DT400 advice needed! Sounds like it is 4 stroking??

[dr30]

Hi everyone,

Me and a few mates recently finished piecing together our dt400 bobber conversion. This site has provided a great deal of info for us during the build!

Our problem is this- we are able to start the motor, but it sounds like it is 4 stroking- just an intermittent "bang - bang - bang" as opposed to the regular idle. If we give it some throttle it just dies. It is blowing an absolute shit load of smoke the whole time it runs.

Our motor is out of a DT400b, with DT400c stator/Flywheel/CDI/Coil.

We have disassembled the carb, re-adjusted the float levels to factory, checked all vents, cleaned the pot, checked it again..and again and again. Still nothing. Having said that, we all have pretty limited knowledge on our to tune carbs.

When it is running it really sucks down the petrol, and we are getting a fair bit of petrol blowing back out of the carb.

We had the timing set at factory, but are yet to chuck a timing light on it. Last night I retarded the timing by 1.5deg but it made no difference.

We do not have the oil pump hooked up, and are just running pre-mix- could this affect the carbs performance?

Could it be an issue with our CDI or timing?

Could or exhaust be stuffed- i.e, expansion chamber is not working properly?

One interesting note is this- we have just been feeding it petrol through the fuel line (tank is still being painted!) - so it just runs until it runs out of petrol. Just before it runs out of petrol, it starts to idle normally. This really makes me think that something is not doing its job in the carby, and there is too much fuel in the pot...

Can anyone help us? Has anyone ever experienced a similar problem? We need to try and get her running by tomorrow for a bike building competition we have entered!

Cheers
Matt

[Viper79]

Is the jetting correct, float not leaking (full of fuel), needle and seat faulty, choke valve not stuck.

Have you checked the crankshaft seal on the gearbox side?  Could be sucking gear oil into the mix..

[dr30]

Thanks for the advice- we will go through and double check all those things again. We hadn't even thought about the crankshaft seal being faulty- good point.

Update: We just tried cutting of the fuel and spraying some WD40 into the cylinder, but it was still not running correctly.

[dr30]

I should also make mention that compression feels good- the arches of my right foot are nicely bruised from kicking it over so much (decomp pin is'nt hooked up).

[tony27]

Get a new float valve assembly if the float is ok, should make sure that the fuel flow stops when there is enough in the bowl rather than continually flowing

[dr30]

Thanks Tony27- we will give it another test.

If the carb is faulty though, wouldnt it run ok when we sprayed some WD40 into the cylinder and cranked it?

[tony27]

Wouldn't think so.
Have had quite a bit of experience of the sinking float problem lately with my yz465, 1 float set had a hole rotted in the bottom of 1 float, the other set cracked on 1 float.
First thing I replaced in the carb when I bought the bike was the float valve which had been not sealing off properly

[dr30]

Ok- definitely have to re-check the floats when I get home tonight. I thought we had it set, but maybe they are still faulty...

[tony27]

Give them a shake then drop in hot water to confirm if they're ok, they'll either sink or start fizzing as the air & fuel trapped inside heat up

[dr30]

They were floating up fine last night, so if there is an issue there it must be the valve itself

[mitch75]

I had a dud pulser on my DT 360 it sounded like it was firing on every fourth stroke. Do a resistance check on the green/white red/white wire. Not sure of specs though

[MX?]

Can't help with your problem, but.....A DT 400 Bobber?
That's got me intrigued, so I had a wee Google and came up with a DT250.
Do you have any pics of yours?

[oldyzman]

Does the carby have an overflow pipe? If there is no overflow then you will not know its flooding either. sounds like its flooding due to the float or needle and seat.

[pancho]

 Does that model Yam have the "O" ring around the outside of the discharge nozzle?
pancho.

[dr30]

mitch75- I hope its not the pulser- but i have a feeling it might be. We are getting a good spark, and timed it as per the workshop manual. Just need to get a strobe light on it, as ours wasnt working.

MX?- yes- thats the Gravel Crew DT250- very nice bike!

oldyzman- yes, there is an overflow pipe, and it has been leaking every now and again- mainly when it doesnt start. When it does fire up, it doesnt leak.

pancho- I am not too sure- I will have to check..

Well, we couldnt get it going for the bike building competition on Saturday, but we still had it on display. Here are some pics-




We had a lot of people at the show saying we would never get it going with the exhaust- however underneath that big diameter stainless tip we were still running the much smaller diameter outlet of the original expansion chamber. And I would think it would still run much better than it is now even without a muffler?

Another thought we had was that maybe because the stator and flywheel were off a newer motor, the timing was out (due to the key on the crankshaft being in a different location)- but i have measured 2.9BTCD and the timing marks on the stator and flywheel are all lining up??!