Mikuni 38mm roundslide issues

jgimar

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I have a ‘77 650 and i am running a 2 into 1 manifold sporting a 38mm round slide. I have outfitted it with a hugh’s pma and a pamco ignition. Thing cranks right up but i’m having trouble getting it to run smooth. Currently working through any pilot jet issues, following suggestions that if you bump the throttle and it fails to settle down, stays at hi idle, pilot may be to large. So we have changed it twice since it fails to back down, but then we noticed that if you shake the carb briskly it will come back down to normal idle. Thinking the slide was sticking, we checked that to no avail, changed out the slide with another one with same result. Throttle cable seems to run smoothly. Kinda at a loss. Any advice appreciated.
 
Hey - so for what it’s worth I think the 38 might be a little big. But anyway, I had the same issue. I couldn’t figure it out by jetting, kept polishing the slide thinking it was sticking, finally tried two springs on the slide and it worked. The signal is so strong on the slide that it was stopping it from moving. Anyway, I’m sure that will fix your problem
 
Love your answer, except i never can get a straight answer as to why it’s too big. No offense but when i search for logical explanations to it’s size as compared to others i get dead end responses like throw it in the woods to it’s old an wore out. At least you only said might be a little big which is the closest valid response, but in reference to size my ignorant question is, why? If with proper jetting, how would it differ from a 36 or a 34. I’m not a guru on carbs by any means so i always look for someone smarter. As for your other comments, de ja vue! We have polished on that slide hoping to solve the problem, our next move is to do as you did, add an extra spring. Glad to hear i’m not nuts. Get back to you with further results. Thank you!
 
So, initially I thought going bigger to compensate for going single carb makes sense, but only one cylinder is drawing at a time so it really doesn’t matter. 34 is the stock size that came on these. I’m sure you can get the 38 dialed in but it may be a little more frustrating. If your engine is stock, I think the 34’s perform the best. 36’s give a little more in the high rpms, but you aren’t usually up there enough, but will perform better on a 750. The carb size will somewhat correspond to the engine size. So you’ll end up jetting that carb down.
 
OK, jgimar, you asked the question so here's the simple answer. Vacuum is required to properly atomize fuel and move it to the port. After a certain point a carb throat (venturi) becomes too big to breathe for the motor it's hung on. Some guys think a couple or three millimeters of venturi size will be critical on a street motor. Experience would teach them otherwise. Still, there are limits.

But wait, there's more. While Yamaha used both BS38 (through 1979) and BS34 (1980 and after) carbs on the XS650, these are vacuum diaphragm carburetors, and the throttle shaft and plate reduce the free area of the venturi considerably. A VM38 carb with cable lift slide is much bigger in effective area than an OE BS38.

You're going to face a problem. Even with a VM34, a typical 2-1 manifold with its long close radius tracts often causes trouble at idle and in low speed operation; fuel condenses out of the mixture before it reaches the engine. The loss of intake velocity caused by a big VM38 will make that problem worse.
 
I've got two 650's with 2-1 intakes. One a wise guy tracker and the other from Counter Balance. Both have 36mm Mikuni VM's and had the hanging idle problem. Doubled up the return springs and fixed that. Both start,idle, and run great with no glitches.
 
Venturi size is a compromise; to get good torque at low engine revs you need a small diameter to maintain air speed through the venturi. As the revs increase there is a limit on how much air can get through and eventually the motor will run out of puff. To maximise torque at high engine revs (this = power as torque x revs = power) a bigger venturi is required. If you want gains at one end you'll lose it somewhere else.
Yamaha sidestepped this problem on the V Max with V Boost. The carbs gave good torque at low and mid engines speed but were not sufficient to give good top end power. To overcome this they inserted a butterfly between two carbs that opened as the revs rise thus allowing sufficient mixture to satisfy the motors requirements.

As Pete says, size matters (not that I would admit that to my wife :redface:).
The 38's have a 41% larger throat area than the 32's and are 24% larger than the 34's.
Why is that important?

You need to consider how a venturi works.
As the throat narrows the air has to speed up to get through and this reduces the pressure.
It is this lowering of pressure that draws the fuel through the jets.
The faster you draw the fuel through the better the atomisation and the better that is the better the mixture burns and the better the mixture burns the better the bang.
On that basis if you have a bigger venturi there is a smaller pressure drop which results in poorer atomisation = poor running.

Now, to speculate, your issue may not be how much fuel you are getting but how it is being atomised.
If there are big droplets and these are pooling in the inlet they will evaporate and add to what is passing into the cylinder causing the idle to hang.
 
So, initially I thought going bigger to compensate for going single carb makes sense, but only one cylinder is drawing at a time so it really doesn’t matter. 34 is the stock size that came on these. I’m sure you can get the 38 dialed in but it may be a little more frustrating. If your engine is stock, I think the 34’s perform the best. 36’s give a little more in the high rpms, but you aren’t usually up there enough, but will perform better on a 750. The carb size will somewhat correspond to the engine size. So you’ll end up jetting that carb down.
Thank you, thank you! Received several answers and i appreciate all.
 
OK, jgimar, you asked the question so here's the simple answer. Vacuum is required to properly atomize fuel and move it to the port. After a certain point a carb throat (venturi) becomes too big to breathe for the motor it's hung on. Some guys think a couple or three millimeters of venturi size will be critical on a street motor. Experience would teach them otherwise. Still, there are limits.

But wait, there's more. While Yamaha used both BS38 (through 1979) and BS34 (1980 and after) carbs on the XS650, these are vacuum diaphragm carburetors, and the throttle shaft and plate reduce the free area of the venturi considerably. A VM38 carb with cable lift slide is much bigger in effective area than an OE BS38.

You're going to face a problem. Even with a VM34, a typical 2-1 manifold with its long close radius tracts often causes trouble at idle and in low speed operation; fuel condenses out of the mixture before it reaches the engine. The loss of intake velocity caused by a big VM38 will make that problem worse.
Thank you very much. Finally am getting what i need to hear. Thank you again
 
I've got two 650's with 2-1 intakes. One a wise guy tracker and the other from Counter Balance. Both have 36mm Mikuni VM's and had the hanging idle problem. Doubled up the return springs and fixed that. Both start,idle, and run great with no glitches.
Thank you very much . Very helpful.
 
Venturi size is a compromise; to get good torque at low engine revs you need a small diameter to maintain air speed through the venturi. As the revs increase there is a limit on how much air can get through and eventually the motor will run out of puff. To maximise torque at high engine revs (this = power as torque x revs = power) a bigger venturi is required. If you want gains at one end you'll lose it somewhere else.
Yamaha sidestepped this problem on the V Max with V Boost. The carbs gave good torque at low and mid engines speed but were not sufficient to give good top end power. To overcome this they inserted a butterfly between two carbs that opened as the revs rise thus allowing sufficient mixture to satisfy the motors requirements.

As Pete says, size matters (not that I would admit that to my wife :redface:).
The 38's have a 41% larger throat area than the 32's and are 24% larger than the 34's.
Why is that important?

You need to consider how a venturi works.
As the throat narrows the air has to speed up to get through and this reduces the pressure.
It is this lowering of pressure that draws the fuel through the jets.
The faster you draw the fuel through the better the atomisation and the better that is the better the mixture burns and the better the mixture burns the better the bang.
On that basis if you have a bigger venturi there is a smaller pressure drop which results in poorer atomisation = poor running.

Now, to speculate, your issue may not be how much fuel you are getting but how it is being atomised.
If there are big droplets and these are pooling in the inlet they will evaporate and add to what is passing into the cylinder causing the idle to hang.
Outstanding! After all these great responses, i have somewhere to work from know. Thank you again.
 
Venturi size is a compromise; to get good torque at low engine revs you need a small diameter to maintain air speed through the venturi. As the revs increase there is a limit on how much air can get through and eventually the motor will run out of puff. To maximise torque at high engine revs (this = power as torque x revs = power) a bigger venturi is required. If you want gains at one end you'll lose it somewhere else.
Yamaha sidestepped this problem on the V Max with V Boost. The carbs gave good torque at low and mid engines speed but were not sufficient to give good top end power. To overcome this they inserted a butterfly between two carbs that opened as the revs rise thus allowing sufficient mixture to satisfy the motors requirements.

As Pete says, size matters (not that I would admit that to my wife :redface:).
The 38's have a 41% larger throat area than the 32's and are 24% larger than the 34's.
Why is that important?

You need to consider how a venturi works.
As the throat narrows the air has to speed up to get through and this reduces the pressure.
It is this lowering of pressure that draws the fuel through the jets.
The faster you draw the fuel through the better the atomisation and the better that is the better the mixture burns and the better the mixture burns the better the bang.
On that basis if you have a bigger venturi there is a smaller pressure drop which results in poorer atomisation = poor running.

Now, to speculate, your issue may not be how much fuel you are getting but how it is being atomised.
If there are big droplets and these are pooling in the inlet they will evaporate and add to what is passing into the cylinder causing the idle to hang.
Max midnite, let me ply your brain? I have two of these 38’s, and i know they are big, but would i solve anything if i ran the two of them in a more normal 2 into 2? Eliminate the long 2 into 1 manifold? Reduce the distance gas and air have to cover?
 
Max midnite, let me ply your brain? I have two of these 38’s, and i know they are big, but would i solve anything if i ran the two of them in a more normal 2 into 2? Eliminate the long 2 into 1 manifold? Reduce the distance gas and air have to cover?
Oops, forgot one more thing. I’ve noticed these rounslides advertised for 2 stroke and 4 stroke. Do they use the same carbs or is there a difference, just curious, to look at them i notice little to no differences.
 
2-stroke motors require richer jetting. VMs come from Mikuni with very rich jets. For our application the only piece of brass that doesn't absolutely have to be changed is the slide, and some change that as well. Plenty has been posted on baselines, seek and you will find; start by clicking on the Tech button at the top of the page. The 38s will need richer jetting than the more commonly used 34 and 36 mm. VMs. gggGary dialed in a pair of VM38s on one of his rides. Maybe he'll jump in here.
 
2-stroke motors require richer jetting. VMs come from Mikuni with very rich jets. For our application the only piece of brass that doesn't absolutely have to be changed is the slide, and some change that as well. Plenty has been posted on baselines, seek and you will find; start by clicking on the Tech button at the top of the page. The 38s will need richer jetting than the more commonly used 34 and 36 mm. VMs. gggGary dialed in a pair of VM38s on one of his rides. Maybe he'll jump in here.
Thanks to you again!!!!!
 
2-stroke motors require richer jetting. VMs come from Mikuni with very rich jets. For our application the only piece of brass that doesn't absolutely have to be changed is the slide, and some change that as well. Plenty has been posted on baselines, seek and you will find; start by clicking on the Tech button at the top of the page. The 38s will need richer jetting than the more commonly used 34 and 36 mm. VMs. gggGary dialed in a pair of VM38s on one of his rides. Maybe he'll jump in here.
i've been reading everything you guys can print. i'll figure it out some day, but one thing i haven't seen, has anyone entertained the idea of modifying one to a double venturi? is it even possible or a bone head idea? thanks again.
 
Max midnite, let me ply your brain? I have two of these 38’s, and i know they are big, but would i solve anything if i ran the two of them in a more normal 2 into 2? Eliminate the long 2 into 1 manifold? Reduce the distance gas and air have to cover?

IMO no plus you would also introduce the added complication of having to balance them.

Inlet length is important as it will affect where you get maximum torque.
Think if it like (say) an organ pipe. It is the length and diameter that mainly determine which note you get and which is caused by sound waves moving at a specific frequency.
At maximum amplitude the air pressure will be higher than at minimum.
These pulses (used in concert with the same effect from the exhaust) can be used to ensure the maximum charge goes into the cylinder.
Now the bad news, as the inlet length is fixed the 'max' effect only happens over a relatively small number of revs. By altering the length of the inlet you can move this up or down the rev range.
On multi cylinder bike manufacturers have used inlets of unequal effect to spread the torque over a wider range but at the expense of a lower value and some use variable length inlets to get the same effect.

To answer your question, I don't know. I suspect it won't overcome the fuelling issues but it could improve ridability. The only way to establish this is by experimentation.
 
IMO no plus you would also introduce the added complication of having to balance them.

Inlet length is important as it will affect where you get maximum torque.
Think if it like (say) an organ pipe. It is the length and diameter that mainly determine which note you get and which is caused by sound waves moving at a specific frequency.
At maximum amplitude the air pressure will be higher than at minimum.
These pulses (used in concert with the same effect from the exhaust) can be used to ensure the maximum charge goes into the cylinder.
Now the bad news, as the inlet length is fixed the 'max' effect only happens over a relatively small number of revs. By altering the length of the inlet you can move this up or down the rev range.
On multi cylinder bike manufacturers have used inlets of unequal effect to spread the torque over a wider range but at the expense of a lower value and some use variable length inlets to get the same effect.

To answer your question, I don't know. I suspect it won't overcome the fuelling issues but it could improve ridability. The only way to establish this is by experimentation.
Once again, indebted to you!
 
Not trying to tell you what to do with your bike, but you should be aware that bigger carburetors are touchier to tune, and you're right at the limit. Apart from gggGary's experiment, the only VM38s I've seen or heard of on XS650s have been on full race 750 motors set up for mile flat track, fast road race tracks, and drag racing. Low rpm operation will definitely suffer, no matter how carefully you tune. You might think about looking for a buyer or for someone who'll swap for a VM34.
 
It's been a while, the bike wears VM34's now but the griz' call is how I remember it. An iron fist in a velvet glove, a pussycat til 5K RPM then a wild rush to redline (and beyond:sick:).
 
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