YZ400F backfires, wont idle, dies

[SledParty]

I just got this bike on trade 1999 YZ400F and when I rode it on the test ride it backfired on deceleration and while reving it was popping.

First let me say I know I'm a NOOB here but I know forum edicate and I have been reading for about 5 hours about this issue. I'm posting this thread in hopes that I might get some good advice and also help out anyone else who has the same or similar issues. I will post my findings once I figure this out :-)

So I cleaned the carb completely today, 175 main but I didn't check the pilot. Stock pipe so I seriously doubt they changed that but I'll go back in if I need to. The carb was pretty clean no real dirt or build up. I completely drained the fuel tank and put in fresh 92 as well.

Symptoms:

- Kick start takes a LOT of kicks when cold. fires up with choke out and runs pertty high RPM's.

- Once warm(er) after maybe 30-seconds to 1 minute I turn off the choke and it sputters and dies immediatly.

- Will start again without choke but will not idle at all, adjusting the big black knob on the side is very hard to turn and seems to have no real effect on the idle.

- If I rev the bike it pops pops pops, and loud. once I let off the gas it dies.

It pops through all of the above, even with the choke on. I put a brand new plug in it today and verified gap, the old one was black and looked like it was rather rich with lots of carbon buildup.

Tomorrow morning I'll check the pilot screw on the bottom, I'll start with 2 turns out and work my way from there. I'm assuming idle screw should start at around 1 1/2 turns out and then adjust to right RPM range.

Thanks!

[Steel Panther]

Exhaust leaks can cause decel pop and backfiring, so check the exhaust for any leaks. then mess with the jetting. Try looking at the jetting database to get an idea of what works with the bike for your altitude/weather. If it is dying right away at idle you probably have a clogged pilot jet, so check it out and then get a new one.

And it wont hurt to check the valves while your at it.

[Gunner354]

Sounds like a classic plugged pilot jet.

[dgcars]

Exhaust leaks can cause decel pop and backfiring, so check the exhaust for any leaks. then mess with the jetting. Try looking at the jetting database to get an idea of what works with the bike for your altitude/weather. If it is dying right away at idle you probably have a clogged pilot jet, so check it out and then get a new one.

And it wont hurt to check the valves while your at it.

+1. What he says. It wouldn't do any harm to do a quick 'timing' check though....

[baxterj787]

1) Clean the carb and passages inside the carb properly. SEARCH the FAQ on how to do this.

2) While the carb is apart, verify that the small oring and flat washer are present on the tip of the fuel screw. Sometimes they hide inside the carb body when the fuel screw is removed, then either disappear or get damaged when the fuel screw is put back in.

3) Put the jetting back to stock. Should be around a 162 - 165 main and 42 pilot. The black plug you have is from a too-rich mainjet that the PO likely installed to cover up a different carb issue. Stock jetting was OK for most everywhere below 3000 ft elevation.

4) is the hotstart knob closing all the way?

5) Check your valve clearances. Again, do a search. Pretty easy to check clearances.

[Gunner354]

I would start with the pilot jet. This is what happens most of the time especially if it sat more than a couple of months.

[SledParty]

Thanks for the replies so far, even more frustrated now.

Pulled the carb and cleaned it again including all the jets (3 right bunch together in the middle of the carb) and the pilot needle. When I took out the pilot needle there was no washer or oring attached but I can see something in the hole but cant get it out.

Put it all back together and now it WONT start at all.. checked the plug and still getting good spark, shot of starting fluid with no carb on and it fires up for a second or so...

So now the carb is off... again.. I'm really starting to get frustrated I traded a spotless 95 Shadow VLX for this turd and it's turning out to be the worst desicion I ever made... I still have not rode this bike once except for the test ride.

It fired up this morning and ran with lots of pops and backfires before I tore down the carb. I've watched all my pieces carefuly doing the work in a nice clean work area....

Checked the header and the gasket is still good, the curved pipe was dinged but I dont think it was bad enough to restrict the exhaust gas.

[SledParty]

ok just came back from the dealer with the spring, washer and o-ring for the pilot needle screw. Slapped that in and also while I was at the dealer I had them inspect all my jets and needles, and the carb itself.

The bike now does start but it's still the same issue... popping and backfiring and will not idle.

I bought a new gasket for the exhaust manifold pipe but the old one looked perfectly fine and there was no sign of leaking around the head.

So, maybe some progress since it runs again but it still wont idle and dies instantly if you let off the gas. Cranking the idle screw (throttle stop) isn't doing much if anything at all. I'm OOF for the next 4 days so I guess I'll tackle this when I get back. Next time I'm buying new

[SledParty]

Ok thought I would add to the thread what I found tonight when I went into my garage...

About 2 gallons of gas poured out of the overflow tube onto the floor of my garage.. Came home from the grocery store and just happened to come in through the garage to see this huge puddle.. Crappiest part is that I just put a bottle of "yamaha ring free" in the tank and now that + the $4 a gallon super is now slowly evaperating away.. Hard to believe but that was just $15 POOF gone!

So now from my experience with motors it's either a needle and seat issue or the float. Any thoughts?

[Steel Panther]

if you don feel any air escaping from the exhaust you probably don't have a leak, but even with a good header gasket if the header is not tightened down evenly it will leak. Also, pilot jets are so small it can be very hard to clean them properly, that being said its best to just get a new one, and maybe try going back to the stock jetting. Also, being that it is a 1999 it is very likely the valves are off so check those, its very easy to check. Shimming them is a bit more involved but still doable for the average person. Also, make sure you turn the gas off .

[SledParty]

After reading a bunch I probobly toasted the oring on the valve needle seat. I pulled the seat out to inspect and clean it and I'm sure the oring had gone flat, it literaly just "fell" in without me having to push it down. My petcock wont stop the flow 100% it just slows it to a trickle so I elevated the overflow hose but that's not stopping it either since gravity is doing all the work. I'm heading out of town I'll monitor the thread but I wont be able to tinker til likely tuesday next week.

I'm also going to try next to disconnect the TPS sensor. I've been reading and seems like a lot of guys who have the same issues I do cure them with disconnecting it.

Thanks for the help, sorry I got so negative earlier I've been working on this bike for two days solid and feel like I haven't gotten anywhere!

[dgcars]

Ok...you got this bike as a trade and when you test rode it, bacfiring & popping. You have started at the carb, so sort that properly and take it out of the equation.

Replace the pilot jet....don't clean it!!!

When you installed the fuel screw did you get all the old 'O' ring out?

Check the valve clearance next. Re-shim if necessary.

Check your timing...it only takes a couple of minutes.

Chillax & have a good weekend

[SledParty]

Ok so I'm back from my trip, today I went to 3 different dealers and got NEW main and pilot jets (stock) also picked up a 50 pilot just in case. I bought a new gasket for the exhuast manifold pipe and new needle and seat.

Good - Bike fired right up with new parts, doesn't seem to leak fuel anymore through the overflow. In general it is running crisper.

Bad - STILL pops and backfires, WILL NOT IDLE if I crank the idle screw ALL the way down it will idle for about 1/2 a second then die. I've found a sweet spot on the fuel screw with minimal backfiring but again will not idle at all.

So to sum up, I cleaned the carb completely with carb cleaner and compressed air from my compressor, new jets, new needle and seat, new exhaust oring. I tried with TPS disconnected and it runs better but still wont idle.

I still have not checked the valves I suppose that is next. Any thoughts???

[SledParty]

Also important note, my header pipe is twisted righ where it goes into the engine. It makes nearly a 1/4 turn twist, is this normal and could this be causing back preasure issues that would mess things up in the combustion chamber?

[BD2715]

Also important note, my header pipe is twisted righ where it goes into the engine. It makes nearly a 1/4 turn twist, is this normal and could this be causing back preasure issues that would mess things up in the combustion chamber?

It can restrict flow A LOT. It happened to me on me 250f years ago. Its caused by a fall (I think It happens on Ti Pipes not on stainless? I cant remember maybe other way around) anyways, yes that could be restricting it enough to cause back pressure!

[SledParty]

Well I bought a clean, straight pipe off ebay today, it's coming from portland so it should be here by friday at the latest. I'm hoping this is the cure all but something tells me it wont be the solution to the problem. My bike is REAAAALLY loud, abnoxious I think my neighbors are gonna call the cops on me every time I start it just to fiddle with the carb. It appears to be stock muffler is it possible to remove the baffles on them or is LOUD just the norm for the 400?

[BD2715]

Well I bought a clean, straight pipe off ebay today, it's coming from portland so it should be here by friday at the latest. I'm hoping this is the cure all but something tells me it wont be the solution to the problem. My bike is REAAAALLY loud, abnoxious I think my neighbors are gonna call the cops on me every time I start it just to fiddle with the carb. It appears to be stock muffler is it possible to remove the baffles on them or is LOUD just the norm for the 400?

IDK if it'll be the cure but it should help, if not anything it'll eliminate it:) since you need to replace it anyway...

My 426 is WAYYY loud. IDK much about the 400's

[grayracer513]

I still have not checked the valves ... Any thoughts???

Yes, but...

[SledParty]

today I put the 50 pilot jet in that I bought along with the 45 (just so I wouldn't have to make another trip to the dealer) Low and behold it almost cured the backfire problem and I do get an idle now. it wont stay idling though, it will idle low for about 5 seconds then die. If I crank the idle screw in too far it will start to backfire again so I've been trying to find a "sweet spot" for the pilot and idle screws. My header pipe will be here tomorrow when I get that I'm going to go back to stock 45 pilot and see what happens. Also I WILL finally check the valve clearances tomorrow. Anyone know of a good write up with pics? The sticky one is good but I'd like to see some pics.

Getting there :-)

Thanks!

[SledParty]

Also, I took my TPS sensor off the second or so time I had the carb off the bike (so many times now I've lost count.)

If I plug the TPS sensor in I get backfires and popping, is there a good write up on how to adjust the TPS sensor anywhere? I spent a couple hours last night digging through TT and google and most people either gave up and left it disconnected or they took it to the dealer for about $100 TPS tune.