Right cylinder not firing on idle

Ok, here we go, guys.

I've recently bought a slightly neglected '81 XS 400 SE with 50 thousands kms on it. I've done just about everything I could to make it run.



1) I've cleaned the carbs several times, checked the diaphragms, float levels (I've got plastic floats, couldn't find any definite information on it) and bench synced them. In went new spark plugs.

2) I've cleaned the points, gapped them at 0,35 mm, set the timing using the strobe, spot-on.

3) I've replaced cracked boots with gaskets, shouldn't have any leaks.

4) Adjusted the valves

5) Tank is clean, replaced fuel lines and added fuel filter.

Now the bike starts perfectly with full choke but for what it's worth I can't get the right cylinder to fire on idle. The right exhaust is popping. Whenever I twist the throttle, the cylinder picks up and works perfectly. It rides very smoothly. But it just won't idle.

I've gone through the carbs three times, making sure the pilot jet and its circuit is free, I've set the mixture to 2,5 turns out, it must be getting fuel. I also tried spraying carb cleaner all over the carb to see if there's a vacuum leak, but to no avail.

I honestly don't know what to do. I've tried swapping the caps, even the coils themselves. The points don't emit sparks, so I suppose the condenser is fine. The battery is new. I'm just lost. I've fixed about a dozen of these XSs since 2014, split countless of these engines, and never did it cause me this much trouble.

If anyone could step in with something, I'd be glad... Thanks!

compression test..?

Years ago mine would foul the left plug all the time and would only fire once the RVs got up. I always carried spare plugs and would change them as needed. I know that doesn't help you at all. I wish I had an answer for you but unfortunately I guess I'll have to watch your thread so that maybe I'll learn something from it.

That's the only thing I couldn't do, but using my thumb I'm sure there's loads of compression as it flew very, very far, and the kickstart is pretty firm, too.

50k is a lot. My 360 has piss poor compression at 23k.
I'd do proper compression tests, wet and dry.

broken idle mix screw tip? Just throwing it out there....

Ok, I went throught the carbs once more and found out the o-rings on the needle valve seat were not sealing properly. I replaced them and everything seems to be working fine. By the way, compression was about 165 psi on both cylinders.

Just to give this the final update: The mixture screws had to be backed down full FIVE turns out to achieve sweet idle mixture. It is now as silky an idle as it gets.

Thank you for the updates, and I'm glad you found the problem.
I'm currently having the exact problem on the 79 xs400 that I'm restoring.
I've converted to electronic ignition and that's not the problem as there is good spark in both cylinders.
So you are talking about the o-rings under the main needle valves on the front of the carbs ?
I'll have to try that. I rebuilt the carbs, and had them off at least 3 more times since, because I knew it was a carb problem, yet I never even touched those needle valve screws.
I'' try that now for sure.
Thanks

Are your needle valve seats the thread-in type that have a sealing washer underneath? If so, I hardly think it's your problem as mine were the push-in type with an o-ring. I'd double check the jet sizes. Make sure your pilot jet (42,5) has a screw plug with an aluminium washer and properly clean the pilot circuit from all possible intakes and orifices and always blow compressed air through to see it's actually free-flowing. Set the float height to +- 26 mm if you have brass floats. Next thing, make sure your idle mixture screws are actually intact and their tip isn't broken-off in the carb body as that would make your idle extremely lean and your life miserable. If they're fine, set them to about three turns out from LIGHTLY seated. Check if your vacuum hose leading to the petcock is connected properly. Consider trying removing it and plugging it instead just to see if the petcock is working properly and not losing vacuum. I suppose you have new intake manifolds (including the paper gasket?), but check for air leaks anyway by spraying carb cleaner or WD-40 around and see if RPMs pick up. If nothing else helps, try to really richen things up by giving the mixture screws a few more turns out - as I said, somehow, mine ended up at 5 turns out from lightly seated, but 3-4 should usually do it. Good luck. Don't give up and keep this thread updated, I'll be watching and try to guide you through! If there is something about these carbs, I should already know it, they've brought me days of misery but taught me well.

The needle valves are the screw in type, and the basic carb rebuild kit I used from mikes came with the float needle valve and seat and the brass washer for the main jet, aluminumn washer for the other, and gasket. I´m debating if I should get the full rebuild kit off ebay that includes alot more, including new jets, nozzle, mixture screw, and more. I am currently not using the gas tank, just a small container of gas with a hose to the carb, just for testing purposes. I did replace the intakes and have the vacum ports plugged so no vacum leak issues.
Two days ago, after going through the carbs again, it was running ok - both cylinders were firing. A day later it was back on only one, and it was the opposite side than was originally giving me the problem. Both carb bowls are always full when I drain them, so I figure the floats are ok.
Iĺl just dig deeper, removing and checking the metering screw, main nozzle, and other parts that I didn´t check previously, and can get out.
I do thank you for all the helpful information and will advise.

Just a quick update... After watching a few you tube ¨how to¨ videos (professional ones) on the bs34 carb, most common carb problems, and on carb cleaning, I was ready to dig deeper with my new found knowledge.
I removed the pilot jet from under the cap screw, and the needle jet tube from under the main jet, which did not come out easily, and when out, I saw why. OMG they were so badly crudded up with fine black debris, I´m surprised it was able to run at all. I don´t think there was an open orifice on either out of the bunch. I am the original owner and don´t recall ever getting that deep into the carbs, and Iḿ glad I did now.
With a new can of compressed air and carb cleaner, and a very fine piece of wire, I got them cleaned up nice. I still have to check the mixture screws but will get it back together soon for another test.
I am totally confident now that I have found the problem that has been giving me problems for some time, and that it should be running great for my next update.
Thanks again for your information and for the motivation to dig deeper.
KM

stormtracker said:

Just a quick update... After watching a few you tube ¨how to¨ videos (professional ones) on the bs34 carb, most common carb problems, and on carb cleaning, I was ready to dig deeper with my new found knowledge.
I removed the pilot jet from under the cap screw, and the needle jet tube from under the main jet, which did not come out easily, and when out, I saw why. OMG they were so badly crudded up with fine black debris, I´m surprised it was able to run at all. I don´t think there was an open orifice on either out of the bunch. I am the original owner and don´t recall ever getting that deep into the carbs, and Iḿ glad I did now.
With a new can of compressed air and carb cleaner, and a very fine piece of wire, I got them cleaned up nice. I still have to check the mixture screws but will get it back together soon for another test.
I am totally confident now that I have found the problem that has been giving me problems for some time, and that it should be running great for my next update.
Thanks again for your information and for the motivation to dig deeper.
KM

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Oh yeah, that's definitely it. I thought you had done the emulsion tubes. ;-)

You should also consider resyncing the carbs. The idle screws seem like they are really far out if you left them at 5 turns imo.

NewHavenMike said:

You should also consider resyncing the carbs. The idle screws seem like they are really far out if you left them at 5 turns imo.

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Yes, I'm sorry, I forgot to do the FINAL update on my side. After syncing the carbs (they were WAY out of sync, I could just hear how the bike stabilized once they were getting synced) the idle would drop below the set RPMs after a slight rev-up, indicating too rich an idle mixture. So I kept going back with the turns until the bike would return to its proper idle after revving but not pop on deceleration. To conclude, I actually ended up at 3.5 turns out, after which I resynced the carbs once again, giving me better-than-factory tuned bike! It's a real joy to work with the Morgan Carbtune Pro. Once you get your hands on it, you never go back.