New Hawk 250 - Flat Spot Between 3/4 - Full Throttle. Is This A Rich Condition?

[HawkTwoFifty]

So I have a new Hawk 250 which now has a total of 11 miles. I have decided to do a 'hard break-in' on the engine of my new bike (thanks to the advice of very intelligent and informed individuals) and according to recommended procedure, tonight was the night for which I was ready to finally work my bike up to a couple full throttle accelerations. My stock bike idles great, has a clean and quick accelerating mid-range, but then it seems to fall flat on its face from 3/4 to full throttle! For lack of a better description, it feels like a struggling, unclean, un-crisp acceleration from 3/4 to full throttle!

I have heard many times now that the Hawk 250's come from the factory with a lean carb condition. But researching rich vs lean carb conditions, I have been reading that what I am experiencing may be a rich condition! Can this be a rich condition from the factory that I am experiencing? Or is this just a normal condition of a stock Hawk 250 where the engine has not "loosed-up" yet from being so new?

I have a new VM26 30mm upgraded carb that I will soon install on my bike and have ordered the recommended pilot and main jets which will not arrive for several days. But If my stock carb is actually running rich (which seems to go against what I am hearing from other Hawk owners) I am wondering what pilot and main jet I will actually need to run in the VM26 (30mm) carb. Can anyone help on this one?

When I bought the VM26 30mm carb, I also bought a main jet kit consisting of multiple main jets for this carb which I currently have right now. I would like to get this VM26 carb on ASAP and if I am in fact running rich, can someone please let me know which pilot and main jets would be worth experimenting with first on my VM26 carb?

I was guessing that my bike would be lean from the factory like everyone else has mentioned, but if my bike is actually running lean, what pilot and main jets would be best to install in my new upgraded VM26 carb?

I am so dumb when it comes to carbs and jetting as I was lucky enough to never have to adjust jetting in any of my past bikes and atvs. Can anyone please let me know what I should be running in my new VM26 carb on my stock bike in the event that my stock carb is actually running rich??

Or... is this condition that I am experiencing just the result of a new Hawk engine that isnt broken-in and not loosened up yet?

Thanks!!!!!!!

[BlackBike]

Unless you are saying it's like a drastic power loss, I would give it time since you are reporting only 11 (something like) miles so far. Get out there for 47 miles (or some random distance) and enjoy.

[dacs57]

Hello!
Make sure the choke is off by push lever down on the left side of carb.....

Hope it helps

dacs57

[HawkTwoFifty]

Originally Posted by BlackBike

Unless you are saying it's like a drastic power loss, I would give it time since you are reporting only 11 (something like) miles so far. Get out there for 47 miles (or some random distance) and enjoy.

Thanks for your reply BlackBike! The power loss is definitely noticeable from 3/4 to full throttle. It goes from a healthy low end and midrange, to a dirty/stumbling/out-of-breath top end with poor top end acceleration. I imagine that this must be a main jet issue. I am just shocked that it seems that this may be a rich jetting issue from the factory when so many others report a lean carb issue from the factory. I now want to slap on my new VM26 (30mm) carb asap but confused now as to where I start with jetting.

Thanks again for your reply!!

[HawkTwoFifty]

Originally Posted by dacs57

Hello!
Make sure the choke is off by push lever down on the left side of carb.....

Hope it helps

dacs57

Thanks for your reply Dacs57. The choke was definitely off during all of my rides so far. With people reporting lean conditions, I was thinking that the choke on would make my bike run better but it doesnt. When my bike is warmed up, it definitely doesnt like the choke on as I have tried it for sure. I wish that I ran the bike today with the choke on at 3/4 to full throttle to see how it reacted, but I didnt think of it.

Thanks!!

[HawkTwoFifty]

Does anyone know if the VM26 carb upgrade runs more lean than the stock carb on the main jet? I am really wanting to swap the stock carb for my new VM26, and seriously hoping that the swap would be more lean on the main jet than my stock carb. Assuming that my issue is a lean main jet on my stock carb which I am still trying to determine. I always performed an easy break-in on my earlier bikes and quads so its still up in the air if this is just a typical issue for a bike that isnt fully broken-in right now. I am so confused right now!

Thanks!!!!!!!!!!

[HawkTwoFifty]

I am at about 700 foot land elevation.

[Megadan]

Originally Posted by HawkTwoFifty

Does anyone know if the VM26 carb upgrade runs more lean than the stock carb on the main jet? I am really wanting to swap the stock carb for my new VM26, and seriously hoping that the swap would be more lean on the main jet than my stock carb. Assuming that my issue is a lean main jet on my stock carb which I am still trying to determine. I always performed an easy break-in on my earlier bikes and quads so its still up in the air if this is just a typical issue for a bike that isnt fully broken-in right now. I am so confused right now!

Thanks!!!!!!!!!!

Did you change the main jet out on the mikuni? If not, stop trying to go wide open. The main jet the Mikuni comes with is way too lean. Even if it was the same size as the stock carb, the pilot the mikuni comes with is leaner than the stock carb as well.

Also, you say the choke was off, but lets verify that for sure. When you have the choke off, is the choke lever all the way up or all the way down? It is the exact opposite of the stock carb.

I say this, because the condition you are describing is almost exactly what happens when you are WAY too rich. Falling flat on its face like that, especially at wide throttle opening, is a symptom of a bike at half choke having the choke plate slam shut from the engine vacuum, which would not only cut off airflow, but also cause a too rich mixture.

[Megadan]

Also, it seems like the advice already given in other threads must have been forgotten or ignored.

So I will state this again....

A completely unmodified bike with a Mikuni carburetor.

Pilot jet, at least a 25 is recommended - a 27.5 would probably be closer to ideal - this is what myself and MANY others run at near similar altitudes to you. The 20 that comes in the bike CAN be made to work for now, but if you setup the main and needle jet around that too lean pilot jet, the second you up the pilot jet size you will have to re-tune the main and needle jet again.

Main jet. For a completely stock bike, at minimum a 105. I would suggest a 110 main jet - especially if you don't change out the pilot jet. This might end up being a little rich, but that is better than lean.

Needle clip - 1 notch down from center to start.

Lastly, choke lever all the way down for the choke to be completely off. Not all the way up like the stock carb.

[HawkTwoFifty]

Originally Posted by Megadan

Did you change the main jet out on the mikuni? If not, stop trying to go wide open. The main jet the Mikuni comes with is way too lean. Even if it was the same size as the stock carb, the pilot the mikuni comes with is leaner than the stock carb as well.

Also, you say the choke was off, but lets verify that for sure. When you have the choke off, is the choke lever all the way up or all the way down? It is the exact opposite of the stock carb.

I say this, because the condition you are describing is almost exactly what happens when you are WAY too rich. Falling flat on its face like that, especially at wide throttle opening, is a symptom of a bike at half choke having the choke plate slam shut from the engine vacuum, which would not only cut off airflow, but also cause a too rich mixture.

Thanks Megadan for your reply, youre always a HUGE help here for me! I am still running the stock carb right now and have it jetted as stock during my break-in period. I am looking to put the Mikuni VM26 upgraded carb on it soon but am unsure as to what I should jet it to before installation.

When I warm up my bike, I have the stock carb choke at the top position. Once its warmed up, I put it to the lowest position. For some reason, there seems to be a middle position between top and lower. Not sure what that middle position is, but I only use top and bottom position. When I ride it after warm up, its always placed in the bottom position.

When I run my bike at 3/4 to full throttle, it is lousy so I am looking to put my new VM26 carb on but first want to jet it as close to proper as possible for a land elevation of 700 feet. I am just confused because most owners say that their Hawk 250 runs LEAN on the STOCK carbs main jet, while my STOCK carb seems to be running rich on the main jet from my factory carb. Any ideas!

Thanks in advance Megadan, you have been a HUUUUGE help here for me! Again, I am still running the stock carb with the stock jetting during my break-in period. But I am looking to put the VM26 on asap to resolve my 3/4-full throttle issue, and looking for pilot/main jet ideas on the new VM26 carb which I plan to install soon, due to my situation with the stock jetting which seems to differ from others experiances, thanks!!!

[Megadan]

Originally Posted by HawkTwoFifty

I am still running the stock carb right now and have it jetted as stock during my break-in period.

When I warm up my bike, I have the stock carb choke at the top position. Once its warmed up, I put it to the lowest position. For some reason, there seems to be a middle position between top and lower. Not sure what that middle position is, but I only use top and bottom position. When I ride it after warm up, its always placed in the bottom position.

When I run my bike at 3/4 to full throttle, it is lousy so I am looking to put my new VM26 carb on but first want to jet it as close to proper as possible for a land elevation of 700 feet. I am just confused because most owners say that their Hawk 250 runs LEAN on the STOCK carbs main jet, while my STOCK carb seems to be running rich on the main jet from my factory carb. Any ideas!

Okay, so now that I have all of the details, let me go over this in order.

First, do not do a hard break in with the stock carb jetting.

Second, you are in the correct choke positions. That said, the middle position you are talking about is known as half-choke. Starting it with full choke is fine, but once the bike has run for maybe a minute, or less on warmer days, you should go to half choke - that middle position. Full choke is really only there for starting when cold. Half choke is for warming up, which is why that position exists.

Lastly, when you run your bike up to 3/4 throttle, you aren't even really touching on the main jet. Between 1/4 and 3/4 throttle most of the fueling is done by the needle jet, with the fueling between 3/4 and full throttle mainly being handled by the main jet. I highly suggest following all stock carb tuning suggestions that you can find on this forum. Most people put one or two 3mm washers under the needle clip to increase mid range fueling, and then put in a 105 or 110 main jet from the kit you bought The main jets you got for the Mikuni can and often are also used in the stock carb as the Mikuni and Keihin main jets are very nearly identical. The threads are minutely different, so they may get a little tight, but they do screw in without damaging anything.

Again, do not attempt to do a hard break in with stock carb tuning.

Either wait until you get the Mikuni setup as suggested and installed, or do what I suggested above with the stock carb.

The reason why is very simple. If your bike is going so lean it is basically falling on its face past 3/4 throttle, then even at half-throttle you are likely already in a dangerously lean condition. With the tempuratures getting colder outside, the already lean during warm days Hawk will now be extremely, almost dangerously lean, on cold days.

If you do not tune the stock carb, or setup and run that Mikuni, you will harm your engine.

Lean conditions create a lot of heat in the combustion chamber. Too lean, especially at high RPM's and under high loads (like wide open throttle in a higher gear during a hard break in), can cause so much heat that you will potentially burn a hole in the piston, scorch the cylinders, or damage the temper on the piston rings causing them to fail - OR all of the above. You can also damage the cylinder head and cylinder jug from extreme heat in some cases.

ALL of this will cause you to have a bad day.

If you are not familiar with how these carbs operate, then I will gladly detail it for you. I would rather spend hours typing out a detailed explanation, or even making my own Youtube video on this subject if necessary, just to ensure that you have the bike running correctly and don't destroy it.

You either need to get a handle on carb tuning and take the advice of the people here, or maybe consider a soft break in and stop going wide open. Even with a soft break in, you still need to tune the carburetor, but at least it won't be punishing the engine quite as much.

[JerryHawk250]

Hawktwofifty, Your running very lean with the stock carb and jets. Please read and follow the advise of Megadan and others. For your sake we don't want you to start a "How to Rebuilt your Hawk engine". Your questions on jetting has been asked and answered in a few of your treads. As Dan stated, if you have the jets then swap out the main on the stock carb to at least a 110. Go back through all your thread and read them again. The answers are all there. We are all hear to help you.