Chaising the infamous hanging idle - Carb tops possibly?

[peanut]

well strictly speaking you are correct 2m but
I was trying to find a simple way of explaining why a slide stays up when you cover the top vent without resorting to science , formulae and big words. There is simply no point in going into great depth about potential and kinetic energy, vacuums and pneumatics etc, nothing would be achieved

The easiest way I can think of demonstrating the action of this phenomena to those that do not understand how a CV slide works is to take a bicycle pump and invert it so the outlet is facing up and the handle is facing down . Push the pump handle all the way in and then close off the outlet with your finger.

The handle does not fall ............it stays where it is despite its weight pulling it downwards due to gravity.
Why is this ?

Now keeping a finger on the outlet pull down hard on the pump handle and see what happens ......

It resists your pull and when released jumps straight back up to its former position.
Why did it do this.?

When you pull on the bicycle handle what is it that resists the combined energy of the weight of the handle and the energy of your pull ?

See its already got far too complicated for most folk to maintain any interest ....suffice it to say that vacuum is involved . The slide is held up by the air seal created in the void above the diaphram. The weight of the slide is potential energy pulling down on the air pressure in the sealed void above the diaphram.

If the void was hermatically sealed the slide will stay up indefinitely because there is insufficient force applied by the weight of the slide to overcome the air pressure in the void. If you were to apply more force on pulling the slide down it would begin to create a vacuum in the void because something has to take the place of the extra space created in the void when the slide drops . that something would be a vacuum.
So very simply put that is how the carb slide (and pump handle ) stays up and how vacuum is involved in the process although in hindsight vacuum is probably not the best word to choose to explain this process.

To further complicate things all slides have a tiny hole drilled vertically through the body which forms an air jet. This hole allows a small amount of air to transfer from the void above the diaphram to atmosphere and thereby allows the slide to rise at a pre determined regulated rate due to the vacuum created by the engine .
Tuners vary the diameter of the hole in the slide to vary the rate of lift of the slide to effect throttle response characteristics.

This is why the slide should fall very slowly in a set amount of time provided that the diaphram is perfectly sealed.
if there is any sudden drop of the slide that indicates that there is an air leak somewhere or an additional air source is available to the void above the diaphram .

 

[Sci85]

@peanut is right. I retested the diaphragms and got them to drop equally from top to bottom but it took a few tries. I guess I wasn't getting that crescent shaped hole covered properly initially. It appears you really have to seal that are good with your thumb and it's a little but tricky.

I went ahead and pulled the head off because of the air pressure test showing air getting past 3 of the valves. I'm going to re-lap them and hopefully that will seal them up properly.

If i pour lacquer thinner in the chamber and none leaks past the valves, is that a valid test? Or do you really need air pressure to test them sealing properly?

 

[peanut]

@peanut is right. I retested the diaphragms and got them to drop equally from top to bottom but it took a few tries. I guess I wasn't getting that crescent shaped hole covered properly initially. It appears you really have to seal that are good with your thumb and it's a little but tricky.

I went ahead and pulled the head off because of the air pressure test showing air getting past 3 of the valves. I'm going to re-lap them and hopefully that will seal them up properly.

If i pour lacquer thinner in the chamber and none leaks past the valves, is that a valid test? Or do you really need air pressure to test them sealing properly?

thats an interesting idea ...but I doubt your engine is going to need that level of precision unless its a complete top end blueprint rebuild .

Just use both the course and fine grinding paste and maybe a portable drill at slow speed and you'll soon have those seats nice and matt grey. . Lift the valve every 5 seconds or so and turn it 90 degrees and occasionally push a little fresh grinding paste under the valve seat. When all the valve and valve seat surfaces have the same matt grey colour all over then they are perfectly lapped and will seal well.
Apologies if I'm teaching my G Mother to suck eggs

 

[Sci85]

Alright, I'll relap. I can't recall if I did both the course and fine. I think I may have done just the fine paste originally. I will check the head for flatness (I never did that originally) and then put a fresh head gasket on and hopefully my high idle problem will be solved. We will see...

It was a good thing though to go through this. When I pulled the clutch cover, I noticed some aluminum bits under the starter gear. Uh oh. So i need to pull that and check everything out. The outer gear is a little wiggly which doesn't seem good. I might have missed something going back together. I also found a thread that talks about bending that special spring thingy which I didn't do. Alas, I should do more research before buttoning everything up moving forward. Thank goodness for this forum and you guys' knowledge.

 

[grizld1]

Don't feel bad, Sci. Except for a few liars, anybody here will tell you he had to do most of his learning the hard way.

 

[Sci85]

Don't feel bad, Sci. Except for a few liars, anybody here will tell you he had to do most of his learning the hard way.

Thanks Griz. Sure would be a lot easier if we just knew everything. And cheaper too haha...

Now to go check those butterfly's and throttle shafts one more time...

 

[Sci85]

Alright, so I got the engine back out. Pulled the head and re-lapped the valves properly this time. Also, fixed the starter gear. Now need to change out the left side main seal. Man, so many little things...

I also did some additional exploration around the carbs. I got it dark in the shop and used a backlight and sure enough, something didn't look right. The backlight was not evenly distributed around the butterfly's. So i pulled the screws out and realigned the butterfly's and did the snap thing. Now they finally seated in the carb body and shaft properly. I also noticed that the left carb's buttefly would not fully shut even with the idle set screw backed completely out. Seems like the sync paddle on the left carb (right carb in the pic) got bent or something and nothing I did would get it to close completely in it's natural state. I ended up putting a small copper washer under the sync shaft and that fixed it. So the carbs now close properly and the buttefly's operate in sync. I think this might have been the ultimate issue but that will have to wait until i get the engine back together to find out for sure. Thanks guys, this has been a very beneficial redo. I missed too many things the first time around...

 

[grizld1]

Haven't seen that one before--good work, Sci!

 

[peanut]

well spotted Sci ......
One of the hazards of replacing diaphrams on round slides for the unwary I guess. Its certainly another obscure fault for us all to add to the troubleshoot list, for future reference .
Glad that the videos turned out to be helpful re the butterflys

 

[kshansen]

Hate to make a fool out of myself but would not be the first time.

I'm having a problem understanding what that copper washer is doing to help close the butterfly. Seems it will only restrict how far the spring loaded pin will move when the screw that adjust the synchronizer, if that's what you call that part, wouoldn't just turning the screw in a turn or to put things in the same position?

Or am I looking at the wrong place in the picture?

 

[5twins]

That copper washer isn't doing anything, lol. I think the real problem was improperly installed and centered butterfly plates, which has now been remedied. If the right plate was closed all the way but the left one wasn't, that simply indicates the sync is off. Playing with the sync adjusting screw should fix that. Here's a pic of some freshly bench synced 34s. The spring-loaded pin hangs out the bottom of the bracket on it's own, no need for a shim washer, because the setting on the adjusting screw on top is pushing it out like that .....

 

[peanut]

actually it is doing something and Sci85 has understood and corrected the problem in exactly the same way that I did with my BS38s with the same issue.

When the plastic throttle shaft bushes and spacers wear they allow the throttle shafts to float from side to side slightly.

The throttle springs can apply force from one side of the throttle shaft forcing the butterfly to one side of the ventui so that it rubs to one side of the wall . This friction can prevent the butterflys from fully returning to their idle stops leaving a hanging slow to return to idle issue.

Sci85 's careful study of how the carbs work has given him a rare insight into the cause of the issue and he has found an effective solution.
I purchased a spare used carb and used the plastic bush which I machined to fit my throttle shaft exactly how it would have been when new which cured my issue.

 

[kshansen]

actually it is doing something and Sci85 has understood and corrected the problem in exactly the same way that I did with my BS38s with the same issue.

When the plastic throttle shaft bushes and spacers wear they allow the throttle shafts to float from side to side slightly.

The throttle springs can apply force from one side of the throttle shaft forcing the butterfly to one side of the ventui so that it rubs to one side of the wall . This friction can prevent the butterflys from fully returning to their idle stops leaving a hanging slow to return to idle issue.

Sci85 's careful study of how the carbs work has given him a rare insight into the cause of the issue and he has found an effective solution.
I purchased a spare used carb and used the plastic bush which I machined to fit my throttle shaft exactly how it would have been when new which cured my issue.

Okay then if I understand what you are saying he is using the copper washer to shim the spring loaded pin so that the adjusting screw is just touching the other sides lever and allowing the two throttle shafts to float free from each other.

So now the adjustment for setting the synchronization between the two carbs is done by shimming the pin and not using the adjustment of the screw? The screw is now only controlling the free play between the two carbs throttle shafts, letting them float axially to each other.

 

[5twins]

Once you install the return spring on the butterfly shaft, it is pulled over to the spring side by the spring. That's why you don't tighten down the plates or try to center them until after you put the springs on.

As far as that little copper washer on the sync adjuster goes, it has nothing to do with side to side movement of the shafts. It won't change it in any way. We're not even talking about side to side movement of the shafts, lol.

 

[Sci85]

Hmm, so I went back and looked very carefully, removing the washer and trying it all again and....5twins is right again! I had the idle adjuster all the way out but not the sync screw which was the plates off. Sheez. After all that, it was indeed the butterfly valves not properly seated in the bore. All looks good now. Got the cylinder and head back on and should have her in the bike tomorrow. Will report back after I fire it up.

 

[Bob Kelly III]

there ya go ! experience is by far the best teacher ! Now you know ! good work !
.....
Bob.........

 

[peanut]

Okay then if I understand what you are saying he is using the copper washer to shim the spring loaded pin so that the adjusting screw is just touching the other sides lever and allowing the two throttle shafts to float free from each other.

So now the adjustment for setting the synchronization between the two carbs is done by shimming the pin and not using the adjustment of the screw? The screw is now only controlling the free play between the two carbs throttle shafts, letting them float axially to each other.

NO....that is not what I was saying at all.

If you look at where the red arrrow points you'll see where the washer has been put. It has nothing whatsoever to do with the stops

My point had to do with the lateral float of the throttle shafts not sync adjusment.
The white plastic tubes are shims that control the amount of sideways movement of the throttle shafts so that the butterflys stay properly centred in the venturi.

When these shims wear over time they allow a certain degree of side to side (lateral) movement of the throttle slides.This allows the butterfly discs to be pressed against one side of the venturi and rub either preventing the butterflys returning to their closed positions or slowing the return to idle . Because they get stuck at various degrees of opening this produces a high or hanging idle.

Watch the video it will explain it far better than a few lines of text can.

Anyway its academic now as it appears that Sci hadn't synched the butterflys to close together properly which was the problem not the butterflys rubbing on the venturis as it was with mine.
It is difficult and very fiddly adjusting the sync easily once the carbs are fitted so I took an extended sync screw off of a XS750 carb and fitted that so that the adjusting screw is accessible below the carb .
The screw is a direct screw-in replacement and its a very worthwhile modification.
The screw in the image is one fitted to a set of GPZ500 carbs

 

[kshansen]

Sorry but I was thinking that the copper washer being refereed to was the one that appears to be at the location with the arrow marked "B" in the above picture.

 

[Sci85]

Actually both are kinda right. The little washer did indeed fix my problem but it was covering up a problem that was caused by the butterfly's not being set in the shaft properly rather than the spacer being worn. So this could be a fix for that. But in my case, once I corrected the butterfly I didn't need the spacer anymore. Regardless, I'm just glad it worked. But what is really interesting is that I had the high idle issues before I replaced the shaft seals. So, it will be interesting to see if this fixes the issue or not. It will have to wait until I get the engine back in to find out for sure.

 

[peanut]

View attachment 100400
Sorry but I was thinking that the copper washer being refereed to was the one that appears to be at the location with the arrow marked "B" in the above picture.

Ah.... i see what you mean . apologies kshansen I hadn't seen that image for some reason..... I was looking at a different image .
Sounds like Sci has got the situation well in hand so i'll be watching to see the outcome with interest