Hawk 250-Carb Swap-Need Diagnosis

[FunkyChina]

Hello,

New to the Hawk. Stock carb was fine, but with everything I read, decided to swap it for the mukuni carb and put in a 115 jet. Set the air/gas screw at two turns out.

Idle seems to be better, but there's major hesitation when rolling on the accelerator. When the throttle's wide open, it's like the engine wants to come alive, momentarily, and then it cuts off.

What gives?

[Tashka]

it's probably rich if you're running thr stock exhaust. pull the spark plug, if its black and sooty, install a leaner main jet.

[FunkyChina]

Ok, I’ll try a 110.

Is it kinda just trial and error? Or, is there an exact method to getting it right?

[Lit Hawk]

Yes try 110 with stock but also biggest mistake for us newbies is accidentally hitting the choke up. Make sure it’s all the way down before switching main jets

[FunkyChina]

Swapped out the 115 for a 110. It ran great cold, but when it warmed up, it started hesitating again.

Move down to a 105? Or do I need to make other adjustments?

[JerryHawk250]

if it's hesitating while rolling the throttle try raising the needle one notch.

[FunkyChina]

Ok. Raised the needle one notch. And left the jet at 110. There was a funny looking piece of thin metal sitting on top of the tube in the carb. What is that for? Do I need it?

Worked a little better. Still some hesitation in 5th gear wide open.

[ChrisWNY]

That's a retention clip (the wing-shaped curved circular clip) and it's needed to keep the needle in place when the throttle cable pulls the slide. The needle goes through the small hole and the small clip sits in its position from the slide (you can clearly see that there's a recess in there for the small needle clip)...the larger retention clip you're questioning slides to the bottom of that slide housing, just over the small needle c-clip. If that retention clip is missing, pull your carb apart and reinstall it properly or you'll definitely run into issues.

Hesitation when wide open could be either a too rich or too lean condition. A #110 main jet with the stock exhaust should be just fine. Check your spark plug after running wide open to get a more definitive answer to lean vs rich. Black/fouled spark plug electrode = too rich, gray/tan = just right, white/too clean = too lean. What's your top speed at full throttle in 5th?

[FunkyChina]

I pulled the spark plug and the tip was almost completely black.

Do I need to replace it he spark plug before giving it another go?

The speedometer reads 45-50 maxes out.

[ChrisWNY]

If you're getting 45-50mph on the stock sprockets then that is about as much as you'll get. If you have a 17T/45T sprocket combo you should get into the low 60mph range with a properly-tuned carb. Sounds like you're running rich, but the question is at what throttle range is the mixture too rich. Could be too rich on the air/fuel mixture screw (which is on the bottom of the carb) or your needle is running a little rich. Did you raise the small c-clip up a notch or lower it on your needle? The lower that c-clip on the needle, the more rich you will run at 1/4-3/4 throttle. When you rev your throttle at idle, can you visibly see exhaust coming out the muffler?

[FunkyChina]

I'm getting 45-50mph on the 17/45 setup :(

Here's what I've done:

Moved the clip up one notch on the needle.
Switched to a 100 jet
Replaced the spark plug

Took it for a spin and it ran like a champ. How long will it take to get the spark plug fouled if it's not tuned properly? Would I notice the hesitation right away?

[Tashka]

if you're getting that from the 17/45, you might running lean now. Notice any throttle hesitation? or coughing?

[Tashka]

what elevation are you at?

[FunkyChina]

No hesitation since incorporating those noted changes. Ride about 6 miles and things seemed OK.

We’re right around 1000ft above sea level.

[ChrisWNY]

You were likely running rich but I would've left the main jet at #110. The problem with 3rd-party main jets (especially if you bought "Wingsmoto" jets from Amazon) is that they're not drilled very precisely, there are manufacturing inconsistencies. I've seen #100 jets drilled out larger than the #110's, in other cases they may be fine (for $10 for a whole package of main jets, I guess you can't expect precision). A #100 main jet is about the size of the one that came stock (#98). Moving your needle clip up one notch was likely what resolved your issues - you were running rich at 1/4-3/4 throttle, which is where the needle clip position factors in.