Yam_Tech314's official build thread

[G_YamTech_314]

5twins has more experience than me with the BS38's, so I'll bow out on jetting.
I will say this though... any engine I've dealt with that will fire on choke, but won't run on it, is because it's too rich to begin with and adding the choke pushes it past the point of running. Simple logic tells me that if you're too lean, adding choke richens it so it should run.

So here's my question... If If it's lean enough to pop and sputter, why does adding choke make it die? When the choke richens the mix, it should run without all the fuss. And if you richen the pilots, what's the choke gonna do to it now? if it wouldn't run on choke when it was too lean, how's the choke react when it's richer to start with?

Does the same thing. Shuts off as soon as I choke it. I'm used to a choke raising the idle and enriching the mixture. So this is behaving abnormally. It's quite possible there's an issue with a diaphragm in my choke circuit, but I would have to say I'm probably only gonna figure that out when I get the bikes jetting sorted out. Once it runs at its peak, I'll rip the carbs apart entirely, do one more final clean and polish, and install them for good being sure to fill my tank with ethanol free gas!

 

[gggGary]

Fuel levels in the float bowls?
Will the carbs leak if it's on the side stand?
Guess I'm thinking it's still something other than jetting affecting how it's running.
Hot water dip test done on the floats?
Drop test on the slides good?
Are they raising together?

 

[G_YamTech_314]

Fuel levels in the float bowls?
Will the carbs leak if it's on the side stand?
Guess I'm thinking it's still something other than jetting affecting how it's running.
Hot water dip test done on the floats?
Drop test on the slides good?
Are they raising together?

Floats are new. I had cracked brass ones in the beginning of the year so I replaced them. Both slides pass the drop test. I don't get any fuel leakage from the carbs (though, I don't have a side stand.) I can wobble and rock the bike any way I please and no fuel spillage.

Float height in each carb is 24mmfrom gasket resting surface to bottom of floats. I also made sure they weren't twisted and that they weren't contacting the top side of the carb when the spring in the needle was fully compressed. I haven't checked to be sure they're both raising together. I suppose I could. They are also spotless clean. Has them apart three times in the last week and cleaned them two of the three times just to be sure. Compressed air and all.

I could just be REALLY picky. When it's at operating temp there's zero dead spot whatsoever, but I really am kinda OCD and want the best possible stoichiometry I can get.

 

[gggGary]

I haven't checked to be sure they're both raising together. I suppose I could.

that ask was about the slides.

 

[G_YamTech_314]

that ask was about the slides.

Yeah, I understood ya. I'll pull the filters tomorrow and give the throttle a few twists to see if they move simultaneously.

 

[5twins]

The float level spec for the '76-'77 carb set is 25mm, not 24. You're set a little rich on the floats, still within the spec (25 ± 1mm) but at the rich end of it. I experimented with the float settings on several sets of BS38s, running them at the spec and at the upper and lower limits, and the bikes seemed to like being set at the spec best (they ran the best there). An exception to this is the BS34s. They came set up so lean from the factory that they often like having their floats set at the rich end of the range.

No, the slide needle and needle jet are not the same, they're two separate components. The needle sticks down into the needle jet and the jet flows more fuel as the needle is lifted out of it because it's tapered. The needle jet is that brass tube hanging down from the middle bottom of the carb's main body .....

We don't normally mess with them when tuning simply because there aren't many other sizes available. However, we do check the condition of the o-ring that seals them into the carb body, and for wear. They should be a snug push fit into the carb body. Place a finger on top of the jet and see if it wiggles. If it does then it needs a new o-ring. If the o-ring is bad, gas can flow around the outside of the jet instead of just through it as it should, and that can cause rich running. For wear, it's difficult to tell unless you have a brand new jet to compare to because the metering hole through the jet is so large. Over time, the needle will wear the hole larger, or if the carbs sat all gummed up for a long time, that gas varnish can actually eat away the brass, making the jet richer. You can often tell if this has happened if the clean jet has a textured or sandpaper-like appearance to it.

I recently replaced the original needle jets in my '78 carbs. Over the past few years, I noticed the plugs have been getting progressively darker and darker. Nothing was changed on the bike, no additional mods, so I looked into the carbs. At first I thought maybe the floats were off, set too rich, but they checked out OK. So then I checked the needle jets and, sure enough, they were worn. Here's one of the originals compared to a new replacement. It's pretty easy to see the difference, lol ......

 

[G_YamTech_314]

Freakin 5Twins is taking me to school! I love it. Just when I thought I knew a good bit about carburetion...

Makes me wonder. I'll have to try to wrap my head around how fuel height from the floats can change the air/fuel mixture. I assume it "dips" the jets in a deeper pool of fuel giving it more to take from. Setting my float height to 25 would make the fuel shut off SOONER. Giving the jets less to drink... Am I followin'?

I'll have these carbs apart again tomorrow to really dig around in them. I'm gonna change my float height to 25mm and see what that does. I'm also gonna fiddle with that needle jet. Some interesting stuff.

 

[5twins]

I think Gary was referring to carb sync, and to check that, you need to look at the butterfly plates and make sure they are matched (both cracked open the same amount). Even if they're not, they and the slides will open together because they're tied together with the linkage. So, what I'm getting at is pulling the pods and watching the slides lift will tell you nothing about whether or not the carbs are exactly synced. To check them and to get them very close, you can do what's called a "bench sync". With the carbs removed, start by backing the idle speed screw out all the way so it's not touching the throttle cable arm at all ......

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 .......

If this is what you find, simply adjust the "sync" screw on the linkage between the carbs until the open plate closes all the way. Keep going once it closes and the other plate will begin to open .....

So, work the sync screw back and forth between one plate beginning to open and the other doing the same, and set the screw about in the middle of those two points (with both plates closed) ......

Then screw the idle speed adjustment screw back in until both plates are cracked open just a hair to re-establish an idle. As I said, this will get your sync very close, but the best way to check it is with a manometer while the engine sits there idling.

 

[willis]

I had a mysterious miss at idle on my 75 the last few years that I could not figure out. I recently notice the butterflies were not in sync on my 75 carbs. I cracked the screws and readjusted them equally by holding up to a light as 5t suggests. Fixed my problem of the misfire at idle.

Let me know if your needle jet orings are worn. I bought a surplus of them last year and can send you a few if yours are worn out.

 

[5twins]

And speaking of manometers, you can build your own for just a couple bucks ......

http://www.powerchutes.com/manometer.asp

Now I know your bike doesn't have the vacuum nipples on the intake boots to hook the hoses to, but your carbs have vacuum ports blocked off by a screw. Remove those and screw some sort of hose nipple in there (M6 threads), and you can hook the manometer up then ......

You can get the M6 brass nipples pretty cheap off Amazon ......

https://www.amazon.com/dp/B07Z9QQC3...ting-goods&sp_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWw

 

[G_YamTech_314]

Then screw the idle speed adjustment screw back in until both plates are cracked open just a hair to re-establish an idle. As I said, this will get your sync very close, but the best way to check it is with a manometer while the engine sits there idling.

I did bench sync the carbs the first time I had them apart last week. I didn't use a light but I did use my calibrated eye. I pulled the idle screw all the way off the throttle too. The interesting part is that when idle is re-set after the sync, only one of the butterfly valves move. This could be contributing to my issue.

Interesting that you can make your own manometer so cheap. I might try that.

I've vacuum synced carbs before too. I just don't really wanna spend the money on a nice set of gauges yet. When I rebuild my CB900C I'll need to bite the bullet. But for now I think I can get away without em. I'll try the flashlight method and see just how close I got with my eyeballs.

 

[G_YamTech_314]

Found me a wobbly and worn out needle jet seal on the left side carb. It's the first time I've ever checked for this issue (thanks to you all)

I'm gonna have to message @willis for some of those seals he speaks of. Might fix more of my issues than I know!

I also grabbed #30 pilots. I know... It's two steps to the rich side of things but 1: they didn't have 27.5's in stock, and 2: who knows... Maybe 1 size up wouldn't be enough. Not to mention the resetting and adjusting Id be doing AGAIN if I ever decide to change the full exhaust.

I don't have a ton of time to dig into them too deeply tonight but they're on the tray and ready for surgery. So they'll get a full tear down and deep clean and thorough inspection once more. I'll be sure to set my float height to 25mm also. Dialing this bike in one small step at a time!

 

[G_YamTech_314]

I remember now when I first rebuilt these all those years ago that my brass nipple came out with the fuel line. Well, I guess back then I just pushed it in real good and left it be cause it didn't leak... Now I'm thinking maybe soldering this in would be a good permanent fix. Has anyone ever done this? Should I not? I considered JB weld but I'm not sure how it'd hold up being around or in fuel.

 

[G_YamTech_314]

 

[Jim]

Don't see how you could solder it. The barb is brass and the carb bodies are zamak, which is a zinc alloy. I've never heard of any way to solder the two together.

I've had good luck with JB Weld (original) and fuel stuff. The key to JBW is cleaning it to bare (shiny) metal... like with a red scotchbrite.

 

[G_YamTech_314]

Don't see how you could solder it. The barb is brass and the carb bodies are zamak, which is a zinc alloy. I've never heard of any way to solder the two together.

I've had good luck with JB Weld (original) and fuel stuff. The key to JBW is cleaning it to bare metal... like with a red scotchbrite.

Good call. I'm not too keen on which types of metals adhere well to solder. I was an aluminum mig welder for 2 years .. I know aluminum sticks to aluminum with aluminum hahaha. The rest is kinda lost on me!

I'll give it the JB weld special. Just gonna have to do it after I get these carb bodies nice n clean. I'm thinking I might invest in a small ultra sonic cleaner. Gonna have to read up on how to make em shine again. Since they're apart I'd like to polish them up real nice finally.

 

[willis]

I dropped an envelope in the mail with your o-rings this morning Greg. Also in regards to ultra sonic cleaners, they will clean well if you use the right solution. They will not be shiny though. They come out a flat gray color. If you are looking to have them look shiny then the best results I have seen is having them vapor honed. But that can get a little pricey unless you know someone that has one. My best suggestion if you do use an ultrasonic cleaner is to keep the heat around 90-100*F and use a mild solution in short 20 minute intervals. I used some citrus ZEP with the heat up high and it darkened one of my carbs. There is a whole rabbit hole thread on here about cleaners. Good luck and keep up the good work.

 

[G_YamTech_314]

I dropped an envelope in the mail with your o-rings this morning Greg. Also in regards to ultra sonic cleaners, they will clean well if you use the right solution. They will not be shiny though. They come out a flat gray color. If you are looking to have them look shiny then the best results I have seen is having them vapor honed. But that can get a little pricey unless you know someone that has one. My best suggestion if you do use an ultrasonic cleaner is to keep the heat around 90-100*F and use a mild solution in short 20 minute intervals. I used some citrus ZEP with the heat up high and it darkened one of my carbs. There is a whole rabbit hole thread on here about cleaners. Good luck and keep up the good work.

Thanks man! You guys are awesome. I do actually know a pretty decent vapor honing business within my general vicinity. I may have them help me out, but I may also just call clean and not shiny good enough.

When my busy schedule allows, I will probably be replacing the butterfly shaft seals, a long with the O rings you are sending, and anything else I need to. I'm going to stick with the following settings for a test run once their clean:

#122.5 mains
#27.5 pilots
Stock needle jet (can this even be changed?)
slide needle lowered one slot
air mixture screw 2.5 turns out.

I'm also going to thoroughly inspect the choke circuit to ensure there's nothing causing the bike to not run on choke.

 

[gggGary]

nothing causing the bike to not run on choke.

We can assume the float bowl choke orifice are open?

When it dies on choke have you pulled the plugs to see if one (they) are wet or dry?
These carbs don't have a choke, they have an enricher circuit which creates an alternate mixture path when the plunger lifts. If it's not a mix ie only air it will lean stall the motor.

 

[G_YamTech_314]

These carbs don't have a choke, they have an enricher circuit which creates an alternate mixture path when the plunger lifts. If it's not a mix ie only air it will lean stall the motor.

That's good to know. I haven't pulled plugs to see, but this sounds exactly like what may be happening.

Is the jet that sits in the choke hole removable?

All of my passages are clear (from what I can tell by seeing carb cleaner flow through them. I've ignored removing the enrich sides of the carbs because I didn't want to pull them apart from the plate. No more being lazy though, they're fully apart and laid out on my kitchen table at this very moment, which means now is my time to get it all addressed once and for all. I'll be sure to shine a light through all passages and rule them out AFTER doing an ultrasonic cleaning.