81 XS400 special problem

YoungOne · 07-03-2008, 06:31 PM

I have been fixing up an 81 xs400 special to ride as my first real motorcycle, here is the last thing (hopefully) that is causing me confusion... I cleaned the carbs and got it running, it runs suprisingly well for a bike that had been doing nothing but sitting for 7-8 years up untill last sunday, i had my grandpa (far more experianced) take it for a test ride.... it ran fine, throttle worked well.... only problem was the clutch was "grabby"... so the next morning I took apart the clutch handle, and took the cable out of the tranny and cleaned it... put it back in and ajusted it.... so it now works good.... but when I test drove it today after swapping the ignition switch, the clutch pulls and releases good still... the bike starts from cold with no choke (possible issue?)... with minimul starter usage... but now when I ride it for like 50 feet the throttle will jump to like 4k plus and just stay there... and the throttle (on the handle bars) will be just like normal for idel, and then when I rev it up... it just goes up, and comes back down to like 4k... but if I hit the kill switch, let it shut off...then restart it its fine for about another 50 feet... anyone ever experienced this?... if so help me out

Thanks,
Marshall

dolllar bill · 07-08-2008, 10:16 PM

Please do not attempt any more test rides until you resolve this problem! Serious injuries and death have occured due to stuck throttles. It sounds to me like a frayed cable. I am not familiar with that set up,regardless a frayed cable will bind inside the casing. I would remove the entire throttle sytem from the grip end to the slides and inspect it for defects. Otherwise make sure the top cap on the carburators that the cable goes into isn't loose causing an air leak. Also check the carb boots for cracks or loose clamps.

Good luck,
Dolllar Bill

wallie_s · 07-13-2008, 04:48 PM

Hi Young One,
My 400Special had a tendancy to do the very same thing. I had small cracks in the carb boots. Hasn't happened since I replaced them with new ones.

Wallie

Fuax · 07-13-2008, 09:15 PM

YoungOne,

Before you cleaned the carbs, did the bike want to start at all?? I have inherited an xs400 but it doesn't want to start and was hoping it was just from years of bad gas clogging the carbs.

v/r

OddOne · 08-27-2008, 03:17 PM

Quote:

Originally Posted by wallie_s

Hi Young One,
My 400Special had a tendancy to do the very same thing. I had small cracks in the carb boots. Hasn't happened since I replaced them with new ones.

Wallie

I'm currently having a similar problem with my '81 xs400. It idles great for the first few miles than idles between 3k - 4k rpm after it's good and warmed up.

I'm not in the slightest mechanically inclined so please excuse my n00b question. Why do small cracks in the carb boots affect idling? I have noticed cracks in mine and will replace them to see if it resolves the problem. Thanks in advance.

Gnash · 08-27-2008, 04:57 PM

The cracks are letting air in. Air is regulated inside the carb. Any EXTRA air from the carb boots makes the engine rev. You can spray some starting fluid on the boots, while the bike is idleing. If it revs higher, the boots are leaky. The boots can be fixed with silicone. New boots are a better fix. I have had this problem(Revs staying high after riding) on my 83 xs400. You put in the clutch during shifts and the motor just stays revving high. I have done all these reccommendations and I still have the problem. I am thinking my floats are just not set properly.

OddOne · 08-27-2008, 05:22 PM

Thanks Gnash! Another thought/clarification based on your explanation. Does the extra air entering the carbs cause the mixture to be richer, thus causing the higher idle rpms? or weaker? Also, why wouldn't the idle speed be affected when the engine is cold? Sorry, as I said, I'm not mechanically inclined so I'm trying to understand the overall effect.

However, I'll address the carb boots first to see if that fixes my revving issue. If not I'll have the setting on the floats checked.

Gnash · 08-28-2008, 06:03 PM

You got me there. I would say with extra air, the motor would be running leaner. These carbs are REAL finicky. Any misadjustment and the motor will run bad. There is an air mix screw that you have to get to through the air box. On the intake side of the carbs. I've heard people adjusting this out 2-4 turns. Just remember where it was set to start with. Screw them in until they seat, then back them out 2 turns. Play with that until you like it. I think mine are set at 2 1/2 turns now. You'll have to reset your idle screw after this adjustment.

OddOne · 08-28-2008, 11:32 PM

Okay, I try that after the carb boots if I dont' get any satisfaction. Thanks for the insight. I can tell I have a lot to learn. Glad to see there are people willing (and able) to share their knowledge and experience.

rbrannon69 · 09-11-2008, 07:29 AM

for me, and my xs400 it was all about cleaning the carbs. they are the most uber simple carb i have ever worked with.
the more i clean them the better they worked.

07-03-2008, 06:31 PM	#1
YoungOne Verified Join Date: Jun 2008 Posts: 73	81 XS400 special problem I have been fixing up an 81 xs400 special to ride as my first real motorcycle, here is the last thing (hopefully) that is causing me confusion... I cleaned the carbs and got it running, it runs suprisingly well for a bike that had been doing nothing but sitting for 7-8 years up untill last sunday, i had my grandpa (far more experianced) take it for a test ride.... it ran fine, throttle worked well.... only problem was the clutch was "grabby"... so the next morning I took apart the clutch handle, and took the cable out of the tranny and cleaned it... put it back in and ajusted it.... so it now works good.... but when I test drove it today after swapping the ignition switch, the clutch pulls and releases good still... the bike starts from cold with no choke (possible issue?)... with minimul starter usage... but now when I ride it for like 50 feet the throttle will jump to like 4k plus and just stay there... and the throttle (on the handle bars) will be just like normal for idel, and then when I rev it up... it just goes up, and comes back down to like 4k... but if I hit the kill switch, let it shut off...then restart it its fine for about another 50 feet... anyone ever experienced this?... if so help me out Thanks, Marshall

07-08-2008, 10:16 PM	#2
dolllar bill Verified Join Date: Jun 2008 Location: "east coast" of Wisconsin Posts: 29	safety first Please do not attempt any more test rides until you resolve this problem! Serious injuries and death have occured due to stuck throttles. It sounds to me like a frayed cable. I am not familiar with that set up,regardless a frayed cable will bind inside the casing. I would remove the entire throttle sytem from the grip end to the slides and inspect it for defects. Otherwise make sure the top cap on the carburators that the cable goes into isn't loose causing an air leak. Also check the carb boots for cracks or loose clamps. Good luck, Dolllar Bill Last edited by dolllar bill; 07-08-2008 at 10:18 PM.. Reason: add/sp

07-13-2008, 09:15 PM	#4
Fuax Verified Join Date: Jun 2007 Location: Waldorf, MD Posts: 76	cleaned carbs YoungOne, Before you cleaned the carbs, did the bike want to start at all?? I have inherited an xs400 but it doesn't want to start and was hoping it was just from years of bad gas clogging the carbs. v/r

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07-13-2008, 04:48 PM	#3
wallie_s Motorcyclist Join Date: Jul 2008 Location: Newfoundland, Canada Posts: 7	Hi Young One, My 400Special had a tendancy to do the very same thing. I had small cracks in the carb boots. Hasn't happened since I replaced them with new ones. Wallie

08-27-2008, 04:57 PM	#6
Gnash Master At Arms Join Date: Jun 2008 Posts: 120	The cracks are letting air in. Air is regulated inside the carb. Any EXTRA air from the carb boots makes the engine rev. You can spray some starting fluid on the boots, while the bike is idleing. If it revs higher, the boots are leaky. The boots can be fixed with silicone. New boots are a better fix. I have had this problem(Revs staying high after riding) on my 83 xs400. You put in the clutch during shifts and the motor just stays revving high. I have done all these reccommendations and I still have the problem. I am thinking my floats are just not set properly.

08-27-2008, 05:22 PM	#7
OddOne Master At Arms Join Date: Aug 2008 Location: North Central PA Posts: 102	Thanks Gnash! Another thought/clarification based on your explanation. Does the extra air entering the carbs cause the mixture to be richer, thus causing the higher idle rpms? or weaker? Also, why wouldn't the idle speed be affected when the engine is cold? Sorry, as I said, I'm not mechanically inclined so I'm trying to understand the overall effect. However, I'll address the carb boots first to see if that fixes my revving issue. If not I'll have the setting on the floats checked.

08-28-2008, 06:03 PM	#8
Gnash Master At Arms Join Date: Jun 2008 Posts: 120	You got me there. I would say with extra air, the motor would be running leaner. These carbs are REAL finicky. Any misadjustment and the motor will run bad. There is an air mix screw that you have to get to through the air box. On the intake side of the carbs. I've heard people adjusting this out 2-4 turns. Just remember where it was set to start with. Screw them in until they seat, then back them out 2 turns. Play with that until you like it. I think mine are set at 2 1/2 turns now. You'll have to reset your idle screw after this adjustment.

08-28-2008, 11:32 PM	#9
OddOne Master At Arms Join Date: Aug 2008 Location: North Central PA Posts: 102	Okay, I try that after the carb boots if I dont' get any satisfaction. Thanks for the insight. I can tell I have a lot to learn. Glad to see there are people willing (and able) to share their knowledge and experience.

09-11-2008, 07:29 AM	#10
rbrannon69 Verified Join Date: Sep 2008 Location: Central Indiana Posts: 30	for me, and my xs400 it was all about cleaning the carbs. they are the most uber simple carb i have ever worked with. the more i clean them the better they worked.