Almost got her tuned juuuuust right

mrriggs said:
When I had the BS38's on my bike years ago there was a flat spot that no jetting would cure. The issue was the jet needles. I tried a few different needles that I had from other bikes but none of them were right either.

With the CVK34 carbs I was having similar issues until I replaced the dual-taper needles with straight taper needles from an "offroad" carb. The straight taper needles are available in all different tapers, diameters and lengths so I was able to dial it in perfectly.

I'm not sure if there are any straight taper needles available to fit the BS38 carbs but it's worth checking into. Grizld1 was right when he said you are going to have to open your wallet to do it right. I spent hundreds of dollars on jets and needles to get a $50 set of carbs working well.
Click to expand...

Sounds good. I've experimented with adjusting the needle clip setting, but nothing seemed right. I've not gone done the road in replacing the needle itself. I have a bunch of jets coming in this week. I'm going to play around and see if I can get the right size pilot jet to do the job.

If not, maybe moving to a different needle might be the solution - or fuel injection :). As I said, the bike runs fine and idles perfectly, there are just nasty flat parts out of first, second and third, but once RPMs get up over 4500, the bike pulls nicely.

I'll keep the group posted.

Thanks
 
vincejames, have you looked at the pilot circuit yet? What I was describing is better explained in post #14 of this thread.
 
vincejames said:
The issues continues and this time right off idle in first gear! Cracked the throttle open as I left a red light and the bike was just flat, no power. Once it got going a little, boom, it's like the bike got hit with paddles ( ...
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This has nothing whatsoever to do with your main jets ;)

This is what is called transition . During the transition from idle to 1/4 throttle on a CV type carb You are moving from the Pilot jet to the slide cutaway .
This is the key feature of a CV carb . Flat slide non CV carbs have to have a fuel pump or other means to inject extra fuel when you rip the throttle open to overcome the loss in vacuum .
I expect if you roll the throttle on slowly the engine will probably pick up better.

You need to be looking at your diaphrams , they must be rising too quickly allowing a sudden drop in vacuum which prevents the engine pulling the extra fuel needed . Make sure that the vents are not blocked by the filters at the venturi mouth and make sure that your diaphrams are air tight by lifting the slides and putting a thumb over the vent . The slides should stay up or fall very slowly.
 
peanut said:
This has nothing whatsoever to do with your main jets ;)

This is what is called transition . During the transition from idle to 1/4 throttle on a CV type carb You are moving from the Pilot jet to the slide cutaway .
This is the key feature of a CV carb . Flat slide non CV carbs have to have a fuel pump or other means to inject extra fuel when you rip the throttle open to overcome the loss in vacuum .
I expect if you roll the throttle on slowly the engine will probably pick up better.

You need to be looking at your diaphragms , they must be rising too quickly allowing a sudden drop in vacuum which prevents the engine pulling the extra fuel needed . Make sure that the vents are not blocked by the filters at the venturi mouth and make sure that your diaphrams are air tight by lifting the slides and putting a thumb over the vent . The slides should stay up or fall very slowly.
Click to expand...

Hey Peanut. So I think you make a really good point here. I actually took some advice from what of your posts a while back and replaced my diaphragms before I began this tuning process. They are from JBM. I took a video for you. I would say that the slides are settling back in to position pretty perfect :)..

 
vincejames said:
Hey Peanut. So I think you make a really good point here. I actually took some advice from what of your posts a while back and replaced my diaphragms before I began this tuning process. They are from JBM. I took a video for you. I would say that the slides are settling back in to position pretty perfect :)..

Click to expand...

Now that I take a look at it though, that sudden drop from 8 seconds to 10 seconds is a little concerning.

Thanks - Vince
 
Looks like I ordered one jet each from Jets are Us (I blame this stupidity on ordering stuff off your phone...) :banghead::banghead:

I'll be making some changes during the week. I'll be sure to change one thing at a time so that I can figure out exactly what has caused my issues.
 
there is your problem right there !
great vid by the way.

Your diaphrams are not sealing somewhere. I had a similar problem and couldn't afford new diaphrams so I smeared vasoline around the edge of the diaphrams where they sit in the lip and a little on the covers and now they are perfect.

The reason that your engine is bogging or going flat off idle is because when you twist the throttle open your slides are leaping up the slide well which is creating a huge immediate low pressure in the venturi. This means your engine cannot draw the extra fuel it needs .

You must hold your thumb or finger over the top vents when the slides are raised up before testing the diaphrams for leaks. (thanks 5Twins)
If your slides do not stay up then you have a leak somewhere either round the edge or at the centre
 
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I don't think taping over the slot is good enough. Block it firmly with a finger or your thumb. Do it AFTER you lift the slide. The way you're doing it in your video, you're creating pressure above the slide when you lift it. That may be why it's dropping quick. That pressure is forcing it down.
 
peanut said:
there is your problem right there !
great vid by the way.

Your diaphrams are not sealing somewhere. I had a similar problem and couldn't afford new diaphrams so I smeared vasoline around the edge of the diaphrams where they sit in the lip and a little on the covers and now they are perfect.

The reason that your engine is bogging or going flat off idle is because when you twist the throttle open your slides are leaping up the slide well which is creating a huge immediate low pressure in the venturi. This means your engine cannot draw the extra fuel it needs .

You must hold your thumb or finger over the top vents when testing the diaphrams for leaks.
If your slides do not stay up then you have a leak somewhere either round the edge or at the centre
Click to expand...

Do you have a video of this showing how this properly works?
 
5twins said:
I don't think taping over the slot is good enough. Block it firmly with a finger or your thumb. Do it AFTER you lift the slide. The way you're doing it in your video, you're creating pressure above the slide when you lift it. That may be why it's dropping quick. That pressure is forcing it down.
Click to expand...

So let me try and understand this correctly, and Peanut, please jump in. The proper way to test is to:

1. lift the slide up
2. while the slide is lifted, block the vents with your thumb then let the slide go - if it drops there is a leak if it sticks, it's working properly?

sooo confused...
 
exactly right.
The vent at the top allows air at atmospheric pressure to the top of the diaphrams. If the diaphrams are properly sealed then that pressure above the diaphrams slows down the rate of accent of the slides, therebye creating a high pressure in the venturi which allows the engine to draw the extra fuel needed from the needle jet ...simples.

When you put your thumb over the vent it blocks off the air to the top of the diaphrams which prevents the slides falling due to the vacuum created. If the slides drop you have a leak on the diaphrams.

Let us see a picture of the top of your diaphrams with the caps off as they sit in the carbs. you may not have fitted your new diaphrams round the right way. There is a front and back
 
The slides should drop, just very slowly. Yours are falling too quick. But like I said, that may be because of the way you tested.
 
5twins said:
The slides should drop, just very slowly. Yours are falling too quick. But like I said, that may be because of the way you tested.
Click to expand...
its a good point but I don't think you'd be able to lift the slides all the way up with the vent blocked off ...unless you had a diaphram leak.
I've edited my previous post to make it a little clearer thanks . :rolleyes:
I've just twigged what the black strip is at the top doh ! Vince has used some electrical tape to block the vent before lifting the slides.
 
5twins said:
I don't think taping over the slot is good enough. Block it firmly with a finger or your thumb. Do it AFTER you lift the slide. The way you're doing it in your video, you're creating pressure above the slide when you lift it. That may be why it's dropping quick. That pressure is forcing it down.
Click to expand...
Yes, you can see the change from pressure to vacuum in the video.
Peanut air compresses and you have a rubber diaphragm so why couldn't you lift the slide all the way up?
 
peanut said:
its a good point but I don't think you'd be able to lift the slides all the way up with the vent blocked off ...unless you had a diaphram leak.
I've edited my previous post to make it a little clearer thanks . :rolleyes:
I've just twigged what the black strip is at the top doh ! Vince has used some electrical tape to block the vent before lifting the slides.
Click to expand...

Thank you for the edit. Makes a lot more sense now. Well, can't take a picture as I am currently at work. So let me just make sure I understand this correctly.

The proper way to test my brand new diaphrams that I installed from JDM is to lift the slide with my finger, while it is lifted up, block the vent hole. If it drops, then there is a leak or the seal is not perfect. If it stays elevated in the air and does not drop (or should it drop slowly?) then there is nothing wrong with the seal and it is working perfectly. This would make sense.

I read some older posts about the JBM diaphragms and there being issues with them not sealing correctly and in some cases the slides would twist out of place. It's hard for me to understand HOW I'm going to get the diaphragms to stay put. When the diaphragms caps are off you can easily pull the diaphragms off of the seal as you go in a circle around the diaphragm body. Should I be super gluing them or something? I don't see Vaseline working that well....
 
Use finger nail polish/super glue at the slide and diaphragms junction. Make sure you have the slide oriented before applying.
Use grease/vaseline at the where the diaphragm seals to the body.
 

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