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Old 07-26-2017, 02:39 PM   #1
kilo0195
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Join Date: Jul 2017
Location: Illinois
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300cc tuning and performance

Ok... so i figured I'd start this thread because my other thread about the coil question got off topic and as I drive my 300cc I realize that I,m going to have many more questions. I also dont know how to edit the title of the other post lol. Anyways long story short about my bike 2007 gz250 1.25" exhaust- 1.5 at the Y- and emgo shorty muffler with 1.75 tip. Ported exhaust, rejet 147.5 main and larger pilot, k&n pod filter, decked slightly, nitrogen 320mm rear shocks, 16t front, 300cc big bore kit... and I hope I didn't leave something out. after about 100 miles on it now i can say that it definitely vibrates a little more... just the nature of a thumper, but was enough it made the baffle in my muffler chirp on decel. I had to remove the baffle, which sounds amazing btw. I also changed the stock plug to an E3 but their plug seems a little cold, so I will be putting in a ngk DR8EIX iridium tomorrow. I will then read and compare the 3 plugs. My main concern now is that it seems to be running warmer maybe. The 300cc jug has better fins on it, but would that be enough? I'm not really feeling like being a test rat for an oil cooler mod. Also what operating temps should I be looking for when I measure?



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Old 07-27-2017, 10:31 PM   #2
kilo0195
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Carb is about dialed in now. The main jet is still the 147.5 and the needle is shimmed pretty far. The real change was I had to change the 52.5 mid jet, or whatever it's called, to a 70 with the mix screw backed 1.25 out right now. Plug looks good after driving at this setup. Going to invest in a infrared thermometer and get some temp #s to post.
Here's the plug with about 30 miles on it... damn near perfect. I switched to the ngk iridium. The e3 was too cold and even had a more black base ring and insulator indicating I wasn't burning as well. http://imgur.com/a/oa4Bs

Last edited by kilo0195; 07-29-2017 at 02:59 PM.
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Old 08-04-2017, 09:37 PM   #3
kilo0195
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Ok so outside temp today was 72... I drove the bike for a while and decided to check temps when I got back. Parked it and let it sit and idle for 2-3 minutes before checking. External head temps on the exhaust side were 354F to 362. The intake side was 316F to 320. Of personal note I got to see just how much the intake charge cooled things as when I shut it off the intake side temp jumped from 317 to 332F within about 3 seconds. So does anyone know what a good external head temp should be for the stock gz250?
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Old 08-07-2017, 02:12 PM   #4
kilo0195
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Ok for anyone actually following this I just went up from a 16t sprocket to a 17t on my 300... so that's 2 over. Keep in mind I'm 150-155 lbs 5'6". Wish I knew the rpm of the motor... will have to install a tach at some point... but I'm very pleased with the results. I bought a generic JT sprockets suzuki, Yamaha, etc 17t 520 sprocket on eBay for $12 that had the same tooth pattern. A custom sprocket was going to be $97 and for something I didn't know was going to work... yeh... $12 is do much better. The splines are just a tad loose so I took up the space with some thread tape temporarily. Will go for a custom one or a shop said they can possibly electroplate the inside splines of the sprocket... I'm not a machinist so I have no idea what they're talking about. Back to the point, the 17t was very nice in town. I lost a little bit of that traffic light to light speed it had on the 16, but made up for everything in that my shift points are ideal for what I'm used to on larger bikes speed wise. On the highway doing 55mph in 4th is about where it was in 5th on a 15t sprocket. I still had enough torque to peg the speedometer on the flats and enough to easily maintain 75 on the hills. From experience with the 16t that will suffer drastically if I ride 2 up with one of my kids. My youngest at 50 lbs I'd guess 70 on hills would be easily maintainable. My oldest at 110lbs I would guess around 65ish... this is based on my experience 2 up on the 16t sprocket and scaling it to the 17t and how it rode. For someone much bigger I'd keep it 16t... then again if I was much bigger, a gz250 would not be the bike I'd be riding. I can already keep pace with some of the larger 750s on cruises even when they punch it to pass etc. I'd imagine the lower rpms of the 17t will at least make it feel like I'm not riding on a giant adult sex toy at 70mph.
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