Fuel valve: removed, cleaned and replaced. Found some small pieces of debris on the screen but nothing major. Noticed no obstructions, used compressed air to ensure interior passages were clear.
Fuel line (to carb): removed, inspected and replaced. No obstructions found.
Carb: removed, cleaned and replaced. Used carb cleaner and compressed air. Found a little varnish on the valve seat but no obstructions in the passages or in the jets. Double checked the slide action (to ensure WOT) and that the cable wasn't binding.
Air box: Cleaned air sock as well as the underlying screen - a little dirty but not nasty. Removed the internal air horn and inspected the box-to-carb boot. It was clear. Inspected the action of the carb slide (to ensure WOT) and choke butterfly - no issues.
Engine: Removed all inspection ports to check the cam timing. It seems like it might be off 1/2 a tooth. But, there's no way to resolve that. Valve adjustments are within spec. The images show the timing mark and the cam gear marks. The timing mark is very hard to see but it's where it's supposed to be, with respect to the cam timing marks, according to the shop manual. I enhanced the cam gear marks so you can see them with respect to their reference marks (on the engine case).
Exhaust: Removed entire exhaust system to check for obstructions and/or rust. Found nothing. The inside of the pipe is clean and has very little carbon (fuel-air mixture correct) build-up.
After reassembling everything, I started the NX and noticed a pretty high idle (2200 rpm). Since it was pretty cold (35ยบ F), I let it idle at that rpm and took it for a short ride to warm the engine. I came back home and adjusted the idle to 1300 (+/- 100) rpm per shop manual specs. With the motor warm, I revved the motor, in neutral, in the driveway. It went right up to 7500 rpm without issue. I went for a ride and the motor would spin right up to 9K+ rpm (well, there's a slight hesitation around 7K rpm).
After reassembling everything, I started the NX and noticed a pretty high idle (2200 rpm). Since it was pretty cold (35ยบ F), I let it idle at that rpm and took it for a short ride to warm the engine. I came back home and adjusted the idle to 1300 (+/- 100) rpm per shop manual specs. With the motor warm, I revved the motor, in neutral, in the driveway. It went right up to 7500 rpm without issue. I went for a ride and the motor would spin right up to 9K+ rpm (well, there's a slight hesitation around 7K rpm).
So, whatever I did (and I'd like to know what it was) fixed the problem. I'm thinking it must have been something to do with the carb. Perhaps the slide wasn't sitting in the bore properly? I dunno. All I know is she screams now (like she's supposed to). Now, if I could only figure out what to do with 13 hp....ha!
After thinking about it some, I’m going to guess the carb slide was somehow bound up. I noticed when I unscrewed the slide cap that the throttle cable housing spun with the cap. When I reinstalled, I held the cable housing so that only the cap spun. Something was definitely different for the idle screw to be more than one full turn off.
Special thanks to my good friend Russ (who has ridden bikes longer than I've known him) who provided some insight and a list potential things that might be wrong. And, thanks to Joe at Cycle Tech in Summit, Illinois. Cycle Tech rebuilt the motor for the previous owner. Joe was kind enough to spend a few minutes on the phone with me during a busy (for the shop) Saturday afternoon. He pretty much verified the list of things I was going to check.
Special thanks to my good friend Russ (who has ridden bikes longer than I've known him) who provided some insight and a list potential things that might be wrong. And, thanks to Joe at Cycle Tech in Summit, Illinois. Cycle Tech rebuilt the motor for the previous owner. Joe was kind enough to spend a few minutes on the phone with me during a busy (for the shop) Saturday afternoon. He pretty much verified the list of things I was going to check.
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