? What pilot jet are you running? Can you adjust the idle screw for highest rpm between 1 and 3 turns out?
Yep, sounds like the PO didn't change the needles when the full exhaust was put on... probably the first place I'd start looking.
So it ran fine with a bad petcock? I would be looking at your new petcock. Did you hook the vacuum line back up on the petcock. If the carb was all gummed up having a leaking petcock still wouldn't make it run normal.
I think he said the petcock was leaking fuel via the vacuum line..thus, extra gas getting into the intake, making it run stronger. New petcock and no more leak, and the bike runs weak...to me that says it needs more fuel, or rather, runs lean. Jetting is likely the issue from the description.
I should have checked the jet sizes when cleaning the carb... It ran much stronger when fuel was getting sucked in with the vacuum on the petcock. I put a clear hose on the vacuum line and ran it to diagnose the issue. It was misting/leaking fuel in when pulling vacuum. I've got a fuel/air extended mix screw and tried many different settings between 1-3. It's like it's getting to much fuel on the quickshot causing it to die/bog and not enough when WOT to feed it. If i'm thinking about it correctly... also, lots of popping on decel
Did you plug off the vacuum port on the head when you changed the petcock???????? That will make it run crap.
I'm trying to get mine jetted right now (installing a new/used fcr39). Try this and verify all jets are correct first: http://www.drzparts.com/2010/05/dr-z400-fcr-39mx-carb-install/
Alright, must be marked differently than the only other carb I've worked on (klx250 stock). Didn't notice numbers when cleaning. I'll pull it apart sunday. Thanks
So from all the previous discussion I'm assuming you put a manual petcock replacement on for the leaky vacuum controlled petcock and put a cap on the manifold spigot that the petcock used to get its vacuum from? I'm curious how well it ran with the leaky petcock? I can't imagine it ran well, but better than not getting the leaky fuel supply? I would start from scratch, pull the carb off and disassemble it, let it soak in your paint can of carb cleaner overnight (buy one if you don't have one, they are worth it), pull it all out the next day and proceed to blow compressed air thru every little port/hole/passage on it then put it back together, reinstall with just the tank on so you can run it and see if you fixed it cleaning it. If it seems to work right put your side covers and seat back on, put some seafoam in the tank, and thrash on it and see what it runs like. I would go over to the Thumpertalk DRZ400 section, there is a lot of info on the FCRs and troubleshooting on there. I believe you have to do a mod to the carb to get rid of the coastdown popping, even when jetted right.
While we're here... my FCR39 is kind of a mess. Massive fuel leak at the fuel inlet (ordered the 3 orings to fix this). No vacuum nipple on the head (ordered), so had to leave the petcock on prime. Starts up with choke (carb has two chokes?? One red, one black). What is the red one? Only stays running on the black choke, pops like crazy if I take the choke off and eventually dies. I'm thinking either a clogged pilot, or completely wrong pilot. I should have pulled it apart before installing to take my own advice and double check all the jet sizes...
The red knob is the Air Control Valve: it's used to assist starting a warm engine that isn't cooperating. This was found on the DRZ400 (no suffix) that was a kick start. KevinD
Nels has a LOT of experience with carb tuning dirtbikes, especially with FCR carbs. It's $50 to run a baseline, and he will tell you exactly what to do with the jetting. For a little more, he can rejet it on the dyno, or you can do it in the parking lot! Also, if you've turned the mixture screw out past 3 turns and it still wont idle below 2200 rpms, you need to go up on your pilot size. Its likely running way too lean to idle.
It ran so damn well with the extra fuel leaking by the petcock and felt like a 450+cc.... the PO might have tuned the carb to the leak/extra fuel without knowing. I'm just going to buy all new jets. Do another clean and put the new/correct sizes in. It's $25 for jets and will hopefully stop me from being forced to pull the carb out again this year.
Wait, who's Nel? When saying my idle is at 2200... i mean that it will die/flood when going idle to WOT quickly if the engine is running too slow. 2200rpm idle to WOT stumbles a little but doesn't die 1500rpm idle to WOT dies instantly
2wheeldynoworks.com I would go up a size or 2 on the pilot, set your mixture screw to 2.5 turns, and go from there.
