irratating problem with carb....maybe?

[billfrombyron]

1977 Yamaha XS360........

I know this is one of those stupid simple fixes, and I will probably kick myself for not knowing, but I will ask away as there is no such thing as a stupid question when it comes to 2 wheels in my book.

OK, I have adjusted and adjusted and adjusted my idle speed until the bike will stay running on its own. -+ 1300-1500 depending upon its mood. Oh by the way I cannot use the choke to warm up the bike as it dies every time.

Bike runs great, power is smooth until I stop at stop sign at which it promptly dies every time unless I roll the throttle a bit.

On another topic, I am still trying to find the magic spot on the clutch, but even in neutral it does this, so its not me.

or the clutch, unless its got some wired interconnection that I am unaware of.

Any ideas? BTW there are no vacuum leaks that I can find so far.

Last week I warmed it up just fine and dandy with the choke and then turned it off and adjusted my idle speed and the bike ran flawlessly. Now I'm wondering if maybe I should lock tight the idle speed screw?

If not that than I am out of ideas and am open to suggestions.

-Bill

[socalyamaha]

Have you been able to tell if the setscrew moves?

Have you cleaned the carbs? Checked your timing?

I had big problems like this on my 550 seca. Which uses similar Mukuni's, but four of them.

And way, the bike liked to die at stop signs and loose power sporadically while riding.

Cleaned the Carbs completely, and rides great now.

Buy jewelry cleaner (ultra sonic cleaner) and go to town.

My slides were filthy.

[fosdyke]

Have you been able to tell if the setscrew moves?

Have you cleaned the carbs? Checked your timing?

I had big problems like this on my 550 seca. Which uses similar Mukuni's, but four of them.

And way, the bike liked to die at stop signs and loose power sporadically while riding.

Cleaned the Carbs completely, and rides great now.

Buy jewelry cleaner (ultra sonic cleaner) and go to town.

My slides were filthy.

Sounds like a carb problem - check and clean your carbs thoroughly and try again.

[Airhead]

Sound like the pilot jet may be blocked to me

[billfrombyron]

OK, with the exception of one bad float everything is clean as a whistle.

I have temporarily mended the float with some double bubble solvent and fuel safe epoxy.

Now I have a question, I was reading my manual again and it states "Don't run the carbs without the air filters in place. The manufacturer has designed and jetted the carb to take into account the restriction of the air filters installed."

Now lets suppose the stock air filters were not present. They were instead replaced with some crappy foam lawn mower type pod filters mounted directly to the carbs with the balance tube removed and all air ducting removed.

They were dirty as hell when I got it and she ran perfect.

I just cleaned the air filters and now it won't stay running and does all of the above mentioned things.

What are the chances that it was caused by the lack of stock type setup?

Does anyone have a brand of pod filters that fit and work well?

-Bill

[billfrombyron]

OK, talked with a friend of mine who is much better than I and he reminded me of the older lycoming aircraft engines that won't run right without the oil applied to the foam type filters due to it causing a lean out condition.

That being said I have squirted K&N air filter oil on the filters and am letting it dry. I will test this tomorrow afternoon and see if that improves things or not by adding some restriction to the filters.

There was oil applied to the filters when I cleaned them, so why not oil them up and try it?

Any other ideas?

-Bill

[billfrombyron]

Well here she is in mostly fully running trim!

It seems the oil did indeed work, now I just have a bit more tweaking to do and she will be perfected.

OH yea here is some pics for yall of her cleaned up.

I still have some work to do on the left side pipe, but there is chrome under all that rust! The pipes sound fantastic for such a small bike, not obnoxiously loud, but not so quiet as to be ignored either.

-Bill

[road_rascal]

Nice looking motorbike!

[billfrombyron]

I think the air filters were a contributing factor, but not the complete answer.

I found out I need the carb mounts for the engine to carb mounting as it seems to have cracked due to age. Any way to avoid spending a hundred bucks on parts at the dealer on this?

-Bill

[Cy Welch]

I think the air filters were a contributing factor, but not the complete answer.

I found out I need the carb mounts for the engine to carb mounting as it seems to have cracked due to age. Any way to avoid spending a hundred bucks on parts at the dealer on this?

-Bill

You can usually get those on ebay for around $30. IIRC the 360 uses the exact same boots as the 400.

[billfrombyron]

OK, time for an AAR on this repair.....

I took the carbs apart last night while on CQ duty(24hrs with nothing better to do) and started cleaning and about 9 hrs later I got board of that, but man what a difference. I thought I had been a bit of a perfectionist before, but I stole some of the wife's nail polish remover and went to town with some pipe cleaners and Q-tips and rags. I broke the carbs down to the housing and just cleaned the heck out of it. I took some dental picks to the float chamber to remove 31 years of crud and now its clean as a dinner plate.

I sealed the carb boots with some make a gasket that Ive had good luck doing similar types of repairs with in the past on my VW, so what the heck if I can save a few bucks for next season they will be replaced then.

I synced the carbs to perfection by eyeball and I am convinced that a vacuum gauge couldn't get them any closer than they are now.

I also found that the float on the right side was "holy" so I sealed it with some epoxy. Next season I will have some spare cash to fix it properly, plus its not worth it for the month or so I have left before going back to IL where it will be cold again.

That and a 4 week lead time on the parts from the dealer made me get creative.

So I will update tomorrow if she runs smooth at long last or if I need to look elsewhere.

-Bill

[billfrombyron]

New Carb floats will be required unless you fellas have any suggestions on how I might save them.

-Bill

[Airhead]

New Carb floats will be required unless you fellas have any suggestions on how I might save them.

-Bill

I assume they are brass floats, The problem with adding epoxy or solder etc in any meaningful quantity is that you will affect the buoyancy and the fuel level in the carb will increase, you will need to take professional advise on this or buy new if you can

[dgrizzle]

I also have a 1977 XS360 and the carbs have given me no end to frustration and trouble over the past year. I actually wore a set of carbs out after removing and reinstalling them about 25 times. I had to get a new set. Here is what I have found.

First thing to check: leaky floats - this bike WILL NOT run correctly without floats in good condition. Get new ones if necessary. Set the floats to the factory recommendation of 26.6mm height. Do this with a caliper. Adjustments of as little as .25mm make a difference in how the bike runs.

Second thing: air filters - it doesn't matter what you use to filter the air, it can be nothing, it can be a k&n, , foam pods, etc. whatever. BUT - it must be placed inside the stock airbox, and attatched with the stock plenum. Without these pieces, you will get good running up to 6k, or good running above 6k, but never both. Making these carbs work with no airbox would involve modifying the air correction jet and slide bleed hole - no thanks.

Third thing: Vacuum leaks - You can't properly synchronize your carbs unless there are NO vacuum leaks. The slightest leak totally throw off the running and the vacuum reading. B/C the carbs on this bike are very large for the engine size, they are very sensitive at small throttle openings, where synch makes a big difference. The three main areas where these carbs leak air are the intake spigots, the throttle shaft, and the choke mechanism. Replace the spigots - they are all cracked - XS400 ones fit and can be purchased on eBay. The choke housing and piston can get leaky as well. Buy/make new gaskets for the housing and make sure that the choke piston seals against its stop properly. The throttle shaft o-rings are the most annoying, and were the most elusive and significant vacuum leak. Because they mount on a rotating shaft, they can seal at some throttle openings and leak at others. This makes the bike run all kinds of funny, with weird, untrackable problems, appearing and disappearing at random. They don't make the little seals that these use anymore, but a good set of viton o-rings will work if you can get just the right size. They need to be a good sung fit on both the shaft and inside the housing. You have to separate the carbs and remove the shafts to get at these. Make sure that you put the throttle plates back in with the numbers facing towards the engine and on the bottom. Put red loctite on the screws that secure the throttle shaft to the throttle plates. Also, before you tighten down the screws, open the throttle and snap it closed a few times to check for free movement and to let the plate center itself in the bore.

Fourth thing: Sychronization - you gotta synch the carbs with gauges. Gotta. It makes a huge difference at idle, off-idle response, and smoothness of running. You probably don't realize it now, but your bike is idling on mostly one cylinder. Synch those carbs!

My final settings:

Stock exhaust, stock intake and air boxes, with UNI foam pod filters inside the airboxes.

Valves adjusted to factory spec.

Timing 4 degrees advanced (very important to match timing cylinder to cylinder)

112.5 Main Jet

15 Pilot Jet

Needle 2nd clip from bottom

26.00mm float height

Idle mixture screws 2.5 to 3 turns out

Idle speed 1500RPM

Results:

Smooth running around town, good response everywhere except for a little dead spot at 6300-6600. Motor comes alive at 6600 revs clean and strong to 8800RPM.

Top speed on the level, crouched in is 95 mph (indicated)