XS400 Seca, running very slow and poor throttle response

My bike has been plagued with issues for the longest time. Just got the most recent issue fixed by my mechanic, and now these issues present themselves.

In short, my bike feels like it's missing about half of its horsepower. Giving it throttle from idle, the throttle bogs for a good second before picking up, then slows its climb above 6k. It feels much of the same in operation, and improves slightly as it warms up. When cold, it basically requires a 3k rpm clutch roll-on to get going, and barely any pre-launch throttle after it's warm. When rolling, the throttle response feels great at around 1/4 throttle, but non-existent when you give it any more gas. Full throttle is almost completely useless as it provides so little gain over 1/4 throttle. As a result, the bike is usable around town but a turtle on the freeway. I am lucky to reach 55mph on the speedo (roughly 70-75mph). I remember reaching around 100mph once when I rode it last year.

Mechanic said the bike is completely in factory spec. The carbs are "spotless", valves adjusted properly, spark plugs gapped between suggested range, and timing set correctly. I also eliminated the gas tank/petcock as the source issue by using a waterbottle of gas directly into the carbs (no change in throttle response). He replaced the entire electrical system to cure the previous issue, essentially pulling it all off a parts bike that ran well but was bent up beyond repair. I'm at a loss as to what else could be causing this. Anyone have any clue what to check next?

Have you tried the carb cleaner test to determine conclusively that there aren't any leaks in the rubber connections between the airbox and the carbs? If you are lucky, you will detect a problem there which ought to be an easy fix.

If it persists though, you will likely get input form several folks on here who have cured this kind of problem - and it will be to clean the carbs - properly.
I've read it over and over on here and it always seems to be true. There are passages which get blocked and which are too tiny to be verified with visual inspection. All the big areas can look spotless and be clean, but gunk can survive in those passages through 3 or 4 boilings before finally opening up. People don't want to believe it and often refuse to - until they prove it to themselves.
If you end up there, make sure to read the carb cleaning threads on here FIRST so you can avoid doing damage to rubber parts, butterfly seals etc.

what do your plugs look like?

upload a picture sounds like you're either running really rich or really lean, in both cases it is likely to be a carb issue like Lou says.

Check for holes in your exhaust as well, and leaks around the headers. The front H-piece of the exhaust (lowest part, below the engine) tends to rust away which causes lean running issues as well. But my prime suspects would be the carbs!

Despite the assurances of your mechanic, the most likely source is carbs and/or timing. The other possibility is some of the parts off the other bike are not perfectly compatible with your bike.

When rolling, the throttle response feels great at around 1/4 throttle, but non-existent when you give it any more gas. Full throttle is almost completely useless as it provides so little gain over 1/4 throttle. As a result, the bike is usable around town but a turtle on the freeway. I am lucky to reach 55mph on the speedo

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Sounds like my story, when my Cam Chain Guide's broke. I was driving at 80mph, then I took a cloverleaf exit onto another highway, then for no apparent reason, I could not get my bike to drive over 50 mph. After taking off my heads, I found the Cam Chain tensioner, had over pressed the one guide, bent the head to 90 degrees. both of them where broke in half, one had 1.5 inches missing, though out the bike.

throttle response feels great at around 1/4 throttle, but non-existent when you give it any more gas. Full throttle is almost completely useless as it provides so little gain over 1/4 throttle.

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This part leads us to think Carbs. try a carb cleaner (seafoam, or something.) I did have an experience where it was acting wonky like what you have here, then I did take it onto the interstate, and it worked its way out on that one run. all of a sudden, it was crappy accelleration, then, pow, then Shazam, and away I went. (proof of the Italian tuneup can work)

Wow, you guys were right! I deep cleaned the carbs, and it fixed the issue. One of the cylinders that slide in the middle of the carbs (don't know what they're called) was locked up, and the fuel inlet thingies (with the mesh filter and loose jet, right above the floats) were pretty dirty. I used well over a can of carb cleaner, and lubed up the cylinder things, she's got top end power now!

I'm still lacking throttle response and, consequently, power below 3k rpm. I think it is either a tuning issue, or because the gas tank internal sealant is flaking off and probably clogging up the petcock. Regardless, I'm just happy to finally be able to use the bike daily!

Thanks everyone for the advice.

suicide bobb said:

... I'm still lacking throttle response and, consequently, power below 3k rpm...

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You may want to check out this thread - similar sounding problem at the low end:

http://www.xs400.com/forum/showthread.php?t=13133

Thanks Ranger.

Unfortunately, the issue is back, and in full force. Amongst other problems, I'm just going to see if you guys can help me identify this one issue.

Right side is definitely the culprit. Feeling the exhaust while it's idling, the right side is weaker and popping lightly every second or two. Popping persists under throttle. To me, that says I'm running lean. Everything on the bike is stock and set to factory specs, so I'm not sure why it's running lean on one side (or if that's even the case). Any ideas?

willem said:

what do your plugs look like?

upload a picture sounds like you're either running really rich or really lean, in both cases it is likely to be a carb issue like Lou says.

Check for holes in your exhaust as well, and leaks around the headers. The front H-piece of the exhaust (lowest part, below the engine) tends to rust away which causes lean running issues as well. But my prime suspects would be the carbs!

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once more

and add an inline fuel filter!!

Ah, thanks. I did install the filter at least, necessary considering my rusted out tank

You may want to look into getting the tank seal on find another one. When you clean the carbs again use compressed air (125-150 psi) to blow through all the holes. It make a huge difference then just using a can of carb spray.

xschris said:

You may want to look into getting the tank seal on find another one. When you clean the carbs again use compressed air (125-150 psi) to blow through all the holes. It make a huge difference then just using a can of carb spray.

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Yessir, that I did. Lots of compressed air and lots of carb cleaner, directly into the passages. I definitely need to fix the tank though, but I think I'll sell it before it gets to that.

Another update. Swapped exhaust for another factory exhaust (don't ask why I have two). Revs much nicer now. Unfortunately, the issue came back.

Here's my best explanation:

1. Runs mostly fine when cold/for around 10min of riding
2. Slight hesitation at low rpm's though
3. Moment of bliss before the issue pops up (rides almost perfectly at low/mid/high rpm and load)
4. All of a sudden, it loses about 75% of its power, feeling as if I'm trying to tow a 2000lb block
5. Backfiring through intake when under heavy throttle
6. If I keep on the throttle hard, it usually "snaps" out of it at around 6k rpm, as if the line that I was towing the 2000lb block with suddenly broke loose
7. Continues to ride great until it drops down below 4k rpm or so, then it repeats the issue
8. Seems to get worse the longer I ride it, until it becomes completely unridable and/or dies
9. At idle, it revs a little more sluggishly and backfires through intake at around 5k rpm (and is subsequently difficult to rev past 5k rpm)
10. Threw in brand new plugs, took it for a spin, pulled both plugs. Both are lean, right side more lean than the other. Right side plug arm is almost pure white, and the center electrode has a small "burn" mark on it

God bless you if you took the time to read through all of that. Hopefully someone has an idea what is causing this?

First pic is left side, second pic is right side. As I said, these plugs were brand spanking new, they only have one ride on them before I pulled them to take these pictures. They are gapped correctly from the factory at about .028

Sound like the carbs are plugged again. Don't remove the plugs with a hot motor. The aluminum head is easy to mess up the threads. If you can't touch the plug it's too hot to remove.

xschris said:

Sound like the carbs are plugged again. Don't remove the plugs with a hot motor. The aluminum head is easy to mess up the threads. If you can't touch the plug it's too hot to remove.

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Sounds like it, but the problem disappears when I let the bike cool down. Issue persists as soon as it warms up again.

Didn't know that about the plugs though, thanks!

As the bike gets hot it will lean out a bit as the motor heats up and things seal better. That's why people get hanging throttles after they warm up the bike. It's also why you sync your carbs with a warm motor.

xschris said:

As the bike gets hot it will lean out a bit as the motor heats up and things seal better. That's why people get hanging throttles after they warm up the bike. It's also why you sync your carbs with a warm motor.

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While that may be true, bikes don't also lose 75% of their power and backfire into the intake rampantly when they get hot. At least, they're not supposed to lol.

Check the valve clearances.