What exactly is the function of the stock H-Tubes?

[blabla]

Hey guys,

I am looking into buying some aftermarket air filters. I am really new to this and do not know all that much about the carb. stuff. I understand that most aftermarket air intakes and filters require upjetting, but from what I understand, this also has something to do with whether or not the factory H-tubes are still used? If I kept the h-tubes, is up-jetting still required?

 

[xschris]

It depends on the filters you get. I would go with a set of foam uni filters that take oil. They will filter well and the mikuni cv carbs work well with them. I have been doing a bit of research on crank case venting into these h-pipes and what it does is cause a negative and positive pressure in the engine (also re-burns un-burnt gases). The pressure also controls foaming of the oil under higher rpms and keeps the oil where it needs to be.

 

[blabla]

Oh okay…that's interesting. Thank you very much for the response. So do you think the UNI filters with the h-tubes would require an up-jet?

 

[willem]

If you get pod air filters, some kinds have an inner lip which can block off one of the air passages on your carbs if you leave out the H-tube. That will make the bike run like cr*p. Some pods don't have this lip and can be mounted directly.

And as Chris said, the crankcase vent can go into the H-tube which is nice. Any oil fumes from your crankcase (it spits out some fumes if your oil pressure gets too high) will be routed into the carbs and subsequently into your combustion chambers. This tiny bit of oil theoretically lubricates your carbs a little, and as the fumes are combustible they could potentially lower your fuel consumption just a tad. Then again, it is quite an uncontrollable, irregular addition of combustible fumes so if you ask me that isn't gonna help any. I believe GTIDave has had trouble with very smelly fumes around his bike when he ran pods without the H-tube.

And to answer your question, yes any type of pods will likely require an up-jet

 

[blabla]

Thanks for your response! So your saying a should leave the H-Tubes? I was planning on that, but i guess my question is would the UNI filters with the h-tubes require an up-jet? I wouldn't think so, but I'm also not very educated on this stuff

 

[xschris]

If you have to re-jet it would be minor. Most likely 1 on the pilot and one on the main. You may be able to get away with just turning the pilot mix screw out a little. Most people need to re-jet for pods because the use the dry/cheap non-oiled ones. I use oiled ones on my 80 and use stock jets. Dave's bike is a dohc model and their case vent is different that the sohc bikes.

 

[blabla]

Awesome, thanks for the info, man. I also had a another random question…what is the point of a crankcase filter…are they really nessecary?

 

[willem]

If you run pods and no crankcase filter you risk dirt coming into your crank case. Better to cap it off with a filter, you don't want anything going in there. It will end up in your motor oil and you'll have to replace that more often to compensate.

Chris is correct, dave's bike is a dohc and that's quite different. My bad.

 

[blabla]

okay, good to know. Thanks man. SO if i buy the UNI filters, what size should I get? What size would fit the factory H-tubes? Same size as the carb? a 34mm?

 

[Sesty]

would depend if you went wanted the filters in the hpipe or around it.

this is the thread u want to look at

http://www.xs400.com/forum/showthread.php?t=11732

 

[xschris]

The outside of the pipe is 52mm.

 

[xschris]

Or if you can fine a set of these that go in where the stock filters go, and they are made to be oiled.

 

[blabla]

Thank you guys very much! I think I'm gonna get me some of these

http://www.motorcycle-superstore.com/25830/i/uni-air-filter-high-flow-filter-kit

 

[xschris]

Looks good just make sure to use the oil designed for foam filters.

 

[Sesty]

chris do you have an Hpipe off the bike? can u measure the OD of where the vent tube connects? need to find a replacement tube

 

[xschris]

I get 15mm.

 

[Sesty]

do u you think the standard clear plastic tubing found at hardware stores could withstand the heat in from the crankcase?

been having trouble finding 5/8 tubing in the thicker black rubber hose.

 

[xschris]

The stock oem hoses are very hard to find. I used a clear one for a while. But then it started to get soft/melt and loose the seal from heat.

 

[Sesty]

yea ill have to go that route till it melts and keep an eye out for 5/8 radiator hose

thanks chris

 

[xschris]

You can try your local yamaha dealer. I lucked out and found two for $15