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aswracing 15th May 2017 20:54

Somewhat interesting 1275 result

2003 XL883R owned by a Valued HAMMER PERFORMANCE customer, equipped as follows:
Very good result for STOCK Thunderstorm heads. Back in the day, when Buells came with these same heads and cams, a good one would turn 90hp, I tested tons of them on this same dyno. So to get 96ish on a Sportster, with it's heavier flywheels to boot, is doing well. The 1275 kit has a bit to do with that.

As is plainly visible in the chart, the pipe is reverting badly below 3000 rpm. The torque is low, and with a small measurement lag, the AFR goes fat. Those are both sure signs of a pipe that's reverting.

What happens is the pipe pushes back during the overlap window, i.e. when the exhaust is connected to the carb. The intake charge that's sitting in the ports and manifold get shoved right back out the carb. Then the piston goes down and sucks it back in again. So you get a charge that's been through the carb three times - in, out, back in again - picking up fuel each time, and the motor goes rich. Plus you get a big hole in the torque curve because the cylinder fill is so bad. One tell-tale sign that you're reverting is a fog of fuel standing outside the carb.

The above chart was done with a 165 main in the Mik. On the right side of the chart, it wanted more main jet. In fact, with more main in place and taking it to 6800, I hit 98hp and 92lb-ft of torque a couple times. However, putting anything bigger than the 165 into the carb made it shoot ducks in the reversion. I didn't want to deliver the bike with that going on, so I settled on the 165. That was the biggest jet it would take without breaking up in the reversion.

Being a pipe issue, the right way to fix this is on the pipe side. There are ways to try to address it on the carb side, for example Thunderjets with less main, but at the end of the day that's a crutch. It's really a pipe issue.

Makes you appreciate EFI. You can't fix the reversion with EFI, but you can at least keep it from going fat.

The pipe follows the reversion with a real hard pull through the mid range, dying on top. In other words, not real great diffusion. A lot of pipes behave this way.

I'd be surprised if I couldn't hit 100 with a Patriot in place, with it's big top end punch.

(Hey Folkie ... there's no sticky reference thread for 1275's ... hint hint)

TT60 11th July 2017 17:37

Looks like my Winter project will be 1275 kit :banana

Just gotta save my pennies now.

Highly-Dangerous 30th September 2019 07:56

Wow!!! Those were very good numbers!!!

I am in the process of ordering the 1275.... I keep wondering about my stock heads... I am worried about regretting it in the future.

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