Carb questions!

[girard]

Hey Guys,

I havn't had the bike 24 hours and Ive already got questions. First question, how do i identify what type of carbs they are? its a 79 special but that doesnt really mean much.
they are definitely mikuni but thats all i can see.

second question, when the slide is all the way down it starts and runs and idles, but when its up the bike stalls right out as soon as throttle is released. carb cleaning time? or could it possibly be a tuning issue?

Thanks!

 

[girard]

ok i think i found that they are BS38's. still confused about the slide tho! If the engine is warm, and I am idling at around 1200rpm with the choke on, would that mean that the idle mix screw is out because it should be higher when the engine is warm and you are adding fuel, correct? I messed with throttle stop screw but couldn't really get a decent rpm with no choke.

 

[retiredgentleman]

ok i think i found that they are BS38's. still confused about the slide tho! If the engine is warm, and I am idling at around 1200rpm with the choke on, would that mean that the idle mix screw is out because it should be higher when the engine is warm and you are adding fuel, correct? I messed with throttle stop screw but couldn't really get a decent rpm with no choke.

The pilot circuit on your carbs is not working. You should not be able to run the engine, with the choke on, for more than about 10 to 14 seconds. If you are unable to raise the rpm, with the choke off, that also means the pilot circuit is not passing enough fuel/air.

Look in the Tech section................find the Carb Guide for CV carbs and read it all several times.

Your next step after that, will be to take the carbs off the engine and take them apart.
Work on one carb at a time. You need to remove the pilot jet and the main jet, in order to read the small numbers on the side of the jets. You then compare the jets you have to the table in the Carb Guide, to see if you have the correct jets.

Important tip: You must have a slot screw driver that fits into the slot of the jets absolutely perfectly. If you try to use the wrong screw driver, that is a poor fit, you will likely damage the jet you are working on.

The pilot jet must be checked to see that it has no blockage.

 

[scabber]

Welcome to the site. Good advice from retired on the carbs. Since you just got the bike it is a good idea to do some routine maintenance. Some pictures of the rest of the bike would be great.

 

[girard]

The pilot circuit on your carbs is not working. You should not be able to run the engine, with the choke on, for more than about 10 to 14 seconds. If you are unable to raise the rpm, with the choke off, that also means the pilot circuit is not passing enough fuel/air.

Look in the Tech section................find the Carb Guide for CV carbs and read it all several times.

Your next step after that, will be to take the carbs off the engine and take them apart.
Work on one carb at a time. You need to remove the pilot jet and the main jet, in order to read the small numbers on the side of the jets. You then compare the jets you have to the table in the Carb Guide, to see if you have the correct jets.

Important tip: You must have a slot screw driver that fits into the slot of the jets absolutely perfectly. If you try to use the wrong screw driver, that is a poor fit, you will likely damage the jet you are working on.

The pilot jet must be checked to see that it has no blockage.

Thx for the help! What do you mean unable to raise RPM? I can raise it, was riding the bike around and it was running fine? I didn't notice that it was runnning around with the choke on until I started looking into it. The guy i bought the bike from never mentioned the problem. Is that what you mean?

Tonight I got my colortune kit out(i previously had an XJ650 with 4!!! carbs, cleaned and sync'd those so I am a bit familiar with the PITA that it can be) and it is running rich as hell! I can't lean it out by adjusting the mixture screw at all. When I pulled the plug they were super black. I am thinking maybe floats are stuck? The PO has the throttle stop all the way backed off, so I adjusted it and can now run without the choke on. Or did I do something incorrect with adjusting the throttle stop?

P.S. A carb rebuild kit is already ordered however I am wondering if that is really the problem?

 

[girard]

 

[retiredgentleman]

Thx for the help! What do you mean unable to raise RPM? I can raise it, was riding the bike around and it was running fine? I didn't notice that it was runnning around with the choke on until I started looking into it. The guy i bought the bike from never mentioned the problem. Is that what you mean?

Tonight I got my colortune kit out(i previously had an XJ650 with 4!!! carbs, cleaned and sync'd those so I am a bit familiar with the PITA that it can be) and it is running rich as hell! I can't lean it out by adjusting the mixture screw at all. When I pulled the plug they were super black. I am thinking maybe floats are stuck? The PO has the throttle stop all the way backed off, so I adjusted it and can now run without the choke on. Or did I do something incorrect with adjusting the throttle stop?

P.S. A carb rebuild kit is already ordered however I am wondering if that is really the problem?

I may have misunderstood. You said you could not get decent rpm with the choke off.

As I said before, you should not be able to drive around with the choke select ON. If you ran for very long with the choke on, that could explain the black plugs. You can adjust the throttle stop to any position you choose.

What is the condition of the air filters? If they are plugged up, that will cause a very rich mixture.

 

[girard]

If the choke is off i could ride as long as throttle was engaged. If i stopped it stalled. I adjusted throttle stop so i have fixed that problem, however i am still running super rich according to the color tune. I screwed mixture screw all the way in and the cylinder was still firing. Air filters are clean. I talkes to PO tonight and he said carbs were clean......but who knows?

 

[retiredgentleman]

If the choke is off i could ride as long as throttle was engaged. If i stopped it stalled. I adjusted throttle stop so i have fixed that problem, however i am still running super rich according to the color tune. I screwed mixture screw all the way in and the cylinder was still firing. Air filters are clean. I talkes to PO tonight and he said carbs were clean......but who knows?

The mixture screw is just a trim control for the pilot circuit. The majority of the fuel/air mixture is delivered directly from the pilot jet via 3 small holes found near the butterflys. If you assumed closing the mixture screw completely would cause the cylinder to stop running, that would be a wrong assumption.

Disregard what the PO said about the carbs being clean. PO's always say that, but it is seldom true.

There's no point in you continuing to post about "running super rich" until you remove the carbs to the bench, as I suggested yesterday. The carbs must be inspected for the correct jet sizes, cleanliness of jets, correct float level, emulsion tube cleanliness, condition of the needle jet O-ring, etc. etc.

Have you read the "Carb Guide"?

With the carbs on the bench, you will also want to do a "bench sync" of the butterflys. I use a slip of paper as a feeler gauge, which is quite accurate. Use the Google Search in the upper left corner to find threads on how to do this.

Wow, looks like you have a ton of work to do. Eventually you will also want to adjust the mixture screws using the "dead cylinder" method. Again use the Google Search.

 

[girard]

Thanks retired,

I plan on taking the carbs apart, however I have not owned the bike 48 hours yet so have not had the time to remove carbs and take them apart and rebuild. also no point in doing it until I have a carb kit in hand to put them back together. Normally I would disregard what the PO said however he seems to be a very meticulous type judging by the cleanliness of the bike and the detailed notes he left in the manual about work he had done to the bike. But yes, I am going to listen to your advice and take the carbs off and apart, that way I know first hand the condition they are in, I would just like to exhaust all methods of troubleshooting before just incase its a simple fix! My previous post was just a reply to your post, not repeating the same problem ignoring your info. Thanks for the help tho, I appreciate it! Will report back after I remove carbs

 

[girard]

looking at this http://www.xs650.com/forum/showthread.php?t=43
on the first chart, would I fall under the 79 xs650F? I have been looking for info because I am an SE but can't seem to find any correlation between SE and A-K or whatever.

 

[scabber]

The carbs are the same on all 79 models.

 

[weekendrider]

Yes, the carb sets don't vary according to model but, by the year.
The first link in that thread is the carb guide RG referenced.
You will want to read it. Being familiar with carb sets helps, you're ahead of the game.
BUT there are several tips, tricks and idiosyncrasies of the XS that save you time and money. You have already committed the costly mistake of ordering a carb kit. Most of what you may need to replace is cheaper by the piece to say nothing of the useless bits you are going to receive in the ordered kit.
Research the vendors, ask on the forum before purchasing anything and remember we do have a classified forum.
A practical tip you may find useful.
The carbs come off easier if you remove the cap nut from the cam chain tension-er.
You don't need to remove the intake manifolds.
And a question about your bike. It looks like the intake vacuum ports are tied together, connected with tubing? Are there any vacuum lines to the petcocks?

 

[Brassneck]

One simple test to see if you're running rich is to pull the vacuum line on one side, and see if that stalls the bike out. If it keeps running (and even revs higher), then you're likely running rich on that side... then try the other side. This is one of the ways to kill the cylinder for the "dead-cylinder" method...but it's a quick test to give you a bit more info.

 

[retiredgentleman]

looking at this http://www.xs650.com/forum/showthread.php?t=43
on the first chart, would I fall under the 79 xs650F? I have been looking for info because I am an SE but can't seem to find any correlation between SE and A-K or whatever.

If your bike is a 79, then its a SF model not SE. My bike is a 78 and its a SE.

There were 3 models sold in 79. The Standard model F,
the Special model SF............and there was the Special II model.

Yes, your carbs would fall under the 1979 XS650F.

 

[girard]

Yes, the carb sets don't vary according to model but, by the year.
The first link in that thread is the carb guide RG referenced.
You will want to read it. Being familiar with carb sets helps, you're ahead of the game.
BUT there are several tips, tricks and idiosyncrasies of the XS that save you time and money. You have already committed the costly mistake of ordering a carb kit. Most of what you may need to replace is cheaper by the piece to say nothing of the useless bits you are going to receive in the ordered kit.
Research the vendors, ask on the forum before purchasing anything and remember we do have a classified forum.
A practical tip you may find useful.
The carbs come off easier if you remove the cap nut from the cam chain tension-er.
You don't need to remove the intake manifolds.
And a question about your bike. It looks like the intake vacuum ports are tied together, connected with tubing? Are there any vacuum lines to the petcocks?

forgive my ignorance but what do you mean by intake vacuum ports? The barbs on the carb holder boot between the carb and engine? Looking at some pics I took, I notice only the left one is connected. The right one is capped off. Could that be an issue?

 

[girard]

One simple test to see if you're running rich is to pull the vacuum line on one side, and see if that stalls the bike out. If it keeps running (and even revs higher), then you're likely running rich on that side... then try the other side. This is one of the ways to kill the cylinder for the "dead-cylinder" method...but it's a quick test to give you a bit more info.

So if i pull the line it should stall out? I have been reading up on the dead cylinder test but it seems a bit confusing, mainly due to my lack of understanding.

 

[retiredgentleman]

Thanks retired,

I plan on taking the carbs apart, however I have not owned the bike 48 hours yet so have not had the time to remove carbs and take them apart and rebuild. also no point in doing it until I have a carb kit in hand to put them back together. Normally I would disregard what the PO said however he seems to be a very meticulous type judging by the cleanliness of the bike and the detailed notes he left in the manual about work he had done to the bike. But yes, I am going to listen to your advice and take the carbs off and apart, that way I know first hand the condition they are in, I would just like to exhaust all methods of troubleshooting before just incase its a simple fix! My previous post was just a reply to your post, not repeating the same problem ignoring your info. Thanks for the help tho, I appreciate it! Will report back after I remove carbs

Yes indeed, the PO took exceptional care of that bike. You may have found the very rare PO that actually has told the whole truth about the bike.

Actually its better to remove and open up the carbs, before ordering parts, as you may very well find that you need to order other unforseen parts. i.e. a defective float, etc. Its the same mistake, the lads make when they find they have to do a top-end rebuild. They order a bunch of parts before they open up the engine, only to have to make multiple orders, once they open it up and find additional parts are needed.

You're a very typical new XS650 owner, in that they always delay as much as possible, the dreaded act of removing the carbs for disassembly. But guess what, its not really very difficult, but there are a few tricks. Trust me, you will have to remove the carbs more than once if you plan to keep the bike for a while. There is no one simple fix.

Adding a little more to what weekendrider said..................yes remove the camchain tensioner nut......................also remove the small bracket that holds the throttle cable on top of the left carb.................last but not least, remove the dip stick and put a rag in the hole to prevent anything falling in. With those 3 things done the carbs will come right out with just a little juggling, no cursing required.

I think I mentioned this in the other thread, but its super important to use a slot screw driver that fits precisely into the slot in a pilot or main jet. I have a screwdriver that I filed and ground until it was an exact fit. If you use a sloppy fitting screwdriver, bad things can happen.

Later on once the carbs are in top notch condition, you can use a little dielectric grease on the rubber carb holders. The carbs will be much easier to push into the holders.

Another smart tip, is to replace the phillips (actually JIS) head screws that hold the float bowl on, with allen head bolts. Makes life a lot easier, and allows for removal of the float bowls, with the carbs remaining on the engine, if you ever wanted to change jet size..

 

[girard]

Yes, the carb sets don't vary according to model but, by the year.
The first link in that thread is the carb guide RG referenced.
You will want to read it. Being familiar with carb sets helps, you're ahead of the game.
BUT there are several tips, tricks and idiosyncrasies of the XS that save you time and money. You have already committed the costly mistake of ordering a carb kit. Most of what you may need to replace is cheaper by the piece to say nothing of the useless bits you are going to receive in the ordered kit.
Research the vendors, ask on the forum before purchasing anything and remember we do have a classified forum.
A practical tip you may find useful.
The carbs come off easier if you remove the cap nut from the cam chain tension-er.
You don't need to remove the intake manifolds.
And a question about your bike. It looks like the intake vacuum ports are tied together, connected with tubing? Are there any vacuum lines to the petcocks?

and in reply to your note about asking the forum about parts. I am in the process of ordering new carb holders from JBM, these do not come with the vacuum ports. I noticed the ones I have now are starting to crack and dry. good idea?

 

[retiredgentleman]

and in reply to your note about asking the forum about parts. I am in the process of ordering new carb holders from JBM, these do not come with the vacuum ports. I noticed the ones I have now are starting to crack and dry. good idea?

I've only heard good things about JBM products. However, having said that, the original carb holders are a very high quality part. If you have not ordered new parts yet, I recommend you keep using the stock holders that you have, unless you detect air leakage/hovering idle, etc. Some minor cracks on the outside of the holders is normal and means nothing.When the carbs are off, apply dielectric grease to the rubber parts of the holders..................its the best thing to put on rubber.

On my bike I'm still using the original OEM carb boots, because they fit well and there is no air leakage. A number of years ago, I bought some boots from Mikesxs, and they were a poor sloppy fit, which allowed a lot of air leakage.............basically unusable junk.

My philosophy is...................don't buy replacement parts until its proven that the stock parts are defective/worn out.

Edit: If you do take off the original carb boots, you can just put them in the mail and send then to me