rshutchinson
XS400 Addict
I meant no disrespect. Was merely a suggestion. What about adding to the plenum you've already built?
Or just do it how you planned and ignore me.
Or just do it how you planned and ignore me.
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81 400xs turbo build
- Thread starter Thedoityourselfguy
- Start date
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- Tags
- chopper
That's the only answer.
The more I look at my '82 carbs the less modifications it seems like they'd need to run under boost.
Seems like everything that needs to see boost/dynamic pressure is already vented from the carb mouth.
The more I look at my '82 carbs the less modifications it seems like they'd need to run under boost.
Seems like everything that needs to see boost/dynamic pressure is already vented from the carb mouth.
Thedoityourselfguy
XS400 Enthusiast
Bbs I like your idea best.... Upon looking at my carbs this is what I have discovered....
You can see the light from the flashlight shining through one of the 2 bowl vents indicating no mods are necessary to pressurize the bowls. Also.... The oval shaped port at the bottom of the mouth of the carb is the venting for the top of the carb indicating again... No mod is necessary.
As for the shaft seals I do not think there will be enough pressure loss to inhibit operation although I will probably just flip the outer two and leave the inside ones as this requires splitting the carbs.
Now the bowls have to see slightly higher pressure than the rest of the carb so die grinding inside the bowl vents to make a funnel shape may be necessary.
You can see the light from the flashlight shining through one of the 2 bowl vents indicating no mods are necessary to pressurize the bowls. Also.... The oval shaped port at the bottom of the mouth of the carb is the venting for the top of the carb indicating again... No mod is necessary.
As for the shaft seals I do not think there will be enough pressure loss to inhibit operation although I will probably just flip the outer two and leave the inside ones as this requires splitting the carbs.
Now the bowls have to see slightly higher pressure than the rest of the carb so die grinding inside the bowl vents to make a funnel shape may be necessary.
I've never run anything under boost yet so it's all guesswork on my part but I'd be cautious of doing too many mods before testing the turbo.
The carbs are already designed to create that pressure differential through the venturi effect caused by the throat diameter reduction.
Increasing that difference ought to increase the fuel/air mixture, which you'll want under boost, but there might be more predictable ways of doing it.
The carbs are already designed to create that pressure differential through the venturi effect caused by the throat diameter reduction.
Increasing that difference ought to increase the fuel/air mixture, which you'll want under boost, but there might be more predictable ways of doing it.
I wouldn't bother messing with the throttle shaft seals. Your engine will spend more time pulling a vacuum than with any significant boost. A reliable idle is going to be more important to you than the slight leakage from those seals.
How are you going to prevent the floats from imploding under boost?
No need to mess with the bowl vents.
How are you going to prevent the floats from imploding under boost?
No need to mess with the bowl vents.
I'm still not convinced that the stock vent holes will work. They do nothing to increase charge in the float bowl. That why guys that use CV carbs in a blow through setup still have to design and use a pitot style system into the float bowls. Under boost, the area in the bowls above the fuel needs to be higher pressure than that of the pressure above the jet in the venturi. Otherwise, no fuel will flow through it. The bikes I have seen that still run without the pitot style setup, run very poorly and are either dead lean, or are only surviving because they are using excess fuel pressure to simply dump fuel into the venturi, not being metered at all.
I didn't mean to say that nothing needs to happen. Something definitely needs to happen. The page linked to above uses a kind of pitot setup.
Grinding the vent holes into funnels probably won't do much. It's the position in the air stream that needs to change. The example above is pretty specific about the pitot hole being placed directly in the middle of flow to see total/dynamic pressure.
I'm thinking there's two options with the stock air vents:
1- block them entirely, plumbing the bowls elsewhere
2- move them into the middle of the air stream
1: Blocking them might actually provide more flexibility. The float bowl cavity extends up quite high behind the Mikuni logos on either side of the carb body to reach the stock vent holes. It wouldn't take much to weld/thread/solder a hose barb right where the mikuni logos are.
Then you can figure out the most ideal location for the pitot and run hose there.
You don't have too many options with that plenum but the tube between the turbo and plenum might be good.
2: Depending on clearances in front of the carb throat in the plenum it would be possible to insert pitot tubes into the stock vent holes.
Drill the stock vents out to 7 or 8mm.
Get matching 7 or 8mm OD tubing, bend an offset into it. Approximately 18mm offsets should get right into the middle of the air stream.
Solder the pitots in place.
Downside here is that depending on their shape and where they are soldered in place they would extend forward into your plenum, where air velocities will be lower anyways. The further towards the plenum they go the less pressure the bowls would see.
Grinding the vent holes into funnels probably won't do much. It's the position in the air stream that needs to change. The example above is pretty specific about the pitot hole being placed directly in the middle of flow to see total/dynamic pressure.
I'm thinking there's two options with the stock air vents:
1- block them entirely, plumbing the bowls elsewhere
2- move them into the middle of the air stream
1: Blocking them might actually provide more flexibility. The float bowl cavity extends up quite high behind the Mikuni logos on either side of the carb body to reach the stock vent holes. It wouldn't take much to weld/thread/solder a hose barb right where the mikuni logos are.
Then you can figure out the most ideal location for the pitot and run hose there.
You don't have too many options with that plenum but the tube between the turbo and plenum might be good.
2: Depending on clearances in front of the carb throat in the plenum it would be possible to insert pitot tubes into the stock vent holes.
Drill the stock vents out to 7 or 8mm.
Get matching 7 or 8mm OD tubing, bend an offset into it. Approximately 18mm offsets should get right into the middle of the air stream.
Solder the pitots in place.
Downside here is that depending on their shape and where they are soldered in place they would extend forward into your plenum, where air velocities will be lower anyways. The further towards the plenum they go the less pressure the bowls would see.
I can confirm that the plastic floats are solid. Shouldn't be an issue at all.
Thedoityourselfguy
XS400 Enthusiast
X2 on the floats
i guess I should remind everyone this is not a 30 psi build......this is a 6-9 psi build so very forgiving compared to others
I did some more digging. Now I feel the stock ports will be ok. The key is having the pressure fed into it, which it would be as is. So long as the fuel pressure is always higher than referenced boost, it will feed fuel through the main jet. What confused me was that I never take into account that the higher fuel pressure adds to the pressure in the bowl. That's what makes it higher pressure than the pressure in the venturi. So that's why you need the vent passage either facing towards the boosted air (as with our carbs) or use a pitot tube.
If you were to use a static style port, the increased pressure in the bowl would try to bleed out the port as well as the main jet, which is why people have lean conditions or surging AFR.
I think you are on the right track. Seems to be all about very precise choice of fuel pump, regulator, and what they are set at.
I hope it ends up running great.
If you were to use a static style port, the increased pressure in the bowl would try to bleed out the port as well as the main jet, which is why people have lean conditions or surging AFR.
I think you are on the right track. Seems to be all about very precise choice of fuel pump, regulator, and what they are set at.
I hope it ends up running great.
Might work all right with our internally vented carbs but increasing the pressure seen by the gas in the float bowl might reduce the severity of jetting changes required to run under boost.
Stick with stock, see how it runs, make changes as necessary.
Stick with stock, see how it runs, make changes as necessary.
Thedoityourselfguy
XS400 Enthusiast
I am going to try running it without fuel enhancement to see what happens. I do have an air fuel gauge for monitoring. If it doesn't run right under boost it will be time to install the fuel system.
Supply and return oil lines done today gonna test pressure soon
Supply and return oil lines done today gonna test pressure soon
It's getting there.
DieselRAT
XS400 Member
Hey did you think about what pressure that waste gate opens..... if your only runnin a few psi boost you may need to do some monkey business with the waste gate pot spring......
Also now i know you got the turbo oil line run already, but someone mentioned running it in series with the top end oiling.... if you have volume problems down the road that may be a viable option.... thinking out loud... i wonder if one could tap the flow to the top end route through the turbo with restrictor removed, then through the top end.... maby cool it after the turbo with a coiled steel line... but then maby you would have too much back pressure on the turbo outlet...... hmmmmmm
Food for thought
Your moving along quickly!
Also now i know you got the turbo oil line run already, but someone mentioned running it in series with the top end oiling.... if you have volume problems down the road that may be a viable option.... thinking out loud... i wonder if one could tap the flow to the top end route through the turbo with restrictor removed, then through the top end.... maby cool it after the turbo with a coiled steel line... but then maby you would have too much back pressure on the turbo outlet...... hmmmmmm
Food for thought
Your moving along quickly!
DieselRAT
XS400 Member
I got to thinkin.....perhaps someone could chime in on this, is the oil pump interchangeable with say an xs650 or some other larger engine... and if so do the bigger displacement engines have higher volume oil pumps?
Maybe higher volume, but not compatible.
This:http://www.partzilla.com/parts/search/Yamaha/Motorcycle/1982/XS650SJ/OIL+PUMP/parts.html
Vs This: http://www.partzilla.com/parts/search/Yamaha/Motorcycle/1982/XS400SJ/OIL%20PUMP%20OIL%20CLEANER/parts.html
This:http://www.partzilla.com/parts/search/Yamaha/Motorcycle/1982/XS650SJ/OIL+PUMP/parts.html
Vs This: http://www.partzilla.com/parts/search/Yamaha/Motorcycle/1982/XS400SJ/OIL%20PUMP%20OIL%20CLEANER/parts.html
levdir
Rounded bolt brigade
Incidentally if you make to Edmonton after this is on the road I'd love to see it up close
Thedoityourselfguy
XS400 Enthusiast
