'79 XS400 - Petcock and Lean troubles

[BurntTimber]

Two problems, one thread. I know I'm giving myself trouble, but want to make sure they're not related.

Problem One: Petcock
So I used the petcock rebuild kit, but am having a significant issue. The only runs out of the petcock when I've got it set to PR (Prime?) but won't flow when set to ON or RES. Now I know this is a vacuum petcock, but even with the line hooked up to the intake, the bike runs out of fuel and dies.

Should I just replace the petcock with one of these: https://www.mikesxs.net/product/20-0019.html

Problem Two: Running lean
So when I set the petcock to "Prime", it starts and runs beautifully. Nice smooth idle, no issues. That said, after I run the bike on the road through the gears. after about 10 min it starts to backfire, run very rough, and eventually just craps out. I know this means it's running lean, but the carbs are running so smooth when it first starts it feels like it's an airflow issue.

• I do have some cracks in the intake carb holders so I've got some on order
• I'm at a high elevation here in Calgary too, so it should be running rich

Anyone have some thoughts on why it would be running so lean with factory set carbs? P.s. the bike didn't run for 30 odd years, but was properly drained, and fired up beautifully once I did the basics.

 

[bcware]

For the petcock read here: http://www.xs400.com/forum/showthread.php?t=4844&highlight=petcock+universal&page=6

I don't mess with "rebuild" kits; it seems like most of the time they do not work or they get installed incorrectly. They also cost nearly as much as a whole new replacement petcock.

If the bike runs perfect when cold, but runs like poop once the engine gets hot (not just at idle, but running and riding) my first suspect is bad coils. Coils are something I would replace no matter what for reliability's sake, but if you want to be thorough you can certainly verify the carbs are properly cleaned, adjusted, and synched before buying a set. You can also adjust the valves and set the timing if you have points. Other culprits might include bad ignition cables (you can trim the stock ones shorter to renew them) or spark plugs caps and spark plugs. Even a weak battery will cause your bike to run like crap so you can verify it is good with a load test and check that the charging system is working. Finally, you want to jet your bike appropriately for any intake or exhaust mods as these bikes do not respond well to modifications without being jetted.

This should keep you busy for a while.

 

[BurntTimber]

Appreciate the feedback BC. I did change the mufflers out and kept the stock pipes, so is likely a big culprit in the whole scenario, and I expect the cracked carb holders are contributing as well.

That said, this is my first bike with mechanical ignition, so checking the points and looking at the timing is likely due course along with the coils... was really hoping to take advantage of the 2 months of summer we get up here. Oh well, back to the garage (and online parts store).

 

[bcware]

Just fixing the carb holders and jets may improve things a LOT! Also, it is not much labor to swap them out. You can get it done in less than an hour easy.

Coils are also super quick and easy to swap out.

Just bite the bullet and get the new holders. You really only need to go one size larger on the pilot jet and probably only one on the main as well.

 

[BBS360]

I don't mess with "rebuild" kits; it seems like most of the time they do not work or they get installed incorrectly. They also cost nearly as much as a whole new replacement petcock.

I can only attest to my rebuild kit and my experience with it, but my stock vacuum petcock works absolutely perfectly after I rebuilt it with a rebuild kit.
Seems like a shame to toss original equipment when they can be restored.

 

[arfstrom]

http://www.xs400.com/forum/showthread.php?t=8033
That link is a how to for petcocks rebuild. I have successfully rebuilt mine. If you don't tool around our website enough you might miss the nugget on the piston o-ring. (my suggestion is to put a smaller one behind it, so mates better. The links on page 2 that goes to our sister bikes with the same petcocks, clearly explain that the Taiko was a shoddy design, and the way they got it to work 100% of the time was by creating a bigger o-ring, seemingly like a pillow, and it will mate up with the mating surface. That O-ring that is being pointed at, is a soft and pliable oring.

That o-ring is no longer being produce, so you get suggestions like stick a nut behind the spring, or (mine) double up on the oring, making sure the bigger one is the one that closes on the mating surface.

My guess is you flipped something accidentally. I strongly recommend going though the maxim site, as it has a lot to offer.
http://www.maxim-x.com/petcock.html

 

[sproutroot]

There are resistance tests you can do to check your coils, wires, and spark plug caps. Check them before you buy new components. I've never done them on an XS, but on my GL1000 and CX500 it's very straightforward with specific tolerances for each component. All you need is a multimeter.
A very quick forum search showed me this post with some tolerances. I'm sure with just a little more looking you could find info on wires and caps. Probably a procedure as well.

Old Post, but this may help someone else:
1. Check all connections, including grounds, in entire ignition system. Clean/tighten as needed.
2. Check battery voltage and specific gravity. 12.7v, 1.265 specific gravity. Recharge/replace as needed.
3. Check fuse and fuse connections. Clean/replace as needed.
4. Check resistance of ignition coil. Primary 2.75 Ohms +/- 10%, Secondary 7.9 Ohms +/- 20%, at 68 deg. F. Replace if needed.
5. Check pickup coils for resistance. 120 Ohms +/- 10% at 68 deg. F. Replace if needed.
6. TCI Unit is faulty. Replace.
This is a simple progression from least-to-most expensive, based on KISS.

I'd also make sure to adjust valves and timing, and give the carbs some attention with respect to cleaning and rejetting for your exhaust mod.

 

[bcware]

I can only attest to my rebuild kit and my experience with it, but my stock vacuum petcock works absolutely perfectly after I rebuilt it with a rebuild kit.
Seems like a shame to toss original equipment when they can be restored.

You know, it may just be that the procedure is tricky. I only recommend replacing for most people due to the cost versus benefit for the average member on here. There are an alarming number of botched petcock rebuild posts on here and as a result I have shifted towards recommending people replace over attempting to rebuild.

Of course, if you're more experienced mechanically or keeping original equipment is important it is not unreasonable to attempt to rebuild the oem petcock.