thanks for that. what's the best way to cap the carb on the right side?from the petcock to the left carb it's a very short length of pipe I found it difficult to fit without it kinking as this would prevent a vacum.im using the prime setting for now with a fuel line running from one carb to the other as this is the only way I can get the bike to idle.ive ordered petcock gaskets etc ill get these fitted and set it up as you have suggested. I've also noticed there is some light blue smoke from left side at start up but vanishes when engine is warm?thanks again for your help.
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petcock question
- Thread starter codyxs400
- Start date
I tried rebuilding mine when I first got it but it still leaked. I went with one off the jungle site for pretty cheap two years in and it's still working fine. Capped both vacuum ports and no issues.
Carbman Petcock Fuel Gas Tap... https://www.amazon.com/dp/B07SWJ4N9J?ref=ppx_pop_mob_ap_share
Carbman Petcock Fuel Gas Tap... https://www.amazon.com/dp/B07SWJ4N9J?ref=ppx_pop_mob_ap_share
thanks for that .when I bought the bike the two carbs were capped .the previous owner used a piece of fuel line from carb to carb ?and run the bike on prime .tiks over good runs ok but hard to get over 60mph.ive took this pipe of from the 2 carbs and tried capping the each carb with a small piece of pipe .bike will start but won't tik over and revs quite rough ?gone service the petcock and see what happens .the bit I don't get is how come it idles when the carbs are connected with the pipe but idles roughly when capped separately?
gpounce32768
XS400 Enthusiast
ebay/amazon has lots of tygon tubing, and vacuum hose if you dont' have a convenient auto supply store nearby. Easiest way to cap the vacuum nipple is a short length of hose with a screw in the end. I had to run new hose for everything including the brake lines on my bike the old stuff was stiff and rotten. I'd suggest new hose, cut to a convenient length.
The carbs could be radically out of balance- there is an adjustment between them to set the closed positions of the butterfly valves when the throttle is closed. If they are out of balance that means one is open more than the other, and one could easily be mostly closed. That line bridging the manifolds could be bypassing air & fuel into a carb with the butterfly entirely closed, allowing the cylinder to fire. With the ports separately capped then each carb is in play with its respective cylinder.
I had different problems with my XS but I did find the carbs easy to balance- but you'll have to make or buy some kind of vacuum measuring device. Essentially, the device show vacuum readings on both manifolds (with the engine idling). THen the balance is adjusted via a screw on the throttle shaft coupling between the 2 carbs so both carbs show approx the same balance. While that adjustment is in progress, the idle screw usually has to be adjusted to speed it up or slow it down because the adjustment effects both cylinders and in the end you want a suitable idle rpm.
I will throw out a word of warning - most auto parts stores sell rubber caps for vacuum nipples, but they don't last very long. Heat kills them and they split.
It really sounds like you have a classic "just bought an old bike" problem - need a carb clean and some gaskets.
It really sounds like you have a classic "just bought an old bike" problem - need a carb clean and some gaskets.
I'm going to agree with gpounce on the carb adjustment issue. The two boots shouldn't be linked. You can do a bench sync of the throttle blades to get them very close and it's not hard to do. Then readjust the idle mixture screws. Several good posts on here on doing that.
thanks for the advice appreciated
thanks fella.thats what I'm starting to think. the carbs are needing cleaning, balancing etc. I think the petcock needs attention but this is not the main issue.gona strip them down at some point soon .
thanks.idle mixture screws .I believe each carb has an air screw . one idle screw to set the rpm.1 and half turn outs on the air screw?
gpounce32768
XS400 Enthusiast
Idle mix screws are covered by brass caps, on the top side between the diaphragm risers and transition to the intake manifold, those will be helpful to refine the carb tune once you have all the bigger issues sorted- so preferable to leave them alone till then. It can be a bit of a pest to get the caps off.
Correct on one idle screw, on the right side of the carb assembly where the throttle cable operates the butterfly valve shaft.
Definitely concur with 1st thing getting the carbs out and cleaned, replacing gaskets etc as needed.
Correct on one idle screw, on the right side of the carb assembly where the throttle cable operates the butterfly valve shaft.
Definitely concur with 1st thing getting the carbs out and cleaned, replacing gaskets etc as needed.
thanks for the much appreciated info
back again .one more thing .the carbs have those stubby air filters fitted. do the carbs need different sise jets ?and does anyone know the float height?
I also have the old air filter housing is it best to put it back on
gpounce32768
XS400 Enthusiast
Yes put the old airbox back on, get rid of the separate filters. Service manual for '82 says for main jets, left carb 127, right carb 117. Mine had 117 on both carbs- runs fine. Starter jet is #35.
check gas level as below- its a somewhat cumbersome sort of measurement to do. I did it by clamping the carb bank in a vise and cobbling up some tubing connected to the bowl drains, then pouring enough gas into the carb inlets to fill the bowls. Float levels on mine were as pictured- no-one had been inside the carbs since day 1 I think.
check gas level as below- its a somewhat cumbersome sort of measurement to do. I did it by clamping the carb bank in a vise and cobbling up some tubing connected to the bowl drains, then pouring enough gas into the carb inlets to fill the bowls. Float levels on mine were as pictured- no-one had been inside the carbs since day 1 I think.
thanks so much.i have the carbs off now .I've noticed straight away the butterfly mechanism is closed tight. should these not be adjusted so they are not fully seated ?a bit worried about dismantling the carbs incase I screw up.
I've checked my manual. says main jet 147 in both carbs,starter jet 30 ?it's 117 for the 360cc
gpounce32768
XS400 Enthusiast
Ah- I have an '82 XS400 Seca, jets in it are as per the service manual. We may have have different carbs- I'd go for your numbers for your bike.
Butterflies are held open by the idle screw so no worries there. No need to seperate or fully dismantle the carbs, the goal here is to get the bowls and diaphragms out and ensure all passages are clear. Then make sure the enrichener mechanism opens and closes properly, diaphragms sealing and sliders moving properly, needles in a reasonable place, float valves operate (and floats aren't leaking). Once everyting is all back together then fuel level in the bowl is tested, then carbs to back in the bike to see how they're working.
At that point I balance the carbs on the bike, with the engine running, and get the idle rpm set.
With all that done and things are reasonable then I go for air/fuel tuning. The effect of that is fairly subtle- I didn't notice much change on the XS.
Butterflies are held open by the idle screw so no worries there. No need to seperate or fully dismantle the carbs, the goal here is to get the bowls and diaphragms out and ensure all passages are clear. Then make sure the enrichener mechanism opens and closes properly, diaphragms sealing and sliders moving properly, needles in a reasonable place, float valves operate (and floats aren't leaking). Once everyting is all back together then fuel level in the bowl is tested, then carbs to back in the bike to see how they're working.
At that point I balance the carbs on the bike, with the engine running, and get the idle rpm set.
With all that done and things are reasonable then I go for air/fuel tuning. The effect of that is fairly subtle- I didn't notice much change on the XS.
thanks for your reply. yeah slightly different bikes.im going to fit original air box.however how does this bloody thing fit in?to adjust the butterfly's do you put a needle into the housing so there's a small gap between the carb housing and butterfly?I also don't know if the previous owner has changed the jets to suit the stumpy air filters.what I can tell you is the fuel that spilt from the float bowl was discoloured, light brown in colour. I'm planning to open the carbs up in the nxt few days.also does the engine breather connect into the air box once fitted.right now it's one of those small stumpy filters .is there another name for starter jet can find any on ebay .thanks
Can you take some pictures of what your airbox and the bike look like? Kinda hard to say whether you even got the right parts.
You can use a strip of card stock. Or not bother, and just get a running synch.
Doesn't sound very good.
Yes.
"Starter jet" is actually not replaceable, it's pressed into a hole in the bowl.
The two replaceable jets are pilot and main.
Hold off on looking for replacements until you see what condition yours are in and what size they are.
thanks.ill get some pictures to u guys .
