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Old 06-12-2009, 04:35 PM   #1
Jamaha
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Default XS400R Rev From Idle Nitpicking

Hey all!

I just got an old 1983 XS400R (the Seca version of the XS400) which is loads of fun!

Everything on it is great, but I had just one gripe while riding it home. Whenever I would stop at a light and sit in idle for a while, then try to get going (open the throttle) the bike would kind of sputter and stay at idle speed for a second and then rev up. However, if I just let the engine sit at idle for a few seconds, it would rev just fine.

Any ideas? I was thinking it might be low on oil. Its in a storage garage at the moment so I can't check it, but I can check everything tomorrow morning.

Thanks!
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Old 06-13-2009, 06:43 AM   #2
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Whether or not it's low on oil should have little bearing on carburetion problems, which is what you're describing.

But don't forget to check (and change!) that oil...........
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Old 06-13-2009, 07:10 AM   #3
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Carb cleaning is on the list right behind a full fluid change and replacing the brake pads. I just thought it was strange to see the problem in such a specific incident.
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Old 06-13-2009, 10:09 AM   #4
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Quote:
I just thought it was strange to see the problem in such a specific incident
"I just got an old 1983 XS400R"

I didn't.

"Runs good - daily rider (when I parked it)" are words I've heard often...........
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Old 06-13-2009, 03:47 PM   #5
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I agree on the old thing, but in comparison to a 71 Mach 3 that I refurbished (and it looked and ran factory fresh when I finished, brag brag) carb problems are...bigger maybe?

Anyways. I have set aside monday for fluid changing, it just got a new battery and cleaning, but one more question. This one is pretty bad (be warned).

I was cleaning it up and I noticed something strange on the bottom of the crankcase. About six inches in front of the gear lever, and a couple inches from the side of the case there is a hole. It just a hole, its smooth, milled, has a lip. I stuck my finger in to see what it might lead to and just ran into some muck (oily dirt and road bits). I suspected that there is something supposed to be in the hole so I referenced my shop manual and some parts drawing, but couldn't find any reference to this mystery hole.

It is not threaded, about an inch in diameter, and in the front left of the crankcase, on the bottom, and has me worried.

Any idea what it is?

EDIT: The oil level is fine, and all the oil filter goodness is there, so its not the filter hole.
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Old 06-23-2009, 08:14 PM   #6
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The hole doesn't seem to go anywhere (just part of the mold I guess) so no worries anymore.

I would like to avoid a complete carb clean this summer (I just don't have a whole day to put into it without being able to get anywhere). So if possible, I would like to tune out the low rpm pull problem.

This is the problem: When opening the throttle after the engine has been idling, the engine is VERY sluggish to respond, but picks up quick afterward.

The problem gets worse as the engine warms up and only occurs after the engine has been idling for 30 seconds or so. Thus I am thinking the idle mixture is rich. The engine accepts full throttle everywhere in the band perfectly. I am a little confused by this one, and very novice to tuning a multi-carb setup like this. Thanks!
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Old 06-24-2009, 04:59 AM   #7
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You'll have to ask mscuddy if he knows specifics about this particular bike (and he probably does!), but it might be a carb-synchronization issue, or some spooge in a jet or bleed hole, or simply a quirk of this particular model - it IS an early-'80s moto, after all!
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Old 06-24-2009, 05:16 AM   #8
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Quote:
Originally Posted by Jamaha View Post
I was cleaning it up and I noticed something strange on the bottom of the crankcase. About six inches in front of the gear lever, and a couple inches from the side of the case there is a hole. It just a hole, its smooth, milled, has a lip. Any idea what it is?
It is a "cashiyawan" (kaa-she-yaw-won), which is Japanese for "engine sphincter." Don't insert Q-Tips or other objects in it, just a gentle rinse with mild soap will do.
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Old 06-24-2009, 07:09 AM   #9
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On the whole I think preperation H feels pretty good
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Old 06-24-2009, 08:15 AM   #10
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Your bike has little caps covering the air adjusting screws on the carburetors. This was started back in the 70s to keep people from adjusting the carbs and committing the most heinous evil dastardly twisted crime in human history.... putting out too much pollution. It is possible that over time one or more of the air screws has turned and your idle air mixture is incorrect. The caps can be CAREFULLY drilled through and removed. The adjustment procedure for the screws may be in the shop manual.

You can mark the position of each screw and then carefully turn them in until they seat gently, counting the turns. They should be between 1 and 1 1/2 turns. You might find one that is way off. But be sure to mark the original positions because getting 4 carbs out of whack will really ruin your whole day.
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