‘77 BS38 running way too lean

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cjv2001

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Just about ready to get er on the road but am having a hell of a time tuning the carbs. Did a bench sync with them and air mix screws are out about 3 turns. Needle is in the 3rd (stock) slot and running UNI pods. Floats are set at 24mm. Replaced old intake manifolds with new ones. Swear I’ve read through almost every bs38 jetting and lean running discussion.

Jetting is 32.5 pilot and 140 main. Everything I’ve read on here is saying that I should be good but I am still getting tons of pops on decel, as well as white tips of plugs and a small amount of hanging idle.

I lubed up the advance rod as well as set points. Had a brief problem with runaway idle above 2k - 3k but solved that by bringing idle down to 1100-1200.

Would it be worth it to set it to the 4th slot on needle? I feel like I should be running rich with all of these conditions I’ve listed.

Please help!!
 
Your first step should be to positively identify the year of your carb set, and verify the jetting inside, especially the needle and needle jet. Here's a chart I made up showing the stock jetting for the various years and carb sets .....


Carb Specs.jpg


If you do have a '77 carb set then yes, your jetting is very rich. On the couple '77 sets I've tuned, I couldn't run any larger than a 132.5 main, and pilots were one up (to a 27.5). But if the pods are your only mod, you may not need mains that big. Also, which UNI pods do you have? If they're the short 3" ones with the rubber end caps, you may not need to re-jet at all because they don't flow much more (if any more) than the stock airboxes.
 
Last edited:
5twins said:
Your first step should be to positively identify the year of you carb set, and verify the jetting inside, especially the needle and needle jet. Here's a chart I made up showing the stock jetting for the various years and carb sets .....


View attachment 336034

If you do have a '77 carb set then yes, your jetting is very rich. On the couple '77 sets I've tuned, I couldn't run any larger than a 132.5 main, and pilots were one up (to a 27.5). But if the pods are your only mod, you may not need mains that big. Also, which UNI pods do you have? If they're the short 3" ones with the rubber end caps, you may not need to re-jet at all because they don't flow much more (if any more) than the stock airboxes.
Click to expand...
Thanks for the response @5twins. I do have a '77 carb set and I am running these pods.

https://www.amazon.com/uni-PK-92-Un...id=1939662225341979778-B000GZMZKC-&hvexpln=73

I am working on it right now and have mix screws turned out to 4 and a half turns which seems like way too much. I also sprayed some starter fluid onto the carbs and the left side seems to rev up. New boots but maybe I will have to try some other brand? I can’t tell if it’s revving from the carb itself or from the boot. These ones are Mike’s XS and have read they aren’t the greatest. Any advice helps thanks again!
 
I’ve rebuild quite a few BS38’s lately, and tested them on my ‘78 stock, they run a little lean on what I jetted them for - as per 5twins I used the stock jetting ( as everything else is stuck) main 135 and pilot 27.5. Mix screw is 2.5 turns out. - I think you are to high on both main and pilot and your mix screw must be just about to come out :). Hope you figure it out - I doubt it’s the carb holder - your intake filter I do not know how tight they run on air flow.
 
Yes, those are the small UNI filters. They have so little surface area that, as I said, they don't flow much, if any more than the airbox did. So, you shouldn't need to re-jet much, if at all. I'd try starting back at the stock jetting and see how it runs. The stock 122.5 main is quite small and while it's fine on an all stock machine, it's quickly out-paced if you start modding. So, maybe a size or two up on the mains (125 or 127.5) might be called for, but no way do you need 140s, lol. The mix screws should dial in within 1/4 to 1/2 turn from the factory recommended setting (1.5 turns out for your carb set).

Did you verify that the carbs have the correct 4M1 needles and Z-8 needle jets? Those large Z-8 needle jets are what limit the main jet size to the mid-130s at most.

What kind of gaskets did you use for mounting the new manifolds? Many come with thin paper ones while the originals were quite thick. I only use original thick type. Besides sealing, I think the thicker gasket was used to help isolate the manifold and carb from the engine heat.
 
5twins said:
Yes, those are the small UNI filters. They have so little surface area that, as I said, they don't flow much, if any more than the airbox did. So, you shouldn't need to re-jet much, if at all. I'd try starting back at the stock jetting and see how it runs. The stock 122.5 main is quite small and while it's fine on an all stock machine, it's quickly out-paced if you start modding. So, maybe a size or two up on the mains (125 or 127.5) might be called for, but no way do you need 140s, lol. The mix screws should dial in within 1/4 to 1/2 turn from the factory recommended setting (1.5 turns out for your carb set).

Did you verify that the carbs have the correct 4M1 needles and Z-8 needle jets? Those large Z-8 needle jets are what limit the main jet size to the mid-130s at most.

What kind of gaskets did you use for mounting the new manifolds? Many come with thin paper ones while the originals were quite thick. I only use original thick type. Besides sealing, I think the thicker gasket was used to help isolate the manifold and carb from the engine heat.
Click to expand...
Will verify the carbs just to make sure what needle jet is in there on thursday. Will order some different jets and update. I actually saw your post how you use the stock gaskets for the manifolds so I did that as well haha.

Could it also be butterfly shaft seals that could be bad? And if so I'm guessing I should try to do butterfly shaft seals first and then jets?
 
the butterfly valves are closed on deceleration, so if you get the popping it is something to do with the pilot/idle circuit. so pilot, idle screw, fuel level in the bowls (float height). leaking air through the shaft pivot would also do that. try spraying wd40 on the shaft pivots with the bike idling. are the boots with vacuum barbs? are the barbs well plugged in? are the air filters/pods well seated and sealing around the carbs?
 
zrx1100 said:
the butterfly valves are closed on deceleration, so if you get the popping it is something to do with the pilot/idle circuit. so pilot, idle screw, fuel level in the bowls (float height). leaking air through the shaft pivot would also do that. try spraying wd40 on the shaft pivots with the bike idling. are the boots with vacuum barbs? are the barbs well plugged in? are the air filters/pods well seated and sealing around the carbs?
Click to expand...
Gotcha, that's good to know. Will try that on Thursday and get back to you. The carb boots are non vacuum barbs and they seem to be sealing, but when I did spray some starter fluid on the left side it started to rev up a bit. I have to spray quite a bit of starter fluid for that to happen. A small amount wont do anything.
 
zrx1100 said:
The other thing is synchronization. Without the barbs is hard to do. You may need to add barbs to carburetors
Click to expand...
I thought I saw somewhere that instead of barbs, you can directly plug into BS 38's by taking a screw off and adding a nipple? Correct me if I'm wrong
 
cjv2001 said:
Gotcha, that's good to know. Will try that on Thursday and get back to you. The carb boots are non vacuum barbs and they seem to be sealing, but when I did spray some starter fluid on the left side it started to rev up a bit. I have to spray quite a bit of starter fluid for that to happen. A small amount wont do anything.
Click to expand...
You have a small inlet leak if the note engine changes when you spray on the inlet rubbers. You definitely need to sort that out. Even a small leak.
 
Its been a long time said:
You have a small inlet leak if the note engine changes when you spray on the inlet rubbers. You definitely need to sort that out. Even a small leak.
Click to expand...
Darn you Mike's XS boots lol. Do you have any recommendations for boots? Every good one i've seen on here seems to be out of stock or they don't make them anymore
 
cjv2001 said:
Darn you Mike's XS boots lol. Do you have any recommendations for boots? Every good one i've seen on here seems to be out of stock or they don't make them anymore
Click to expand...
There's another recent thread for a manufacturer recommended over there. Sorry, I can't think who it is as it's not relevant to me in the UK.
 
The good ones are the Japanese made ARS brand. They supplied Yamaha with them originally. Look for their trademark label or emblem on any you're thinking of buying .....

LabelARS.jpg


LabelARS-2.jpg


The K&L brand, and the TourMax brand are usually ARS ones. Do a Google and eBay search. The K&L part number is 11-4210 .....

https://www.ebay.com/itm/192771269555
 
5twins said:
The good ones are the Japanese made ARS brand. They supplied Yamaha with them originally. Look for their trademark label or emblem on any you're thinking of buying .....

View attachment 336143

View attachment 336145

The K&L brand, and the TourMax brand are usually ARS ones. Do a Google and eBay search. The K&L part number is 11-4210 .....

https://www.ebay.com/itm/192771269555
Click to expand...
Thanks for the suggestions @5twins. I ended up ordering some JBM industries holders a few hours ago. Bit expensive but hoping they are worth the price.
 
Yes, pics will let us positively identify the year of your carb set. Show us the intake bells, and outside bottom and inside top of the float bowls. Also the inner side of each carb, the sides that face each other when mounted. I'm assuming you have a linked set that uses one throttle cable.
 

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