400ex popping and backfiring at startup

[250xfreak]

Just got done rebuilding my 400ex, 10:1 wiseco piston, stage 2 hot cam, new timing chain, full LRD exhaust, Big Gun rev box, it will only start by pumping the throttle a couple of times and after it starts it runs rough, popping and backfiring, checked the plug, its black, timing is correct both marks flush with the top of the cylinder head when the flywheel is on the T mark, cleaned the carb, needle is on the 3rd groove from the top, has a 155 main and 42 pilot, tried a 38 pilot and it made no difference, tried the fuel screw and that didn't do anything either. Going to recheck the valves tomorrow. Anyone have any other ideas?

[Drfat400ex]

Originally posted by 250xfreak
Just got done rebuilding my 400ex, 10:1 wiseco piston, stage 2 hot cam, new timing chain, full LRD exhaust, Big Gun rev box, it will only start by pumping the throttle a couple of times and after it starts it runs rough, popping and backfiring, checked the plug, its black, timing is correct both marks flush with the top of the cylinder head when the flywheel is on the T mark, cleaned the carb, needle is on the 3rd groove from the top, has a 155 main and 42 pilot, tried a 38 pilot and it made no difference, tried the fuel screw and that didn't do anything either. Going to recheck the valves tomorrow. Anyone have any other ideas?

Sounds really lean, even though your plug was black, if you don't ride it for a considerable amount of time at different RPMs the plug will be black when you check it. When you turn it off it cuts the spark so unburnt fuel, which is the black you see, gathers up on the plug because it's not igniting. Go up to probably 170 and work down.

[250xfreak]

I have either a 165 or a 168, I will try that tomorrow, I did ride it around the yard the other day and it seemed to be ok through the powerband, just not at idle or at startup.

[dxcody]

Not to cause debate, but Rev boxes have been known to cause troublesome starting.


However i will agree you are prolly running a little lean.

Full exhaust and 10:1 piston will prolly run you in the lower 160's.

[ben300]

you probably need need a 165ish + main....and id probably bump up to a 45 pilot...


i have a stock bore stock piston 400, hmf slip on, no lid on the air box, uni filter and snorkle removed and im running a 162 main, 42 pilot, and the needle on the 3rd clip...


just my thoughts would be if your are seeing these problems on idle and start up...try 165 on the main, and 45 on the pilot....that'll probably get you close

[ben300]

you probably need need a 165ish + main....and id probably bump up to a 45 pilot...


i have a stock bore stock piston 400, hmf slip on, no lid on the air box, uni filter and snorkle removed and im running a 162 main, 42 pilot, and the needle on the 3rd clip...


just my thoughts would be if your are seeing these problems on idle and start up...try 165 on the main, and 45 on the pilot....that'll probably get you close

[250xfreak]

Thanks guys, I also read to check the carb boot between the carb and head, I did notice that when the marks on the cam sprocket line up on the head that the T mark is about 1/8 past the notch in the stator cover, is this ok, or does the T need to be exactly perfect? I timed this quad the exact same way as my last 400ex and that one had no issues.

[TheAwesomehonda]

That same thing happened to me with the sprocket and T mark not lining up exactly at the same time. That was caused by a stretched/work cam chain.

[250xfreak]

Put in a 165 main & 50 pilot and now it runs perfect, never thought a full exhaust would require that big of jetting.