problems with carbs, xs400 1978

[DrHans]

Hello

during the build I have found a problems I suspect it is carburators:
The bike only starts with Choke On and runs very unstable, when opening the throttle bike turns Off.

what may be a problem ?
jets ? (if jets, what are sizes and names for replacement jets) carbs are: Mikuni side choke lever type (photo attached)

I cleaned the carbs before, but only used petrol to clean. If they require soaking in special carb cleaner, which one is best ?

 

[16VGTIDave]

Vinegar, lemon juice, or any other mild acid.

A search will yield many threads on carb problems and how to clean them. I posted a link to a great tutorial on another site a week or 2 ago.

 

[Pork Chop]

I used dish washing detergent, then kerosene & carb cleaner spray. I used the mrs ultrasonic cleaner too. Fishing line to poke in the holes to check for blocks.

 

[DrHans]

thank you for answers guys, do you know what exact name for pair of carbs I have. Mikuni side lever type ......., and jets sizes for them ? (want to replace them)

 

[xschris]

Those are the 76/77 xs360 carbs ( here in the u.s.) maybe they used them over where you are but thats what they are. Very good carbs but are set up very different than the xs400 ones. Jets should be 17.5 pilot and 135 main. I would measure the inside part of the body that goes faces the engine and make sure they are 34mm and not 32mm I know there where xs250's over there that used that stile and size.

 

[DrHans]

Those are the 76/77 xs360 carbs ( here in the u.s.) maybe they used them over where you are but thats what they are. Very good carbs but are set up very different than the xs400 ones. Jets should be 17.5 pilot and 135 main. I would measure the inside part of the body that goes faces the engine and make sure they are 34mm and not 32mm I know there where xs250's over there that used that stile and size.

Thank you for reply Chris, measured the inside part (that faces the engine) and it appear to be 32mm. so unstable runing of bike is due to wrong carb size ? do I need to replace 32 carbs with 34mm ?

 

[xschris]

I have never run a set of 32mm so can't help there. If you can find a set of 34mm for cheap it would not hurt.

 

[bcware]

Take the pod filters off. Check to see if they have an inner lip that blocks the intakes on the carb mouth.

 

[DrHans]

Take the pod filters off. Check to see if they have an inner lip that blocks the intakes on the carb mouth.

It is not filters, I tried to run the bike with and without them. it is something else

 

[bcware]

It won't run well without a filter either. If they have a lip it won't run well. Without any filters it won't run well. It's a crap sandwich. You need a proper filter before you can move on.

 

[DrHans]

It won't run well without a filter either. If they have a lip it won't run well. Without any filters it won't run well. It's a crap sandwich. You need a proper filter before you can move on.

I have checked the filter, it does not cover air intake

 

[willem]

the air filter also gives problems if air flows through it too easily. You will need to rejet, most likely. Just get a few different sizes of jets, and try out what works best. But only after you know 100% sure your carbs are clean

 

[Pork Chop]

G'day DrHans,

Those carbs look like BS32's. I have them on my xs250. Stock jets are 20 and 117.5 for the xs250 but i would guess 137 or larger main jet for the xs400.

I think they were used on very early xs1 but can't find where i read that.

Posted via Mobile

 

[Scorpio1963]

i would guess 137 or larger main jet for the xs400.

You are correct and 42.5 for the pilot

 

[xschris]

These carbs are not like the 77-79 or 80-82 xs400 carbs. The bs32's came with a 20 pilot jet and a 117.5 main. From left to right 76-77 xs360 (also bs32's) 77-79 xs400 and 80-82 xs400 carbs. The 76-77 xs360/bs32's have there pilot and main jets in the bowls

 

[DrHans]

thank you everyone for answers, so to adapt these carbs to xs400, I should run 137 main and 42.5 pilot ? or any larger combination ? (if so, what size they should be) ?

 

[willem]

there is no way to tell exactly which jets you need, as it depends on a lot of things like how freely air can flow through your system, what the barometric pressures are in your area, etc etc. You'll just have to try and see in the end, but maybe someone else can tell you what worked for them

 

[Scorpio1963]

How does barometric pressure affet the carbs?

 

[willem]

if you're high up in the mountains, you'll have thinner air (lower pressures) so you need to have more of it going into your mixture.

Other way around, with thick air at sea level you'll need relatively more fuel.

 

[Scorpio1963]

Thanks for the explanation I didnt look at it that way.I did know the higher in altitude the thinner the air thus more fuel requirement but I didnt know it had to with barometric pressure also.I wonder if theres anyone who has tryed making a fuel injection system for our bikes.Im at 560ft above sea level here and thats pretty consistant through most of lower michigan