Adjusting Carbs??

[Shifty2932]

Hello,

I'm new to the xs 650 engines and the forum. I just bought a 1977 xs 650 custom bobber. The bike is having some problems running. Seems to have issue in first (not wanting to gain power), flat or power drops through mid range, and sputters at times when throttle is wide open. It also doesn't want to idle down once the bike is hot and been running for a while. Already did some reading on here about that issue. But I have a few question and I'm sure this should fix my problems. I live in southwestern South Dakota and want to know how to properly adjust my carbs. The holes where the adjustment screw are at are still covered. So I am assuming the carbs have never been adjusted and I need to drill the holes out to get at the screws. I have taken the carbs apart and cleaned everything and checks all the seals and o-rings. All good there and the carbs are in sync. Want I would like to know is how to properly adjust the carbs. Procedures and steps would be great. The elevation out here is around 3500-4400 feet. Any recommendations for correct size for jets and what position the jets should be in?

Thanks in advance for all your help.

 

[DogBunny]

All of your carb questions are answered in the Carb Guide which can be found in the Tech section of this forum.

The failure to idle down when hot is possibly due to the advance weights not returning due to springs that don't have enough tension. Check that first.

If it's a bobber with straight pipes or with headers only, then you likely do not have baffles. You will never have good low end power until you place a small baffle, or lollipop, or thumbscrew in the end of the pipes. Google those terms.

 

[jd750ace]

Another recently purchased fine "custom" that don't run for shit but probably has "the look" I'm shocked.
Make note of your symptoms. Download or print out the Carb guide.
Take your carbs back apart with the guide in hand for an inspection aid and make note of what size jets are installed. Easier to drill off the screw plug off the bike anyway. If you don't know where your jetting is currently, then you won't know where to go from here.

Once you are aware of your starting point, If you can read and comprehend, it should answer all of your questions.

 

[5twins]

Your first task is going to be properly I.D.ing the year of the carb set you have. From your description of needing to drill block-off plugs, it doesn't sound like you have the '77 carbs. They don't need drilling to access the mix screws. Only the '80 or newer BS34s require that.

 

[Shifty2932]

Dog bunny, jd750ace, and 5 twins. Thanks for pointing me in the right direction.

 

[aldo5468]

You also may want to search this forum for info on pod filters - they have been discussed extensively in many posts, but I can't remember at the moment whether the issues with them are covered in the carb guide. You didn't note what type of air filters your bike has, but being a custom bobber, I'm guessing it has some kind of pod-type filter. The wrong kind can make it difficult or impossible to get the bike running smoothly.

 

[Shifty2932]

Okay, have some updates. I have I.D. the carb type. They are BS 38 CV. I was misinformed on them. So I have checked the pilot (27.5) and main (135) jets Spec sheet say they should be 25 and 122.5 respectively. I assume they are a size bigger due to the exhaust being just headers/straight pipes. I didn't have a snap ring pliers handy to remove the needle to check the needle jet and needle/clip spot. I will do that tomorrow. Anyways the mixture screw was about 3.5 turns out and the spec sheet says for the jets that are in there to have it at 2.25 turns out as opposed to 1.5 +/- .5 for the stock jets. I ended up putting it at about 1.75 turns out. Seems to idle well there and when winding the motor up it returns back to idle speed the quickest at that position. The airpods appear to be the 54mm KN air filter.

 

[5twins]

Well, I don't think you've I.D.ed your carbs quite yet. Show us some pics and we can probably help. Yes, 122.5 mains and 25 pilots are stock for '77 but a snap ring retained needle is not. That would be a '78-'79 carb set characteristic, as would 27.5 pilots and 135 mains. And to confuse matters even more, rebuild kits for the '76-'77 carbs come with 27.5 pilots and 135 mains, so if someone used one of those kits, that might account for the jets you found. There were major changes from the '76-'77 to the '78-'79 carb sets so it would be best for you to positively identify just which set you've got.

 

[Shifty2932]

5twins,

Good point. This darn this is more complicated than it needs to be. Diesel and jet engines are much simpler. So I checked the needle/clip spot and the needle jet, both are once again fall the standard specs for a '78 carb. After looking at some pictures from the carb guide it does appear to be that these are '78 carbs. Let me know what you think and again, thanks for all your help.

 

[jd750ace]

Those look like my 79 Special II's carbs.

 

[5twins]

Yes, that's a '78-'79 carb set. The "tells" are the large vent holes for the float bowls on the intake bells at 3 and 9 o'clock and the lack of overflow nipples (for a drain hose) on the float bowls. As I mentioned earlier, the needles retained by a snap ring is also a '78-'79 carb set thing. That's the only set that retained them that way.

 

[Shifty2932]

Dogbunny,

The thumb screw idea was great. Cheap and very easy to install. I got everything put back together today. Rewrapped the pipes, carbs, and had to do a little mod to the foot clutch and clutch cable. Took the bike for a spin and the low end power is back and the sputters with throttle wide open is gone. Motor still seems to want to run away with the rpms when the clutch is pushed in and it is very intermittent. So I'm kind of puzzled at way it's doing this. If it does it I just bump the choke quick and it forces it back down to idle speeds imeditately. Don't have to do it very often, but wish I never have to do it. Any suggestions at what it could be.

 

[5twins]

Your advance unit and advance rod probably need to be serviced. The advance rod needs to be greased where it runs through the cam. It may be sticking in the advanced position sometimes which would cause the RPMs to hang.

Your carbs have stock jetting. With your mods, you should probably re-jet. Running lean can also cause the RPMs to hang. K&N type pods aren't the best choice for these carbs. You'll never achieve the best state of tune possible using them.

 

[Shifty2932]

5 twins,

Thanks for the response. Do you have any suggestions for jet and or pods?

 

[XSLeo]

Not just K&N pods but any of the pleated paper element filters. The shorter and more tapered the pod the worse they are.
They cause air flow turbulence through the carb that disrupts the vacuum signal that lifts the slide.
A filter with a foam element works best. There are several brands out there but Uni Filter is the brand most recommended. Their PK-82 fits the BS38 carbs. the PK-92 fits the BS34 carbs.
Leo

 

[5twins]

Yes, the UNI foam pods work best. As for jetting, the usual routine is one or two up on the pilots and 2 or 3 up on the mains. I can't say exactly what you'll need, it will take some experimenting on your part to find what works best on your bike with your particular mods. To start, I'd go one up on the pilots and 2 up on the mains (#30 pilot and 140 mains). You will also probably need to lean your needles a step.

 

[Shifty2932]

5 twins and dog bunny,

I was looking into greasing the advance rod and had taken off the springs and weights. Did some reading and from understanding need to remove cylinder head to remove advance rod. Is this correct? Anyways decided to stop there put things back together as I don't have time to rip the motor apart yet. Thought I put the weights back on in the position I took them off at, however the bike will not start now. Backfires every now and then when I try to start it( kick start only). Any suggestions? Oh and replaced the springs while I was at it, weights return right back into position.

 

[Shifty2932]

Disregard previous post. Found the answer in advance rod stalling thread. Points were off.

 

[XSLeo]

When you remove the weights and reinstall them you may have assembled the weights to the notched disc 180 out of time.
I don't know where you read that you had to remove the head to take out the advance rod but that just ain't so.
On the advance side just remove the nut holding on the notched disc and the notched disc. Now the rod will slide right out to the points side.
Or you can remove the points cam on the other side and slide it out the advancer side.
Either way will work. Just be sure the pins are all pointing the same way when you put it back together.
Leo