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post #1 of 12 (permalink) Old 07-04-2008, 03:05 PM Thread Starter
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For you synchronization Guru's:

The factory service manual on my VT1100C2 Sabre recommends synchronizing the carbs at a steady 1,000 rpm, +/- 100, until both vacumn readings, (or columns of mercury in my case,) are as even as possible. OK...

(The RPM I guess at since Japan is afraid of sinking into the ocean if they put tachometers on their cruisers.)

My question is that if the main jets between both carbs is deliberately off by 5 numbers, and you set both carb readings exactly the same, wouldn't the rear cylinder carb in reality be actuating before the front carb was? Last time I set my front carb reading to be just a sniff higher, (or greater,) than the rear carb reading was and the bike seemed to run like a champ. Best ever.
The vacumn reading is determined by how much fuel the carbs are delivering down the lower boot and into the port, so why set both carbs exactly the same if one carb must of necessity deliver fuel at a slightly faster rate than the other one does? Anybody here set the carb synchronization off just a little intentionally?


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post #2 of 12 (permalink) Old 07-04-2008, 04:04 PM
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Sync'in the carbs doesn't have anything to do with the main jets! What you adjust when you sync the carbs is to adjust the throttle blades. The needles control the fuel passing through the main jets,
The slide pistons controls the lift of the needles,
The pilot jets controls the fuel for the idle circuit, The air mixture screws fine tunes the pilot jets.
The throttle blades controls the total mix of air and fuel coming through all the carburetors.
So to get the premixed mixture evenly (the same amount at the same time) to all cylinders the throttle blades need to be balanced, its not about richer or leaner, its about the balance of the carbs.


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Anybody here set the carb synchronization off just a little intentionally?
NO!
If one cylinder needs a different fuel air mixture you DON'T do that by setting the carb sync one ahead of the other. That would make the throttle blade timing OFF! in other words the carbs would be out of time of each other.
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post #3 of 12 (permalink) Old 07-04-2008, 05:12 PM
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Okay, I called it "Timing" in reference to one being head of the other. When you balance the carburetor throttle blades what you are doing is making sure that the cylinders are running at the same rate, or one cylinder isn't trying to outrun the other. The throttle cable opens the throttle blades, what you need them to do is be "Matched" or "Synchronized"

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Originally Posted by American Heritage Dictionary
1. To occur at the same time; be simultaneous. 2. To operate in unison. --tr. 1.a. To cause to occur or operate with exact coincidence in time or rate: We synchronized our watches. b. To cause to occur or operate at the same time as something else:
But at any rate, you CAN'T change the mixture at the throttle blades!
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post #4 of 12 (permalink) Old 07-04-2008, 06:58 PM
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Yeah....what he said. Jets are fuel delivery throttle blades are air. air has to be synched or u will negate that 5 point difference in the fuel and that cylinder will try and outrun the other...........I think lol

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88 VT1100, 08 HARLEY FAT BOB (traitor) 87 700 intruder, 85 700 MAXIM, I ride the Harley mostly but "still" prefer to hang with metric riders because they're not stuck up. OOps, keep forgetting, AINT stuck up
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post #5 of 12 (permalink) Old 07-04-2008, 07:01 PM
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Hrmmmph just do what the book says.............

AUSSIE SHADETREE MECHANIC EXTRAORDINAIRE
88 VT1100, 08 HARLEY FAT BOB (traitor) 87 700 intruder, 85 700 MAXIM, I ride the Harley mostly but "still" prefer to hang with metric riders because they're not stuck up. OOps, keep forgetting, AINT stuck up
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post #6 of 12 (permalink) Old 07-04-2008, 07:04 PM
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Duh you cant change the FUEL at the throttle blades but you can change the AIR.....hence you CAN change the MIXTURE at the throttle blades........this is fun

AUSSIE SHADETREE MECHANIC EXTRAORDINAIRE
88 VT1100, 08 HARLEY FAT BOB (traitor) 87 700 intruder, 85 700 MAXIM, I ride the Harley mostly but "still" prefer to hang with metric riders because they're not stuck up. OOps, keep forgetting, AINT stuck up
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post #7 of 12 (permalink) Old 07-04-2008, 07:47 PM
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Carb synchonization

The main jets are different between cylinders due to the running temperature of the two cylinders, the rear being hotter running due to less cooling and hot air from the front cylinder. By having the rear cylinder different due to temp the mixture must be different in the rear cylinder to have the same burn rate of the fuel /air mixture.

Now that the power produced by the two different air/fuel mixtures is the same, you must now "Synchronize" the throttle position so that both carbs are drawing the same amount of air. This is measured by checking the amount of vacuum created in the venturi (narrowed area in the throat of the carb) and adjusting the mechanical linkage so this is equal.

Synchonization should be done periodically to adjust for wear and anytime a component of the intake or exhaust is changed or modified (i.e. new pipes, baffles modified, new style air filter, jet change, etc.).

Synchronization is done to prevent surging (in severe mis sync), improve cruising smoothness and prevent vibration. That's right, prevent vibartion.
Even a slight mis sync will cause the cylinder getting the most air to try to outrun the weaker cylinder creating a tug of war. I have smoothed out many a two, three, four, six and eight cylinder engine by syncing multiple carb setups. I had an old 4 cylinder Porche that I had to resync the four seperate down draft webers damn near every time I dove the thing

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post #8 of 12 (permalink) Old 07-04-2008, 11:56 PM
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Quote:
Originally Posted by bananabenda
Yeah....what he said. Jets are fuel delivery throttle blades are air. air has to be synched or u will negate that 5 point difference in the fuel and that cylinder will try and outrun the other...........I think lol
Quote:
Originally Posted by bananabenda
Duh you cant change the FUEL at the throttle blades but you can change the AIR.....hence you CAN change the MIXTURE at the throttle blades........this is fun
Well, okay it sounds funny! but the throttle cable only connects to ONE carburetor, and that is to the shaft that the throttle blade is mounted to. So when you twist the throttle to increase the engine speed you are ONLY opening up the throttle blade. The linkage of both carbs is connected together so when the throttle is opened up at the one carb then it opens the other. The increased engine speed isn't done by changing the mixture.
That's why I say the throttle blades isn't about the mixture, but more about ENGINE speed. But!! if the linkage isn't matched then the carbs would be trying to make the two or more cylinders run at different speeds, BUT! they can't because all of the pistons and rods are connected to the same crankshaft.
So, some thinks I'm off in wonderland, but! find me some information that says you can't "Out OF SYNC" the carbs on a multi cylinder engine and correct a mixture problem with one of the cylinders!! any takers?
MarkC
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post #9 of 12 (permalink) Old 07-05-2008, 10:35 AM
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Volume, not mixture
The throttle blades do change the air flow, but not without increasing or de-creasing fuel, that's because as the air flow increases or de-creases going past the throttle valve's (throttle blades) the pressure changes under the slide pistons and increases or decreases the lifting or lowering of the slide pistons which is lifting and lowering the needles up and down in the main jet chamber's, Volume of the mixture! LOL
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post #10 of 12 (permalink) Old 07-05-2008, 11:33 AM
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Quote:
Originally Posted by MarkC
...you can "Out OF SYNC" the carbs on a multi cylinder engine and correct a mixture problem with one of the cylinders!!
Well you can certainly try! Success, on the other hand...




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