And yet another dumb carb question

Mickster

Mickster

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Mikes sell part number 48-5010 idle mixture screw without the spring. Does that mean that the spring is not needed?

I ask because I have springs in both my carbs but the right carb with the spring will only engage 2 threads/turns before bottoming out where the left carb goes about 5 threads/turns. Now, if I screw in the mixture screw in the right carb without using the spring I get 5 threads/turns instead of only 2.

Do I have a mis-matched set of BS-38's, do some use the spring and some don't, or does my right carb have a thread problem?

Thanks for any insight.
 
After wasting way to many of hours of my life fooling with this BS38 carb set and not being able to get it like it should be I'm ready to replace with new carbs. The carbs I have may just be too worn out to ever get right. Or I just don't have the skill to ever get them right. About the only thing these carbs do correctly is idle, the rest is a lot of popping, coughing, stalling, choking, and on and on.

So I went to MikesXS and see they have several options. One option is to buy the 32mm kit, complete and ready to install. However their ad does not say "Mikuni" like the individual carbs. Maybe Chinese?

The individual carbs are Mikuni but are not advertised as complete and ready to install.

Any experience or advice on what I should do?

Thanks guys, your help is always appreciated.
 
Mickster said:
The carbs I have may just be too worn out to ever get right. Or I just don't have the skill to ever get them right. About the only thing these carbs do correctly is idle, the rest is a lot of popping, coughing, stalling, choking, and on and on.
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Have you completely gone through the ignition system?
 
https://www.xs650.com/threads/carbs-carburetors.43/
Use this section to insure you have the right parts in your carbs, the float is set correctly and you have no airleaks. Tape a little volt meter like this to confirm your system is charging at ~14.3volts. If you wire this into the headlight shell 12V+ compare it to the battery actual measured voltage and expect that voltage drop.
1702249016091.png
 
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Kevin Werner said:
I wish I had the @grizld1 quote on that...
Click to expand...
grizld1 said:
You can play whack-a-mole with symptoms or you can follow a systematic troubleshooting procedure. Back when I first turned a wrench for a dollar, the boss drilled something into my hard young head with these words, often repeated: "Get your fingers off that carburetor! Ignition first, dammit!" Why? Because ignition, compression, and valve train can be directly observed and inspected and fuel/air mixture, in the absence of an exhaust gas analyzer of some sort, cannot.
Click to expand...
;)
 
Thanks for all the input fellas but I still think it's a carb issue and not ignition. I put a new Accel coil and wires on it a few years ago so those parts should still be good. Also has electronic ignition.

Specs: I've cleaned and fiddled with these BS38's several times over the years but what notes I have say a 30 pilot, 142.5 main and the needle in #2 slot. All else is original from the factory I think. K&N pods, oversize headers with reducers and free flowing exhaust.

I just took it for a ride to try and get a play by play of what it does and pass that data on to you guys.

Cold motor, put on the choke, hit the button and it starts right up. Headlite comes on. After a few seconds turn off the choke and the bike settles into a pretty nice, steady idle at about 1200 rpm. Let it warm up for about a minute.

Took it out, went through some gears, did some throttle roll on's, did some cracks on the throttle, hit it up to about 5000rpm in third.

From idle to about 1/8 throttle it's not bad, at 1/8th (give or take a bit) throttle under load it bogs, sputters and chokes, turn it up to about 1/4-1/2 throttle and above it takes off like a rocket. The rpm's don't seem to matter much, all the bad stuff happens at around the 1/8 throttle position.

I'm not a tuning expert so I really need some help with this. Am I back to fiddling with those idle screws or is it something else? And remember, my first post describes the 2 threads/5 threads issue. I looked and could not find anything in there but I'll go back at it again if recommended.


1702414600292.jpeg
 
Mickster said:
Thanks for all the input fellas but I still think it's a carb issue and not ignition. I put a new Accel coil and wires on it a few years ago so those parts should still be good. Also has electronic ignition.

Specs: I've cleaned and fiddled with these BS38's several times over the years but what notes I have say a 30 pilot, 142.5 main and the needle in #2 slot. All else is original from the factory I think. K&N pods, oversize headers with reducers and free flowing exhaust.

I just took it for a ride to try and get a play by play of what it does and pass that data on to you guys.

Cold motor, put on the choke, hit the button and it starts right up. Headlite comes on. After a few seconds turn off the choke and the bike settles into a pretty nice, steady idle at about 1200 rpm. Let it warm up for about a minute.

Took it out, went through some gears, did some throttle roll on's, did some cracks on the throttle, hit it up to about 5000rpm in third.

From idle to about 1/8 throttle it's not bad, at 1/8th (give or take a bit) throttle under load it bogs, sputters and chokes, turn it up to about 1/4-1/2 throttle and above it takes off like a rocket. The rpm's don't seem to matter much, all the bad stuff happens at around the 1/8 throttle position.

I'm not a tuning expert so I really need some help with this. Am I back to fiddling with those idle screws or is it something else? And remember, my first post describes the 2 threads/5 threads issue. I looked and could not find anything in there but I'll go back at it again if recommended.


View attachment 257323
Click to expand...

Have you tried raising the needles one notch? 1/8 throttle is about where the needles kick in.
 
Mickster said:
Thanks for all the input fellas but I still think it's a carb issue and not ignition. I put a new Accel coil and wires on it a few years ago so those parts should still be good. Also has electronic ignition.

Specs: I've cleaned and fiddled with these BS38's several times over the years but what notes I have say a 30 pilot, 142.5 main and the needle in #2 slot. All else is original from the factory I think. K&N pods, oversize headers with reducers and free flowing exhaust.

I just took it for a ride to try and get a play by play of what it does and pass that data on to you guys.

Cold motor, put on the choke, hit the button and it starts right up. Headlite comes on. After a few seconds turn off the choke and the bike settles into a pretty nice, steady idle at about 1200 rpm. Let it warm up for about a minute.

Took it out, went through some gears, did some throttle roll on's, did some cracks on the throttle, hit it up to about 5000rpm in third.

From idle to about 1/8 throttle it's not bad, at 1/8th (give or take a bit) throttle under load it bogs, sputters and chokes, turn it up to about 1/4-1/2 throttle and above it takes off like a rocket. The rpm's don't seem to matter much, all the bad stuff happens at around the 1/8 throttle position.

I'm not a tuning expert so I really need some help with this. Am I back to fiddling with those idle screws or is it something else? And remember, my first post describes the 2 threads/5 threads issue. I looked and could not find anything in there but I'll go back at it again if recommended.


View attachment 257323
Click to expand...
Let us assume you have 14.1 - 14.5 volts at you battery when the engine is revving at 3000 rpm, that is really important. Have you inspected the spark plugs. A quick look will tell you what generally is happening. Plugs and reading them is a big subject I am not qualified to address except white (with a carburetor) is a bit lean and black/dark brown is too rich.
I had similar running conditions when I bought my blue tracker. I was (still am) very much the novice. After fits and starts and a lot of coaching on this forum it became evident I was over jetted on the mains and under jetted on the pilots. If your carbs are 76-77 or 78-79 the 30 pilots might be close but the 142.5 main seems really large. If your BS38s are earlier. 70-75. then the 30 pilot would be really small and the 142.5 main would still be really big.
My '77 with a 750cc kit and very free flowing 1.75 inch pipes and mufflers runs pretty good with 30 pilots and 130/132.5 mains. When I bought it it had 25 pilots and 137.5 mains. The chart below is from the Tech section and it pretty obvious who we can thank for putting it in,
Nice looking bike!
1702417501578.png
 
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The K&N pleated type pods don't work very well on these CV carbs. They cause turbulence in the air flow which messes with the slide lift. Get some straight foam UNIs, for your carbs you would want the UP-4200. That just may cure your issues because your jetting sounds pretty close. But, you could mess with it a little more, maybe try 140 mains and 32.5 pilots. The '78-'79 carb set often needs the 32.5 pilots, but a 35 is usually too big. I also agree that the 142.5 mains may be a bit much, but maybe if you leaned the needles some more they would be OK.

For fine tuning, you can shim the needles and achieve 1/2 step settings. You're already in the #2 needle slot, one step leaner than stock. With a shim washer, you could get the equivalent of a 1.5 slot needle setting if the #1 slot proved to be too lean.
 
I'm going to yank the carbs and give them a good cleaning.

Jim, when you say, Have you tried raising the needles one notch?, does that mean I should put the needle in the 1st notch from the top or the 3rd notch from the top. I'm pretty sure it's the 3rd from the top but I just want to make sure. If it's the 3rd notch from the top that will allow more fuel to pass correct?

The foam Uni's are a good idea too.
 
I don't think of it in relation to the notches, think in relation to what you want to happen. If you raise the needle, that lifts it further out of the needle valve/emulsion tube..... allowing more fuel to pass, thus making the mixture richer.

So, raising the needle makes your mixture richer, lowering it makes it leaner. Just move the clip accordingly.

If you think in relation to raising or lowering the clip, it's easy to confuse the issue because clips go down to make things richer... which is opposite the needle.... which needs to go up to let more fuel in.

So you made this statement... "From idle to about 1/8 throttle it's not bad, at 1/8th (give or take a bit) throttle under load it bogs, sputters and chokes, turn it up to about 1/4-1/2 throttle and above it takes off like a rocket. The rpm's don't seem to matter much, all the bad stuff happens at around the 1/8 throttle position."

About 1/8 throttle is the transition point (or thereabouts) where the needle jet starts having more of an affect than the pilot jet. You said it idles good... which says the pilot jet is close enough, but throw the needle in there and you say it sputters and bogs. That can mean either too rich or too lean, but in my experience it's usually too lean at the transition for pops and sputters. That's why I suggested raising the needle to make things a bit richer.
 
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Oh... and I agree with 5t... the pleated filters can cause tuning headaches. Better to replace them before you get into changing carb setting so you ain't jus' chasing your tail.
He's got a better eye than me, I couldn't tell you have the K&N's on the carbs.
 

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