XS360 idle issues

[pollestevens]

A few months ago I purchased a 1977 Yamaha xs360. The bike was in a very rough condition when I bought it so i spent these last months rebuilding nearly everything.

I'm running stock-ish (not open) exhausts and the original carbs with pod filters. I replaced the #130 main jet with a #135 to compensate for the extra air. The pilot jet is the stock #17.5 ( pilot jets for xs360 carbs are very difficult to find so I couldn't bump up those) and the air/fuel screw is 2.5 turns out. Sparkplugs look good, maybe a little on the black side.

It starts up great and idles very good when the engine is cold but when it warms up it doesn't properly return to idle. It doesn't run bad when i'm accelerating but when i slow down the rpms drop very slowly.

These are the steps I took to troubleshoot the problem:

- New intake boots, gaskets, throttleshaft seals and diaphragms
There are no indications of false air when tested with carb clean. Slide seems to move properly
- Checked the valves
All clearenses were in to spec
- Checked the ingition timing and points gap
Timing was way off (points closed too late), reset the timing but made no difference.
- Carbs are synched

I also tried with velocity stacks because i read somewhere these CV carbs love 'em. It seems to be a little better but still not ideal.
This is as far as my knowledge about diagnosing idle issues reaches, did I miss anything that could cause these issues?

This is a picture of the (almost) finished bike

 

[xschris]

These type carbs don't like pleated type pods at all. They cause too much turbulence. I would put the stock H-pipe back on and get some black foam uni filters if you must have pods. Make sure you sync the carbs with a manometer. With a non-stock muffler/intake setups you will just have to keep tweaking things till you get to a place you can live with.

 

[pollestevens]

The bike wasn't complete when I bought it, that's why i'm running these pleated pod filters since they are the cheapest option. But foam filters do seem to make a lot more sense on CV carbs now that I think about it. I've also been thinking about doing a single carb conversion with a mikuni flatslide or something similar just so that I can stop messing around with synching and finding parts for 40 year old carbs. But I will try foam pods first and see if it improves the idle.

 

[pollestevens]

I messed around with my carbs some more and I found that when I mount my filters, the inside of the rubber might be blocking off or restricting the oblong shaped hole and the other small inlets on the intake side of the carb.

Could it be as simple as finding a filter that fits better and doesn't restricts these holes? Because if this is the problem shouldn't there also be idle issues when the engine is cold?

 

[XSLeo]

Blocking those carb ports, actually vents for the float bowl, vents for air to flow to the pilot and main jets as well as vent for the slide. All these need free flowing air. You can try running without the pods just to see if things change.
As xschris mentioned using the stock H pipe on will clear these vents. Using a long straight foam filter works best with CV carbs.
Leo

 

[pollestevens]

Running without the pods seems to make things a little better, but it's hard to gauge since its getting more air now thereby leaning out the engine more.I'll try to find an H pipe somewhere locally and purchase some UNI filters. And maybe I'll let someone with more experience do the carb synching.

But should I only be focused on the carbs or are there other factors that might be at play? Like overheating for some reason or electrical gremlins?

 

[XSLeo]

Generally when a bike has issues it's best to start with the basics. Go through the basic tune up procedures.
There are basic principles. Mechanical, Ignition and Fuel.
On my XS650's I do them in this order.
Mechanical
#1 Cam chain adjustment.
#2 Valve adjustment.
Electrical, This has two sections, points or TCI.
#3 On points set the gap.
#4 Set the timing.
Now on to the Fuel, This is where you get into the carbs. You need to get the others items done first, some running issues have similar symptoms but are caused by different systems. The ignition and fuel delivery are very closely linked.
Get the other items right before you start on the carbs. If not you may end up fighting ignition issues with the carbs.
I have mentioned this before, if not in this thread but others discussing similar issues.
Over on our sister site XS650.com the carb Guru's wrote this www.amckayltd.com/carbguide.pdf The XS400 uses a BS34 carb very similar to the XS650 BS34 carb. This guide has been found very helpful to those working on carbs. If you follow the steps to tear down, clean, inspect your carbs. The inspection determines if and what parts need changing.
Once you determine what is needed then buy parts if needed. Then follow the steps to reassemble the carbs, making needed adjustments as you go. Float Height is a biggie. Bench Sync is another.
The cleaning part is often scrimped on by newbies because of lack of experience. Best to do it two or three times to start than pull the carbs two or three times to do it.
The clean idle circuits are vital to proper carb function. These idle circuits are composed of small passages and tiny holes. The passage runs from the pilot jet to the mix screw and to the tiny holes in the throat of the carb next to the throttle plates. You can see the holes by opening the throttle plates. They will be up on the top where the plates come near the wall of te throat.
There may be 3 or 4 holes. If these holes are restricted in any way the engine will not run well.
I use Spray carb cleaner. With the mix screw and pilot jet removed, insert the tube of the spray down into the hole where the mix screw was. As you spray watch where the spray comes out. It should spray out where the pilot jet was as well as all the tiny holes. Then spray in at the pilot jet to back flush the passage.
Spray may not come out the tiny holes at first. The passage from the mix screw to the holes doesn't get flushed well unless you block the hole where the pilot jet was. I use a finger tip. This forces more of the spray out through the tiny holes.
Even with that you might still not get spray to come out those holes. Use an air hose blow gun with a rubber tip to blow air in through the pilot or mix screw holes, sealing the other to force the air out the tiny holes.
You might even have to very gently probe those tiny holes. A twist tie from a loaf of bread has a thin soft steel wire inside the plastic cover. This works well to probe those holes.
Be gentle, you don't want to make those tiny hole bigger.
At Idle fuel gets drawn through the pilot jet up into the passage to the mix screw and those tiny holes. The mix screw bleeds off part of this fuel and sends it to the larger hole by the throttle plates, just to the engine side of the plates.
When at idle the throttle plates block off the tiny holes. This lets the engine run on mostly the mix screw. As the throttle plates open up the tiny holes get uncovered allowing the engine to get more fuel through these tiny holes. The fuel flow through these tiny holes is controlled entirely by the pilot jet, the mix screw has no effect on them.
Therefore when adjusting the mix screws you want to minimize the effect on idle by the tiny holes. You do this by lowering the idle speed as low as you can and keep the engine running. Once the mix screws are set you can bring the idle speed back up.
On determining what, if any jet changes are needed is covered in the "Tuning for Mods" section of the carb guide. Your bike maybe pretty much stock but with the ethanol fuel we have to run now you may need to rejet it get right.
Leo

 

[pollestevens]

Thanks for all the info. Now that I have a better understanding of how the passages run inside the carb it might be easier to pinpoint the problem.
Ignition timing, valve timing and valvelash are all set to spec so there shouldn't be any issues with that. Cleaning out the carbs was one of the first things I did when I got the bike. I mostly work on small engines like the ones found on a honda ct70, blowing compressed air through both jets and cleaning out the bowl usually does the trick on those.
But I guess those carbs aren't as finicky as cv carbs so maybe I didn't clean them thoroughly enough.

 

[pollestevens]

So I took my carbs apart for a thorough cleaning. I made sure that the carb cleaner sprayed through those tiny holes XSLeo mentioned. I also checked all the other passages for good flow. But the little hole I have encircled in the picture below seems to be clogged in both carbs. I assume this is the hole where the mixture that is metered by the mixturescrew passes through since it looks like it goes straight into the hole for the mixture screw. But when I spray cleaner through the pilot jet hole or the hole where the screw was it doesn't want to come out of there, not even when blocking the tiny holes.

Does anyone have any tips to clear this hole? Poking in it with a wire doesn't do the trick and I also can't put compressed air to it cause it's a little to deep inside the throat of the carb.
Or am I wrong and does this hole has absolutely nothing to do with the idle mixture screw?

 

[xschris]

Looks like your pilot mix screw tips have broken off into the carb bodies. Look in the holes where they screw in and see if there is any brass stuck in them.

 

[pollestevens]

Yeah you're right, I had a feeling those idle screws looked a little short. there is indeed some brass at the bottom of both holes. Must have been the previous owner who tried to take them apart because I learned the hard way not to force anything when loosening or thightening stuff on carbs.
Is there a way to get these out or are the carb bodies basicly thrash now?

 

[xschris]

You can use some micro drills and try to get them out but with both being plugged you will have a hard time getting the correct size. These are also a tapered hole so you would need to use multiple sizes to try and achieve that. I would start looking for another set and make sure those are good.

 

[pollestevens]

Seems very tricky drilling those out so I'll be on the lookout for a set of carbs then.

Man I'm glad I found this forum, I would have never been able to figure this out without all the experience you guys have. Allthough my carbs are trash now I'm glad I found the problem after many frustrating rides. Thanks for all the information, I learned a lot and that's what working on bikes and the community around it is all about, at least in my opinion.

 

[pollestevens]

A little update on my carbs, in case someone has the same problem as I have.
Yesterday while I was trying to sleep an idea came to me on how to get those broken of screw points out. It is worth noting that the method I used will only work on the bs34 carbs found on a 76-77 xs360 and the old style bs38 carbs on a xs650. The mixture screw is located horizontally on those carbs.

I actually study design and production technology so I'm learning how to design and machine metal parts. I started thinking about the manufacturing process of my carburetors and then it occured to me: to make the mixing chamber where the tip of the screw seats they have to drill al the way trough. And indeed, on the opposite side of the mixture screw they capped off the mixture chamber with an aluminum plug.

So here's what I did: I carefully drilled out the plug using a 4mm drill bit and made very shure not to go to deep. When I looked inside the hole I just drilled I could see the tip of the screw and was able to push it out with a needle. I then tapped the hole for a short M5 bolt.

I then put the bolt in and a rubber washer to seal off the hole.

And there you have it, I just need some new mixture screws, but I think I just fixed my carbs!