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Any Idea What Happened Here?

Mr.humbucker

Mr.humbucker

Forum Mentor
Past Site Supporter
You'll see by the photo that I have a main pilot jet tube (I think that's what it is called) that is busted in half. I was tearing apart a set of 550 carbs I picked up on eBay and had some difficulty pulling the diaphragm/needle jet out of one of the carbs. After some pulling and prodding it finally released with the top half of the tube attached to the needle. The bottom half was still in the carb. It is clear from the break that this happened long before I started to tear these guys down. There was also a dark soot like stain on the underside of the large cylinder that attaches to the diaphragm. Almost looks like there was an ignition and cracking of the main pilot jet tube??? What could cause such a break?

Also ... for ten GS points ... what's the small item that is the photo accompanying the broken jet tube? I found it in my parts box where I am storing my bits and bobs for the bike I am working on.

DSC_0049.jpg
 
We have a wonderful mix of (mis-)terminology going on here. :-k

Your "main pilot jet tube" is actually a "needle jet", and is sometimes called an "emulsion tube".

Your "large cylinder that attaches to the diaphragm" is what we call a "slide".

If the "dark soot like stain" starts at the needle and continues to the 'downwind' edge of the slide, that is somewhat normal.

Can't say that I need an "GS points", but that extra item appears to be a "cable stop ferrule". A cable will go though the small hole on the side, a small screw threads in the end to clamp the cable into place. Think of it as an adjustable cable end.

.

 
I've seen some gummed up needle jets, but never one broken in half.

I agree with Steve that it's a cable ferrule, but it isn't a stock piece. I'm wondering if someone made their own choke or throttle cable and used that as the end. Either way, it's not needed.
 
Thanks for the proper identification of the stuff in my description. I referred to the Parts Diagrams from Partzilla to try to get it right. You'll notice I am in the "forum apprentice" category ... ain't no "sage" ... maybe someday ... maybe.

The answer I am looking for is what could have caused the "needle jet" or "emulsion tube" to break? The patina, oxidation, whatever ... on the break clearly indicates this happened sometime ago and not during my extraction. odd.

I believe you're correct on the "cable stop ferrule" not being sock, seem to remember someone jimmyrigged it on the end of the clutch cable mmmmaybe?

Thank for your input help ... again.

Steve said:
We have a wonderful mix of (mis-)terminology going on here. :-k

Your "main pilot jet tube" is actually a "needle jet", and is sometimes called an "emulsion tube".

Your "large cylinder that attaches to the diaphragm" is what we call a "slide".

If the "dark soot like stain" starts at the needle and continues to the 'downwind' edge of the slide, that is somewhat normal.

Can't say that I need an "GS points", but that extra item appears to be a "cable stop ferrule". A cable will go though the small hole on the side, a small screw threads in the end to clamp the cable into place. Think of it as an adjustable cable end.

.
Click to expand...
 
The needle jet is in fact at the top of the emulsion tube and the main jet screws in the bottom.
The assembly is intended to have the main jet removed and then comes out of the carb body upwards - towards the slide.
Looks like someone has tried to pull it out downwards - towards the mainjet end. Possibly by putting something in the mainjet threaded hole and pulling....
 
GregT said:
The assembly is intended to have the main jet removed and then comes out of the carb body upwards - towards the slide.
Looks like someone has tried to pull it out downwards - towards the mainjet end. Possibly by putting something in the mainjet threaded hole and pulling....
Click to expand...
It would be really hard to pull it out downwards. There is a flange around the top that prevents that. And, since there is nothing that protrudes at the bottom, you would have to use a main jet or bolt with similar threads to try to pull it out the bottom. Again, very unlikely because the needle jet is made of brass and the threads will strip long before there is enough force to cause the tube to break. There is the outside chance that there was a casting flaw, and the 'normal' tension of the main jet caused the crack.

Either way, that is the first time it has been seen on this forum.

.
 
I am ok if this sounds goofy cuz I very much consider myself to be a learner, but since there was a heavy "soot" stain on the bottom of the cylinder (hope I go the part name right Steve - I am trying) above this jet/tube that is fractured and only the one cylinder is stained, is there any chance there could have be an ignition in this area for some reason?
 
Mr.humbucker said:
I am ok if this sounds goofy cuz I very much consider myself to be a learner, but since there was a heavy "soot" stain on the bottom of the cylinder (hope I go the part name right Steve - I am trying) above this jet/tube that is fractured and only the one cylinder is stained, is there any chance there could have be an ignition in this area for some reason?
Click to expand...

I'd doubt it. If by cylinder you mean the slide and it has a dark stain on the lower surface, that's almost certainly because with a broken emulsion tube there would be quite a lot more fuel in that carburettor bore. Not uncommon to find fuel stains where a tank for instance has been leaking.
 
There was also a dark soot like stain on the underside of the large cylinder that attaches to the diaphragm. Almost looks like there was an ignition and cracking of the main pilot jet tube??? What could cause such a break?
Click to expand...
Don't know about the break, but couldn't the soot stain be caused by a valve that has lost all clearance? If a valve isn't closing all the way, won't there be gases pushed back at the carb on the cylinder with a valve that isn't closing?
https://www.youtube.com/watch?v=SQEFWeFeqUo
 
I will go with the "fuel stain" theory.

Unless the stain is starting at the needle and going to the airbox. Then, I'll go with some sort of wrong combustion theory.

.
 
It will remain, likely, a mystery. I suppose oneway to debunk the ignition or combustion hypothesis is to ask where could/would an ignition source come from?
 
Mr.humbucker said:
It will remain, likely, a mystery. I suppose oneway to debunk the ignition or combustion hypothesis is to ask where could/would an ignition source come from?
Click to expand...
It would come from an intake valve that is not closing all the way. Combustion flame would travel past the intake valve instead of staying in the combustion chamber and forcing the piston downward. I think someone would have been ignoring that popping sound for a long time to build up soot on the slide. You'll be able to tell fairly easy by whiping the slide. If it's soot, it should wipe of fairly easily. I'd go with a fuel stain idea as the most likely.
 
It took some carb cleaner to remove the stain. Thanks for the flame explanation, I more fully understand the manner in which the valve likely play a role and the trouble they can cause if out of adjustment. Lening how to adjust the valve clearance is on my spring bucket list. Thanks to all who contributed here. I have learned a lot.
 
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