Hawk 250 Throttle Hesitation

[DHH 32]

Hello everyone. I just did a valve adjustment (.002 in, .004 ex) and installed a mikuni carb with 110 main jet. The bike started up great and I could hear the valves tapping slightly. The only problem is that there is a slight hesitation in the first 1/4 inch of throttle. What are the most likely causes of this? It never happened with the stock carb. FYI I live in the Houston, TX area, elevation ~100ft. I have done the airbox mod but still have stock exhaust.


Question on the valve adjustment. It was a completely cold engine. I adjusted the valves to where the desired feeler gauge had slight resistance. However, I could squeeze the next size up in the gap with a little force. Two sizes up could not fit at all. Is that ok, or are they too loose?


Thanks!

[JerryHawk250]

Raise the needle one notch from the center notch. Have you adjusted idle mixture? May need to richen up a little.

[DHH 32]

I set the mixture screw 1 3/4 turns out. So you’re saying make it ~2 turns?

[ChrisWNY]

I went 2 turns out on mine (Mikuni VM26), it's a good starting point. Idles very well. 1.5-2 turns out is ideal because you're right in the middle of that range.

Don't adjust the idle until the engine has fully warmed up. 3 turns out on the fuel mixture screw is fully open, so if you find yourself turning that screw more than that you probably need to change out your pilot jet.

If you adjusted your valves on a cold engine, you may need to re-check them soon because chances are your gaps are too wide on the valves.

[DHH 32]

So you think the engine should be warm for valve adjustment? I researched it for a week and found 1000 different answers on proper valve lash and whether or not the engine should be warm. Haha. What are the symptoms of valve lash being too big?

And just to make sure we are talking about the same thing, I turned the air/fuel mixture screw on the bottom of the carb out 1.75 turns. I left the idle screw on the side of the carb alone because the bike idled fine.

So the hesitation is caused by being too lean? Should I go with 115 main? Does the nearly 100% humidity of Houston affect this?

[ChrisWNY]

The main jet only applies when you're up above 70% throttle to wide open so it won't really make a difference idle to 25% throttle. If you've opened up (modified) your airbox and added a wider performance exhaust then I would say a #115 main jet will give you more power when your throttle is opened up...smaller jets will run things too lean with that extra airflow through the engine. If you're using the stock main jet on the stock exhaust, for example, I'd still go up to a #105 or #110 if you're at lower elevation (below 1500 ft or so). Better to run a little rich than lean.

Symptoms of too much lash (gaps are too wide): Your valves will tap loudly and you'll have noticeably less engine performance/power and top speed if the gaps are too wide. There seems to be a consensus though that too much lash is better than too little. If the valves are too tight they can literally overheat and burn out. If they're loose there might be a little extra wear & tear plus power loss but valves will seat better and tighten on their own over time. At least that's what I've read.

I had the exact same experience as you with valve adjustment and research - I read 100 different schools of thought on 100 different websites. Ask someone whether to adjust valves on a warm vs. cold engine and you'll likely get different answers. From my experience, when I adjusted the valves cold, the gap was WAY too large and the valves were noisy afterward, that was going with a 0.004 intake/0.005 exhaust gap. I readjusted the valves with the engine warmed up and everything seems perfect now. Valves can be heard tapping but it's a quiet tapping compared to the hell'ish rattle I was hearing before lol...it literally sounded like my chain was rubbing over the engine

[Azhule]

My China Bike valves are happiest getting an ice cold chiropractic adjustment

[Gh426]

So has anyone done an ice cold valve adjust at .001 / .002 ? and gotten happy results?

I hate working on hot / warm stuff, its oily etc.

g

[roundhouse]

I did my adjustment with the bike 100% cold. both valves at .003 and it runs like a champ with just a hint of ticking. WAY better than it was from the factory.

[DHH 32]

So just an update. I haven’t adjusted anything since my original post. The bike idles great and does ok at low rpms while cold but as soon as the engine fully warms up it starts bogging pretty bad. It actually died completely once. Based on some research that indicates a too rich condition. Is that right? There are so many different opinions online that I’m getting more and more confused. Haha.

[Digger]

Lot of people on this site have more experience with Hawks than I do...,but in my years of adjusting valves .001-.002 is too tight. On my Hawk i set mine to .005 intake and .006 exhaust. Stone cold engine, engine runs perfect.

[DHH 32]

I adjusted my valves to .002 and .004.

I just took my spark plug off and there was a lot of fouling that wasn't there 2 days ago before my new carb/110 jet. Definitely too rich. Now the question is how to fix it. Do I drop down to 105 main jet, adjust the mixture screw on bottom of carb or adjust the needle?

[DHH 32]

What size pilot jet comes on the Mikuni VM 26? Where can I find replacement pilot jets? I can only find main jets. Thanks.

[Ariel Red Hunter]

Originally Posted by DHH 32

I adjusted my valves to .002 and .004.

I just took my spark plug off and there was a lot of fouling that wasn't there 2 days ago before my new carb/110 jet. Definitely too rich. Now the question is how to fix it. Do I drop down to 105 main jet, adjust the mixture screw on bottom of carb or adjust the needle?

It depends on your riding style. If you are mostly riding around the neighborhood at 25-40 mph, needle position is the most likely culprit. Another is are you sure the choke is fully open? My advice is only buy genuine Mikuni main jets and pilot jets. The others are not accurately made...ARH

[ChrisWNY]

Agree with ARH - if your plug is fouled up after a general check (most don't do the wide-open throttle test and kill their engine, then check the plug) it's likely the Mikuni needle position. You may be running rich from 25-70% throttle and be fine at wide open. A #110 main jet is a pretty good size and should work well in most situations. Anything smaller than that is likely to result in lean running conditions at full throttle.