Iellei Posted July 22, 2008 Share Posted July 22, 2008 Here's the background~ 1981 xs 400 SH, 17k original miles I've used two old Mikuni carbs for their parts to make one good set. I also bought and installed kits in both sides, and otherwise have not made any internal adjustments so far. The problem~ She fires right up, and idles perfectly at 1200 rpms without using the choke. The speed limit on my highway is 60mph, which is pretty fast for this bike. The tach shows just over 5k rpms at that speed in 6th gear. After only a few miles at that speed, the rpms won't come back down when I let off the throttle. I've checked the throttle cable and it's sitting right where it should be. I'm going out right now to take off the rear boots so I can see if the butterflies look right when this happens. I'll edit my post once I have that info. Anyone have ideas on what is causing this and/or how to resolve? *edit* so I can't see the butterflies real well, but it does look like they were fine. This time my husband just rode it in a figure 8 on the street in front of the house and it stuck at the rpms he was using in 2nd gear, about 2k. I was able to reduce the rpms to a normal idle by turning the adjuster screw whereas when it is stuck at the higher rpms this isn't possible. Link to comment Share on other sites More sharing options...
road_rascal Posted July 22, 2008 Share Posted July 22, 2008 You may want to look around the carb boots and air box boots for vacuum leaks. The best way (that I know of) is to spray carb cleaner around the boots while the bike is running. Any leakage will show an increase in idle speed. Usually the clamps are a bit loose or the rubber has deteriorated enough to cause air/vacuum leaks. Link to comment Share on other sites More sharing options...
ginger0195 Posted July 22, 2008 Share Posted July 22, 2008 If the connection between the throttle cable and the butterfly valve is external, then pull the throttle right open when the bike is not running and then watch it as your let off the throttle. If you see the throttle cable become slack and the valve not move then it is most probably a faulty return spring, or the valve is getting stuck somehow. I would also suggest checking that the inside of the cabereutter is smooth and free of lumps and/or scores that the valve may become stuck on. Adam Link to comment Share on other sites More sharing options...
yoda Posted July 22, 2008 Share Posted July 22, 2008 I think that you should be very careful with this bike until you have found the cause. I you detach the throttle cable from the carb end, you will be able to exercise it and see if it is sticking at all. You will also be able to see if the the throttle butterflies are closing without sticking. If after reassembling them it is still doing it, it is either the throttle return spring not pulling back or the cable is jamming. The carbs should be OK from the checks that you have made. If there was an air leak it will give the same sort of hang up but that will clear in a second or two. One other thing to check is the carbs float height. You might have one float low or filled with fuel causing this problem. Link to comment Share on other sites More sharing options...
Cy Welch Posted July 24, 2008 Share Posted July 24, 2008 Here's the background~ 1981 xs 400 SH, 17k original miles I've used two old Mikuni carbs for their parts to make one good set. I also bought and installed kits in both sides, and otherwise have not made any internal adjustments so far. The problem~ She fires right up, and idles perfectly at 1200 rpms without using the choke. The speed limit on my highway is 60mph, which is pretty fast for this bike. The tach shows just over 5k rpms at that speed in 6th gear. After only a few miles at that speed, the rpms won't come back down when I let off the throttle. When mine did that it turned out that the mixture was actually very very lean, and the idle speed had been adjusted to compensate. After properly adjusting the idle mixture (under the plugs on the top of the carbs by the boot) it has never done that again. Link to comment Share on other sites More sharing options...
Iellei Posted August 4, 2008 Author Share Posted August 4, 2008 So I finally figured it out, one of the jets had debris stuck in it so even though it worked, it was sticking! Thanks for all the responses. It still runs slightly lean, but the jet on the other side is iffy looking so I don't want to try to remove it. I'm afraid I will strip it and will really have a problem... overall a big improvement! Now for an oil change, and to hopefully find a halogen replacement headlamp Link to comment Share on other sites More sharing options...
Gnash Posted August 4, 2008 Share Posted August 4, 2008 Nice! I was having issues like this until I cleaned the carbs again and put a fuel filter on. Link to comment Share on other sites More sharing options...
Cy Welch Posted August 5, 2008 Share Posted August 5, 2008 So I finally figured it out, one of the jets had debris stuck in it so even though it worked, it was sticking! Thanks for all the responses. It still runs slightly lean, but the jet on the other side is iffy looking so I don't want to try to remove it. I'm afraid I will strip it and will really have a problem... overall a big improvement! Now for an oil change, and to hopefully find a halogen replacement headlamp Even if you can find a halogen bulb, you may have trouble with the charging system as these things are marginal as it is. I am slowly replacing all my standard bulbs with LED bulbs to lower current levels. I found the bulbs for the turn signals but I'm going to need to modify some wiring to make them work right (gotta put diodes at the indicator to keep both sides from flashing together), but I will have gotten the drain down to the headlight and ignition with almost nothing for the other bulbs, that may let me get away with a brighter headlight and turning the fronts into running lights (assuming I can find the right sockets to convert them with). Link to comment Share on other sites More sharing options...
Moderator drewpy Posted August 5, 2008 Moderator Share Posted August 5, 2008 Even if you can find a halogen bulb, you may have trouble with the charging system as these things are marginal as it is. I am slowly replacing all my standard bulbs with LED bulbs to lower current levels. I found the bulbs for the turn signals but I'm going to need to modify some wiring to make them work right (gotta put diodes at the indicator to keep both sides from flashing together), but I will have gotten the drain down to the headlight and ignition with almost nothing for the other bulbs, that may let me get away with a brighter headlight and turning the fronts into running lights (assuming I can find the right sockets to convert them with). and a different flasher relay http://www.norbsa02.freeuk.com/goffybulbs.htm do hals for older bike fitment. he attends stafford shows in uk as well. Link to comment Share on other sites More sharing options...
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