400 ex smokes

[catch2otwo]

alright another brain buster for you dirty guys.


Brother's 400ex smokes a lot. We put new valve seals in it and it seemed to smoke just the same. is there anything else besides rings? backfires alot when you let off, and likes to stall when coming to a halt. its got an aftermarket header and some crappy hollowed out can. any ideas?

[KB]

Do a compression and leakdown test. That'll tell you what's wrong.

KB

[butcher bergs]

Brother's 400ex smokes a lot. We put new valve seals in it and it seemed to smoke just the same. is there anything else besides rings?

Too much oil?

backfires alot when you let off, and likes to stall when coming to a halt. its got an aftermarket header and some crappy hollowed out can. any ideas?

Running too lean. Fatten up the main jet.

[636]

backfire on decel is air/fuel mixture is off....if it smokes on decel it would be blow by

[catch2otwo]

k compression check will be in order. whats a leak down test? im assuming both of these tests will indicate whether the ring needs to be replaced?

636: i know it smokes on acceleration, dont remember if it smoked on decel. ill have to check that.

[636]

if it does it on both then rings or scored cylinder

[catch2otwo]

if it does it on both then rings or scored cylinder

gotcha. i havent run it since the summer so i dont remember. hes got the 416 kit sitting in his room, so im sure that would take care of the problem. id just like to know for my own knowledge what it could be.

[R1]

That is exactly what happened to my 400ex before i seized it. Ill take pics of the old piston tomorrow, but it turned out to be a scored cyliner and bad rings.

[catch2otwo]

hows your rebuild coming along? almost done?

[R1]

hows your rebuild coming along? almost done?

as of right now its not, i am really busy, i just dont have the time to put into it right now. I figure i might have some time in april.

[SSearchVT]

A leakdown test is where you blow compressed air into the cylinder with it at TDC - then listen for where air might leak out. If you hear it through:

Oil Fill hole - bad rings, scored piston, scored cylinder
the Carb - bad intake valve
Exhaust Pipe - Bad Exhaust valve

Can you put the stock pipe and can back on? If the backfiring goes away - It's the jetting. Otherwise, pull the spark plug and see what it says. White - means lean condition (backfiring). Dark means rich condition.

When does it smoke, and what color is the smoke? Constant blue smoke is from the piston/rings/cylinder. Puffs of blue smoke when you let off, or get on the gas is valve guids/seals. White smoke that smells woody is coolant, white smoke with no smell can come from gunked up pilot jets, or bad leaks in the intake.

[gmdboston]

k compression check will be in order. whats a leak down test? im assuming both of these tests will indicate whether the ring needs to be replaced?

636: i know it smokes on acceleration, dont remember if it smoked on decel. ill have to check that.

A leak down test measures the pressure differential between an air source (you will need a med-lg air compressor for the source) and the pressure retained in the cylinder. It's expressed as percentage. i.e. 100% means the combustion chamber retains 100% of the inlet pressure. Readings of 80% or more are ok, over 90% is best, race motors we look for 98%. As stated earlier you can also pin point the faulty sealing surface by finding where the pressure escapes. I have one if you need to borrow it.

Before you tear into it, start with the basics. ( I know it goes against every thing the internet stands for....) Clean the air filter, disassemble the carb, remove and clean EVERY jet and passage with carb cleaner and compressed air. Most after market pipes and mufflers do not require rejetting to run well. Check the jet sizes, pilot screw setting and float level and make sure they are stock. (although rejetting can help it run better, it should run fine just bolting on the exhaust to an otherwise good running motor.) Also check the needle and slide for wear.

Good call on the oil level, drain the oil and refill properly.

If none of that changes your problem, then it's time to look inside the motor

[catch2otwo]

thanks for the info guys. ill have to run it before the weekend. but as i can remember it it was bluish white smoke. im definately doing all the simple things first. made that mistake with my rm, tore the whole thing down just to find out it just needed a smaller pilot jet.

will the exhaust matter all that much? i dont have the stock pipes because the bike came with the exhaust its got on there now.

Pete do you guys do rebuilds if we do find out its in the motor? got a 416 kit ready to go.

[gmdboston]

If I'm not too busy I will take on work other than suspension. Let me know what you find.

[catch2otwo]

will do. ill take a look at it tonight and report back.

Another thing is that my brother might want to just put the 416 in it for a bit more get up. Would it be a bad idea to put the kit in before this problem gets fixed?

[SSearchVT]

It's never a good idea to just throw parts at something without knowing what's causing the problems. If the problem turns out to be a scored cylinder the big bore kit will take care of that. BUT - what caused the cylinder to score in the first place (bad oil pump?), and was anything else damaged at the same time?

[catch2otwo]

ya SS thats what i was thinking. The idea isnt to throw the kit in there just to fix the problem, he got it for Xmas and hes been wanting to get it in. Either way ill be going over all the basics you guys talked about and hopefully its just something simple like rings.