Need help!! 250sx still fouling plugs

[86trizinger]

I have been trying to get my 1985 250sx running for a while now with no luck.
Here is what is has:
-Top end is rebuilt 1mm over
-head completely rebuilt with new valves an valve springs
-Bassani Exhaust
-UNI Filter
-350x carb - rebuilt with new jets

I know other people have run this carb with no issues at all.
If I put a new spark plug in, it will fire right up with no issues at all. It will run nice and smooth. Once the engine cools down it will not fire. I pull the black and it is black with fuel. If I replace the plug again at this point it will fire right back up.

So far I have tried adjusting the carb with no effect. I checked the spark as well, with a new plug it is a very strong blue spark.

The one thing I noticed strange is that every once in a while if I rev the engine a few times and then shut it off fuel will drain drain out of the hose off of the bottom of the carb. It only does it for a second and only once in a while. I didn't see any way to adjust the float for this carb.

I am thinking something is going on with the carb. Any help would be great.

[Dirtcrasher]

Well, your float valve may be hanging up but you seem to be too rich anyhow.

What main jet are you running?

Where is the air screw set at?

Have you rebuilt the carb? Like you said, the floats are not adjustable. Just the rubber tip that wears out on the valve can be an issue.......

[86trizinger]

I am running the stock 350x jets. My 250x jets were different. Looking but had the same size on them. The float needle is new.

I tried setting the air screw at different amounts. Right now it seemed to run the best at 1 3/4 turns out.

[Dirtcrasher]

Without an exhaust an airbox mod, what makes you think 350X jets will work for you? Ever race an SX against a 350X?? :D They can't be the same size.

Seriously, I know when I popped on a 350X carb on my 87SX I was able to jump the SX jet sizes up a bit due to my Cobra (uggh) and open air box; But the 350X main was far too rich.

That 1.75 turns out on the air screw may be great for starting and idle, but you've still got a needle/pilot and main jet to deal with.........

[86trizinger]

I am running a bassani exhaust with no air box top. When I got the 350x carb rebuild kit the jet numbers were the same as what the stock carb had in it. I thought that was a little odd. Only difference was one jet was longer.

Should I order a smaller main jet or just order a 250sx kit and start from there.

[Dirtcrasher]

I'd change that jet that didn't match and drop the main a couple/few sizes and read the plug. That one long jet may be a problem.

That is if your certain it is not oil fouled from rings or valve seals........

[tri again]

About the 3rd time thru my qa01 85 carb I noticed a little
emulsion tube Under one of the jets that I kept missing.

About 1/2" long with holes in it and clogged to death.
Not to second guess a complete carb cleaning but I missed it
2x.
Also good to blow out internal passages, just because jets are clean or new doesn't
account for the internal passages.

It was good because I had a good carb and the misbehaving one so I
could blow air into all the jets and passages under water and look for bubbles.

That one clogged carb was a real pain but I had a known working unit to
compare it to.

Now you say 'other people' have run that carb without problems?
THE same carb? or one like it..

If so, then I'd be tempted to heat up electrical components with a hairdryer until something craps out
to duplicate your hot/cold situations. or cold spray from the first aid section at the drugstore...you know what I'm trying to say.
Compressed air works well if it's cold out.

There's also a common electrical connection near the battery on the sx's that comes loose.

A far as overflowing for a second, sometimes.
I have a couple that do that but they run good and the extra fuel is burned to sorta 'compensate.'

I've also noticed 2 different length float needles in identically marked carb kits.

Feels like going in 2 separate directions at the same time.
See if you have good spark around the time you know your plug is going to come out wet.

Intermittent spark could be it too.
You're also in that zone of trying to match a trick carb and exhaust so jetting is not a specific science.
That float overflow has me wondering too.

These are the kinds of problems that interfere with my linear logic.
so I apologize for the scatter approach.
Maybe it will trigger some useful sequence for you.

You'll get it.

We always seem to somehow.

[86trizinger]

The emulsion tube was replaced. I think I am going to start with a new rebuild kit to make sure o got right kit.
Should I start with the 250sx carb kit and get some bigger main jets for that?

[86trizinger]

Just gonna give up on this for now, I have a camping trip in 3 weeks I need this bike for.

I found a rebuilt stock 250sx carb with choke and throttle cable with the throttle housing for under $50 shipped.
I'll tune that carb for now. It has enough power for right now.

[Thorpe]

How did the new rear shock pan out in the backend of your SX?

[86trizinger]

the rear shock is all mounted on. A little stiff at first so I adjusted the setting on the shock. That helped some. With my weight it feels a little on the stiff side but a lot better then my stock shock was. My youngest son rides with me most of the time. His extra 50 pounds makes a huge difference in how it feels.

I plan on making a rear rack for it so that should make the suspension perfect for me. I went over my sons jump a few times and it springs up the back end a little too much but lands so smooth. I really don't race or jump it. We just got permission to ride on the 800 acre farm behind my house so it's just trail riding.

Once I get the carb sorted out I will be able to give a better review on the rear shock. I haven't had enough time to ride it since I rebuilt everything.

[RubberSalt]

When putting a carb on a bike that the carb wasn't designed for, usual everything needs rejected.

Put a new plug in. Get on the bike and hold it wide up in 3rd gear or higher for 30 seconds or so. Kill the bike at speed and then pull the plug. If the porcelain around the electrode is black or wet, get a smaller main jet. If it's white then get a bigger main jet.
Keep doing this until it comes back brown, like cinnamon.

Do this all again with 1\4 throttle. If black, lean out the fuel/air screw. If white, richen up the fuel/air screw. Crack the throttle idle to 1\4 - shouldn't bog. After 2.5 turns out, look for a new pilot jet. If less than 1 turn out, look for a new jet.
Do all this again only giving it 1\2 throttle, this tells you where where you need the needle. If black, hen lower needle, if white, raise needle. If needle all the way on 1 side and still not right, then you need a new needle. If the needle is all the way up, then you need a skinnier needle. If it's all the way down, you need a fatter needle.

After tuning, you should be able to get a nice crisp throttle response. Crack the throttle from idle to 1\4, make sure it no bog, then from idle to half, then idle to wot. If it bogs, usually too rich. If it hesitates, usually too lean.

[RubberSalt]

Also, make sure you don't have a leaky intake boot and your head is torqued to spec. New top ends can rattle bolts loose. Usually that causes a lean situation.

Do you have the air box on? if so, try removing the lid or something, what ever makes it less restrictive. I know with the stock air box on my YT, my blaster carb was rich. Without the air box and with a clamp on foam filter, it was ultra lean. Air boxes can rob tons and tons of power.

[86trizinger]

Everything on the top end was taken apart and re-torqued. It literally only has 15 minutes on it. I am pretty sure there is an issue with the carb but I am just going to put a stock carb back on it for now to see if that helps.

I run the stock air box with UNI filter and no lid.