Stock carb tuning

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Sourmilk

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Sup, newbie here. Got a question. Just bought my first xs400 from a friend. It idles but bogs down when you give it some throttle and dies. Up till this point I've only done moped builds. It ran fine when he bought it, he only owed it for a month before selling it. Wanted a Honda CB, what ever. So he cleaned the carbs and replace the oil filter, sold it to me. But since cleaning the carbs and filter it started this bogging problem.
I tested the cylinder compression which is 140 for both. So now I want to check the fuel level to make sure its 1 to 3 mm under the carb body?, correct that if its off. Figured I build a sync tool to check their suction balance. I believe I read that the pilot jet is stock set at 2 1/2 turns out?
Does anyone know what stock tuning is? It a 1978-79 model. I figure I dont need to mess with rejetting or needle postions, not sure if xs400 have that adjustment? Also I wanted to replace the airbox filter with a smaller crome top/mesh filter. What do you guys thinks? Thanks for the help, support.

Matt Sour
:bike:
 
My guess is that that carbs are still a bit dirty, and after cleaning them....again....try 3.5 turns out and adjust from there. Doesn't sound like anything serious.
 
If you go with pod type filters you will have to rejet the pilot and main jets.
 
Along with cleaning the carbs EXTREMELY well, you will also want to check the timing. I had similar symptoms when I bought our project bike. The bike would idle but the carbs were a mess. After a thorough carb cleaning and rebuilding the bike still would not rev up with the throttle. Adjusted the timing and all was well.
 
So what is stock tuning?
Clean.. again. check float level. set pilot jet.
pod type filter? Sorry but metal mesh high flo? No air box. What kinda rejetting an I looking at?
 
I would say a 45 pilot and a 145 main with the idle mix screw set 3 turns out would be a start you might have to raise the needle up a slot as well.
 
Sourmilk said:
Sup, newbie here. Got a question. Just bought my first xs400 from a friend. It idles but bogs down when you give it some throttle and dies. Up till this point I've only done moped builds. It ran fine when he bought it, he only owed it for a month before selling it. Wanted a Honda CB, what ever. So he cleaned the carbs and replace the oil filter, sold it to me. But since cleaning the carbs and filter it started this bogging problem.
I tested the cylinder compression which is 140 for both. So now I want to check the fuel level to make sure its 1 to 3 mm under the carb body?, correct that if its off. Figured I build a sync tool to check their suction balance. I believe I read that the pilot jet is stock set at 2 1/2 turns out?
Does anyone know what stock tuning is? It a 1978-79 model. I figure I dont need to mess with rejetting or needle postions, not sure if xs400 have that adjustment? Also I wanted to replace the airbox filter with a smaller crome top/mesh filter. What do you guys thinks? Thanks for the help, support.

Matt Sour
:bike:
Click to expand...

Yes you'd need to rejet if not stock airbox, problems at higher rpms espec.
You said it ran fine before carb cleaning? hmmm.
Float level should be adjusted to 32mm or 1.26", importance of float level described in link (courtesy andrew) http://www.factorypro.com/tech/carbtun.html
Float height adjusted with carbs removed and upside down.
Is everything AIRTIGHT upon reassemby?
Turn fuel off and remove float bowls and check for debris, clean out dirt. push the floats upwards a few times to possibly dislodge any dirt in the float valve needle/seat.
Wouldnt hurt to make sure petcock is nice and clean too.
There are three tiny bypass or transition holes that are exposed just as butterflies open, the port to those holes needs to be clean or opening throttle will do what you describe.
Do each of the slides if pushed upwards return smoothly?
Opening the throttle from idle and engine hessitates, probably adjusted too lean and need to richen by turning pilot air mix screws out.
Fuel mixture can be checked by inspecting the color of spark plugs - if blackened on the insulator then too rich, if plugs are white crusted then too lean. I could learn alot, can someone tell me if that is a good test only when going full throttle and checking the color, would that apply to idle speed mixture also if starting with cleaned plugs?
If removing the plugs check electrode condition and gap too.

Before he sold it to you and cleaned carbs and changed oil filter did he do anything with the timing and points?

I think you can roughly sync the carbs by placing hand near exhaust and feel if aprox even for both sides. For a manmadeometer and some good basic info on my learning level:
http://yamahaclub.com/forums/topic/21942-xs-400-adjusting-mixture/page__p__120362
http://yamahaclub.com/forums/topic/21270-1980-xs400/page__p__116845
 
Thanks Ill check it out. Appreciate all the great feedback. I'm uses to moped forum where people trash talk and jerk around, but this forum is kill'n it!
 
Get a manual too(You can download it from this site) and read it cover to cover a few times. Soon you will be a pro at tuning...
 
drewpy said:
only for brass floats, plastic floats +/-2mm using clear tube as sight gauge!
Click to expand...

I noticed you asked if he had the later plastic floats. And concerning Haynes info inadequacies, the manual I have downloaded is Haynes 250, 360 and 400 ranging from 1975-78. They give the one figure only from what i've seen.

In other free downloaded manual (don't know who it is) (generic), it is more specific with year (page 1285):
XS400 D E F/2F = 32mm/1.26
XS400 2E = 25.7mm/1.0
XS400 G SG H = 27.3mm/1.1

Whatever that download manual is it is a good supplementary and nice to have the 3 but feel lacking not having the FACTORY manual :shootme:
 
Sourmilk said:
So he cleaned the carbs and replace the oil filter, sold it to me.
:bike:
Click to expand...

Forgot to ask, did he do you the courtesy of washing the bike before selling it to you; possibly just drain floats if there was water in them, starting with simple stuff first :)
 
I've downloaded the manual posted thru the forums. The bike came with the Haynes manual. I havent had the time yet to try anything but Im about to go to the shop and start working. Figured I first check the idle screw if that doesnt help, reclean check float. Check petcock. My friend who sold it to me said he didnt play with the floats. also check that the valves in the intakes are plugged. Hope it runs.
 
HoughMade said:
The manual on this site IS the factory service manual.
Click to expand...

SWEET! That's why pages in the thousand digits? I will have to revisit and see if I can find more of it maybe. Thanks :)

edit: nonetheless, great to know so I don't have to always call it the "unnamed" manual, ha
 
Last edited:
Mikesxs has them. They use the same pilot and main jets as the 80-82 carbs:thumbsup:
 
Running up my carbs and am a little confused. When you say point jet do you mean pilot air (inside carb near pod air filters) or pilot fuel(on top near carb boots) do I unscrew both x turns or is Air all the way in?
 
Pilot Air jet - forget about it.
Pilot jet - in float bowl, controls idle and part throttle fueling.
Pilot Screw - used to adjust idle mixture. In = lean, Out = Rich. minimum 1.5 turns, max 4 turns from GENTLY bottomed. Outside that range, resize Pilot Jets accordingly.
Pod filters - :doh:
 

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