Hawk 250 Rpms jump and bike jerks while cruising

[Swoleosaurus Rex]

I recently bought a 2015 hawk 250 off Craigslist with 2000 miles on it. The guy I bought it from said he did his own maintenance on it but I'm not sure what exactly he did to it.

Recently, I've been having problems with it jerking, backfiring and the rpms jumping while cruising at about 50mph is 5th gear. This only happens when the engine is warmed up and I've been riding for about 5 minutes. I took the carb apart (I believe it's a makuni aftermarket carb) and cleaned it. There is a 100 fuel jet in the carb at the moment.

I live in florida so this is my main source of transportation for the next couple months so any suggestions will be much appreciated!

[Azhule]

Most around here like to use a 110 or larger main jet on the HAWK

Also, might be a good idea to check and adjust the valves

https://www.cscmotorcycles.com/Articles.asp?ID=293

Oh, and Welcome to the forum

[Megadan]

I will start by seconding what Azhule said. Check the valves.

Also, if the carburetor is the "VM26" clone like the one myself and many others use, it's jetting is a bit too lean even for a stock bike.

There are too many variables to give you definitive numbers, but as a whole both the pilot and main jets are too lean.

To start with the pilot jet. For the Mikuni carbs the jet is a VM28/486 Link: https://www.jetsrus.com/a_jets_by_ca...-486_pilot.htm

The Mikuni clome comes with a 20 jet. In my own experience, on a stock bike a 25 pilot jet with the needle set in the 4th position (second from bottom notch) will get you extremely close, With you being so close to sea level you may need to step up to a 27.5 or maybe even a 30 with mods like an intake and exhaust.

The main jet is a N100.604 Link: https://www.jetsrus.com/a_jets_by_ca...arge_round.htm

A 100 mikuni main jet is also a little too lean for the Hawks. At minimum a 105 main jet is needed, with again some people at lower altitudes find the 110 main jet to work the best on an unmodified bike. I ended up finding a 107.5 to work well for me. I would suggest starting with a 110 if the bike is stock, and doing a plug chop to determine if the main jet is the right size for you.

It would be best to buy at least 2 or 3 jets of each to help tune in the carb for your altitude, and if you can afford it, use genuine mikuni jets like in the link provided.

I would suggest at least starting with a 25 and 27.5 pilot jet, and a 105, 107.5, and 110 main jet. These should allow you to find the right jetting combination for you.

If the bike has any exhaust or intake mods then you will be looking at upping the main jet a couple of steps.

[Adjuster]

Too lean or air leak in the system also making it too lean. You need more gas and less air.



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[Azhule]

Also, check the "needle" on the carb... I have a feeling it is set to the "Fully Rich" setting to get the bike to run on the jets shipped with the "Mikuni" carb...