2000 400ex backfires through carb/won't idle

[ontario400ex]

Bought a 2000 400ex, all stock.

I've been through the search feature, one by one, and I've checked/fixed/replaced things based on those suggestions. I'm an auto-mechanic by trade, and I've had 2 stroke bikes and jetski's throughout my younger years.

Anyhoo...
Bike ran fine when I demo'd it before buying, but when I got it off the back of my pickup truck, it would barely start, and then it wouldn't idle without playing with the choke, and then it would backfire through the carb. It backfired so badly that it blew the boot apart that bolts to the cylinder head.

None the less, I replaced that, thinking maybe it had been loose and allowing air in (leaning it out), no different. Still backfires and won't idle.

I took the float bowl off of the bottom of the carb and blew carb cleaner and air through each of the jets. I took the jets out next and made sure they were clean. My pilot jet read something like 138, and the main was 148. I'm not 100% on those numbers, more like 98%. Jets were clean so I reassembled the carb. No difference. It still sputters, backfires, and won't idle unless I play with the choke, and even then it will still die after 2 or 3 seconds.

Pulled the sparkplug.... CAKED with black powdery carbon, and looks like had been in there since... well, quite a long time. I replaced the plug and gapped it to .035 (as per the NGK catalog), Still sputters, coughs, backfires, etc.

I removed the plug again to check the spark. It looks faint compared to what I'm used to, but it sparks consistantly. The end of the spark plug after 30seconds of sputtery running is slightly black and runny looking. i.e. severely rich)

I then unscrewed the sparkplug cap from the coil wire, and snipped off 3/8" so that I could see fresh core wires. I held the wire a 1/2" away from the cylinder head, and it would jump that gap. The spark was by no means super strong bright blue, but it was blue and I could see it jump the gap. I screwed the cap back onto the wire, reinstalled the plug, hooked it all back up, its still coughing, sputtering, etc.

I was going to test the compression while it was still slightly warm, but I don't have an adapter for the goofy tiny plug thread on these engines. I managed to get my thumb down the plug well and it very easily blew my thumb off with hot-compressed air.

I'm very wary on ignition timing or valve timing on these engines, because of past bad experiences.

If there's anyone who can direct me on this forum, or if anyone is in my area (Hamilton ontario/canada) please help me out...

I just want to ride this thing, even if it's around the parking lot at work..


Much thanks in advance for ANY useful replies,

-Scotty

[GPracer2500]

I assume the air filter is clean? No obstructions in the intake tract? Fresh gas in the tank?

BTW, 38 pilot jet and 148 main jet is the stock jetting.

[ontario400ex]

I'll replace the fuel tomorrow... Air filter is new.

I should also add that it black-smokes like it's hammering the fuel to it, (probably due to the choke) but that's the only way she'll run.



-Scotty

[GPracer2500]

I'd remove the carb and clean it properly. The pilot circuit being clogged could be your trouble. The rich symptoms could simply be from running it with the choke on.

[ontario400ex]

it most definately sounds like pilot related...

if I can manage to rev it up past the stumble phase (1/2 throttleish), it will pick up and rev, but it won't idle worth a Fart and doesn't stay running.

But would the pilot problem cause it to backfire through the carb?

If I close the choke to keep the engine running, the 'backfire' flap on the choke plate oscillates about 45° out while it's running...

I know engines have a 'pulse' that comes out of the intake, but that seems pretty violent.

Carb-cleanout tomorrow as well... if the dealer phones with my clutch cable, I'll see if they have one of those magic 42 pilot jets as well...

What should I ask them for? Do I have to tell them what carb?

Thanks for the help so far!
-Scotty

[GPracer2500]

Stock pilot jet type is a Series 26. They are not as common as the slightly longer Series 21. The Series 21 will work fine though. The "Series" is the style of jet and is independent of the actually "size" (e.g. 38, 40, 42, 45, etc).

Have you touched the fuel screw at all? Make sure it's not screwed in all the way--that would cause it to run like that circuit was clogged. It should be somewhere around 2 turns out from lightly seated.

[400eXr1d3rZ]

From what i'm hearing, the pilot sounds like it's clogged. Replace the 38 pilot with a 40 pilot and it'll start like a dream. Carbon buildup is from running rich, and like GPracer2500 said, it's probably from running with the choke on all the time.

[jrm03]

I'm roughly in the same boat you are. When cold it does it terribly. I adjusted the needle by raising it one notch on the clip since it was around the half throttle position when the carb was bucking around from the backfiring. And also played with the fuel screw from the idle position and it helped but I have not cleaned it up completely on a cold engine. Your not alone here guy.

[christophere251]

cam timing is a breeze on thes motors. Just take the rocker cover off then look on the case by the shifter and youll see two plug wholes Remove the big plug and it has a bolt in there u can turn. Use this to turn the crank. (18mm wrench i think) Then there is another sight plug right above it u turn the crank till the tdc mark lines up. Then look at ur cam and it will have 3 hash marks on it one points straight up the other two are horizontal. The horizontal ones line up with upper surface ofthe head where the rocker cover bolts on!@ timing should be dead on.

[ontario400ex]

got it runnin great now!! :)

brand new (looks updated) intake boot from honda, seal.

Pulled the float bowl back off, blew carb cleaner through the pilot and gave her everything I had.... Took the 38 pilot out, went to the nearest dealer for a 42, but it was the 'longer' style pilot. They wouldn't sell it to me, but they sold me a 45 instead... It looks the same as the 38 I took out.... I figured, what the hey, for $5, I'll give it a shot....

Drained the fuel tank, went and bought a tank worth of new gas, mounted it temp. on the frame.

BLAMO!!!

Fired right up, no choke, no nothing. No black smoke at idle or during acceleration.

Pulls smooth all through the powerband, no ticky noises, no chain rapping on the cases, nothin...

For a second I was ready to drive this quad back through the guys' shop..... 'cept it wouldn't run... lol

Can't wait till this weekend. Thanks for all the help :)
-Scotty