Damn you Carbs.

[Socopunk]

73 TX. Ran great for a few weeks. Had a pinched wire that was grounding out my left turn signal. Traced all wires and found the problem. Fixed it. All good, except when I start it now. Bike starts easily, but will not idle. If I don't hold the throttle it dies. Cleaned the carb and all jets twice. Where do I buy new BS38's? These ones are cursed.

By the way, open exhaust with pods, but it ran great on this setup before.

 

[TeeCat]

Oh, my. Jimmy... I have been there and got all those tee shirts!

Speak to us more about what you have done with your air and idle speed screws (you have two independent cables, right?). And are your carbs synched (that is to say, do your butterflies open at the same time?)? Are the idle screws in sufficient contact with the arms on the carbs to open the flies a bit?

Some variable changed, and you have to track that down before you commit to new carbs.

TC

 

[Socopunk]

Mixture screws are 1 turn out. Idle screws set to each other with paper resistor test and then dead cylinder method (though while holding throttle open a little bit, or else bike will die). Two independent cables are releasing from idle screw point at the exact same time. I did find a little bit of residue in float bowls of both carbs. Cleaned it all out with carb cleaner. I suppose I will pull carbs and clean the jets AGAIN.

I know this bike is almost 40 years old, but I would like to daily ride it and can't even grasp that right now with how fickle it is. I know, I know it's always operator error.

By the way. Is there a conversion from dual throttle cables to one on these BS38's? Is it easy? Is it worth it?

 

[Travis]

I'm not all that familiar with the BS38 carbs but what happens when you just try to increase the idle speed with the idle screws?

If you thought you cleaned the carbs only to find junk in the bowl, then you either didn't clean them very well the first time, or have junk in the tank finding its way into the carbs again. You do have an inline fuel filter in place don't you?

What is the lowest rpm you are able to keep the bike running at? We need to figure out if this is a pilot jet and dirty carb problem or idle screw adjustment problem.

 

[TeeCat]

I'd be interested to know if it drops a cylinder during the misbehavior, too. If it were a contaminant issue purely, it seems to me that one cylinder might not be affected.

 

[gggGary]

how does it idle if you have a battery charger on it or have it jumpered to a car battery?

 

[Socopunk]

Turned both idle screws all the way up and no change in idle (with a little throttle) speed. Junk was in the tank but I then cleaned the carbs twice. RPMs will go down to 1200, but I need to keep a slight twist on thethrottle to keep it running. Bike acts the same way with a 12v trickle charger on it. Not sure about the dropped cylinder, but it does seem like there is a little more exhaust release on the left side.

 

[nightowl49]

i dont think you need new carbs think the ones you have are not yet set right or idles still clogged, check your timing and adj cam chain ie never seen a set of carbs that couldnt be redone and run properly

 

[Gordon]

Sound's like the pilot jet circut's are gummed up. Very small passages, and also very easy to 'think' that they are clean. Take a can of carb clean, put the straw on it and spray very sparingly around the base of the carb holder's, then around the throttle shaft's, while listening for the engine to speed up. When it speed's up, there is where you have a vacuum leak. Very common on old machines to have bad throttle shaft seals and/ or bad intake's. If you do not find a vacuum leak, spray close to the pod's and see if it will idle then. If so, pilot circut's plugged.
Nightowl is correct, carb's can be redone almost indefinately

 

[5twins]

If you want a single throttle cable, you'll need to switch to '76 or newer carbs. There's nothing inherently wrong with any of the carb sets that came on the 650s. If there was, after all these years it would have come to light, like for instance "the '71 set is no good". It's fine, they're all fine, but yes, some years are better than others. You're missing something on your set, either in the cleaning process or in the tuning part. Or maybe they are bad, eaten away inside from being all gunked up at one time. That does happen and, well, then that set is junk. And of course, it may have nothing to do with the carbs. If your motor is worn (low compression, bad valves, etc.) or the timing is off, no amount of carb tuning will fix that.

 

[TwoJugs]

Like someone else mentioned, I found that low voltage gave a weak spark and can act like a carb symptom at idle. It sounds like you did an electrical repair then this started? I would look into the spark a bit more.

 

[twinsarehot]

like John used to suggest, take some guitar string, bend the last cm or so at a 90 degree angle and clean the crap out of those small passages. Dirty tanks are TERRRRRIBLE. Also check your battery health.

 

[Socopunk]

Only half kidding on getting the new carbs. Gonna jump back into them tomorrow. Took the petcocks off and there was a little residue in there. cleaned it out good and sloshed the tank. Replaced new inline fuel filters, just in case. Took carbs off and got in all the little jets and holes and nooks and crannies with guitar string and carb cleaner. Took out the main jet and pilot jet and cleaned in there as well. I'll try to get it all back together tomorrow. One more question. If I want to make a manometer for these BS38's, is it a problem that the plastic exhaust nipple is busted on one of them (Circled), or can I use the port above (arrow)?

 

[gggGary]

the manometer needs to read vacuum from between the engine and the throttle butterfly. I have not done a set of the early BS38s (yet) so I am no expert. The later ones have a screw in a threaded port on the front right of the carb. I will have to go look to see if the early carbs have that port.
The broken off elbow is the overflow tube on the float bowl, eliminated on the later bikes when they switched to vacuum petcocks since overflowing carbs were then impossible. I have forgotten the function of the upper hose but I am sure it will come to me or more likely someone else will jump in with the answer. It appears to be a float bowl vent, on later carbs it is routed to inside the intake bell?

 

[OakBehringer]

From what I recall of my '73 carbs, there is no vacuum screw for syncing the carbs. It's a real shame.

-------------

Travis said "I'm not all that familiar with the BS38 carbs but what happens when you just try to increase the idle speed with the idle screws?"

You replied: "Turned both idle screws all the way up and no change in idle (with a little throttle) speed."

I think there's some confusion here. You're playing with the mixture screws. These have one correct, optimal spot and should not be used to modify your idle speed. The idle speed screws are the larger, completely exposed screws located at the end of each throttle cable. Once the carbs are synced, you can turn each of these in (the same amount to maintain sync) to increase your idle speed. If turning your throttle increases idle speed, so will this.

- Adam

 

[gggGary]

Looked and no vacuum port on the early carbs, don't think the manifolds had em either. On the real early engines, 70 71? The head has a cross port so it doesn't matter much, Manifolds with vacuum ports is probably $implest. You could drill and tap a passage that is molded into the body on both carbs. That is what the later BS38s have already done.

 

[Gordon]

As gggGary stated, you need a vacuum sorce between the carb and the intake valve on each carb. Also, the barb in your photo with the small hose on it is the vent for the bowl. If that wasn't there, the fuel would not fill the bowl. I don't recall what year you said, '73? I think there is a manufacturer that makes intake's that have the barb's on them for that year, not sure, though. A guy here named T-CAT would know. I recommend that you go to the tech page's here and read up on the BS 34 carb's to become familiar with the nomenclature of the carb's. See what part, screw, float setting ect. work's best. From what I've read, you do not know or unsure which screw doe's what. On screw is for idle SPEED adjustment, and another for idle MIXTURE adjustment. The two are in totally different location's, and serve two very different function's. In the tech section, there are method's for syncronizing the carb's using household item's, instead of going through all the hassle of replacing the intake boot's and such. From what I've read, that the screw you turned all the way in and out with no real difference is the mixture screw. If you turned the idle speed screw's all the way in the engine would be screaming at you. So go ahead and read up on them. If you can't find the info there, go to Mike's home page, to make a positive I.D. and see what is available. Good luck,,,,Gordo

 

[Socopunk]

I have the mixture screw 1 turn out. When I turned the idle screws all the way down, it was don't changing the idle at all.

Oak - I meant that I had to keep my hand on the throttle a little bit to keep it from dying and then adjusted the idle screw Clockwise and then let go of the throttle to see if it would idle then, or if it still died. It still died.

Is this picture labeled incorrectly? Am I wrong?

 

[Gordon]

You have them labeled correctly. While the carb's are off, open the butterfly's with the tang that holds the cable end. Throttle plate opening? I thought so, from your previous post's. Now let it rest and see how much the throttle plate is open now. Then turn your idle speed screw in all the way until it bottom's.Are the throttle plate's open more now? It should be. If the plate is not opening, You have to remove that small nut that holds the small lever & see why it's not turning the throttle plate shaft. Check both carb's the same way. If no problem's are found there, then you got to have a massive vacuum leak, possibly the carb holder's themselves. Not uncommon, esp. with the Methanol that is in the gas now. All US gasoline has methanol in it, the amount varies in different parts of the country. Some have 10%, like here on the east coast, but goes as high as 85%, like in corn country. Methanol eats away at the rubber compound of the holder's, making them porous , enough to cause some major problems. I think that there is a passage in the head between the two intake ports that links them together. Have you tried spraying the carb cleaner around like I suggested previously? What was the result's of that test? Did the engine speed up, or even idle on its own breifly?

 

[xjwmx]

take some guitar string.....and clean the crap out of those small passages.

That could be taken two ways. He doesn't mean be aggressive with the guitar string; he means clean the crap out with the guitar string. Be delicate with the string to avoid enlarging the holes.


Gordon - good to see you! Wondered where you were.