Need help sync'n carbs!

jerseyjew · Full Member Joined: Mar 28, 2008 Posts: 48

Ok, first I have a 78 Special. The bike has vacuum operated petcocks which have a hose that run from the petcock to the a barb on the carb holder which is sandwiched between the carb and the engine. My friend just picked up a nice sync gauge with some misc barbs for different bike setups. I have searched and am having trouble figuring out exactly where the hoses from the sync tool need to be hooked up to. (I was thinking maybe just the barbs that provide vacuum for the petcocks, but then I wouldn't be getting any fuel) I also need to figure out where I make the adjustments once I'm getting a good vacuum reading. Some sort of DIY tutorial or picture of the carb would be very helpful! The haynes manual doesn't help at all!

weekendrider · Posted: September 7, 2008, 8:30 pm Post subject: Re: Need help sync'n carbs!

Set your petcock on pri that gives a direct flow of fuel. Balance them with the idle screw for the right carb. The one in between the two carbs.

jerseyjew · Full Member Joined: Mar 28, 2008 Posts: 48

weekendrider wrote:

Set your petcock on pri that gives a direct flow of fuel. Balance them with the idle screw for the right carb. The one in between the two carbs.

So just hook the sync tool up to the barbs that provide vacuum for the petcocks? And the only screw I am looking to make adjustments on is the middle idle valve which changes both carbs at the same time?

killtrip · Posted: September 7, 2008, 9:20 pm Post subject: Re: Need help sync'n carbs!

I think you use the middle one for the right side carb, then you use the idle screw for the left side carb..

someone correct me if I'm wrong? I remember reading somewhere that you should do the right side first, then the left side.. again, someone correct me if I'm wrong please? I dont want to be giving false information.

weekendrider · Posted: September 7, 2008, 9:39 pm Post subject: Re: Need help sync'n carbs!

Yes hook your tubes to the intake manifold barbs. Use the idle screw in the middle to match/sync the carbs. Then the one the outside will be used to adjust the idle for both as they will now be in sync/tune/matched.
This is if you have stock carbs on a 78 where they are linked together.
If you have independent carbs the procedure is different.

The devil is how do you get to the middle (right carb idle screw) with the tank in place. For a dollar I'll tell ya. Wink

jerseyjew · Full Member Joined: Mar 28, 2008 Posts: 48

weekendrider wrote:

Yes hook your tubes to the intake manifold barbs. Use the idle screw in the middle to match/sync the carbs. Then the one the outside will be used to adjust the idle for both as they will now be in sync/tune/matched.
This is if you have stock carbs on a 78 where they are linked together.
If you have independent carbs the procedure is different.

The devil is how do you get to the middle (right carb idle screw) with the tank in place. For a dollar I'll tell ya. Wink

Umm do you accept paypal? Lol, I have no clue how I am going to tackle this.. Especially with the engine running!

weekendrider · Posted: September 7, 2008, 10:39 pm Post subject: Re: Need help sync'n carbs!

Well I'd rather have you owe than beat me out of it. Get 3-4" of fuel line and a double male barb to connect this line to the carb line then sit the tank on a saw horse/table etc. to the side. DON'T think it will balance on the rear fender with the bike running like I would. It won't.

jerseyjew · Full Member Joined: Mar 28, 2008 Posts: 48

weekendrider wrote:

Well I'd rather have you owe than beat me out of it. Get 3-4" of fuel line and a double male barb to connect this line to the carb line then sit the tank on a saw horse/table etc. to the side. DON'T think it will balance on the rear fender with the bike running like I would. It won't.

Thanks for the help! I am looking forward to seeing exactly how far my carbs are out of sync! I will post back with results.

Retiredgentleman · Posted: September 8, 2008, 12:32 am Post subject: Re: Need help sync'n carbs!

On my 78SE I am able to adjust the carbs balance screw, with the tank in place,...............long bladed screwdriver.

jerseyjew · Full Member Joined: Mar 28, 2008 Posts: 48

Retiredgentleman wrote:

On my 78SE I am able to adjust the carbs balance screw, with the tank in place,...............long bladed screwdriver.

I am definitely going to give it a valiant effort before I try removing the gas tank.

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

Motion Pro has a plastic gas tank used just for tune-ups. Take the stock tank off the bike. Put gas in the plastic tank, hang it up high and connect it to your carbs. Then you can access that hard-to-get screw and have some light to work by.

There are two models of that Motion Pro tank. I think the el-cheapo (the one I've got) sells for around $22. Might as well get one. The price of the hassel you save yourself will add up over time. You'll save enough time after two or three tune-ups to make it worth the investment. Your emotional health will improve. You'll become a more stable human being. . . . Cool

xsjohn · Posted: September 8, 2008, 3:38 pm Post subject: Re: Need help sync'n carbs!

Really only need one vacuum guage.......fish tank adjuster (walmart) and some tubing........get it idoling decent and adjust the right to match the left..........taking it back and forth........and messing with the center adjustment........

If you really want to get cheap you can use a cheap tire pressure dial guage...like at autozone........take it open and bend the wand to 10 lbs.....it will read backwards...........

xsjohn

SRDESIGNER · Full Member Joined: Jun 05, 2008 Posts: 31

Never done anything like this before so this is NOT a challenge. What does the valve do? Is it not possible to simply slip the hose on the manifold port straight on to the vacuum gauge?

y current plan is to pick up two guages from harbor freight glue them both on a piece of wood and hook them straight onto the hose barbs...if I need valves, I'll certainly get them but I'm just curious to know how they play so I can do it all correctly when I've got it built.

Thanks a million

xsjohn · Posted: September 8, 2008, 5:50 pm Post subject: Re: Need help sync'n carbs!

Really only need one guage when it is all said in done.......the valve trims the vacuum pulse so the wand doesn't jump all over the place.......trims it out nicely.........for a couple of bucks.......

Only rebuild what is necessary......most of this stuff lasts a long time.....later carbs trash everything to please the epa and makes things even worse if not delt with.....and even before 80 the right cylinder stresses things because it is lean after the thing warms up.........no body believes me but what the heck........run it for 50 thou and see....it won't smoothh out when it's warm when one cylinder is 30 degrees hotter and being fed the same and running lean ......like the fat girl and the skinny girl....pick one.... but not at the same time for smooth running....especially if the wife is anywhere near the activity or you are riding it..........

Right side just plain and simple for sure runs hotter that the left......done deal as far as I am concerned........simple fix really after all the getting the rest together is solved first.......another balance factor for sure........

xsjohn

kingwj · Posted: September 8, 2008, 7:12 pm Post subject: Re: Need help sync'n carbs!

I just stick one end of a length of 1/8" ID tubing into the carb and the other end in my ear. When the pitch is the same in both carbs the flow is the same. Works on my MGA, XS650 and XS750. It would also work on a 275GTB with 6 Webers with 2 barrels.

weekendrider · Posted: September 8, 2008, 9:39 pm Post subject: Re: Need help sync'n carbs!

A plastic gas tank can usaully be scrounged from your local lawn mower repair.

kingwj wrote:

when the pitch is the same

well now that's a handy tidbit that makes sense
too bad I can't carry a tune in a bucket Sad

jerseyjew · Full Member Joined: Mar 28, 2008 Posts: 48

kingwj wrote:

I just stick one end of a length of 1/8" ID tubing into the carb and the other end in my ear. When the pitch is the same in both carbs the flow is the same. Works on my MGA, XS650 and XS750. It would also work on a 275GTB with 6 Webers with 2 barrels.

Hahaha.. that is a pretty insane tuning method. I like it!

Tomterrific · Posted: September 9, 2008, 8:28 am Post subject: Re: Need help sync'n carbs!

I've got points ignition. One benefit is that I can pull each plug cap for a three count. The engine goes from smooth to dut-dut-dut as it runs on the oposite cylinder. It's easy to tell an imbalance and I adjust accordingly to get each cylinder running the same speed. I use this method on any twin carb, twin point bike. It's fast, it works and I carry the tools with me (fingers and ears).

Tom Graham

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

A variation on the "dead cylinder" method that Tom mentioned is to make up a couple "special tools" out of old spokes. A spoke nipple has the same threads as the top of your spark plug so it will screw right on. With the "special tools" installed on the plug tops, you can then start the bike on both cylinders. Alternately short one cylinder, then the other, with a long screwdriver to the jug. Compare the sound and adjust as necessary.

Retiredgentleman · Posted: September 9, 2008, 2:16 pm Post subject: Re: Need help sync'n carbs!

This is a very simple,very accurate and very cheap method to sync carbs. Buy 22 feet of 3/16 ID, 5/16 OD plastic tubing ( Rona/Home Depot/Lowes) and fasten to any board that is handy. Just form a loop that is about 4.5 feet long, and fasten the loop to the board. That leaves 6.5 feet to run over to the bike intake barbs. Put one end in some "Sta-bil" and suck the other end until about half the loop is filled with fluid. Requires only a few onces of fluid. Any fluid can be used, I used "Sta-bil" because I had some handy, its bright red so easy to see, and its engine friendly if it accidently got sucked into the engine. Total cost about $6.00 to $7.00 . I thought I would need a restrictive orifice in the 2 lines, but turns out it works fine with just the tubing connected.
Picture attached, shows engine running at 1200 RPM and sync completed. If you think your carbs are badly balanced you may want to use a clamp on each tube as a restriction until you can correct the balance. My carbs were very close so I did not need to use clamps.

jerseyjew · Full Member Joined: Mar 28, 2008 Posts: 48

Well I just got finished sync'n the carbs and boy were they way off. I appreciate everyones help on the board. I ended up being able to adjust the middle screw with a long blade screwdriver. No need to remove the fuel tank!

nj1639 · Posted: September 9, 2008, 9:47 pm Post subject: Re: Need help sync'n carbs!

Well.....which method did you use? Inquiring minds want to know!

jerseyjew · Full Member Joined: Mar 28, 2008 Posts: 48

nj1639 wrote:

Well.....which method did you use? Inquiring minds want to know!

A friend has a 4 carb sync tool made by motion pro like the this one.

I simply used that by hooking the lines up to the vacuum ports on the carb holder and adjusting the screw in between the carbs with a long blade screwdriver.

nj1639 · Posted: September 11, 2008, 5:56 am Post subject: Re: Need help sync'n carbs!

After adjusting with that sync tool I'd just HAVE to check the ' tube in carb, to ear' sound comparison.

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