Misfire, backfire, shift issue

[Joshwuh]

I have a 1977 XS400 and am new to the MC world so any help would be greatly appreciated. I am currently having 3 issues:

1. When I hit 7K RPM's and above my engine misfires a good amount but it still has a good amount of power. Although, when I ride on the highway at 7-8K RPM's I don't hear it misfire.

2. When I decelerate especially while in gear and/or engine breaking my engine backfires.

3. Sometimes when I go to shift into 6th it goes into neutral or will engage and when I accelerate will slip out into neutral. Along these same lines I often times come up to a stop light and won't be able to shift down into first or neutral. I'll have to slightly let the clutch out to right where it starts to engage then I'll hear a "click" and I'll be able to get it down into first.

I am new to MC's but eager to learn so any help and as much detail and explanation as possible will be greatly valued. Thanks for your time.

 

[HoughMade]

Well, the first 2 sound like a lean condition, but the reasons may be different. For the 7,000+ rpm issue, you will be on the main jet. If your bike is running stock air filters and exhaust and the slide needle is in the stock position, the diaphragms are good and the valves are in spec, I'd look for air leaks around the intake boots or cracking of the boots themselves. If the diaphragms are bad, that could be it. If you are running a more open intake or exhaust, you may need to rejet, but I would make that a last resort. You'd be surprised at what can be done with stock jetting.

The second issue sounds like a lean idle circuit (or pilot or low speed- same thing). If everything is clean, what are you idle mixture screws at. My bike pops on deceleration when the screws are less than 3 turns out and there is none at 3.5 turns out.

Good luck!

 

[Joshwuh]

Wow, that's impressive my air intake boots ARE cracked all over but I was hoping that it was purely cosmetic. I will look into replacing them asap. I'll check my mixture screws and try to set them at 3.5 and see if it does anything. Thanks for the help.

 

[dellrio]

I have the same shifting issues as you, I have found that if I am "stuck" in neutral at a stoplight, releasing and reengaging the clutch lever allows it to shift to first gear. As for the shifting to neutral between 5th and 6th - I am clueless, I have experienced this as well, its almost like the gear is not there sometimes, a quick downshift, then reshifting to 6th has resolved the issue every time I have had it. Only had the bike for about a month though, so dont know the cause or resolution to these issues only the workarounds. If anyone has more info on this - I would be interested to hear.

 

[Joshwuh]

Yeah I do the same things to deal with it but lately it has been getting a little worse. Sometimes I have to pull and release the clutch 3 or 4 times before it will shift out of neutral. When mine misses 6th I just hit it again and it usually engages.

 

[Joshwuh]

Where is/are the idle mixture screw(s)? I have the Haynes manual but the only thing I can find in it is the throttle synchronization screw. Is it the plastic white screw (of which there are two, one on each side) in the upper right corner of the pic? I also saw a knob/screw in between the carbs that I would have to remove the battery to get to, could it be this? I can't find it anywhere in my Haynes manual.

 

[Joshwuh]

Nevermind Hough I found the answer to my question on another forum and it was answered by HoughMade lol. For anyone else who is asking this same question the answer is here http://www.yamahaclub.com/forums/topic/21942-xs-400-adjusting-mixture/

 

[HoughMade]

I'm everywhere you want to be! At least everywhere people are discussing XS400s.