78 BS38 Syncing

[hjpaul7]

78 BS38 that I just cleaned and rejetted.

I should first note I haven't done the ignition timing yet, I skipped the order, so I am tackling that first today, THEN the carburetors. I have done plugs, plug caps/wires, can chain, and valves. But I'd still like to know how to so this.

I made my own sync tool with a tall piece of wood, tubes with 2 cycle oil in them. I hooked them up the valves that attach to the carbs. Fired it up (after its warm). It is really hard to adjust these because the bike does not want to idle for long, I had to have my neighbor hold the throttle. Immediately the levels were way off. One started shooting up and the other down, I had to kill the bike so the oil wouldn't reach the valves

Now, I'm adjusting the screw in the front end of the carbs, that's in between the two carbs right? Pretty big flat head screw. If it is, it is insanely hard to reach. I had to use a bendy screwdriver attachment but that barely stayed in place. Any how, I adjusted the screw both ways while it was fired up and no change. Am I doing this correctly?

It's also popping out of the exhaust only when the choke is OFF and revved. I turned the mixture screws IN to tighten and it helped a bit, but I feel like I may go to far. It's really put putty in low RPMS in 1st and 2nd.

 

[5twins]

This is the sync screw .....



You really should have done what's called a "bench sync" while the carbs were off for cleaning. That's nothing more than visually adjusting the butterfly plates so they're the same, open the same amount. Then a final sync once they're mounted and with gauges hooked up usually requires no more than a quarter turn either way of that hard-to-access sync screw.

Your sync seems pretty far off. I would pull the tank and give the sync screw a full turn in one direction or the other. Start the bike and observe if things have improved on the gauges. The bike will run for a minute or so on just the gas in the carbs. If you chose the wrong direction on the screw, go back the other way and check the results. You should be able to get it pretty close this way, then install the tank and do the final sync. The sync screw shouldn't need much turning then. I use a long shaft 3/16" tip Stanley screwdriver something like this to reach in from the side and tweak the sync screw setting .....

http://www.ebay.com/itm/3300-Stanle...470?pt=LH_DefaultDomain_0&hash=item51be7554ee

 

[hjpaul7]

How do you do the bench sync? I have a screw drover that length, but it doesn't work from the side since the screw faces up.

 

[5twins]

It does work from the side. You stick the blade in from the side and twist. The sync screw will turn a small amount. Lift the handle as much as possible so the shaft angles down towards the screw. I know it works because I do it that way all the time.

For a bench sync, the carbs have to be removed. Hold them up to a good light and look in the engine side at the butterfly plates. Back the idle speed adjusting screw on the left carb way out so it's not touching the cable arm at all. You want to see a space between the screw and the arm. That will close the butterfly plates completely, both of them if they are synced, and you would see no light showing around their edges. In your case, since the sync is off, one plate will be closed up but the other will still be open some. Simply adjust the sync screw to close that open plate and match it to the other one. Keep turning the screw and the other plate will begin to open. Work the screw back and forth watching for each plate to just start opening. Sync will be midway between those two points. Turn the idle speed screw back in until both plates just crack open a hair. You want to see just a sliver of light beneath both. Once started, you can dial the idle speed in better.

 

[weekendrider]

You can remove the bolt on the back of the tank and lift it up for easier access to the sync screw.
"Bench sync" is to match the throttle plates while they are off the bike.
Some like to put them over a light and looking through the bore try to match the amount of light they see coming the gaps.
Others like to put something (a small drill bit, popsicle stick, piece of paper) under the throttle plate to match them.

If you add a valve to one side of your sync tool tube it makes it easier if you are working by yourself.
If you add a reducer to the tubes (pilot jets work well) it helps to steady the pulsing of the fluid.

 

[hjpaul7]

It does work from the side. You stick the blade in from the side and twist. The sync screw will turn a small amount. Lift the handle as much as possible so the shaft angles down towards the screw. I know it works because I do it that way all the time.

For a bench sync, the carbs have to be removed. Hold them up to a good light and look in the engine side at the butterfly plates. Back the idle speed adjusting screw on the left carb way out so it's not touching the cable arm at all. You want to see a space between the screw and the arm. That will close the butterfly plates completely, both of them if they are synced, and you would see no light showing around their edges. In your case, since the sync is off, one plate will be closed up but the other will still be open some. Simply adjust the sync screw to close that open plate and match it to the other one. Keep turning the screw and the other plate will begin to open. Work the screw back and forth watching for each plate to just start opening. Sync will be midway between those two points. Turn the idle speed screw back in until both plates just crack open a hair. You want to see just a sliver of light beneath both. Once started, you can dial the idle speed in better.

Thanks I'll take the carbs off and give this a shot. Is the popping related to them not being synced?

 

[5twins]

It can be, also your mix screws may be closed up too much. The spec for the '78-'79 carb set is 2.25 turns out but many of them like more, say 2 1/2 to 2 3/4 turns out.

 

[hjpaul7]

It can be, also your mix screws may be closed up too much. The spec for the '78-'79 carb set is 2.25 turns out but many of them like more, say 2 1/2 to 2 3/4 turns out.

I had them at 2.5 turns out, but it was popping like crazy just idling, I'll mess with it again. When I tightened the scews all the way, I tightened it till I got resistance without really cranking them down, then 2.5 turns out.

 

[5twins]

That's correct. You don't want to crank the mix screws in super tight. They have a pointed end and the bottom of the hole is tapered. You can damage that seat if you crank them in too tight. If opening the mix screws up to near 3 turns doesn't help with the popping and you installed a larger pilot during your re-jetting, you may have an air leak. It shouldn't pop like that with larger pilots and the mix screws open that much.