Need help with carb tuning/jetting...

connor1

connor1

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Trying to get my BS38's tuned. XS2 starts fine, idles nice and low, but as soon as you hit the throttle, it bogs down and dies..seems like a too rich condition and this is my assumption. Feel free to correct me if I am wrong. When I remove the carbs the throttle plate/valve is soaked in gas supporting my thought that this is a mixture issue.

The mixture screws are turned in all the way in an attempt to compensate, and the carbs have been completely torn down, soaked and ultrasonic'd. Mikes XS rebuild kits installed, with pilot jets at 42.5. Appears from the "carb guide" that these jets are on the rich end of the spectrum. I have also moved the main needle up to the top to position 1. Not that that will help, but desperate times...

As I have spent hours removing, tearing down and reinstalling these carbs and have not accomplished much, I am considering ordering new pilot jets. Any ideas on what size to order?

I am not sure how many sizes to go down to in this scenario..Should I buy 30's, 35's. 37.5? All are available on Mikes XS.

Thanks in advance for your expertise!:banghead:
 
First off the carb kits from mikes are no where near correct from what I have read, I have never used them but that is what I have read. You need to correctly ID the year of the carbs since there were changes every couple of years. Then you need to use the carb guide to determine what the stock jetting and float height was for the year of carbs not for the year of the bike. After you have correctly identified the year of carb and the stock jetting use the carb guide to "tune for any mods done to your bike".

The carb guide is going to be like your bible at this point for getting your system sorted out correctly. It will help you identify the carbs and tell you all of the stock settings for the carbs and also give you a baseline for any mods that have been done.
 
Thanks for the reply!

Per the carb guide chart, these BS 38s use with the 42.5 pilots, but my bike dies when you hit the throttle.

Per the guide-
"A very rich condition usually causes break-up or stumbling which gets worse as you
apply more throttle. Just off idle, this would indicate too large a pilot jet or to a lesser
extent, too rich a mix screw setting. Try leaning the mix screws first but you will probably need a smaller pilot - that's the usual cause."

So, in following the guide I am trying to get some suggestions on what size pilot to go down to, as that is what the guide is indicating I need to do.
 
Yes, you have BS38 carbs but what year carbs? As Ippy said the carbs changed every two years. So you need to know the year of the carbs.
As Ippy said the brass parts in Mike's or most any rebuild kit are wrong. Do you still have the stock parts, if so they will work better than the parts from the kit.
Depending on the year the pilots can be anything from a 25 to a 45. Mix adjustments from 3/4 to 2 1/4. Mains from 122.5 to 135.
I quote all this to help you understand how important knowing what carbs you have is.
Leo
 
Thanks Ippy & Leo.

Please educate me. I do not know how to ID the carbs, and. Have not come across a resource to help with that, maybe I missed that.

Am trying to upload pics to help with the ID.
 

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To be clear .....moving the needle up by putting clip lower richens. Lowering needle by moving clip towards the top leans.

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Judging by your pics I would have to agree these are early carbs. Your jet sizes should be 42.5 pilot, 130 main, 4JN19-4 needle and z-6 long needle jet if there have been no modifications done to the intake or exhaust. Which kit did you buy, which parts did you use from the kit and did you save the old brass? The pilot jets changed in 76, they will screw in but they flow different and do not work in the earlier carbs. And chizler is correct if you raise the clip it drops the needle and leans the mid range mixture.
 
Twomany, one of the bodies has 306 2x5 stamped into it a code I could note find in th carb guide or on the mukuni website. I will print out your attachment. Thanks.

Chiller, yes, I moved the clip up put more needle in the hole which I thought would lean. I also rested that plastic disk on top of the clip.

Ippy, the pilots are 42.5 and the main is 130. Not sure about the needle, but with that setup running way too rich if you just give it any throttle. Could float height cause that? I was not confident I did that adjustment correctly.

Angus, what is a jet nozzle oring? Which jet does this go on? I don't think ay of the jets have an oriing, except for the enriched circuit, and that one seals up nicely.

Thanks to all in advance...
 
Did you replace the pilot jets with jets in a kit????
Did you replace the needles with ones in a kit????
If so which kit and did you save the old brass????

The reason why I ask this is that the early carbs (70-75) used BS30/96 style pilot jets and the later carbs (76-79) used VM22/210 style pilot jets and often times the needles in the kits are a generic Y-22. If you have the old brass put them back in and double check your float height because if the float height is too high (measurement was to short when you set it) it can cause it to run rich through out the rpm range.
 
Yes, float settings are important and can make the bike run rich if they're off. You shouldn't need smaller than stock jets. Something else is wrong. When you check the floats, you need to measure each one twice. You need to measure both sides because the float can be twisted, one side sitting higher or lower than the other.

The problem could also be ignition related. If the coils are getting weak, they would produce a weak spark and black plugs.
 
Did you replace the pilot jets with jets in a kit????
Yes
Did you replace the needles with ones in a kit????
Yes
If so which kit and did you save the old brass????
No all. Appears I only have a 42.5 pilot a 130 main and a y22 needle in my extras box.

Tore down the left side carb tonight. Verified that the pilot is a 42.5, the main is a 130, and the needle is a Y22. Measured the float height at 22mm, so I bent the tang down to raise the float up to 24mm as measured from the gasket. Noted the slide falsl too quickly, takes just over 1 second to bottom with the oval hole blocked. On the right side carb, takes just over 6 seconds.

Will tear down the right side carb tomorrow.
 
the nozzle, scientifically called an emulsion tube, is the long brass tube that the needle rides in. if the oring is bad, fuel will be pulled from around the nozzle. To check it, pull out the nozzle from the bowl side. If it just falls out, new orings are in order. It may be difficult to get the nozzle off, but it will come out. Even if its tight, the oring might still be damaged.
If you find you need some I could mail you a pair for free. They are the good McMaster Viton orings. I needed 2, but minimum order is 25, so I have a few extras. Pm me your address.
Don't ever throw away stock brass. most of the time, the only good parts in a kit is the bowl gasket. Ive rebuilt about 6 sets of carbs, and the bowl gaskets still works, No need for replacement unless it leaks.
 
That "jet nozzle" or "emulsion tube" is actually your needle jet. It's the large brass tube hanging down off the center of the carb bottom .....

PilotInletBS38.jpg


It is a snug push fit into the carb body, being made even tighter (and sealed in place) by a good o-ring. Place your finger on top of the jet and see if it wiggles in its hole. Also check how easy it is to pull out. If it wiggles and pulls out easily (or basically just falls out), the o-ring is shot and needs replacing. These are some of the later needle jets with a shorter top (8mm long), but they illustrate the o-ring needed .....



The top portion on yours will be longer, about 12.5mm.
 
That Y22 needle is wrong. It should be a 4JN19 needle. The Y22 came in the kit. Put the original needle back in and throw the Y22 away.
On setting the float you don't measure from the gasket, you remove the gasket and measure from the gasket surface.
Leo
 
I use a very thin 6" steel rule graduated in MM down one side. It can be pushed down between the gasket and the lip on the outside of the carb body, reaching the gasket surface on the carb body, so the gasket doesn't have to be removed.
 

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