Hunting Idle and Idle hang/slow idle help

[jakobgoodnessgracious]

Hello everyone, this is my first post on here. I just got my bike a little over a week ago. It's a 1974 TX650.

I was having trouble with it dying at low range when I was trying to engage the clutch and take off. Also, after it would die I couldn't start it up but once more otherwise it wouldn't start after that. I decided to clean the carbs and I bought a tender charger to reinvigorate the battery.

I ended up getting some fuel in the oil so what I did was changed the oil, bought an oil filter (got the wrong one so I still have to exchange that for the right one), and changed the spark plugs (they were a dry grey color). I also replaced the float bowl gaskets as they were both cracked.

This got the engine starting up just fine.

To the issue:
Now however after taking it out for a ride around the block I noticed that when I disengaged the clutch the engine would hang at its current rpms. If I downshifted and engaged the clutch that would help slow it back down. Maybe it's because I'm new to motorcycles, but in my car when I disengage the clutch the engine returns back to idle. I feel like that's how it's supposed to work.

After riding it around the block and coming home to park, the engine was in neutral but it was doing the sort of 'hunting' for idle. It revved up on me for no reason. The throttle was disengaged.

I feel like I cleaned the carbs well and put everything back together correctly. I have looked online and found that my bike is probably running lean. Adjustments-wise, I don't know how I can fix this. I looked at the sync in the manual and apparently to adjust the sync, you just adjust the throttle cables. I think they should be fine from observing them just by looking at the cables anyway.

As far as I could tell when cleaning the carbs, the butterflies were working just fine. I am thinking maybe the vacuum in the diaphragm isn't pushing the needle/piston housing back down. But I don't see why it would be having issues either as everything looked fine to me when I cleaned it.

I saw that you can check the boots by spraying carb cleaner on them while the engine is running and listen for a difference. However, I don't see much of a boot with this bike. It seems just like the intake is directly connected to the back of the carb.

Sorry for being long-winded, I just want to be thorough as this is bothering me and I want to fix it as soon as possible.

Posted a bunch of pictures so you can see what's going on. Not sure if any of it helps, but I figure it can't hurt.

Thanks for any help!

Jakob

 

[weekendrider]

Shee it that is a nice looking survivor! You have adjusted the cam chain, valves and checked timeing? Re-visit the carbs make sure when you spray carb cleaner through the passages it comes out the little holes next to the butterfly in the throttle body. The mix/pilot screw may be set a little lean. You will want one of these.
It includes all you need lever, washers and screw.
http://www.ebay.com/itm/Yamaha-XS65...ash=item33c26226b0:g:pr4AAOSwZ1lWdGoD&vxp=mtr

 

[5twins]

You probably need to service your advance unit too, or more correctly, the advance rod that runs through the cam and that it connects to. It's supposed to rotate back and forth in the cam as the timing advances and retards. When it's lube dries out, it can stick. The timing can hang in the advanced position and that will cause the RPMs to hang high. And yes, this can all happen without you even touching the throttle.

 

[Max Midnight]

Can you confirm that the carbs are balanced?
Can you confirm that there are no air leaks into the inlets?
Can you confirm that both the throttle slides return freely from maximum opening?
Can you confirm that the throttle cables move freely in both directions?
If yes, then I would start by richening the idle mixture screws and see if the condition improves.

 

[retiredgentleman]

If the advance unit is working properly, I suspect you have some air inleakage around the carbs or carb boots. You should look carefully at the carb boots. Its takes very little air leakage to cause the high hovering idle.
Another thing to check. With the carbs apart, remove the needle jet and inspect the needle jet O-ring. If it looks old and crusty, you need to replace it. The needle jet should be a tight fit.................you should not be able to wiggle it.

 

[grizld1]

Also click the Tech button, find the link to the XS650 Garage USA Carb Guide, print it off, and read it. 5twins and I wrote the thing once. I don't know how he feels about it, but I stopped writing it again on demand a long time ago.

 

[jakobgoodnessgracious]

Wow, you guys are quick.

@weekendrider I will check by spraying some carb cleaner through the pilot holes and I'll check the pilot screw.

@5twins I'm going to look into how I can lubricate the advance and I will take a look at it here shortly.

@Max Midnight I will check for any leaks. I'm not sure how to check all inlets, but I will check where the air intake meets the back of the carbs. I will double check the throttle cables though as of now I'm pretty sure they return from tension just fine. A second look may prove otherwise.

@retiredgentleman Now that you say it, I remember not even noticing the o ring because it looked embedded (not in a good way) into the jet. I will order replacements for both. One of the jets was a looser fit than the other so I will make sure to be more aware of that. I'm guessing the new o ring should help with that.

Also, I know this is a stupid question, but the engine should return to idle with the clutch pressed in right? Right now the engine continues its rpms depending on how fast the wheels/bike is moving in that particular gear. . .

Thanks for the help!

 

[jakobgoodnessgracious]

@grizld1 I will check that out. I think I saw it earlier today or yesterday. I'll have to print it out.

Thanks!

 

[DanielBlack]

I recently purchased a packet of those itty bitty o rings. I'm happy to drop a couple in the mail if you send your address in a PM.

I had a bear of a time sourcing them locally and bowed to Grainger.

 

[Orion61]

Beautiful 74' Brother, wish it was mine!
When you open the throttle and then just let it go, does it snap back closed? Make sure you don't have a bad spring on one carb that is sticking open.
Another thing I noticed the rips in your seat cover on the sides there can be repaired with Black silicone, also the Rubber glue they use to recondition new foam surrounds on Speakers works well, it dries quickly and stretches really nicely without giving way.
It will help to keep the seat going longer. I'm the Master of Cheap fixes...

 

[littlebill31]

If you are riding and engage the clutch, the engine should go to idle unless something is hanging.
Make sure you read the carb guide and tune the bike before messing with the carbs or you'll be chasing your tail.
As others mentioned..cam chain tensioner, valves, timing, them carbs.
Carbs are number 4. Its pointless to mess with them until the other items have been attended too.
Also, to check your diaphrams, lift the slide up with your finger, cover the top kidney bean shaped hole with another finger and then release the slide keeping the port covered. The slide should stay up or almost up. Remove your finger from the port and the slide should drop.
Make sure the carbs are sitting up as mounted if bench testing them.

 

[jakobgoodnessgracious]

@Orion61 Haha thanks I hope I got a good deal on it, and I certainly love the way it sounds, rides, and looks. I think it is in pretty good shape from what I can tell so far. I'm also certainly green, but just what i've done taking apart the carbs alone and learning so far has taught me a lot more about an engine than I ever knew. It is certainly going to be good for me. I think getting a bike like this is a great place to learn. And there's something about it I can't put a finger on that just brings a smile to my face. Thanks for the tip on the carbs and the seat too!
Yeah I'm gonna start adding things to the list, and then adding more, and then . . . I have to get used gauges as well for her. Someone I talked to locally said it is a b**** to rebuild those.

@littlebill31 I will continue to do more research on carb tuning. I read that at idle and just after, setting the pilot and idle speed will help but outside of that it won't affect much. Is that true? Anyhow, I'm having a tough time judging the idle rpms because the tach is wacked out which is a pain. I didn't realize how important the others were. I will attend them first.

I didin't have time to get to what I wanted today, but for what it is worth, I rode around a little today and noticed that the engine was going back to idle fine for about 10 to 15 minutes of riding, but started acting up and sticking after that. Not sure if that's important or not to note. I downloaded a user manual and I will look into what it says about valves, camchain tensioner, and timing as well. I'm not sure where to start with that stuff but i will research it.

Thanks again for all the help so far.

 

[weekendrider]

jakobgoodnessgraicous (from now on you will be JGG 2 me) your intake manifolds are on the other end of the carbs where they meet the head. If the lip is torn or the are cracked bad is where the air leaks happen. Also the throttle shaft seals get hard and don't flex to seal so it might also be a part of the problem. But I'm guessing 5twins may have solved the problem with the advance rod tip.

 

[angus67]

Is possible the intake boot clamps arent tight enuf. Seals fine cold, but as the engine heats up, the rubber gets softer and flexes, letting air in. This might be it if it idles fine when cold then has problems when the bike is hot. Tightn the clamps more when the bike is all warmed up. You can usually get another full turn on it

 

[littlebill31]

I will continue to do more research on carb tuning. I read that at idle and just after, setting the pilot and idle speed will help but outside of that it won't affect much. Is that true? Anyhow, I'm having a tough time judging the idle rpms because the tach is wacked out which is a pain. I didn't realize how important the others were. I will attend them first.

There are circuits that work off of throttle position on these carbs, not engine speed. The mix screw and pilot run the lower portion of this (it's all explained in the carb guide).
If you have an air leak from the shaft seal, boot, ect, then it will affect more of these circuits like the needle and main jet portions.
You need a tach. I don't care what anyone says, you cannot tune a carb without a tach. Dead cylinder method and manometers are the way to go and need a tach. Any bike will run with air, fuel and spark, but if it's not tuned correctly, it's crap.

 

[grizld1]

Uh, Bill, one thing that we explained in the Carb Guide is that with vacuum carbs, especially early vacuum carbs with heavy round slides, throttle position can be deceptive. Unlike mechanical carbs, they have no direct connection between the twistgrip and the slide; as 5twins has frequently observed, under light load, slides in OE BS38's don't even start to lift before 4K rpm. That's something to keep in mind when you look at component-and-range charts. BTW there are two circuits in these carburetors: the pilot circuit and the main circuit. The needle, needle jet, and main jet are main circuit components.

 

[littlebill31]

Thanks, Grizld.
I have always looked at this http://www.iwt.com.au/mikunicarb.htm as well as the guide Twins and yourself wrote.
The link above has served me well and I have little issue tuning my carbs, both BS and VM.
Of course it's also a good idea to learn from people such as yourself.

 

[jakobgoodnessgracious]

Hello again everyone,

I just wanted to post an update.

I took my carbs to a local shop to make sure I wasn't missing anything with regards to them being clean and up to par. I don't feel like I received a lot of feedback in that area. When I originally mentioned that I researched and had been given the possibility of the problem being my advance I was assured that this was not the case. I know that my help there at the shop meant well, but I don't feel like I got as far as I'd hoped talking to them. I came home today and opened up the cover for the advance and noticed that after opening up and then releasing the 'weights' I think (or the attached components), they returned very slowly and then sort of stopped short of a full return. I believe this is a significant finding. I am terribly new to all of this but I am going to research further and continue on this hunt for the solution to my problem.

I noticed a small crack as well in one of the intake manifolds. If the clamp is tight enough around it I am thinking maybe, just maybe it will be okay. I want to start it up and spray some carb cleaner around it and see if that changes the engine sound at all before I go and buy new ones.

I ordered some yamalube molybdenum disulfide grease for the advance and it is on the way to me. Alas, I got my main jet o rings in the mail from DanielBlack whom I thank very much for that. I will affix those to their new homes in the carbs soon.

Side note:
I've been working on getting some stuck pistons out of the calipers and had a victory today with the help of my dad and a grease gun on one of the pucks that was frozen. As of yet I only have the back drum brake working. The front was not on when I bought the bike. I am also looking into buying some used guages as well so I can finish my endeavor to make her roadworthy.

Thanks again to all for your help so far.

I will continue to post updates.

Jakob

 

[jakobgoodnessgracious]

Hell, I might as well post this pic from the side for fun:

Still gotta fix that rip @Orion61! Gonna look online for some glue tonight.

 

[5twins]

The fact that the shop told you the advance unit couldn't be at fault tells me they know very little about older bikes, and even less about 650 Yamahas. Next to air leaks, a sticky advance on a 650 is probably the most common cause of a hanging idle. It is hard to find shops that really know what they're doing on older bikes. All that relates to them was once common knowledge but most of it has now been forgotten. Even though keeping the advance unit and advance rod properly lubed is required on these, one can't really fault any of the otherwise diligent previous owners. Yamaha failed to note this required maintenance item in any of their manuals. Most of the bikes were fine their first 5 to 10 years out of the factory, but now, 30+ years later, these parts are in dire need of some "lube love", lol.