No Air Box, No Pod Filter, A Third Alternative?

Bellerophon

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I am rebuilding an XS400 engine (see signature) and I do not have the Air Box, H-Pipe or Filters and they are expensive to get here in the UK (and they are ugleeeee!). I also don't want to try pods (K&N, Uni or otherwise) as this is my first engine build and I haven't even seen the thing running yet, so I think that would be too much of a leap into the unknown.

Has anyone tried or heard of a third alternative that I haven't mentioned? Maybe a compromise off-the-shelf product or an air box from a different 400cc Japanese bike (cheaper / better looking) that can be retro-fitted?

Thanks
 
Stock is the best setup for overall performance and long life. Uni's on the H-pipe is the next best thing. K&N on the H-pipe is next. The H-pipe is a big key with tuning as it helps the carbs balance and smooths out turbulence that these carbs hate. It also vents the crank case gasses and pressure back into the motor. The oil in the gasses also helps keep the carb slides lubed. If you must, uni's on the carbs is about the last thing to do. Make sure they are black dense foam and long. I have never heard of any other bike that has a stock air box that fits the sohc xs400 bike. I would worry more about fuction over looks any day as I would rather be riding then tuning any day. I have been down the pod road before. I have used four different ones in many configurations. I run all my bikes stock now and will never look back. Some pods work better in certain rpm ranges but none improved all of them. OEM setups work very well for overall.
 
Any CV carb will tend to like the OEM airbox as it is designed to complement the pulsing in the intake tract. The carbs tend to help each other lower the air pressure in the box which allows the slides to open easier. Remove that sharing and the carb slides slow down in action and bike loses a lot of the peppiness it had.

Pods work fine on carbs that are not CV type.

I have one of the worst CV/pod combos, Honda DOHC 750 with CV carbs using mechanical labyrinth instead of the rubber diaphragm, they are super b-tchy about putting pods on them and typically do not like them at all. Even changing the OEM paper air filter for a freer flowing K&N stock replacement filter will make them lay down and die.
 
I just got my 80 XS400 going. it did not have any of the factory air intake. I put 137.5 mains in the stock carbs (cleaned ultrasonically) and installed Uni UP4182 filters on each carb. It was basically a parts bike brought from the dead.
It runs surprisingly strong with no issues. Stock exhaust on this also.
I also changed to euro type bars and built a custom seat. Very cool and great running cycle now.
 
They will seem to but often not open the slides 100% up, take it out on say a mile long straight and see if it maximizes top rpm out in top gear, most won't.

Service manual off this site calls for 142.5 jet in XS400...................???
 
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Stock 80 jets are 135. The 77D was the only one that had the 142.5 main jets. Other sohc bikes used different size mains also as emissions, exhaust and carb models changed through the years.
 
They all say that until they realize top gear won't hit redline. Run pods on CVs for 25 years, you learn some things. Like by not sharing the airbox to draw down to a slight vacuum you lose a torque hit that otherwise happens at low throttles and sudden snapping open of them. Put airbox back on and you feel it instantly. You can get some of it back by drilling the slides if pods are where you intend to stay.
 
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Thanks guys. I'm going air box for now, managed to find some parts that weren't extortionate, I think rebuilding the intake as stock for now is the best. Especially as I'm hoping to run a non-stock exhaust.
 
The factory spends thousands designing the airbox to work with the correct gulp factor of the carbs depending on crank firing order and the needed vacuum increase to induce the CV slides to work faster, only to have people dump it to go to pods or the like and then spend more time and $$ trying to emulate the airbox they just dumped.

That's on CV carbs of course, you can pretty much do anything using direct lift carbs.
 
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