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christoph21

xs400 not fast enough!

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Upon trying to really top out my 81 xs400, I got to 71 MPH and just couldn't get anymore. When trying to rev the throttle any higher than 6500 RPM in 6th gear, the engine boggs down and can't go any higher. The bike is really quick up until that and the engine runs strong. I know most of these can do 90-95 mph with stock components. What's going on here!?

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Sounds like the carbs needed to be cleaned. I had the same problem when I picked up my 400 Maxim. Ran fine until about 60-65 then wouldn't go any faster. I would also check for vacuum leaks on the carb boots.

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I understand that this could be the problem; but i have a hard time believing it. I have cleaned the carbs extensively. All the jets are spotless as of June this summer. One of the venturi diaphrams may be a little questionable, but I don't think it's creating a problem. The intake boots have all been sealed with silicon as well.

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I understand that this could be the problem; but i have a hard time believing it. I have cleaned the carbs extensively. All the jets are spotless as of June this summer. One of the venturi diaphrams may be a little questionable, but I don't think it's creating a problem. The intake boots have all been sealed with silicon as well.

If the diaphrams are not perfect you WILL have problems. With the age, it's likely they need replaced. You can get the piston with diaphram from MikesXS (1980 and later 400's and 650's use the same carbs) or you can you can get new diaphrams from siriusconinc on ebay for about 1/3 the price. However, it's tough to get them right, if I had to do it again I would get the ones from MikesXS. High throttle settings are right where you will get problems if the diaphrams are leaking as the slides won't be opening enough.

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.............My bike goes 90 mph?

:o

If it's running right it should be capable of it.

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Another question - If I were to do pod air filters, what jet numbers would i need to go up to to compensate for the increased air flow!?

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I understand that this could be the problem; but i have a hard time believing it. I have cleaned the carbs extensively. All the jets are spotless as of June this summer. One of the venturi diaphrams may be a little questionable, but I don't think it's creating a problem. The intake boots have all been sealed with silicon as well.

Hi Chistoph,

did you manage to find the solution to this?

I've got a 1987 XS400 12E, it seems to run fine, but when in 6th gear the engine seems to stall and it's quite difficult to take it further than 6000rpm. In lower gears it is full of power (up to 5th) but revving it up it seems to have difficulties beyond 7500 rpm. I will try check more accurately where the limit is today if I can. (in lower gears, as I'm a newbie and I'm not confident enough to check what's its max speed, although I think it's not going much further than 81mph/130kph)

Regards,

Asier.

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The official manual assures a 'safe' top speed for the 400xs at 81mph

as we all know we can tune cars/bikes to get at least 10% extra

but do you really need to drive at 90mph???

I get frikkin terrified

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Well that means I'm actually at top speed. I didn't find that figure anywhere.

It was more about curiosity and ensuring my bike's running fine rather than trying to break any record.

Actually I _already_ get frikkin terrified at 81mph when the stability of my rather-narrow tyres starts to advise me on going slower...

For me, right now, 81mph is a more than acceptable figure, even more knowing I can get there full of power and therefore +/- safe in the highway.

Thanks for the answer,

Asier.

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Yes, I do know what my problem is. I do have a vacuum leak in one of the venturi slides. I'm in the process of ordering a whole new set of carbs but its $$$$$$...

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What are the speed limits in Spain?

Im doing a eurotrip soon, 90kmh? 100kmh?

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Christoph21, sorry to hear that.

I've been testing my bike on the road today and maybe the "problem" is the driver :unsure:

'cause I've been pussing it a bit more and it's able to reach at least 9000 rpm; in fact, it reaches 81mph in 4th gear, so I guess it will pass that figure in 5th. To be honest I didn't test it, it was a bit windy, I'm newbie and the bike becomes a bit instable at that speed... maybe other day.

Someone at a different forum with an XS400 has told me his maintains a cruise of 81-88mph, and that 6th is only for maintaining speed rather than speeding up.

So probably my bike is working properly.

CharlyG, as far as I remember:

- Toll highway and freeway: 120kmh

- Roads with at least a 1.5m shoulder: 100kmh

- Roads with a shoulder smaller than 1.5m: 90kmh

- City: 50kmh

Anyway, remember to check traffic signals :D

In the last years they are pushing quite a lot wrt speed cams so be careful, they are everywhere, and depending of the speed you can go to jail.

If you come along the North of Spain contact me and maybe we can drink a beer together :D

If you need any further information or I can be of any help feel free to send me a PM.

Regards,

Asier.

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...Someone at a different forum with an XS400 has told me his maintains a cruise of 81-88mph, and that 6th is only for maintaining speed rather than speeding up...

That's just fast enough to go back to the future! ;)

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Yes, I do know what my problem is. I do have a vacuum leak in one of the venturi slides. I'm in the process of ordering a whole new set of carbs but its $$$$$$...

Why order new carbs? Jus order new slides from MikesSX at $60 each, or just order new diaphrams from Siriusconinc for about $20 or 30 each.

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