carb tuning question

[pa23driver]

So i'm trying to tune the stock bs38's on my 1977 standard.

my current setup is:
mikes 1.5 inch pipes with commando mufflers
new JBM diaphragms and manifolds
needle and seats replaced last year
UNI filters
130 mains (pulls really nicely up to redline)
needle dropped a step (#2 position)
carbs freshly synch'd this morning
valves adjusted (.003 intake, .006 exhaust)
pamco iggy timing reset
carbs were cleaned last year using the carb guide, also installed fuel filters.

here's the problem:
i'm getting the flat spot just off of idle and the idle is hanging when i blip the throttle, all indications of too lean pilot jet - makes sense since i dropped the needle.

i pull the plugs to double check my reasoning. the right cylinder is showing what you'd expect: a light smoke ring at the base of the ceramic with the rest of the ceramic white and the electrode slightly white/grey. i pull the left cylinder plug and its black/grey with just a little white showing on one side of the ceramic.

what could be causing the difference between the plugs? float level? dirty something-or-another? trying to figure this out before i start changing jets again.

 

[5twins]

Yes, you need a larger pilot. Go up one size from the #25 stock to 27.5. Your reasoning is correct. When you leaned the midrange by lowering the needles, you also leaned the upper part of the pilot circuit which created that off-idle flat spot. The 3 circuits in your carb overlap each other a bit so when you change one, it has some effect on the one next to it. In the case of the midrange, it effects both the lower main and the upper idle circuit.

The dark plug could be float level related or it might be residue from starting the bike using the choke and not running long enough before the plug check to burn it clean.

 

[pa23driver]

dont think its choke related. heres a couple pictures of the plugs after about 20 miles of high rpm, full throttle test runs. first picture is the right cylinder spark plug, second picture is the left cylinder spark plug.

anything else i should be checking for while i got the carbs off (other than float height)?

 

[5twins]

Well, runs like that should have cleaned any choke residue off. You might check the o-ring condition on the needle jet. Supposedly they can pass fuel around the outside instead of just through the jet if that's bad. I imagine it would happen worse pushing hard like you were than running easy. With the bowl removed, place your finger on top of the needle jet and see if it wiggles in it's hole. If it does, the o-ring is toast. Also check how tightly it fits. It should be a rather snug push fit.

 

[pa23driver]

that got me thinking...

i replaced the needle jet o-rings last year when i had the carbs apart, they were toast. i remember having a hard time getting the needle jet to seat completely after i replaced the o-ring, maybe i didn't get them pushed all the way in.

thanks 5twins

 

[xjwmx]

If that doesn't solve your problem - I don't know if your '77 has dual coils and I'm too lazy to find out, but it if does a bad coil on that side could cause that. If so, switch coils. If no joy, triple check your float condition and setting, as well as your mix screw setting. Half millimeters of float setting matter, as you're probably aware. I see about a 1mm float difference on the plugs, if it was a BS34, which I know it isn't.

 

[pa23driver]

i switched to a pamco with a dual output coil last year. i was having coil issues before i switched which made carb tuning a confusing endeavor.

i found out the hard way with float settings. was cleaning up the carbs on the gf's 1981 special, did a quick float adjustment and was off a little bit. didn't know that much fuel could flow out the back of carbs. the really amazing part is the thing still fired up and ran.

what do you guys find is the easiest way to check float level? i've been using the depth guage side of a caliper (with the slide locked) with obvious hit-or-miss results.

 

[XSLeo]

That's how I set my floats, just check both bulbs of the floats to be sure they are even, if not twist them till they match.
Some people cut a u shape out of cardboard or light sheet metal so it sets over both sides of the float as it sets on the carb body. That way you see the setting is right and that both bulbs match.
Using a clear hose oof the float bowl drains held up beside the carbs as the are on the bike, held straight up. The fuel in the bowl fills the clear hose up to match the level inside. The level in the hose should come up to 1 mm below the carb body sealing surface on the BS38 carbs and at the sealing surface on the BS34 carbs.
If you have Factory repair manual it shows how to do this. Biker.net has them as a download.
Leo

 

[xjwmx]

Using a clear hose ...

I doubt that method is accurate enough. Plus, if you lower the hose and then raise it it seems like it would show a false level. Unless the motor was running in which case you'd be dealing with the vibrations. 1/2mm, 1mm certainly is a significant difference with the bs34s, if you're trying to tune the carbs to the degree that people here seem to want to.

 

[XSLeo]

Move the hose all you want. The level in the hose is the same as the level in the float bowl. Liquids in a hose like that always match levels.
It is more accurate than measuring. You can actually see the level. When you measure you are just approximating what you see with the hose.
I measure just because it's easier, and accurate enough.
Leo

 

[weekendrider]

zjwmx,
imho, the clear tube method is the best. It allows you to verify that the float setting (dry) is working correctly.
Done while running you can actually see when the bowl is filling.
My experience is it will pull the volume down about 1mm, running vs. non-running.
And that 1mm is the "working" area of the float needle seat.
If it drops 3-4mm before refilling something is sticking.
If it doesn't maybe it is getting fuel from someplace besides the float valve (o-ring maybe).

 

[retiredgentleman]

The clear hose on the bowl drain, works just fine for me. I never use the measured float height method. I think you can get error from either way, but I like to see the actual fuel level.

 

[weekendrider]

If you lower the hose it will make-up volume and rise to the same level.
But if you lower it, it fills and the float valve shuts it will still have that volume and it will need to go someplace if the hose is raised. It sure ain't going back up the hoses into the tank.
Place your hose and clip/wire it place or you change the varibles.

 

[xjwmx]

Ok, then look at it this way.

!. Let the hose fill.
2. Lower the hose some amount.
3. Gas will run in through the seat and will fill up some empty tube and seek its new level.
4. Raise the hose again. Some gas in the hose runs back into the bowl, and now the level in the bowl is higher than the floats would have allowed.

If you do it with the engine running, then it's self-correcting eventually. But it will be shakin. Just measure with a mm ruler. You really need 1/2mm accuracy in my opinion.

 

[xjwmx]

Because with 1/2mm accuracy you can tune the stock BS34 so well that it keeps you from wanting to play with the jets and needle. Keeps me from wanting to anyway.

 

[pa23driver]

was able to work on the carbs today. dropped the bowls and reset the float height, they were both a little off - took my time and got them as close as possible to 24 mm.

checked the needle jets and it turns out there was a little of the old o ring stuck in the groove preventing the needle jet from completely seating - i'm thinking this was my problem since the floats were set more towards the lean side.

looked outside and noticed the rain had stopped for a minute and the temperature was a nice balmy 65 degrees so decided to take it out for a spin.

of course when you spend all day in a garage with the door closed, working with gasoline something is bound to go wrong. forgot to tighten down the float bowl plugs. she ran great until the left plug fell out.

can't find the plug. am i able to source this part locally or did i just earn myself a ridiculous shipping charge from mikes?

 

[jchrisk1]

I went to Cycle-Moore in Interlochen and picked some up. They have a bin full of old float bowls. Just take your other one with you to compare it with.

 

[pa23driver]

awesome, didn't even know about that place. i'll head on over there tomorrow

 

[jchrisk1]

They are Harley folks, but they aren't prejudice. They don't care what you ride and will work on anything. They are good people. I've been ordering my jets from them and they get them next day.

 

[pa23driver]

usually i deal with fox powersports near acme, the service/parts department is pretty good there - one of the guys rides a XS650. but i do like dealing with small shops when i can