syncronizing Carbs

Gmaan · Posted: January 21, 2009, 1:51 am Post subject: syncronizing Carbs

I built a synchronizer this weekend, and it work perfect. I do have one question, I can set the carbs and synchronize them at idle but when I rev it they go out of sync. I set the carbs in the middle. What is going on with my fuel system. The bike runs perfect.

Retiredgentleman · Posted: January 21, 2009, 3:36 am Post subject: Re: syncronizing Carbs

If you are using the U-tube manometer with oil or ATF, they are extremely sensitive and accurate. There are bound to be differences in the size of the carb venturies/inlet manifolds/head intakes etc.. So when you open up the butterflys and increase air/fuel flow, you are just seeing the differences between the two sides. Its nothing to be concerned about. When set at idle, sync. is done, and as you have seen the bike runs perfect.

xsjohn · Posted: January 21, 2009, 3:54 am Post subject: Re: syncronizing Carbs

The mix screws have to be dialed in for each carb correctly....each side set independently......if not when you come off idol they won't be the same.......slow runs of a few miles setting the mix screws and watching the plugs can get it right....there is more to it but that is a good beginning.....checking carb sink as you improve the mix.....

xsjohn

Gmaan · Posted: January 22, 2009, 12:25 am Post subject: Re: syncronizing Carbs

Thanks for the info, I feel I have the carb mixture pretty close. I need to verify the exhaust temp still. The plugs have a perfect light brown color, and most of my exhaust back fire when decelerating is gone.

Gmaan · Posted: January 22, 2009, 12:31 am Post subject: Re: syncronizing Carbs

Oh forgot a couple pics

xsjohn · Posted: January 22, 2009, 12:34 am Post subject: Re: syncronizing Carbs

Is the speedo cable touching the pipe.....or just looks that way......xsjohn

Gmaan · Posted: January 23, 2009, 2:25 am Post subject: Re: syncronizing Carbs

just looks that way.

pamcopete · Posted: January 23, 2009, 9:58 am Post subject: Re: syncronizing Carbs

Well, back to the subject, if only briefly..... Rolling Eyes

After syncing the carbs with whatever method suits you, warm up the engine and do the fine tuning by adjusting the center sync screw to get both exhausts to sound the same. This final tuning takes into account all of the dynamic differences in the two cylinders, and you can hear the difference off idle and all the way up the power curve as well. If you have done a good job with the conventional syncing, then you shouldn't have to twiddle the center sync screw more than +/- 1/8 of a turn to get both exhausts to sound the same, but you will be surprised at the difference it makes.

This final tuning, as well as the initial syncing, should be done with the mixture screws set to the nominal factory setting. I think a lot of tweeking of the mixture screws just masks the real problem......the carbs aren't synced, even after being synced! When I do this on my '81/H, I do not have to touch the mixture screws.

Obviously, this "dynamic" tuning is easier to do if you have a set of real dual exhausts withourt a crossover.

For a final touch, if you have vacuum barbs, connect the vacuum barbs together with a length of vacuum hose to equalize whatever remaining difference there is in the two cylinders. Amazing what this simple "mod" does to smooth out the idle. It will also extend the time between syncing sessions as well. More riding...less screwing with the carbs... Razz

If you have vacuum pet cocks, then just put a tee or two in the crossover vacuum hose to operate the petcock(s). Cool

xsjohn · Posted: January 23, 2009, 2:14 pm Post subject: Re: syncronizing Carbs

The mix screws have to be set to the proper low speed mix (fuel air ratio)......most of the cruise (low throttle) riding is on this mix....and decel.....since no 2 carbs are exactly identical this may require slightly different settings for each carb....connecting barbs seems to lean the mix....

xsjohn

pamcopete · Posted: January 23, 2009, 3:00 pm Post subject: Re: syncronizing Carbs

xsjohn,

Well, it's my contention that when people adjust the mixtures differently on each carb they are really compensating for a sync problem.

I have had the vacuum barbs on my '78/E cross connected for years with no evidence of the mixture being lean.

Have you tried this method of fine tuning the sync, John? Why not give it a try if you haven't already and let us know what you think. Start with both mixture screws set the same......humor me....

xsjohn · Posted: January 23, 2009, 3:27 pm Post subject: Re: syncronizing Carbs

Have connected the barbs in the past and just did it again to double check......my BS34's are tuned dead on...this required a larger pilot jet to make cruise and decel correct.....and when I introduced the barb connection a few minutes ago the idol went up 2 hundred rpms indicating a leaner mix......don't supose connecting them hurts anything if the mix is compensated for but correcting the mixes to me would be the best........

Also cylinder temp differential can create a cruise imbalance .....and the higher stock compression ratio makes cruise tuning more difficult ......since I dropped a 1/2 CR forcing the engine to have a bit more load on it a curise it rkuns dead smooth......(for a change)

And some mix screws can be set identical on some bikes and some can't....differences in carbs I suppose......I tune each cylinder to what it want's.........don't know of a better way......

xsjohn

pamcopete · Posted: January 23, 2009, 3:47 pm Post subject: Re: syncronizing Carbs

xsjohn,

The reason the RPM's go up with a vacuum barb cross connect is because you effectively have two throttle openings feeding each cylinder at idle, not because the mixture is leaner. You have to close the throttle plates to reduce the flow of air/fuel.

Have you tried slightly adjusting the sync screw with the mixture's set the same to get the same sound from the pipes? You can hear it at all RPM's.

xsjohn · Posted: January 23, 2009, 4:27 pm Post subject: Re: syncronizing Carbs

Just got back from an either or test...more than once 61 degrees....on mine it leans the cruise just a bit too.....mine runs smoother without the connection......could tune it out but don't see the necessity...........my carbs are not stock......and have already been dialed ........but from prior experence with BS34's it would have made the stock jetting even leaner...already being way to lean to begin with......maybe someone else can give a different anaylsis.......and all the imbalance has already been dialed out of mine .......

In all fairness I did try it......would be a step backwards for mine.....maybe someone else can find a different result .....and give a different test result.....shouldn't hurt anything though and if tuned with it on it may pan differently.....don't want to change mine to find out........

xsjohn

pamcopete · Posted: January 23, 2009, 5:59 pm Post subject: Re: syncronizing Carbs

xsjohn

The RPM's go up because each cyclinder is being fed by two carburators via the vacuum cross feed at idle, not because they are leaner. So, you just trim the idle speed down after you install the crossfeed.

And I have it installed on my stock '81 with stock BS34's......you remove the cross feed and plug the barbs when syncing.

xsjohn · Posted: January 23, 2009, 8:09 pm Post subject: Re: syncronizing Carbs

Pete.....Anyone that is that persistant deserves another chance......tried tying the barbs together again..........does seem to idol a bit smoother......will leave it on and see what happens........plugs were perfect before but wouldn't think it would effect that with my mixes....will give it a few hundred and see how they look......

The breather one way valves that some members got me to try do seem to make it run better too (made my own and use the stock air boxes modded) and my oil consumption seems to be way down now......

...thanks.......

xsjohn

pamcopete · Posted: January 23, 2009, 8:48 pm Post subject: Re: syncronizing Carbs

xsjohn

That's great, John.....I think you will like the difference. Yamaha did something similar on the early XS1 models with a passageway right in the head casting joining the two intakes together with a small orfice for each carb. Problem was they didn'y provide a way to block the passageway so there was no way to sync the carbs.

The Yamaha pre '83 XJ550, 650 also has a passageway that interconnects all four carbs for the same purpose. They called it the "Yamaha Induction Control System" or "YICS", beleive it or not. You have to have a special tool to block the passageway to syn the carbs, so they discontinued it after '83. I have an '82 XJ550 and the difference with and without the tool is very noticeable.

Using the vacuum barbs on the XS650 makes it easy to remove the crossover when you sync the carbs.

xsjohn · Posted: January 24, 2009, 4:31 pm Post subject: Re: syncronizing Carbs

Pete......With barbs joined the cold start this morning was much smoother......proof that you can teach an old dog new tricks.......now if you could just figure a way to make tires last....you should hear this thing idol with 8-1CR...dead smooth.....not one sound in it......except exhaust.........love it.

xsjohn

pamcopete · Posted: January 24, 2009, 8:45 pm Post subject: Re: syncronizing Carbs

xsjohn

The reason the engine starts better cold with the vacuum barb crossover is because each cylinder is drawing fuel / air from both enrichers. Even better, the first cylinder on the intake stroke effectively "primes" the other cylinder by drawing fuel / air into the carb throat all the way up to the barb so the second cylinder on the intake stroke has fuel / air "waiting" for the intake valve to open. Its enricher has already started to flow.

This means that you will probably get a good cold start just using the second notch that you provided on the primer rod. This eliminates the ultra high RPM's that occur when you use the full "choke" notch. The RPMs will probaly be higher, but not as high as when using the full "choke" notch.

The smoother idle is due to the fact that with both cylinders drawing from both carbs at idle, there is a smoother flow of fuel / air mixture. Twice as many intake strokes means that there is always a residual mixture "waiting" in the carb throat for the intake valve to open.

xsjohn · Posted: January 24, 2009, 9:18 pm Post subject: Re: syncronizing Carbs

I added a notch between the middle and wide open choke on the rod.....best if it is just a bit off center towards the middle position......just a file notch is all it takes to hold it there......with my mixtures 46 pilot 2 turns and the custom needles it runs perfect.........you should try the ones I sent you.....no shim......

xsjohn

Gmaan · Posted: January 26, 2009, 2:37 am Post subject: Re: syncronizing Carbs

This is great information. My old RD400 I added a cross over pipe and it runs real soothe. I have a cross over pipe and uni pod filter, and chambers on the RD. I had to redo the jetting when I originally built it, but is is still a single kick starter. I did not think this could be done on a four stroke. I will need to try it. I drove my bike to work last week, 65 miles one way. I got back into Napa, and it was running so sweet! I think syncing the carbs, and taking it out for a good freeway run made the whole thing come together. I double checked my ignition timing and it is still spot on, your ignition module is great too.
I love this forum so much good information to read!

tacoswild · Posted: January 26, 2009, 7:09 am Post subject: Re: syncronizing Carbs

Pamcopete,

That bit about the XS1 head caught my eye. Did the XS1B have this too? I have one and I've been wanting new carb holders with barbs so i can sync the carbs better but it sounds like it might not work? So far I've been using the dead cylinder method from the manual then listening to the exhausts to make sure they sound the same.

xsjohn · Posted: January 26, 2009, 10:18 am Post subject: Re: syncronizing Carbs

Connecting barbs on VM carbs ..........wonder if cold start with one choke might make things easier........?

xsjohn

pamcopete · Posted: January 26, 2009, 10:55 am Post subject: Re: syncronizing Carbs

tacoswild

I got that information from 71xs1b over in the garage. He is very knowledgeable about early XS650's. You might try contacting him.

pamcopete · Posted: January 26, 2009, 11:01 am Post subject: Re: syncronizing Carbs

xsjohn

Well, I'm not familiar with the VM's, but if you only have one enricher working, then the carb without the enricher would still draw air / fuel from the idle circuit, which would be leaner, so when mixed with the carb with the enricher through the crossover, the result would be a leaner mixture for both cylinders.

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