Carb tweak questions - Hawk 250

ChrisWNY · 08-15-2018, 08:06 PM

Since adjusting the valves to .005 exhaust and .004 intake, my top end speed seems limited compared to before (17T/45T sprocket combo). The first time I did an adjustment, the valves were tight as can be from the factory, but I was reaching 60-65mph wide open on a #120 jet (was definitely running a little rich). I dropped down to a #115 after the valve adjustment, top speed was barely 50mph. I have a 2nd Mikuni carb (brand new), so I installed a #105 jet into the new carb, turned the fuel mix screw 1.5 turns out, clip on the middle of the needle, and swapped the carbs (identical VM26 Mikuni carbs...will be nice to have 2 to swap them instead of having to pull them apart to change main jets).

On the #105 jet, I get up closer to 55mph, but RPM's seem to top out at 6000-6500...RPM's top out at about 6000 or so on the #115 main jet. I haven't pulled the spark plug lately but overall the engine still seems to be running a little rich. The engine idles perfectly at about 1200 RPM's...doesn't die, sputter, or hesitate anywhere in the throttle range. Acceleration is smooth but not what I would call powerful. Valves are definitely "tappy". Engine only has about 200 miles so perhaps it just needs more break-in time.

Should I re-adjust the valves so that the gaps are tighter, or drop down again in main jet size to a #100 main jet? Seems like it would run lean going down to a #100. I've modified the air box and have a performance exhaust installed. Any suggestions? Thanks.

08-15-2018, 08:06 PM	#1
ChrisWNY Join Date: Jul 2018 Posts: 212	Carb tweak questions - Hawk 250 Since adjusting the valves to .005 exhaust and .004 intake, my top end speed seems limited compared to before (17T/45T sprocket combo). The first time I did an adjustment, the valves were tight as can be from the factory, but I was reaching 60-65mph wide open on a #120 jet (was definitely running a little rich). I dropped down to a #115 after the valve adjustment, top speed was barely 50mph. I have a 2nd Mikuni carb (brand new), so I installed a #105 jet into the new carb, turned the fuel mix screw 1.5 turns out, clip on the middle of the needle, and swapped the carbs (identical VM26 Mikuni carbs...will be nice to have 2 to swap them instead of having to pull them apart to change main jets). On the #105 jet, I get up closer to 55mph, but RPM's seem to top out at 6000-6500...RPM's top out at about 6000 or so on the #115 main jet. I haven't pulled the spark plug lately but overall the engine still seems to be running a little rich. The engine idles perfectly at about 1200 RPM's...doesn't die, sputter, or hesitate anywhere in the throttle range. Acceleration is smooth but not what I would call powerful. Valves are definitely "tappy". Engine only has about 200 miles so perhaps it just needs more break-in time. Should I re-adjust the valves so that the gaps are tighter, or drop down again in main jet size to a #100 main jet? Seems like it would run lean going down to a #100. I've modified the air box and have a performance exhaust installed. Any suggestions? Thanks. __________________ 2018 RPS Hawk 250cc Mods so far: 1. Mikuni VM26 Carburetor w/#115 main jet 2. 17T/45T JT sprockets 3. JT 428 X-ring chain 4. Air box mod 5. Hawk Digital Cluster 6. Aftermarket IMS shift lever 7. Performance Aftermarket Exhaust 8. 295mm Nitrogen Gas Monoshock, 20W oil front forks
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