Yam_Tech314's official build thread

[5twins]

Yes, Vessel makes very nice tools. I like their 2" power bits and these are what I use most ......

For really tight stuff like old jets and these butterfly plate screws, I use a T-handle bit holder. You can exert the needed downward pressure quite easily with this and also exert tremendous turning torque .....

 

[G_YamTech_314]

This will allow the butterfly plates to close up completely if they're in sync. If they're not then one plate will be closed but the other will still be open a little. Hold the carbs up to a good light to "see" this

Im rebuilding my carbs tonight and got my butterfly valves back in no problem. I'm noticing that when 100% closed there's still just a tad bit of light that shows through. Like a REALLY small amount of light showing along the sides of the circle. From what I can see, top and bottom don't have light coming out of them. I assume there needs to be a tad bit of a gap to allow slight side to side movement of the shaft? This picture makes the gap between the carb body and the butterfly look massive. It's likely a human hair width or less.

 

[Jim]

That looks pretty excessive to me.

 

[G_YamTech_314]

That looks pretty excessive to me.

Like I said, it looks way worse in the picture somehow. I have a SMART phone. It uses "night mode" when things are dark and fills in voids using AI. It's not as big a gap as it seems. I'll try to take a better shot of it here quick

 

[G_YamTech_314]

It's really hard to capture accurately. For comparison purposes here's what it looks like from afar... Not nearly as much light as it looks like from inside the bore.

 

[5twins]

OK, when you re-installed the plates, did you "center" them before fully tightening their mounting screws? When you install the return springs, the shaft and plate are pulled towards them. That can bind the plate against the side of the bore. To avoid this, you need to "center" the plates when installing them. With the mounting screws still loose, open the plates completely and let the return springs snap them shut. Do this several times and then you can tighten the screws. In both your pics, the plate looks off center a bit.

 

[G_YamTech_314]

I did this AFTER sending the 2nd pic. I was struggling to get them to center and realized they'd like home best when snapped shut on their own. I'm glad I caught onto it and kinda happy I figured it out myself before reading your reply lol. The entire assembly is working really well now that the mating bracket is installed. The carbs are back in the bike and ready to be tested in the next 10 minutes. Will be sure to update on whether it runs on choke or not. Will also update you all on how it sounds when warmed up and whether or not my little hiccup is gone. I F$@#ING hope so!!!

 

[G_YamTech_314]

Bike still stalls when choke is turned on but I noticed if I "blip" the choke quickly the idle increases for a moment before returning to normal... I havent ran it enough to know if it's going to stutter at cruising rpm's yet or how it'll act under load. I wanna figure out why it don't idle on choke.

For reference: my pilots are 27.5's and I installed new seals in the throttle shafts, as well as new rings around the needle jets. I also changed my float height to 25mm as recommended. I'm wondering now what else may need tweaked.

EDIT: I turned the idle mixture screws 3 turns out and tried choke. Still nothing.

the bike still stutters hard at 2500 rpm's. But idles like a dream and has fantastic 1/4 throttle response.

Bike won't run at all at 1.5 turns out. Can get it to fire off but not idle. This tells me that 3 turns out is at least in the right direction.

I'm still trying to fully understand the science behind float height changing the mixture of the fuel and air... Would changing my float height to 25mm make it so I once again have to raise my needle back up to the center setting...? Or is 27.5 on the pilot still not a big enough jump... Hmmm.


Also, I heard that if a mixture screw is on the intake side of the carbs you're adding air when turning out. But if it's on the engine side then you're adding fuel... Is this a good rule of thumb or total b.s.?

 

[5twins]

That's a very good "rule of thumb" and applies to like 99% of the carbs out there. These 650s have a true "mixture" screw so turning it out makes the idle circuit richer. But it's only for fine tuning and only controls about 25% of the mix coming from the pilot jet. There are 4 tiny holes that let the air/fuel mix from the pilot jet into the main bore. Three are clustered together at the top of the main bore about where the butterfly plate closes and the fourth is off to the side. This fourth hole comes from the mix screw and you can "regulate" it's flow. On the other three, the only way to change the mix strength they put out is to change the pilot jet size .....

When you clean these carbs, one of the things you check is that all 4 of these little holes are clear. I start with something mild like WD40, blowing it through from the pilot inlet on the bottom of the carb's main body. I only resort to carb cleaner if I find any of the holes plugged .....

First I block off the mix screw hole on the outside of the carb and it's inlet into the bore, and blow the WD40 through. This tests the three little holes. Then I block them off along with the mix screw hole on the outside of the carb and blow more WD40 through. This tests the mix screw inlet. Then you can "back flush" the circuit by blowing WD40 in from the mix screw hole on the outside of the carb while blocking off all 4 of the little holes in the bore. It should come out the pilot inlet on the bottom of the carb body. You can alternate which holes the WD40 comes out by blocking all the others. For instance, block the pilot inlet and the mix screw outlet, and the WD40 will come out the three little holes.

You may need to revisit cleaning the pilot circuit because you shouldn't need 3 turns on your mix screws. It sounds like they're not flowing enough. On the several sets of '76-'77 carbs I've set up, they all only needed around the spec (1.5 turns out).

When I jet a set of carbs, I start on top with the mains because they influence all the other circuits and settings. I increase the mains one size at a time until I get stumbling in the upper midrange, in the midrange to main transition area. This happens eventually because of that circuit overlap I mentioned to you. Then I'll lean the needles a step to "fix" the upper midrange "stumbles". The stock pilot is usually fine until I lean the needles. Then a larger pilot may be needed because leaning the needle often creates a flat spot in the idle to midrange transition area. The larger pilot "fixes" that.

So, you seem to be going at this ass-backwards, lol, starting with the pilot jet and needle setting. What size mains are you running?

 

[G_YamTech_314]

That's a very good "rule of thumb" and applies to like 99% of the carbs out there. These 650s have a true "mixture" screw so turning it out makes the idle circuit richer. But it's only for fine tuning and only controls about 25% of the mix coming from the pilot jet. There are 4 tiny holes that let the air/fuel mix from the pilot jet into the main bore. Three are clustered together at the top of the main bore about where the butterfly plate closes and the fourth is off to the side. This fourth hole comes from the mix screw and you can "regulate" it's flow. On the other three, the only way to change the mix strength they put out is to change the pilot jet size .....

View attachment 349561

When you clean these carbs, one of the things you check is that all 4 of these little holes are clear. I start with something mild like WD40, blowing it through from the pilot inlet on the bottom of the carb's main body. I only resort to carb cleaner if I find any of the holes plugged .....

View attachment 349562

First I block off the mix screw hole on the outside of the carb and it's inlet into the bore, and blow the WD40 through. This tests the three little holes. Then I block them off along with the mix screw hole on the outside of the carb and blow more WD40 through. This tests the mix screw inlet. Then you can "back flush" the circuit by blowing WD40 in from the mix screw hole on the outside of the carb while blocking off all 4 of the little holes in the bore. It should come out the pilot inlet on the bottom of the carb body. You can alternate which holes the WD40 comes out by blocking all the others. For instance, block the pilot inlet and the mix screw outlet, and the WD40 will come out the three little holes.

You may need to revisit cleaning the pilot circuit because you shouldn't need 3 turns on your mix screws. It sounds like they're not flowing enough. On the several sets of '76-'77 carbs I've set up, they all only needed around the spec (1.5 turns out).

When I jet a set of carbs, I start on top with the mains because they influence all the other circuits and settings. I increase the mains one size at a time until I get stumbling in the upper midrange, in the midrange to main transition area. This happens eventually because of that circuit overlap I mentioned to you. Then I'll lean the needles a step to "fix" the upper midrange "stumbles". The stock pilot is usually fine until I lean the needles. Then a larger pilot may be needed because leaning the needle often creates a flat spot in the idle to midrange transition area. The larger pilot "fixes" that.

So, you seem to be going at this ass-backwards, lol, starting with the pilot jet and needle setting. What size mains are you running?

This is a lot of useful info. I'm gonna re-read everything you've posted to this thread that is carb related and start on a game plan for them first thing tomorrow morning.

I'm almost certain (I need to be better and record this stuff on paper so I can be 100% sure. Hard to go the right direction when I don't know where I'm starting)

I'm running stock size main jets. When I had the needles set to the middle setting it was so rich it was fouling plugs. When I dropped the needles one setting it made the plugs look great. I'm pretty sure you're right about my pilot circuit having a clog in it somewhere...

 

[Wingedwheel]

Make sure there is no side to side movement in that butterfly shaft or it’s not centered.

 

[5twins]

Well, if you were running so rich that you were fouling plugs with stock jets then I'd say the jets were worn, eaten away from sitting in gas varnish for a long time. That would make them bigger and richer than they should be. In particular, the needle jet can be eaten away inside big time. Since that controls the midrange (where you spend most of your time), it has a big effect on plug color.

Years ago I rebuilt some BMW carbs that were all varnished up. They worked fine afterwards but the plugs were black and my gas mileage was horrible (in the low to mid 30s). When I took a closer look at the needle jets, I found they were all eaten away inside, and that made them much bigger and richer. Replacing them fixed the issue and those carbs were fine from then on.

 

[G_YamTech_314]

Your experience is really helpful @5twins. They're coming back apart this morning. I'll be sure to look at things much closer.

 

[G_YamTech_314]

Make sure there is no side to side movement in that butterfly shaft or it’s not centered.

Perhaps the butterfly's wear out over time. There was always side to side play in them even when the original peened screws were in place...

Unless you're referring to side to side play when they're CLOSED. When they're fully open there is play and there always was. When they're closed there is no play whatsoever

 

[G_YamTech_314]

So. My official re-recording of my setup is complete.
Both carbs run identical components.

Main: 122.5
Pilot: 27.5
Needle Jet: Z-8
Needle: 4M1
Float height:25mm
Clip position 1 up from center
Mixture set to 2 full turns out.
UNI POD foam air filters

I have confirmed that fluid flows through the pilot inlet into and out of each of the 4 holes in the venturi of the carbs. So my ports are not clogged.

Both main jets are shiny and free from wear or grit.

Both pilot jets are brand new

The needle jets are old but new o-rings were installed yesterday and the step portion of the jets are symmetrical all around.

No wear seen on the stepped part. Seems to only show any kind of shape or bore diameter difference deep in the jet where the bleed holes are. Pictures below show this.

The 4M1 needles are showing a portion that is shiny at the very top of the tapered portion. See picture attached. I can catch this spot on both needles with a thumbnail and wonder if this is causing me any issues.

 

[5twins]

Well, needle jets do wear out. It's hard to tell by looking at them because the hole through them is so big. You really need a new jet to compare to.

The plugs on my '78 were getting progressively darker over the last few years. Nothing was changed in the bike's set-up so I figured it had to be the carbs. At first I thought maybe the float levels had changed, gotten higher due to wear or just the constant pressure on the float tang bending it a little. But, they checked out fine. So then I looked at the needle jets. I had managed to score some N.O.S. ones off of eBay and when I compared a new one to the old one, it was pretty obvious the old one was worn out ......

So, replacing your needle jets would be the next logical step. You shouldn't need to run the needles leaned a step with a stock main, unless the mains are worn too?

 

[G_YamTech_314]

Well, needle jets do wear out. It's hard to tell by looking at them because the hole through them is so big. You really need a new jet to compare to.

The plugs on my '78 were getting progressively darker over the last few years. Nothing was changed in the bike's set-up so I figured it had to be the carbs. At first I thought maybe the float levels had changed, gotten higher due to wear or just the constant pressure on the float tang bending it a little. But, they checked out fine. So then I looked at the needle jets. I had managed to score some N.O.S. ones off of eBay and when I compared a new one to the old one, it was pretty obvious the old one was worn out ......

View attachment 349609

View attachment 349610

So, replacing your needle jets would be the next logical step. You shouldn't need to run the needles leaned a step with a stock main, unless the mains are worn too?

It's so hard to say without having a ton of carb trouble shooting experience. I took it for a ride today because I needed it. Badly. It's too nice of a day not to ride and it runs good enough to take a few miles here and there. It backfires on decel VERY slightly. So that points to a fuel mixture issue also. I'm going to order new needles AND new needle jets... The next step after this is gonna be cable round slide dellortos or something

Just kidding... That may be in my future, but getting the bike to run right like it was built is part of my educational journey. I just wish it wasn't the same outcome no matter what I try. You're probably right... And if I were more patient I'd wait to ride til I got new parts in. But I'm not that patient.


The head gasket also still weeps. Which is shitty. But it's another lesson I taught myself about building bikes... Do it right, and triple check all my work before moving forward...

Onward to the next teardown LOL

 

[Jim]

Make sure you get genuine Mikuni brass.

 

[G_YamTech_314]

Make sure you get genuine Mikuni brass.

I'm gonna look for it .. I looked earlier and had a hard time finding genuine mikuni needle jets...

 

[5twins]

Yes, they may not be available. You may have to settle for repops. Mains and pilots are readily available though.