Riderundead
XS400 Member
Background
Me: I'm new to carbs. I'm new to motorcycles. I'm not afraid to fix it.
The bike: 81 xs 400 barn find. 6600 miles on the odometer. Completely stock. Did nothing to the engine except a couple of oil changes and fresh gas. (side note: I ditched the vacuum petcock in favor of a manual one that doesn't leak.) Ran ok but lacked top end and didn't run super smooth (subtle backfire) above 4000 rpm. Disassembled carbs and cleaned in ultrasonic cleaner. Lots of crap on mains. Removed brass plugs from mix screws. Reassembled and installed carbs with mix screw at 3ish turns out. Balanced with clear hose and ATF method. Tried to follow tuning guides but never quite feel like I got the tune right because I don't know what I'm doing. After the rebuild the bike runs/ran much better. Was happy to find that the engine would blast past 5000 rpm and beyond with no problem. Bike was happy to sit at 6000 to 7000 rpm and cruz... My test ride ended up going long... 100 miles long.
The Problem
The next day I went for a short ride and the idle hung at around 3000 rpm mid ride at a stop but not at every stop. Seemed to run ok otherwise. Same thing this evening on a short ride. Once at home the idle would hang at 3000 after a blip of the throttle. If I kill it and restart the bike idles at 1500. I decided to try to readjust the carbs. I pulled a plug wire and set about trying to tune the opposite carb. It doesn't seem to me that it makes much difference if the mixture screw is out 2.5 turns or 3.5 turns... When I moved onto the 2nd carb I noticed is that one cylinder won't idle as low as the other. In other words I had to adjust the idle set screw a couple of turns to keep the bike running while adjusting the 2nd carb. I decided to set both mix screws at 3 turns out and call it a night. What bothers me about the whole process is that the two sides seem to idle at different throttle positions and I never really felt like turning the screws 1/4 or 1/2 turn here or there made much difference. I need some guidence.
Thanks!
Me: I'm new to carbs. I'm new to motorcycles. I'm not afraid to fix it.
The bike: 81 xs 400 barn find. 6600 miles on the odometer. Completely stock. Did nothing to the engine except a couple of oil changes and fresh gas. (side note: I ditched the vacuum petcock in favor of a manual one that doesn't leak.) Ran ok but lacked top end and didn't run super smooth (subtle backfire) above 4000 rpm. Disassembled carbs and cleaned in ultrasonic cleaner. Lots of crap on mains. Removed brass plugs from mix screws. Reassembled and installed carbs with mix screw at 3ish turns out. Balanced with clear hose and ATF method. Tried to follow tuning guides but never quite feel like I got the tune right because I don't know what I'm doing. After the rebuild the bike runs/ran much better. Was happy to find that the engine would blast past 5000 rpm and beyond with no problem. Bike was happy to sit at 6000 to 7000 rpm and cruz... My test ride ended up going long... 100 miles long.
The Problem
The next day I went for a short ride and the idle hung at around 3000 rpm mid ride at a stop but not at every stop. Seemed to run ok otherwise. Same thing this evening on a short ride. Once at home the idle would hang at 3000 after a blip of the throttle. If I kill it and restart the bike idles at 1500. I decided to try to readjust the carbs. I pulled a plug wire and set about trying to tune the opposite carb. It doesn't seem to me that it makes much difference if the mixture screw is out 2.5 turns or 3.5 turns... When I moved onto the 2nd carb I noticed is that one cylinder won't idle as low as the other. In other words I had to adjust the idle set screw a couple of turns to keep the bike running while adjusting the 2nd carb. I decided to set both mix screws at 3 turns out and call it a night. What bothers me about the whole process is that the two sides seem to idle at different throttle positions and I never really felt like turning the screws 1/4 or 1/2 turn here or there made much difference. I need some guidence.
Thanks!
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