Take a gander at my spark plugs.

[CaptCardboard]

In messing with my toy today, I found my left side plug is dark and sooty while my right side plug seems just like a spark plug should be.
I bought some brand new NGK BPR7ES plugs, set the gap to .030, and got kicking. It took a few more tries than normal, but then I realized I forgot to set the choke. Derp.
I had some fun pulling plug wires off the bike while it was running and found the left side cylinder is firing much stronger than my right side. Pulling the right side plug wire showed a slight decrease in volume and idle strength, but pulling the left side brought the bike way down and eventually stalled.
I thought I might mess with the carbs and see if I should try to lean out the left side since the plug was so black, but maybe the right side could benefit from running a tiny bit richer.
I read a super handy thread on here about adjusting carbs without a manometer or anything and just adjusting the air mix screws, but here's the part where I show my inexperience.... Where are the air mix screws on my carbs? I understand the idle speed screws, how the left side is the master and the right side is the balancer. But how and where do I adjust my air mix?

Thanks e'rybody!

 

[Jim]

I read a super handy thread on here about adjusting carbs without a manometer or anything and just adjusting the air mix screws...

Using a manometer... or by sound and feel... is to adjust the sync between the carbs, NOT the mixture.
And going by your description of one cylinder being weaker than the other, I suspect the sync is the cause. Try syncing the carbs.

 

[CaptCardboard]

Sounds like a solid case of my overthinking a simpler issue. Thanks!

 

[JJames]

I would never disagree with Jim so sync those carbs. Always helpful to tell us what year the bike is. Different models, different carbs. The mixture screws are important so if you post this info., someone can tell you where they are located. The varnish stains on the float bowl tells me that gas is above the gasket when it sits on the side stand. This means it may be seeping out the bellmouth of the carb, possibly soaking the filter and causing a very rich condition. If float level is to high, fuel is constantly "sloshing " over. This negates any other adjustments made to the carbs. Your plugs are a good example of this. Get busy and get back to us.

 

[gggGary]

PS those cheesy pods have a garbage can waiting. In addition to gaps, screening you can toss a cat through, the flange/hole has a small ID and square shoulder where it attaches to the carb, cutting off good airflow to the various vents and passages on the intake bell. A decent UNI foam filter will improve your ability to tune those carbs.

 

[Wingedwheel]

Also you are running resistor plugs. Do you still have the OEM style resistor caps?

 

[CaptCardboard]

Thanks guys. The bike is a '75.
I figured a pod filter discussion would come up at some point. I'll look into ordering uni filters. Luckily I still have the original metal side covers for the bike, so no one will even see the filters.
A good cleaning is definitely on the list for my carbs. Just hoping to mess with a few other things first and know my baseline before making a big change like that.
I just replaced the plugs with the same number that was already on the bike. I don't know if I'm running resistor plugs or how to tell if I'm running OEM style resistor caps. I'm going to guess not since one side plug boot fits over the threaded post on the plug, and the other side boot fits over the little nub that comes screwed on the threaded post of the spark plug.

 

[JOHNDADDY]

PS those cheesy pods have a garbage can waiting. In addition to gaps, screening you can toss a cat through, the flange/hole has a small ID and square shoulder where it attaches to the carb, cutting off good airflow to the various vents and passages on the intake bell. A decent UNI foam filter will improve your ability to tune those carbs.

The pods is not really an appearance issue, they have effect on the carbs.

 

[Wingedwheel]

If you are running two separate caps, one resistor one non, that might explain some of the difference in color.

 

[gggGary]

NGK BPR7ES plugs

The R is for resistor.

OEM style resistor caps

Easy enough to measure the caps with a multi meter. Will be either 5K or 0 ohms

 

[Melnic]

Anyone who services their own bike has to buy the tools to do so.
Even a cheap sync gauge set from Amazon will do the trick. Just make sure after its synced that you swap the gauges to compare. the cheap ones often come to you not agreeing with each other.
There are other choices for non resistor plugs, champion, denso brands people run in these bikes and the XJ650 which use the same plug specs.
You Could have:
Out of sync carbs
one float bowl set higher than the other
vacuum leak
Idle mixture
Dirty carb
and more choices.

Just have to eliminate them.

 

[5twins]

Oh my, where do I start, lol. Well first thing is your carbs are not the originals for that bike, they're a later linked set. Not a bad thing really. The single throttle cable and linkage is easier to adjust and sync, and it holds that sync longer. Also, your carbs have been modded, the choke lever was removed and a knob of some sort put in it's place. In your pic, that knob is blocking the view of the mix screw. It is just above that big flat head screw. That screw, by the way, is blocking a vacuum port. Replace it and the one on the other carb temporarily with a hose nipple of some sort and you could hook up a manometer or vacuum gauges to sync the carbs .....

Here's a better pic showing the mix screw, this one on the right carb ......

And also in your pic, notice how long the rear carb top screw is? It's in the wrong spot. It should go where there's a bracket, either in the front where the throttle cable bracket is, or in one of the 4 inner holes where the butterfly shaped bracket mounts.

 

[CaptCardboard]

Thanks for all the details, 5twins!
I just made the connection today that I don't have the original carbs for this bike. I guess these came out a year later than mine. After what I've read so far, I think I'm going to just leave those screws alone until I'm ready to pull and rebuild the carbs. I don't quite understand the big knob on the left side. The dude I bought the bike from included a roach clip to prop up the lever on the right side to act as my choke. It works, and I chuckle at a roach clip being a part of the regular operation of this bike.
Good eye on the screw on top of the carb. I noticed today I'm missing one of those.

I basically followed the instructions that DogBunny posted on here a decade ago about carb syncing and that made a noticeable difference in how it runs. I'm looking forward to my ride to work tomorrow!

 

[5twins]

You will need to determine what year the carbs are. Then you'll know what specs to use for float setting, mix screw setting, and jetting. There were two sets of linked BS38s used, the '76-'77 set and the '78-'79 set. Easiest way to tell which you have would be to look at the bellmouth where the air filter mounts. The '78-'79 set has 4 round holes along the sides, the '76-'77 set only has two .....

 

[CaptCardboard]

Super helpful!
I'll pull my pods (and likely replace with uni) on my next day off and determine which carbs I have.
5twins, I'm gonna go ahead and guess that you have a resource to find the specs for float, mix, and jetting once I know what carbs I have...

 

[5twins]

Yes, I put together this chart of the stock carb specs on the U.S. bikes .....

As you can see, just about everything differs between those two sets of carbs.

 

[5twins]

The UNI pod you want/need is the UP4200. It's 2" I.D. mounting flange fits the BS38 like a glove and the 4" length is the longest you can fit in front of the battery box .....

And there's no need for the fancy foam filter oil, it's actually too thick and can clog them. Just use a 50-50 mix of motor oil and gas.