Carburetor Troubleshooting - BS38 Flooding

[Leams]

Hey All,

I have been reading as many threads as possible on troubleshooting BS38's. I am still struggling.

So i appeal to the forums expertise.

- The bike kicks first time no trouble
- Give it a little bit of throttle it sputters and dies
- A small amount of black smoke comes out of the right carb
- The spark plugs are black indicating that it is rich/flooding


Steps i have taken to solve the problem:

- Disassemble both carbs and thoroughly clean them using carby cleaner (followed a youtube instructional on this)

- Checked for dirt and grime blocking jets

- Checked for air leaks in the diaphragm

- Checked to ensure the float height is correct. As far as i can tell it is near exact to the recommended height.

- Checked for perished gaskets and seals, all appear to be fine.

- Re-synced the carbs using the "dead cylinder" method.

- Checked petrol is flowing freely to the carb.


Any help or advice would be great!

 

[650Skull]

Have you identified which set of 38's you have and are the carbs jetted properly for the correct year.

If the bike is not running the plugs will probably be black from the soot anyway. Cold engine won't burn the fuel efficiently

have the pilot holes been properly cleaned.

 

[Leams]

Great ok, i haven't identified which 38's or if they have been jetted differently. Previous owner has swapped to pod filters. I will check this out.

That being said the bike has been running great for almost 12 months, the problem developed recently. What would your recommendation be for properly cleaning the pilot holes? Is carb cleaner from repco etc. sufficient?

Thanks heaps.

 

[scabber]

Welcome to the site. BS 38 carbs changed jetting over the years. Not knowing what year carbs and jets it could be jetted wrong. The carb guide is a good place to start. Sounds like the problem is a dirty idle circuit would be my guess. Mixture screw adjust ?No first hand experience with BS 38 carbs other than taking them off my 79.

 

[peanut]

welcome Leams

these old carbs can be a bit of a challenge at times .You need to be really meticulous with the cleaning and adjusting

A little more information could be helpful in diagnosing the problem.

Assuming that your timing and advance are set correctly and there are no faults in your ignition system.......

You may need to adjust the air mixture screws to help throttle pickup off idle but until you get the engine warmed up you'd be wasting your time trying to do it . Bit of a catch22
Initial pick up off idle is mostly governed by your air /fuel mixture from your pilot jets and the slides.(also choke enrichment when the engine is cold obviously)

Make sure that your choke valves are seating properly and the choke jets are clear... use a guitar string to carefully rod the choke pickup tube. Blow it through with compressed air. A bicycle pump would do if you have no compressor.

Check that the tiny choke and pilot exit holes into the venturi are clean and blown through. I use a compressed air spray with a flexible fine spray tube like the computer cleaners
You'll find this post by 5twins very helpful
http://www.xs650.com/forum/showthread.php?t=19671

What happens if the engine is idling and you apply just a small bit of choke say 1/4 choke?

If the engine picks up it would indicate that the pilot circuit mixture could be lean and visa versa if the engine immediately falters and or dies it might indicate you need to screw your air screws out to enrich the pilot circuit or you have a slight air leak somewhere on the carb bodies or inlet stubs

 

[650Skull]

What year is your bike? Saw a 75 XSB in you profile.

It is important to identify the year of the carbs because the later 38's have a jet in the inlet throat.

If the bike was running well it could be a bit of dirt, easy to miss, re clean the carbs making sure all the jets are clean and make sure the choke inlet hole is clean. Be very careful poking things into the jets, they are brass and even a small enlargement will affect the fuel flow, if using a guitar string make sure it is plain wire and not the wound string it can act as a file on the jet. If the blockage is recent try to clear the jets using the compressed air first.

Compressed air for blowing out the jets after cleaning to get rid of dislodged dirt. If you don't have a compressor maybe buy a can of compressed air. Blow back against the fuel flow, better still take out the jets and when you do make a note of the sizes for reference
purposes.

Here is a thread that may be of some use as well.
http://www.xs650.com/forum/showthread.php?t=28547

 

[gggGary]

Brass floats frequently crack and leak. Get a cup of HOT water hold a float with a pair of plier submerse it in the hot water for just a couple of seconds. If you see bubbles rising, the float is junk, replace it. I find more bad than good floats in these old carbs.

I need to redo this vid with better focus.





This one is pretty obvious but they can be hard to see, just a crease or series of creases after cleaning, viewed in good light. I think it comes from decades of temp swings flexing the brass from internal pressure changes.

 

[grizld1]

Do you have any particular reason to blame the carburetors, or did it come to you in a vision that they're the cause of your trouble? Make sure you have good compression and ignition. A defect in either/both will cause poor combustion, and poor combustion will foul your plugs, which could be oil fouling as well.

 

[peanut]

Do you have any particular reason to blame the carburetors, or did it come to you in a vision that they're the cause of your trouble? Make sure you have good compression and ignition. A defect in either/both will cause poor combustion, and poor combustion will foul your plugs, which could be oil fouling as well.

he hasn't blamed the carburettors ?

Do you actually read peoples posts before insulting them ?

Leams is a new member and as such should be welcomed not insulted and ridiculed.

He has taken the time and trouble to set out fully all that he has done so far to cure the problem which is really helpful and asks for help & advice. How about cutting the guy some slack

 

[TwoManyXS1Bs]

Sudden onset richness can also be caused by dirty/blocked air jets (air bleeds).
The sizes in this pic may not be the same as yours.

 

[grizld1]

Yes, I read the post. Did you read the title, peanut? The assumption is that carbs are the source of trouble, and that may very well not be the case. You don't think it's helpful to suggest inspecting ignition and compression? Just why is that--because I credited the OP with the good sense and ability to research those procedures, since he appears, from his first post, to have both?

You found light humor insulting, and I wonder why. Is it that I didn't clutter the post with moronic emoticons, or because you're looking for confrontation? You're thin skinned enough on your own account. Taking offense by proxy is a bit much.

Here's how I learned troubleshooting sequence. In the late 1960's I had a part time job as riding instructor and light maintenance wrench in a little rental-and-repair shop owned by a certified Norton mechanic. Every time he caught me starting to fiddle with a carburetor, he roared "IGNITION FIRST, YOU MORON!" It's a lesson that stuck and has proven extremely useful.

 

[weaselbeak]

Yes, I read the post. Did you read the title, peanut? The assumption is that carbs are the source of trouble, and that may very well not be the case. You don't think it's helpful to suggest inspecting ignition and compression? Just why is that--because I credited the OP with the good sense and ability to research those procedures, since he appears, from his first post, to have both?

You found light humor insulting, and I wonder why. Is it that I didn't clutter the post with moronic emoticons, or because you're looking for confrontation? You're thin skinned enough on your own account. Taking offense by proxy is a bit much.

Here's how I learned troubleshooting sequence. In the late 1960's I had a part time job as riding instructor and light maintenance wrench in a little rental-and-repair shop owned by a certified Norton mechanic. Every time he caught me starting to fiddle with a carburetor, he roared "IGNITION FIRST, YOU MORON!" It's a lesson that stuck and has proven extremely useful.

Absolutely correct. The problem lies not with your advise, but your delivery. Not everyone gets trained by a certified Norton mechanic.