Need feedback on my carb tuning plan of action

[blackbetty]

Sorry for being wordy....

I've read through the detailed carb guide and am just wanting the carb pros to review my plan of action, make any suggestions if you have any.

Here's the details:

76-77 BS38's
Uni pods
Straight pipes
Carbs are completely rebuilt with new diaphragms

Valves are set, .004 exhaust, .002 intake
Timing is set (Boyer ignition)
Timing chain adjusted

Rejetted to:

125 main jet
27.5 pilot jet
Dropped needle 1 slot
Mix screws a little over 1.5 turns out.

I have not synched the carbs yet (anyone know where to get m6x1.00 threaded nipples for my carb set?)

Was planning on using dead cylinder method to adjust mix screws and to synch since I don't have threaded nipples to use in the vacuum ports for my carb set just yet.

Right off idle it stumbles badly; gently rolling the throttle helps reduce the break up.

Right cylinder is popping occasionally out the exhaust at idle. It's honestly a little disconcerting, but this seemed to go away as the motor got warm.

Other than that, it starts and idles very nicely.

Since I haven't ridden the bike, I can't do plug chops, but after idling for a few minutes, they are blackish. I do know there is a small valve seal leak on the right cylinder, but the motor doesn't smoke and the exhaust doesn't smell rich Regardless, the carbs in this state make the bike unrideable anyway...

My plan of action is to first make sure the syching is on (I did not play with the synch screw during the rebuild, and the carbs were tuned when I bought the bike). I plan on syching using dead cylinder method, unless I can figure out where to get some threaded nipples for a manometer.

Second job is to bring the mix screws back to 1.5 turns out and use dead cylinder method to tune these.

After this, the carb guide suggests using a smaller pilot, but I only went from the factory 25 to a 27.5 with open pipes and pods, so I'm assuming my worries are mix related.

What could be causing the intermittent pops on the right cylinder? The pipe on the right cylinder is slightly longer than the left, would baffling help reduce this?

Any suggestions are appreciated; I also have a short video of the bike running if necessary.

 

[5twins]

I would open the valves up a little, use .006" for the exhausts and .003" or .004" for the intakes. Then I would go bigger on the mains, maybe to a 130. You're only one size up now so lowering the needle may not be needed yet. With 130s it probably will be but with 125s, probably not. Put the needle back to the stock position and it will probably run better now, eliminating that bad spot just off idle.

You can get the M6 nipples from 650Central. Until then, you can do a pretty close bench sync by visually inspecting and comparing the throttle plate opening. The carbs must be removed for this but you can get sync very close with this method.

You can set the mix screws by ear. Start them at the 1.5 spec then work each one (do one side at a time) in/out 1/4 turn at a time until that cylinder starts to stumble. Best setting is somewhere in between the two stumble points. I run mine to the rich side of this range.

 

[retiredgentleman]

This link has some good info;
http://www.xs650.com/forum/showthread.php?t=9181

You can use a strip of paper as a feeler gauge. Adjust the sync screw until you get equal drag under each butterfly. After checking with a manometer, I found this method was very close.

When you do the dead cylinder tuning of the mixture screw, you will not likely hear any rpm change if at 1200 rpm. Lowering the rpm down to 300 to 600 rpm will allow you to detect a change.

 

[blackbetty]

Very good feedback, thank you 5twins and RG.

I actually did a bench sync before I put the carbs on the bike in a pitch room with an LED flashlight shining through the backside of the throat, and they looked pretty even; the exhaust tone sounds decent too. RG, I read throguh that thread this morning, I will also try the strip of paper technique to check my baseline.

Here's a quick vid:

 

[blackbetty]

FYI everyone,

If you only need the 6mm brass barbs for synching BS38 carbs, motion pro sells them:

http://www.motionpro.com/motorcycle/partno/08-0219/

 

[XSLeo]

Go to most any hardware store or auto parts store. Get a grease zerk with the 6 mm thread. Use a samall drill to open the hole where the grease goes in. this releases the ball. This makes the grease zerk straight through. Now use these to thread into the carb and hook the manometer too, the homemade one of course. Cheap and very effective.
I didn't come up with this greasezerk mod fitting deal so I can't take credit, I just can't recall who did.
Leo

 

[blackbetty]

So to update, the first thing I did was adjust the mix screws and I surprisingly got really good results. I was able to take the bike around a few blocks and it ran pretty well save for a few pops out the left cylinder. I also readjusted the valves to .004 intake and .006 exhaust.

I took it home that night, and when I came to ride it the next day, I got it cranked and rolled it into the street. It died, and I started messing w the mix screws again before realizing I was out of gas. After I messed w the mix I couldn't get the bike to idle even after putting the screws back in to 1.5 turn baseline.

So I put the needles in the middle slot and it seemed to run too rich. I went back and dropped them a slot this morning, as well as do a bench sync, and I still can't get the bike to run right or idle well enough to get a manometer on there.

At times the left cylinder will smoke, and I can't seem to keep it from backfiring, it's shooting flames. Plug on left cylinder is sooty.

I have the screws back at 1.5 turns out and it still is difficult to even get a proper idle, and the popping out the left pipe is an issue. I need to also note that the idle will race occasionally, and it's like a runaway idle, I have to hit the kill switch. When I went to adjust the needle position the slide was wet w fuel.

At best, I can manage to get the bike to idle crappily, but the left cylinder shoots fire, and if I crack the throttle too fast off idle it'll simply snuff the motor out.

The strange thing is I can get each cylinder dialed in pretty well using dead cylinder, but as soon as I fire the bike back up on both cylinders it runs like complete shit.

 

[ProSimex]

your timing is off I think, also check your advance assembly and make sure its working properly.

 

[angus67]

how clean is your gas tank? running a fuel filter i hope. Running out of gas could have sucked shit into your carbs, blocking your pilots. Thats what it sounds like to me. could also relate to dying motor with quick throttle release, the sudden closing of the plates slams shut the needles in the jets, and if your pilots are plugged, they cant keep up unless you close the throttle slowly.

runnaway idle...after the bike was warmed up, did you re-tighten the carb boots? you can get them tighter when they are hot.

 

[blackbetty]

Tank is new, running dual petcocks and each carb has its own filter.

The left carb is running way too rich, it's dumping fuel into the pipe which seems to be causing my flamethrowing problem. When I went out and pulled the slide to readjust the clip, it was wet w gas. Plug also wet.

But again, when I use dead cylinder and get the carbs to run one at a time, I can wrangle the idle in a bit and the popping seems to go away.

On another note, I put my compression tester on the motor and it read 90psi on both sides ....BUT, I think my tester is bad so I'll do a leakdown test to make sure my top end is holding.

And my issue is that I can get the bike to a rough idle, but throttling off idle will kill the motor. A gentle roll can keep it going, but if I snap the throttle off of idle, it gasps and dies.

I re tightened the intake boots as well.

Prosimex -

I've got a boyer unit and the timing is dialed. Since I was getting some popping out the carbs I dialed it back a bit, and that seems to have helped.

 

[5twins]

You've made no mention of float levels. Are they set exactly to spec (25mm for your carb set)? When you tested compression, did you have the throttle wide open? You'll get a low reading if you don't.

 

[blackbetty]

5Twins......oh man....

I did not have the throttle open! I can't believe that. I'll report back tonight after I retest compression.

I'll also reset the floats this evening, see if the left carb is out of spec. I set them to 25mm +/- 1mm when I rebuilt them, but I'll reconfirm their positions.

 

[5twins]

Measure from each side on the floats. In other words, take two measurements per float. Many times, the float can be twisted or bent on it's mount. If you only measure from one side, it could still be off. The 38s are nice because there's no main jet tube in between the float bulbs. You can place the ruler on the far side and sight evenly across the tops of both float bulbs to get an accurate reading. Here's a '76-'77 float set correctly at 25 .....

 

[blackbetty]

Very helpful sir, thank you 5twins!

I'll report back this evening...

 

[XSLeo]

I think it was xjwmx did a little testing on float hieght. If I remember right he set them exactly to the spec on his carbs, the BS34's as I recall, 27 mm. bike ran great. Reset to 1 mm off and was to rich or too lean. So it needs to be very close, and both sides of the float has to be right.
I think it might have been gggGary that built a gauge, u shaped so it sat over both floats at the same time. That way you could check the heiht on both floats at the same time.
You might find it in the Homemade tools thread in the TECH section.
Leo

 

[blackbetty]

Thank you for the suggestion Leo ^

I went ahead and pulled the carbs last night to go over them. Good news is I did the compression test correctly this time and got just under 150psi on each side, so I'm happy we can rule out an engine issue.

After I got the carbs off and pulled the float bowls, I noticed the floats were WAY off. Measuring each side, they seemed to sit about at about 35 or so millimeters. I readjusted to 25mm per 5twin's instructions using a small ruler that came with an edelbrock rebuild kit.

I also reconfirmed the jet sizes and made sure all passages were clear. After I pulled the carbs, there was raw fuel in the left runner on the head as well as on the slide and on top of the diaphragm? Is it unusual to see fuel on top of the diaphragm?

Tonight I'll get the carbs back on the bike and put in some new plugs, see if it runs better.

Thanks everyone for your patience and feedback while I sort this out.

 

[gggGary]

Please no rubber fuel line, tygon or other branded fuel line. Clean and buff the float valve and float valve needle, be sure the needle spring is free and has decent "spring" to it.

 

[blackbetty]

Thanks for the suggestion gary, here's what I'm using:

Helix fuel line here:

http://www.oemcycle.com/Item/product/6547

The float valve and needle were new replacements from the mikes 76-77 BS38 rebuild kit Part #48-1920

Everything inside the carbs seemed to be in good order save for the whacky float adjustment. But my understanding is a float adjustment that was too high would cause a lean condition, when clearly it seems like the motor is way too rich?

 

[gggGary]

New parts from mikes (or anyone else) doesn't mean they don't need attention or close inspection, just sayin............

This might be bragging a bit here's set going back on the bike today. We will see if my practice matches my advice. Had to tweek and set the "appear to be new" floats. These were "pretty good looking" carbs when I started. They had fuel line chunks and a PO's fuggled screws and adjustments. At least 6 different jets, passages and orifices were plugged or restricted. I would guess I am 6-7 hours into these in my fits and starts where is the ???, buff this, polish that way. Pretty much everything checked but the throttle seals, the shafts are snug so I 'll leave em for now. The PO fuggled one butterfly plate screw head and quit, WHEW.

 

[blackbetty]

Please report back to us if those carbs perform as intended after 6-7 hours of work!

I have a few hours in my set polishing, cleaning, etc, but certainly not 7 hours!

I replaced everything on mine including the shaft seals, but you are right in that mikes certainly has a less than stellar reputation for their carb rebuild kits. I'll actually recheck the float valves and see if they match up with the ones I removed from the carbs.