Carb sync-ing

Are the washers exactly the same size?

Washers both the same Bill. I placed each washer on both carbs. 

I did swap the Snails over thinking that maybe there not reading the same. That proved me wrong, still the same readings. 

The snails should read the same.....If not, you have some adjusting to do. I have always found it better to come down to the lowest reading...ie....bring the 18 down towards the 12. The 12 will probably go up so watch closely..... There are others on here that have the same system you do so they can give better advice.   I use a Vintage Speed "PUSH" linkage system which allows slack at idle but allows sync adjustments after all the slack is taken up and set at about 2500 rpm. The Hex Bar System is junk IMHO ......Bruce

Everybody here (and over on TheSamba) loves, Loves, LOVES the hex-bar, Bruce. If they can't love it anymore, they move on to a Scat linkage, or CSP, or anything (ANYTHING!) but a sync-link. Apparently, because a sync-link costs 2x what the CB linkage does, guys feel it's over-priced. It's cheaper than buying 3 different linkages, and feeling less than satisfied.

Stan, I'm with you but I've got it it now so need to persevere.

I was using the centre pivot linkage that Raby's sold when l ordered the Dtm many years ago. I did find that good in the last engine set up. For curiosity l did shelve the Sync recently and put the Raby unit back on and noticed the Sync was much smoother even with this problem. So I'm back with the Sync-Link again. 

So putting that aside, this problem I have once the carbs are dialled in at idle without any linkage connected. As soon l move  it of idle which l used a washer to hold both carbs exactly the same from idle. I'm getting differant readings, 12 and 18. If l pull the washers the carbs return to idle and both read 5 on the Snails. If l put the washers back in place and adjust bringing the 18 down to 12, once l pull the washers and it returns back to idle they're out of sync and runs uneven.

In the past I've always sync'd them at idle and left it at that. Presuming  the'd be synced of idle also.  So can be causing this and what can l adjust to keep them sync'd both on and of idle??? 

I did think maybe its down to differant compression on both banks. So l took a reading and got these. Could it be the 5psi causing this?? 

Cylinder 1 140 psi
Cylinder 2 135 psi
Cylinder 3 135 psi
Cylinder 4 140 psi

One other thing that I've not tried as I'm writing this is the Air adjusting screw??. Would this be an option?? If not I'm stumped, and any input or direction to go in would be greatly appreciated.

Wait, what is "your" sync procedure? 

Here for reference,  https://www.speedsterowners.com...ure%20Adjustment.pdf

Bill, that's basically the way it's been done.

I'm thinking this has been a long term problem that no one has picked up on until now.

Richard,

Sorry for last night's post. I love the sync-link, and it's the only way to fly. You've got the best possible part, so on to the question.

You've arrived at the deep end of the pool here. There's probably never been a dual carb set-up (or dual throttle-body setup, for that matter) that draws in perfect sync all the way up to WOT. There are too many variables-- how the throttle butterflies mount on the shafts, etc. Fortunately, it doesn't matter as much as you would think. What matters is what happens at idle up to about 1/4 open.

The good new is that it's pretty much impossible for a sync-link to pull the throttles in anything but a linear manner, as opposed to a hex-bar, which has so much slop as to be pretty much a guess. If I'm understanding the nature of your question, you're in perfect sync at idle, just not at a partial throttle opening.

The thing about idle is that you can (and should) be in sync without any throttle linkage at all. Adjustments to idle are made with the throttle-stop screws with the linkage out of play. Your linkage should always allow the throttles (both of them) to go back to the stops at idle. It sounds as if when you adjust the carbs to pull in sync at slight openings, the throttles don't return to the stops at idle. You can't have it work that way.

What you are looking for is very slight movement of the accelerator cable producing a linear movement on both carbs. I don't think I'd be looking at it (initially) with a snail, I'd probably be using a feeler gauge or something similar. I'd set up the linkage so that a tiny amount of movement on the accelerator cable (which will pull the 3/4 butterfly) has a corresponding pull on the 1/2 carb.

You could do this without the engine running, by holding the 3/4 throttle open with a feeler gauge between the adjustment screw and the stop, then seeing if the 1/2 carb is open the same amount (you'd check with a second feeler gauge). The only real adjustment you are going to be able to get is just off the throttle stops- making sure the throttles crack at just the same time. Once you've got it set up where they are on the stops at idle, and just cracking together, then I'd check with the snail.

Good luck.

Stan and I disagree on hex bar vs. sync link, but here that really doesn't matter. Hex bars are sloppy unless modified with heim joints. I helped Lenny set up his sync link, and he is happy. I agree, it's a nice system.

Richard, you're going about this all wrong. Throw away the washers, they aren't helping you. Ok, you have sync at idle using the throttle stops. Great.

What you need to do is pull the throttle open very slightly, and hold rpm around 1500 to 2500. Even 3000 works, which is what I used to do. A hand throttle helps if you have one, I rigged one up. Doesn't matter much what rpm, just pick an rpm and STICK TO IT. Have a friend hold it there, using the throttle pedal, because that's how it will work when you're driving it. IMHO, 1500 is best as you want it perfectly synced just off idle, this is where your foot is when you are cruising down the road at 55-65.

Use the snail and the adjuster between the carbs to bring them as close as possible. It should be possible to get them spot-on. For reference, my synchrometer reads about 16-18 at this rpm. Take it for a drive, you're done. It is impossible to get them perfect at all rpm as Stan says. Idle and just off idle matter, once the plates are 1/4 open and beyond doesn't.

We agree to disagree on linkage. Everything else is just two ways to skin the same cat.  

About hexbars, but not the Synch procedure, correct?

Everyone I know, including me, synchronizes the carbs with linkage off or disengaged.

Then install the linkage so both carbs engage at the same time and you're finished.

...once you've squared away jets and venturi sizes.

DannyP posted:

About hexbars, but not the Synch procedure, correct?

Correct. 

Guys thanks for the feedback. 

Danny l used the washers purely to hold the revs, in this instance @ 2500rpm. I did not want to connect any linkage until they are reading the same or similar.   If l can't get them reading without the linkage then no point hooking everything else up.

I've been doing some digging on the web regarding the air bleed screws. A lot of question with uncertain answers but nothing concrete. 

Decided to call a carb importer and parts supplier here in the U.K.. after a indepth chat etc he advised not to adjust the airbleed screws to sync the 2 carbs. He did advise to swap the carbs over and see if the problem persisted on the same side or if it had swapped sides. Indicating a carb or an engine problem depending on results.

So this afternoon l got my tools out and pulled both carbs. Took the opportunity to give them a quick blast with carb cleaner, not that they were dirty. Upon doing this l noticed the length of the air bleed screws protruding from the locknuts was different from carb to carb. I cracked open the lock nuts on one carb and found that one screw was 2 turns out and the other on the same carb was 1 1/2 turns. Proceeding onto the other carb, both locknuts cracked, l found that both air bleed screws were in the fully closed position.  Interesting...as l never touched them...

So decided not to swap carbs from one side to the other at this moment, instead l adjusted all the air bleed screws so that they were all 1 1/2 turns out as a starting point. I've now put the carbs back as they were without any linkage connected and got the engine started and warmed up. Running lumpy but once warm and l re synced the carbs at idle again things smoothed out as before. now for the part throttle bit. Decided to sacrifice a feeler gauge to be 100% cut in half trapped between the idle screw and throttle stop arm as before to pick up and hold the revs. 

Surprisingly both carbs with the Snail read the same reading of 13 on the Snail. Pulled  the feeler gauges out and they returned to idle reading the same 5.5 in the Snail. It's a great feeling making progress and finally getting to the bottom of something.

Just need to take it for a run and see how it goes. Not sure how much the air bleed screws should be turned out or in. Alot have mentioned about flat spots if out or in to much so trail and error guess at this stage.

Well Done Richard !  Interesting idea about switching carbs too.........Bruce

Close the air bleeds unless you need them to balance front to back on the same carb.

Cheers fellas. Your input all helped.

What's your reason behind to close the air bleed screws Stan. Not questioning your knowledge, just curious.

Richard, close the air bleeds. Listen to Stan, he knows a thing or two. They are there merely to balance a low cylinder. This is maybe caused by a slightly bent or misaligned throttle shaft, maybe a throttle butterfly isn't 100% centered. Whatever, you bring the low cylinder up with the bypass screw. Once off idle, the bypass ceases to work. FYI, mine has one low and is balanced with the screw, the rest are closed.

Please, throw away the washers and the cut feeler gauge. Useless.

You need the airflow synchronized when you are driving, when you are pulling on the linkage with your right foot. PERIOD! Any other method is a fools game.

I am glad you figured out the difference L/R and didn't resort to swapping sides. But that is a good method for troubleshooting.

I'm with Stan.  I've always started with the air bleeds fully closed and then open one on a low (front to back) cylinder just to balance that side.  Never heard the folklore about air bleeds causing a flat spot.....That is usually caused by something entirely different.

I have also made a manual throttle similar to Danny's, but mine is a rod attached to one of the case studs near the distributor.  The rod has a turnbuckle on the other end and a way to clamp onto the throttle cable arm on my (gasp!) heim-jointed, hex bar linkage.  Once assembled, I can dial in whatever engine speed I want by turning the turnbuckle to get me to 1,500 - 3,000 rpm to make sure things are OK on the mains.  I always disconnect the linkage at idle to sync things there.  After that, I use the same process as Danny and Stan.  

Looks like you found the culprit in that opened air bleed.  Those little buggers can make a big difference.  That's probably why people recommend to close them unless you need to balance both throats on the same carb - that's what they're there for, but 12 to 18 is a big difference on one carb.  As mentioned, I would turn them all in closed, then only open one on a low throat on the same side carb to bring it up to sync on that side.