Hawk 250 decided to retune itself?
 

So about a week and a half ago I ran out of fuel earlier than expected (guess I'm getting more comfortable on it), and had to switch to the reserve. I went straight to a gas station and filled her up and didn't think anything of it. seemed to run normal the rest of my ride to work (had spots of weak power like it was running a bit lean). on the way home later that day the weak power spots seemed to be worse, so I took the stock carb off and sprayed it down with carb cleaner I never removed the bowl(didn't want to drill it). It was late so I didn't test ride it.

The next day we were heading out for a ride and it started driving like absolute garbage. seemed to be able to throttle through it, but don't have any useable mid range, super low or WOT. We hit some trails and did some wheelie practice other that limited throttle range the bike rand "fine".

The next day we had plans to pull the carb and drill the bowl off. Did that, cleaned the carb, had problems with the gasket swelling (anyone know how prevent this?) ended up having to remove some of the O-ring bowl gasket to shorten it.

To make a week long story shorter:

After putting it back together and many disassembly/reassembly iterations later I have diagnosed that my needle jet has grown in size all by itself.

The bike Idles fine. From 1/4-5/8 the bike is rich as all get out. 5/8 and up feels fine(might be a little lean, or only make 14hp).

I have made many adjustments to the pilot screw and none seem to have any effect.

Im almost positive my problem lies in the transition from pilot circuit to main circuit. But what would cause an unmodified bike to suddenly change like that?

I have a rebuild kit for the keihin clone on the way, and a mikuni clone coming as well. Its a race to see what gets here first. This has been so frustrating, and the worst part is I can't even go for a ride to relax :cry:



Problem Update:
Check your air Filter for restrictions. Mine was soaked in oil starving the engine of adequate airflow.

Sounds to me like the pilot jet is still clogged. I'd take it back apart and reclean.

You had to shorten the bowl gasket? I believe the bowl has to be completely air tight. Did shortening the gasket cause a possible air leak.


/

By the way its interesting you mentioned the bike retuned itself. The same thing happened to me. My idle speed dropped way low and the fuel mixture appeared to be way off. I think maybe its because we had a weather cold front come through. Went from mid 80s and extreme humidity to 0 humidity and mid 70s. Could a weather low pressure front cause this?



/

A word to swollen, gasoline or solvent-soaked gaskets and o-rings.
These shrink back into their proper size, after airing and drying thoroughly.
The same goes for diaphragms, for example, in cv-carburators. (That good quality parts shouldn´t swell and soak at all or that much... is a different matter...)

Carburator bowls don´t have to be airtight. In fact they have vents...though these days, they´re led into some emission control systems.

Of course, the bowl should be liquid-tight.

I go along with the thesis of a clogged idle (primary) jet. (..but the needle valve may be cleaned, adjusted and tested as well.)


regards

Panic-Pete

Slide or diaphragm carb?
 

I'm not at all familiar with your bike, so only speculating here.

When you took the carb apart, is there a chance the slide got installed backwards (turned around)?

Is it possible the needle got dislodged and pushed up higher in to the slide?

Every thing that Pete says holds true, too.

Sorry for the late response. So I've gotten a rebuild kit for the stock carb, that has an adjustable needle. I also received my mikuni clone. Both carbs have the same effect. The mikuni came with a 20slow and 100 main. Whenever I do a plug chop the plug is black as all get out. Everything is where it's supposed to be. I'm soooo confused. Earlier this evening I was futzing with it and it idled best on full choke, and would eventually die after I opened it up. Anything after idle, choppy sputtering and backfiring. Could it be ignition related? Arrrrrgh

As for shortening the gasket, i let it sit overnight and even tried putting it in the freezer there was just too much material. I took the material out of the straight sections on the side and it holds fuel just fine.

The stock Mikuni jetting is way too lean, unless you're at 10000 feet.

My concern and what is so confusing, is at any amount of throttle when it gets all choppy the plug is black and sooty as can be.

Black and sooty? The stock jetting should be showing lean....REALLY lean. Like Weld says, 10,000 feet lean. Did you by chance knock the float setting when the bowl was off? They're very delicate and can be tweaked easily. Black and sooty usually means dumping fuel.

I know. Float level is in spec. This is why I'm so confused. Nothing is making sense.

What RPM was the plug chop performed at (roughly)? Idle, part throttle or WOT?

The engine usually starts breaking up 1/8---1/2 throttle. I've done a few of them, the most recent were at about 1/4.

That could strictly be clip position. Clean the plug, and try a plug chop at idle.

Any chance the jet needle was installed with the clip above the spring thing that holds it in place rather than below it?

No the needle is installed correctly. I took it down the road today and found that once it gets through the lower rpm's ( whenever it can stumble it's way through them) it seems to run good, albeit lean as all get out. I'm beginning to wonder if this is an ignition related issue. Hnu13 has one that I can borrow a cdi from for testing purposes. While I was tinkering with it I noticed that a part of the wire harness was pinched between the fuel tank and the frame and has rubbed a bare spot in the power wire running from the ignition to the cdi( no break just bare) so I reinsulated the wire hoping that was the problem, it wasn't.

While searching the interwebs for a nugget of information to help me solve this dilemma I ran across a guy with a 2stroke who was having a similar issue. In the comments below his vid he posted what he discovered a bolt had backed out of his stator allowing it to wiggle back and forth throwing his timing off. Has anyone else heard of this happening?

So that's my update. Oh yeah, weldandgrind I did a plug chop at idle and the plug looked fine. Not sooty not lean.

Thanks for your help everyone.

Good job on finding the bare spot. That certainly saved you future trouble.

Have you tried a high-rpm plug chop?

It wasn't quite a plug chop, but when I took it down the road earlier I had to run at high rpm/WOT. When I checked the plug after that it definitely looked to be lean. It's all the middle rpm's that are giving me trouble, near as I can tell throttle position is not having s much effect as rpm range is. Idle is fine and higher rpm's are good too but all those revs in between are not smooth at all. I'll try and get some video this weekend, I recorded my ride earlier but I had it in low light mode so it's kinda useless.

I'd move the clip down one notch, and fatten the slow and main jets.

Gonna try and get some jets tomorrow.

I figured I should update this since I hate when op's never post their solution.

I finally gave up and just took it to a shop. For just over $200 they jetted my mikuni carb. I was also informed that my air filter was soaked with oil and would need to be replaced.

IMO this information about the air filter has been my problem all along. With an air filter that is soaked in oil my engine was being starved for air. Causing the unexpected rich condition. I have since installed a cone air filter. I put a rag in the hole where the intake tubed was attached to the air box. Im not sure what I'm going to about the breather tube. I might just make a new "air box" That functions better as a tool box. and has an integrated crankcase breather/catch can. We will see.

I just cut a piece of open cell foam (about 1/2" thick) to fill in the 'trapzoid' shape at the bottom of the air box that the breather tube goes to. The breather tube is still attached. You can find small cone air filters to go on the breather tube/outlet, but I never found one that fit the location without interfering with either stock setup or the cone/foam filter alternates. I also found that my breather tube kept splitting so I replaced it with clear tubing that was thicker and can now see if I am getting significant oil suction from the crankcase.