TOPIC:

I synced my carbs today and got into a debate with the friend who was helping me. I was using a mercury manometer and when the manometer showed that all four cylinders had the same vacuum the engine was running slower than when I had started. My friend told me that I need to find the sweet spot where the engine speed is highest. When I started syncing them there was about a 3.5 cm spread on the manometer and when I had all 4 cylinders perfectly at the same level the engine was idling lower and rougher than when I started. Is my friend right? Any input on this?

There is no "sweet spot" that will maximize the RPMs, when syncing. The "sweet-spot" comes into play when you set the mixture screws.

If you synced 3 carbs to the one that had the lowest reading, it should rev up. If you sync 3 carbs to the one with the highest vacuum, the revs will go down.
The roughness could be because the RPMs are too low now.

The lower the vacuum reading, the more open the throttle is on that carb. That is, assuming the engine is ok, there are no vacuum leaks, and the manometer is calibrated correctly.

Either way, all you are doing is making the throttles openings the same. If you do it by closing some down, you must compensate by opening all of the carbs a tiny bit by turning the idle-speed screw. This will open all of the throttles by the same amount to regain the RPMs you lost during syncing.

On the other hand, if you sync by opening some throttles, the RPMs will go up and you compensate by turning down the idle-speed.

It is important to calibrate the manometer. You do this by using a cheap plastic 4-to-1 vacuum manifold. It's available at most auto parts stores. You can find it amopng the windshield washer fluid fittings.

You connect the 4 manometer hoses to the manifold, then connect the manifold to one cylinder of the bike. Since all of the manometer tubes are connected to the same vacuum source, they should all have the same reading. If they don't, you must adjust the restrictor in that tube. There should be a restrictor at the end of the hose that connects to the engine. It is usually a plastic plug with a pin hole in it. To increase the reading, you poke a pin into the hole to make it a tiny tiny bit bigger.

Just a joke but it sounds like to boobs working on a bike. By the way ditto to Loudhvx

I go to two boobs around two in the morning. There, they're thier homonyms. A-huh huh huh, he typed homo. Thnaks for the Manometrical clarifications Loud Sir. Man o metric. Homoerotic inuendo notwithstandig is this some type of weenie measuring device?

zn, sounds like you did just fine. Now adjust the idle using the idle adjustment. On my engine it's the knob below and center of the carbs.

When u get really good you leave the pin in the cushion, Nanuk gets to stay home, and you suck through the front of each carb and feel the resistance. Then you adjust each slide till all four are the same. This is the most sensitive method of all and I would not advise this to anyone but me, because I am supersyncher, the perfect manometer. All small dogs stay home on delicate hunt. FEEL the passion I say, and live and breath with the devices. Be part of them. Zoom zoom zoom......puuurrrrrr....idle idle idle idle......