Setting carbs up

kenb · Posted: June 22, 2008, 4:53 pm Post subject: Setting carbs up

Hello guys,

I've managed to get the bike running since we last talked about the carbs, and the engine seems to be running OK based on a couple of rides I've taken around the block. I'd like to take it out on the open road to see how it really runs, but pesky little details like no insurance, plate, or motorcyle rating on my drivers licence tend to inhibit that type of thing. Although the bike is ridable now, the behavior at idle isn't what I'd like it to be.

Things have changed a bit since my last post about the carbs. I picked up a pair of incomplete parts carbs (BS34's) with the intent of using the parts to rebuild my original pair, but I changed my mind about that once I got them apart and noticed that they were in much better shape than the 1980 ones that came on the bike. Once I noticed this little detail, my originals became the parts donors and I rebuilt the others instead.

So here's what we've got on the bike now:

-Identical BS34's, from 81 or later since they have plastic floats. These carbs have been thoroughly cleaned out, not that they seemed to need it since they were very clean when I got them. I set the float levels on the bench to roughly 2-3mm below the top of the bowls. See the picture below to see the level.
-K&N pod air filters
-Main jets changed out to 135
-Pilot jets changed out to 45
-Aftermarket mufflers of unknown origin to replace the rusted out and leaking stock units that came with the bike. Although you can see right through these mufflers when looking in from the ends, they weren't as loud as I thought they would be. See them in the pictures below.
-Idle mixture screws opened 2.5 turns from bottomed out
-No leaks at the rubber carb connections to the head, spraying them with carb cleaner made no difference in idle speed.

And here's what the bike is doing:

-Bike starts right away with no choke, suggesting a rich condition.
-Slightly uneven idle (roughly 1100 rpm) after warmup, will always stall if left alone for a minute or two.
-Dead throttle zone between 2000-3000 RPM, almost impossible to maintain steady RPM within that range.
-More smoke out of right pipe than left.
-Occasional chuffing & popping.
-Engine generally doesn't sound too happy at idle, prone to vibration, suffers from inexperienced motorcycle mechanic.
-Bike runs fine out on the road once out of idling mode, lots of low end torque and good acceleration.

I'm a little buffalo'd about where to proceed, maybe you guys can help me figure out where to go from here.

Ken

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

On the BS34s, there's supposed to be a rubber plug or cap over the pilot jets. Do you have that in place? If not, the pilot jet will draw too much fuel and the idle circuit will run rich.

You'll also want to check your sync and play with different mix screw settings to see if that helps. Try the mix screws another quarter turn in/out, run the bike a bit, and see if it helps or makes things worse. If worse, go in the other direction. Eventually, you'll find the best setting.

xsjohn · Posted: June 22, 2008, 8:26 pm Post subject: Re: Setting carbs up

Good luck on all that guys.......will be interested to see how far it gets down the express.......mine 10 years ago would make one exit in desperation....now there has been thousands....5 years ago with the big main jet it only cost me a top end.....have to get all the quivers out and the temps controlled.....and all mid range mixtures correct as possible watching the right temps and geared properly so it doesn't rev it's self to death even with all else being correct........then there is a chance....but never with the stock carb parts of any....for sure....if not it makes one hell of a good beer runner.......slide mine out and 200 milles later slide it back in all is cool.....till some bearing craps which seems to be a real problem if these are worked instead of looked at and is fun work for all given enough use and the inclination......another of the many catch 22's for sure.....xsjohn

kenb · Posted: June 23, 2008, 9:44 pm Post subject: Re: Setting carbs up

5twins wrote:

On the BS34s, there's supposed to be a rubber plug or cap over the pilot jets. Do you have that in place? If not, the pilot jet will draw too much fuel and the idle circuit will run rich.

Hi 5twins,
Yes, the plugs are in place.

5twins wrote:

You'll also want to check your sync and play with different mix screw settings to see if that helps. Try the mix screws another quarter turn in/out, run the bike a bit, and see if it helps or makes things worse. If worse, go in the other direction. Eventually, you'll find the best setting.

The bike is running much better tonight! I made up a manometer today out of clear plastic tubing, using my old stock set of of pilot jets as restrictors. I made a point of keeping my thumb close to the kill-switch when I started it up the first time (which was a good thing!) just in case one side was pulling harder than the other. Sure enough, the second the engine fired up, the fluid in the tube connected to the left side shot up to the top almost instantly. Since I still had a good six feet of tubing between there and the manifold there wasn't much danger of it making it into the engine, but it definitely convinced me that there was a balancing problem between the carbs.

I guess it took about 3/4 of a turn on the adjusting screw on the linkage between the carbs before the vaccum draw was evened out between both sides, and the difference in the way it runs is outstanding. Once I got things balanced out it was much easier to observe the subtle differences made by adjusting the mixture screws, and I ended up turning both sides back in again to their final setting at just over two turns out from the bottom.

The engine will idle relatively smoothly now at just a hair over 1000 RPM, and the flat spot between 2000-3000 RPM is still there but it has been substantially reduced. I'll see if it needs the choke to start tomorrow, as it's still going to be a bit on the warm side after running it tonight.

The only thing that still concerns me is the position of the idle set screw, which I had to turn all the way in against the compression spring to maintain the idle at 1000 RPM. The screw didn't have to be turned in that far before, so I'll have to revisit that little detail tomorrow when I get back from work.

Ken

jimdi · 650Rider Supporter Joined: Oct 27, 2005 Posts: 512

I have been there - you have an over rich lower end- the idle speed screw is turned in all the way to let in enough air to compensate for all the gas pouring in.

If you are that rich you flood the plugs it kills the lower end. Try 1.5 turns out to start - on the idle mix - and you may have to drop those canadian needles...

Over rich can cause stumble if it is fouling the plugs.

Also not being able to ride it effect the warm up and all these settings should be done warm....

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