Carbs are off again. What now?

[5twins]

The needle jets in my '78 carbs were very loose when I got it, to the point of falling out under their own weight (the o-rings were shot), but it ran OK. I replaced them of course, but based on that experience, I don't think yours being loose is the issue.

 

[Its been a long time]

The needle jets in my '78 carbs were very loose when I got it, to the point of falling out under their own weight (the o-rings were shot), but it ran OK. I replaced them of course, but based on that experience, I don't think yours being loose is the issue.

Ah, OK. That's a big disappointment then.

I'm going to know one way or the other soon enough. If this doesn't fix it, I don't know of anything else to do.

I have been suspicious about the brass pilot mixture tube that runs up the carb behind the throttle plate. Blowing with a compressed air line today proved that isn't a problem.

 

[Its been a long time]

@5twins Could I ask please, do you have a view of a BS38 carburettor body looking directly into the port with the mixture screw removed? This is what I see looking directly down there with the tapered mixture screw removed. You can clearly see a little brass plug pushed in there. I think there should be a hole there and what I can see is the tip of a screw that's broken off in the hole? Thanks.

 

[Jim]

If it's anything like the 34's, there's supposed to be a hole there. That looks like someone smoked the screw tight in it's threads and sheared off the tip of the needle. Unfortunately that's a fairly common occurrence. If that's indeed what we are looking at, I'd call it a smoking gun for your problem.

 

[Its been a long time]

If it's anything like the 34's, there's supposed to be a hole there. That looks like someone smoked the screw tight in it's threads and sheared off the tip of the needle. Unfortunately that's a fairly common occurrence. If that's indeed what we are looking at, I'd call it a smoking gun for your problem.

I agree. The tapered screws I removed were bent at the end so previous owner has form. Problem being I have no idea what drill size I need to use to recover the carburettor. Oh dear

 

[Jim]

There's threads here dealing with removing the remnants. Do a search and a few of them should turn up.

 

[Its been a long time]

There's threads here dealing with removing the remnants. Do a search and a few of them should turn up.

Thanks. I'll have another look, I couldn't find anything a few minutes ago.

Edit to add - This thread tells me enough. Go in from behind, not down the mixture screw hole. Makes sense, less risk that way. Link -

https://www.xs650.com/threads/plugged-mixture-screw-hole.32696/post-331842

 

[Jim]

Here's one...

https://www.xs650.com/threads/plugged-mixture-screw-hole.32696/

 

[zrx1100]

Does the other side carb have this hole plugged also?

 

[Its been a long time]

Here's one...

https://www.xs650.com/threads/plugged-mixture-screw-hole.32696/

Cheers. Our posts crossed, that tells me what I need to know.

 

[Its been a long time]

Does the other side carb have this hole plugged also?

I will look in the morning now. My guess is that the answer is maybe 75% yes. When I removed the screws from both carburettors back in January, both screws were bent at the end

 

[Jim]

Iirc, @gggGary has a pretty good removal method also.

 

[Its been a long time]

Iirc, @gggGary has a pretty good removal method also.

Thanks Jim. Reading that other thread, you are drilling into a void so if I go carefully I think I have a good chance of rescuing this. Actually, it's really good that with the help here I have eventually landed on what I think is 99% the problem. Will check the left side carburettor tomorrow.

 

[gggGary]

Yeah the BS34's are simple cuz the screw tip is sticking out into the bore making it fairly easy to put pressure on the tip and pop it back out the way it went in.
pushing back out from the other side the only reasonable method.
It's benna what year is your carb set?

 

[Its been a long time]

Yeah the BS34's are simple cuz the screw tip is sticking out into the bore making it fairly easy to put pressure on the tip and pop it back out the way it went in.
pushing back out from the other side the only reasonable method.
It's benna what year is your carb set?

1977.

I will have a go at it in the morning.

 

[Its been a long time]

This morning's voyage of discovery. Drilled the mixture screw port from behind as described in the previous posting about broken screw tip removal. Decided to drill with a 11/64" drill on the basis that the broken screw tip is a small target and drilling bigger makes no difference really as it's going to be plugged off anyway. Once drilling as I got near the broken tip, it fell out. I have to say, another mm or two at the most and the carburettor would be scrap. Not a procedure to be taken lightly.

What to do now with the drilled port? In a perfect world I would make an alloy or brass plug with an O ring on and press it in. But my lack of ability to machine anything here precludes that. So I have made a blank out of a screw that drops in the hole and covers where I have drilled. I have cut the screw off short so that dropped in the hole head first I can fill the space with epoxy. If the carburettor now works it would still be possible to remove the epoxy plug and fit a proper metal one. (Unfortunately, I ruled out threading the boss that I drilled. It measures 9.8mm OD and around 6mm ID. So it's not going to support a 8mm threaded plug without losing robustness. The wall thickness would be less than 1mm.)

Pictures show the mixture screw inserted and bottomed before the broken tip removal. Then the screw inserted after broken tip removal. And a view into the hole showing the tip of the mixture screw set 1.5 turns out. That's where I'm going to plug with a modified screw and epoxy.

 

[Its been a long time]

Does the other side carb have this hole plugged also?

Yes it does. I am going to drill this one out next. You can easily tell if the hole is plugged with a broken mixture screw tip. If you refer to the pictures in the previous post, the mixture screw goes in about 3mm further if the pilot mixture screw port is open. Which confirms I need to do the left side carburettor next.

 

[Wingedwheel]

Didn’t the other side carb run fine?

 

[Its been a long time]

Didn’t the other side carb run fine?

It ran much better than the right side one, that's a fact. Whether it was running properly is another question. I haven't been able to get the bike running well enough yet to ride it. Left side carburettor has now been drilled per the right side. Mixture screw now sitting several mm more into the carburettor now.

Final roll of the dice for me. If it doesn't run right now, then I'll be looking at a set of VM34 Mikunis.

 

[jetmechmarty]

Final roll of the dice for me. If it doesn't run right now, then I'll be looking at a set of VM34 Mikunis.

Read the threads on this. Lots of people run VM34 Mikuni carburetors “Like a boss!” Those who struggle with the stock CVs also struggle with setting up VMs. FWIW, I recall @grizld1 recommended VM32.
CVs work great on the street once you get them right. Stay the course. Persevere.