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carburetor options for 81 xs400 and other issues


bznchris
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Hey all,

I recently picked up an 81 xs400 that the previous owner had begun converting to a cafe racer. I'm continuing with the conversion, but need to straighten out some other issues first. One cylinder will fire when in higher rpms but not while idling. I thought this would be an issue with the main jet, but after a thorough cleaning of the carbs, it still happened. Then I turned to the leaky exhaust, fixed the leaks, and it still happened. I have noticed that the diaphragm of the corresponding carb has some pinhole leaks, so maybe at idle there isn't enough vacuum for it to run? Anyways, replacement diaphragms are as expensive as complete used carbs, so i was considering other options as far as new carbs- aftermarket? The head also leaks oil substantially, could this contribute to the problem? And yes, the cylinder is getting spark.

Second, I'm having charging issues. After riding for 20 minutes or so, the battery will completely lose it's charge, and the bike will die in lower rpms and not have enough juice to start back up. I replaced the battery thinking I had a bad one but it still happens. AC generator problems?

And I have all the manuals associated with this bike, but i'm having a hard time pinpointing the exact cause of these problems. Any help or ideas would be appreciated!

Chris

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I can't imagine XS400 carbs being expensive, they'd come up cheap at either a breaker's or on ebay. But if the diaphragms have pin holes in you need new ones my friend!You could replace them with a pair of mikuni VM carbs, but by the time you bought the carbs and all the bits you need you'd be well over the cost of a replacement XS400 carb assy.

The 400 had a weird rotating field magnet and stationary current windings similiar to the 650. Unlike the XS650, which has slip rings to take the power to the rotating field coils, the 400 uses mutual induction to a slotted piece of metal, very odd. I seem to remember issues with the wiring around the gear selector - worth having a good look at the wiring and check with a multimeter before lashing out the cash.

The lack of charging MAYBE be adding to the running problems, I remember one of my old XS400s running very rough when the regulator gave up on it.

Do you have a solid state rectifier/regular or the seperate rectifier and vibrating contact regulator?

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Do you have a solid state rectifier/regular or the seperate rectifier and vibrating contact regulator?

Couldn't answer that at this point. I'm still very new to this bike, and bikes in general really.

Thanks for the good info though.

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Couldn't answer that at this point. I'm still very new to this bike, and bikes in general really.

Thanks for the good info though.

My 81 XS400 had electronics: http://cgi.ebay.com/ebaymotors/1981-Yamaha-XS400-special-rectifier_W0QQcmdZViewItemQQhashZitem3ef97eb32cQQitemZ270473802540QQptZMotorcyclesQ5fPartsQ5fAccessories#ht_500wt_956

Mounts around/under the battery. I replaced that and my battery and I have no charging issues any longer.

The holes in the diaphragm will certainly screw things up. The new diaphragms are expensive, so is gambling on whether an ebay special is going to contain a decent set. I'd go by the condition of your carbs in general whether that's a good idea or not.

I haven't seen aftermarket carbs for the XS400, just her larger sisters. MikesXS has a lot of parts that will fit in our carbs without issue: http://www.mikesxs.net/products-35.html#products < was vital while I was rebuilding my carbs.

EXCELLENT overview of the BS34 carbs on our bikes (while some of the specific jet sizing info may be off, in general this guide is amazing): http://drop.io/jn5sqkc

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My 81 XS400 had electronics: http://cgi.ebay.com/ebaymotors/1981-Yamaha-XS400-special-rectifier_W0QQcmdZViewItemQQhashZitem3ef97eb32cQQitemZ270473802540QQptZMotorcyclesQ5fPartsQ5fAccessories#ht_500wt_956

Mounts around/under the battery. I replaced that and my battery and I have no charging issues any longer.

The holes in the diaphragm will certainly screw things up. The new diaphragms are expensive, so is gambling on whether an ebay special is going to contain a decent set. I'd go by the condition of your carbs in general whether that's a good idea or not.

I haven't seen aftermarket carbs for the XS400, just her larger sisters. MikesXS has a lot of parts that will fit in our carbs without issue: http://www.mikesxs.net/products-35.html#products < was vital while I was rebuilding my carbs.

EXCELLENT overview of the BS34 carbs on our bikes (while some of the specific jet sizing info may be off, in general this guide is amazing): http://drop.io/jn5sqkc

BIG help. Thank you.

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  • 2 weeks later...

My 81 XS400 had electronics: http://cgi.ebay.com/ebaymotors/1981-Yamaha-XS400-special-rectifier_W0QQcmdZViewItemQQhashZitem3ef97eb32cQQitemZ270473802540QQptZMotorcyclesQ5fPartsQ5fAccessories#ht_500wt_956

Mounts around/under the battery. I replaced that and my battery and I have no charging issues any longer.

The holes in the diaphragm will certainly screw things up. The new diaphragms are expensive, so is gambling on whether an ebay special is going to contain a decent set. I'd go by the condition of your carbs in general whether that's a good idea or not.

I haven't seen aftermarket carbs for the XS400, just her larger sisters. MikesXS has a lot of parts that will fit in our carbs without issue: http://www.mikesxs.net/products-35.html#products < was vital while I was rebuilding my carbs.

EXCELLENT overview of the BS34 carbs on our bikes (while some of the specific jet sizing info may be off, in general this guide is amazing): http://drop.io/jn5sqkc

You can use plastidip to fix the diaphragm and it will work pretty well. Otherwise while they are expensive, replacement carbs are going to have old ones too, and once again plastidip may help there.

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