high rpm after carb clean

L

Line

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• Mar 12, 2016

• #1

hello everyone.
just got a hold of an xs400D from '77 (from what i could tell)

I cleaned the carbs and now it's easy to start. problem is that it idles at really high rpm's
the idle screw is all out and the carbs should be somewhat syncronized.
is there something vital im missing?
the bike is stock by the way. no aftermarket stuff. and it didn't have this problem beforhand. then it would die off ass soon as a little throttle was givin.
could an airleak in the old intake have such a huge effect?
please help.

 

XS400NJ

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• Mar 12, 2016

• #2

Sounds like air is leaking... First I would check that vacuum lines are connected to petcock and are not leaking, then that butterflies close all the way when 'idle screw is all out' , your throttle cable may be holding them from closing. If they close all the way - air may be leaking through the gap between carbs and cylinders, or there is a leak in butterfly's shaft (those rubber rings can go bad). Also, if you cleaned idle mixture screws (the ones on top ) - check there is no air leaks there, I think there is a little 0-ring there as well.

just a few thoughts through Saturday morning headache...

 

MrStubb

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• Mar 12, 2016

• #3

I can't agree more. Sounds like a vacuum leak. Make sure you check your carb boots where they slip onto the manifolds. The rubber on mine was in bad shape & after I cleaned the carbs the 3rd time I found a few splits in the boots. I used a good silicone sealer to temporarily fix the issue. Also make sure you have the needles & throttle adjusted right. You could also have an issue with the enrichment (choke) circuit in 1 or both carbs.

 

Lego

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• Mar 14, 2016

• #4

If you say idle screw is all out, you mean pilot screws? (one for each carb) If yes, they should not be all out, but 2 and half, or 3 turns out.
If problems are on idle to 1/4 throttle, reason can be pilot circuit, or butterfly valves, as other guys are telling you. So probably check this also out, together with air leaks and other stuff.

 

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razornightman

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• Jul 13, 2016

• #5

I am having the same issue. How do you search out a vacuum leak? I replaced the boots earlier this year. Replaced hose from petcock to carb also. Still wants to idle at around 3000 rpms. It only happens when the engine is hot.

 

akirasan

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• Jul 14, 2016

• #6

Spray carb cleaner or wd40 around the carbs and listen for the revs to change and you have found a leak.

I do not have any o rings on the pilot screw look on parts list and there are not any on that

I have a 4002e 1977

 

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razornightman

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• Jul 14, 2016

• #7

Akirasan. Thanks for info. I thought that I had read somewhere about doing that. Other post suggest to lean of a mixture. I will pull plugs and have a look at them also.

 

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xschris

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• Jul 14, 2016

• #8

Did you sync them with a manometer.

 

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razornightman

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• Jul 14, 2016

• #9

Yes, not sure what is going on. I just came back from a 3 mile ride and it is not doing it. I have a slight sputter at around 5000-6000 rpms,but thfast idle didn't happen.