Just Bought a 2004 Suzuki Z400
 

So i got the atv for $1700 which is under book value. I noticed it needs new rear brake pads, but other than that it is in average shape. The previous owner had bypassed the radiator temp sensor and direct wired it so the fan kicked on all the time. I rewired it to factory, bought a new fan, new sensor, and some engine ice. Will install all this weekend since the fan stopped working on a play ride an the radiator started bubbling over (overheating). The bike is ridiculously fast compared to my brother's '08 400ex, but there are a few issues maybe you could help with:

1. Doesn't want to stay idling for more than a minute or two, shuts off.
2. Yoshimura pipe backfires a little upon idle and when you decelerate off the throttle when riding.
3. When you turn the engine off it does one final backfire after the engine cuts out
4. When the fan kicks on the idle changes (goes down)

I was told the atv was jetted when the team suzuki Yoshi was put on, but I don't know when last time plug changed or carb cleaned. Are any of these things major issues? I was almost thinking about selling it and getting a 400ex even though they are slow cause they already seem like way less maintenance. Thoughts and any help would be appreciated.

Derek

Welcome! It sounds like with a little work, you can get that Z back to running well. Don't give up on her yet!

1) Not sure about this, but let's address some of the other stuff and see what develops. This could be an electrical component overheating, but it also may go away after fixing the other stuff.

2) Off throttle backfires are caused by a lean condition. Chances are the either you have a small intake or exhaust leak, or the carb needs to be jetted better. If you don't find any leaks, I would check your jet sizes and make sure the passages are clean. Give Yosh a call and get their recommendations. You could just go up one size on the pilot and see if it gets better. After that try one on the main. There are a few good carb tuning tutorials on the net if that doesn't get ya fixed.

3) I think the fix in 2 will alleviate this also.

4) I don't have a Z400, but this is common with sport quads. They really don't put out much wattage. It probably isn't a big deal, but maybe someone with one of these will chime in.

I would change the plug and oil now. With your other issues cleaning the carb is also a good idea. I am suspect that there is an issue with the fan not working which is why it was hardwired. It may be a bad sensor or wiring issue. The engine ice will help with overheating, but I would also make sure the radiator is clean, no mud in the fins.

Thanks very much for the advice! I checked the radiator and it is clean. It also isn't cracked or losing fluid. The hoses are good too. Opened cap and watched fluid moving so don't think water pump bad. Maybe they just thought it would be easier than replacing the sensor. checked the leads going into temp sensor and it seemed at about 10v which is lower than the 11.8 I measured at the battery, but don't think that's the issue. I think the sensor went bad and they decided to do the easy thing. I'm going to flush the fluid out from the drain below water pump and run water through to rid any buildup before replacing sensor, fan, and putting ice in.

I don't think there's anything serious wrong with the engine or it wouldn't run well once it gets out of idle. I think messing with the jets may be a bit too advanced for me maybe, but I hate paying $89/hr. that the dealer charges. I tried messing with the fuel screw but that didn't seem to help. I would hope maybe having someone who knows their stuff help clean the carb and jet properly would solve the issue along with new plug and a good oil change. Anything else I should look for?

also, when you say intake leak, what and where do you mean? I assume exhaust leak would be where pipe meets header?

Update: I flushed radiator and head, added new temp sensor, new Moose Racing fan, and Engine Ice. Rewired sketchy butt connectors with proper 14awg wiring and sealed so no water getting in. Got new battery since found that was the issue with voltage drop. Ran some Seafoam with 93 octane and the thing came to life. Runs amazing, had to adjust the idle down a bit, but starts on first try and needs no choke. Backfiring stopped.

Did an oil change and switched to Amsoil 10-40 with new filter. There were a few small metal dust/shavings attached to the drain plug magnet when I did the change, but nothing bigger like schrapnel pieces. Is that okay? Also, anyone have a front wheel they want to sell or trade me for a pair of hardly used rears?

Sounds good! A little junk on the drain plug magnet is OK. No telling how long it has been there so I wouldn't sound the alarms just yet. It is good to keep an eye on that sort of thing though. Don't forget the clutch will normally wear over time and you will get particles in the oil from that. Just another good reason to stay current with oil changes.

Good deal, thanks for the heads up. I figure to change it again at end of summer or November as it would be in Florida. BTW, I've read dozens of silly posts about what gas to use. I just switched from regular pump 93 that had apparently always been run in it to 89 ethanol-free and what a mistake. at the point 3/4 through the gear when the thing usually starts hitting the torque curve it starts pinging and that's all you get. Switching back to Mobil 93 with some added maxima fuel stabilizer tomorrow night and day difference.

2) My ltz does the same. The atv may need different jets or the jets may be clogged and not letting the correct amount of air to mix with fuel causing a crackling noisebecause of to much or little back pressure through the exhaust port. The atv may need different jets for the yoshimura exhaust as mine does with same exhaust..