Help with 78 special bs38 carbs please advise needed.

[Mlourigan83]

Some background on the bike, it’s all stock and I recently rebuilt the carbs and synced using a digisync. Bike seems to make a “puffing” noise and die while pulling to a stop sign at near full stop. Bike will be impossible to start for awhile until I let it sit for after that. I did not set the float height while cleaning them could this be the cause?

 

[5twins]

Is this after the bike has been running for a while and is warmed up or hot? If so then you may have an electrical issue, specifically the coil or coils. When coils go bad, they often do so when hot. After they cool down some, they work fine again.

 

[Mlourigan83]

Is this after the bike has been running for a while and is warmed up or hot? If so then you may have an electrical issue, specifically the coil or coils. When coils go bad, they often do so when hot. After they cool down some, they work fine again.

No I don’t believe it’s electrical because it does do it sometime without leading to the “no start” condition. Sometimes it’ll just make a hesitant noise then stall and I can start it right back up almost as if when it’s really bad when it won’t start very much strikes me as flooded in that case. Thanks for the reply I’ve seen your name on these forums a lot so i appreciate the help no doubt.

 

[Mlourigan83]

No I don’t believe it’s electrical because it does do it sometime without leading to the “no start” condition. Sometimes it’ll just make a hesitant noise then stall and I can start it right back up almost as if when it’s really bad when it won’t start very much strikes me as flooded in that case. Thanks for the reply I’ve seen your name on these forums a lot so i appreciate the help no doubt. It’ll maybe not start back up like 10 percent of the time it dies.

 

[Mlourigan83]

Is this after the bike has been running for a while and is warmed up or hot? If so then you may have an electrical issue, specifically the coil or coils. When coils go bad, they often do so when hot. After they cool down some, they work fine again.

If you know of any ways I could maybe confirm or not it is a fuel issues like maybe I could bring a wrench and empty the fuel bowl or something when this happens I’m not sure. Thanks again in advance to anyone with advice.

 

[650Skull]

First off Welcome to all things XS650.

Second off, the more information about yourself, or more precisely, your experience around bikes, mechanical and electrical, the better for us to understand if a reply to an answer is based on knowledge or an assumption or hunch.

A saying carved in wood from one of the most experienced members, (RIP Griz'd1), used to say Fuel problems should not be confused with electrical problems and 90% of fuel problems are electrical. Don't dismiss advice because your mind is leaning/blinkered towards another reason, 5Twins does know his stuff. Maybe wrong in this case but to not check and eliminate his answer because you don't think it is the reason is not a reason to and we do not know what knowledge/experience level your at

A recent members problem similar to yours where the bike would die on deceleration ended up being an electrical issue with a loose connection that only seemed to show up on deceleration.

From your pic.....................Your speedo is from an 80-83 model...............the engine side-cover is from a 70 or 71 model. Early 71 model have a smaller oil pump which Yamaha increased. Something to keep in mind...................I see fuel has been leaking on to the top of the carb.......................You header ha an unusual amount of disscoulouring caused by???? Factory headers are double walled and should never stain unless the inner wall has rusted through or the carbs are in very poor tune................I see no hose clip on the vacuum hose on the manifold barb. I have had a hose heat up, get loose causing the bike to stop by not using a hose clamp.............

There are things that have been changed on the bike, so can't take things for granted.

Nice bike, I love those Blue 78's, should be a lot of fun once sorted

 

[BowlingSS]

I had a problem with a gas cap not being vented properly once and it would run for some time and then stop. Had to wait some time and then it would start right up.
I also had a bad CDI box go bad on my 81 that caused it to die. Yours should have points. Condenser bad maybe.
I would also check your coil connections.
Good luck on finding the problem. This is a great site for help and info.
Bill

 

[Mlourigan83]

First off Welcome to all things XS650.

Second off, the more information about yourself, or more precisely, your experience around bikes, mechanical and electrical, the better for us to understand if a reply to an answer is based on knowledge or an assumption or hunch.

A saying carved in wood from one of the most experienced members, (RIP Griz'd1), used to say Fuel problems should not be confused with electrical problems and 90% of fuel problems are electrical. Don't dismiss advice because your mind is leaning/blinkered towards another reason, 5Twins does know his stuff. Maybe wrong in this case but to not check and eliminate his answer because you don't think it is the reason is not a reason to and we do not know what knowledge/experience level your at

A recent members problem similar to yours where the bike would die on deceleration ended up being an electrical issue with a loose connection that only seemed to show up on deceleration.

From your pic.....................Your speedo is from an 80-83 model...............the engine side-cover is from a 70 or 71 model. Early 71 model have a smaller oil pump which Yamaha increased. Something to keep in mind...................I see fuel has been leaking on to the top of the carb.......................You header ha an unusual amount of disscoulouring caused by???? Factory headers are double walled and should never stain unless the inner wall has rusted through or the carbs are in very poor tune................I see no hose clip on the vacuum hose on the manifold barb. I have had a hose heat up, get loose causing the bike to stop by not using a hose clamp.............

There are things that have been changed on the bike, so can't take things for granted.

Nice bike, I love those Blue 78's, should be a lot of fun once sorted

Hey thanks for the direction I’m really not familiar with posting on threads so any direction is appreciated. And your right I should not be so quick to dismiss that problems after all I’m here to learn.
About me: I’ve been riding for about 18 years and my experience with fixing bikes is intermediate roughly, electrical problems can give me trouble no doubt it’s not something I have tackled a lot. I currently have 6 bikes 3 that’s run. 78-cb550, 78xs650, 83-gl1100, 80cb650, 86cn250, and 65-s90. I’ve owned about 6 or 7 other bikes in the past. All my stuff being vintage I would watch my friend and work with him on a lot of my projects but he was no doubt the one with experience. So I’m no noob but certainly no expert. In response to the intake nipples I have capped them off now with clamps leaving the petcocks on “prime” I’m currently ordering some gravity petcocks to replace. Nice spot on the side cover that I didn’t know just noticed it didn’t look like others based on the filter cover. The headers I can’t say much about I’ve had the bike for awhile but haven’t driving it much more then a few miles at a time due to the unreliability of it just randomly Turing off when I stop. As long as I’m moving the bike has no issues driving and if I blip the throttle while at a stop it also will not die. It’s almost as if it just hiccups sometimes and it makes a little puff noise then dies.

 

[Mlourigan83]

I had a problem with a gas cap not being vented properly once and it would run for some time and then stop. Had to wait some time and then it would start right up.
I also had a bad CDI box go bad on my 81 that caused it to die. Yours should have points. Condenser bad maybe.
I would also check your coil connections.
Good luck on finding the problem. This is a great site for help and info.
Bill

Thanks for the reply I have had problems with vaper lock in the past I’ll try to open the cap and see if it’s releasing any pressure thanks. I’ll try to do a YouTube on condenser and coil test for the bike can’t be to hard.

 

[Mlourigan83]

Thanks for the reply I have had problems with vaper lock in the past I’ll try to open the cap and see if it’s releasing any vacuum thanks. I’ll try to do a YouTube on condenser and coil test for the bike it’s nice to at least start a list of things to go through.

 

[Mlourigan83]

So in recap it doesn’t always die when I stop, just sometimes and not right away every time it’s very inconsistent. I am making a list of things I can try to diagnose and I will update for anyone interested or who stumbles across this thread in the future. It has helped me many times. Also I will work on my typing to make it easier to read sorry.

 

[650Skull]

Earth connection??

 

[nj1639]

The header discoloration can be a tell. Shaking the cobwebs out thinking yellowed headers are a sign of a rich condition, blue headers are a sign of lean. It costs nothing but time to check the float height.
Skull pointing out the early side cover says the bike isn't totally stock for its year. Check the engine numbers against the frame numbers and see if they're the same - it'll give you a known base to start with.

 

[5twins]

What do you have your idle speed set at? The factory spec is 1200 and this motor likes that. Set it much lower and the bike will randomly stall at stops. Being an "old school" machine, the idle speed should be set when the bike is fully warmed up. If you set it cold, it will idle too high once warm.

 

[Mlourigan83]

Ok so after giving it some riding I’ve realized it is only when I brake hard very distinctly. I have turned the idle up to maybe 1400 to compensate a bit and with a blip I can keep it going every time it’s just a fickle process.. I’ve noticed a small leak in either manifold or possibly a throttle shaft seal with a small bird chipping noise. so I think that should hopefully fix once I get time to address it. Thanks for everyone’s reply I am really interested in the motor now that it’s been pointed out the older model engine cover.

 

[Mlourigan83]

I’ve attached some photos of places I know can be distinct to certain years with engine code sites and the rear cylinder head maybe someone can share some insight and wisdom please.

 

[nj1639]

Pic 0353 above, is where the engine numbers should show, to the left on that flat behind the lower motor mount.
That motor shows no engine serial number that I see.
Out of curiosity, what shape is the left, front valve cover? Answer should be semi rectangular or round like the right front valve cover.

 

[Wingedwheel]

Just a question but you say you rebuilt the carbs. Did you use a kit, just clean or replace with genuine Mikuni parts? Did you change anything as far as needle setting?

 

[5twins]

Well, you do have the '78 type cam chain adjuster so I'm going to guess someone just changed out that right engine cover.

What you're describing (hard braking causing a stall) is often related to the float levels. In this case, they may be too low.

 

[Mlourigan83]

Just a question but you say you rebuilt the carbs. Did you use a kit, just clean or replace with genuine Mikuni parts? Did you change anything as far as needle setting?

I used the rebuild kit from mikes-xs the sizes were all the same but the pilot jet did have a different hole pattern and I decided to go with the new one. I left the needle setting as the bike to my knowledge was stock and I made sure to count the turns on the air screw at about 2 1/2. I disassembled them and dipped the whole unit into a chem bath however I was unable to remove the throttle shafts so I’m concerned the o rings might have gotten damaged. I did not remove the intake manifolds and they appear to have some silicone so those should be addressed as well.