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Good morning..

I did my first carb sync on my 'new' 1100 after receiving them back from Pistol Petes.. I followed the Randaak procedure to the letter and got them as close as possible at idle... It sounded and felt good, no hesitation and no other issues that I could see.
OK, then out of curiousity, I reved it to 1500, then 2000, then 2500 while still measuring the sync. I noticed that while all were equal at idle, they changed quite a bit on cylinders 2/4 in relation to 1/3, getting more pronounced the faster it reved.
Is this normal? And if so, wouldn't there be a performance hit the faster you reved the engine? I want an engine that idles decently, but wouldn't it make more sense to have it the carbs synced at cruising speed? Maybe it's just a tradeoff... What do you think?

thanks,
Paulie
 

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You may have other problems..slides that aren't working correctly..vacuum leak, clogged main or secondary jets..float setting issues. Was o your tank clean and a new filter put on?
 

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From a carbtune manual
The engine should be warm and running at just enough above idle speed so it doesnt falter and stop during adjusments.
Don't try to balance at high engine speeds.
 

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When you are syncing your carbs, you're measuring how "closed" each throttle slide is, so that each carb is allowing the same amount of fuel/air through, and each cylinder runs with the same amount of power. That's why you do the sync at idle, when the throttle slides are closed. When you open the throttle, there's nothing to adjust - the slides are opening, coming off the stops, which is what you are adjusting when syncing. When the throttle is open, and the engine is making lots of power, any measurement you take with your vacuum gauge is measuring the efficiency of your air filter and intake system, and is a useless measurement as far as syncing carbs.
 

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Hi Paulie

Traditionally you sync them at idle and walk away. I have experienced what you are talking about. I have a V twin that normally vibrates around 60 mph therefore on that bike I will sync it at high RPM this smoothes out the engine on highway speed. But the sync is out of wack on idle. I belive like you said it is a trade off. Under normall conditions I would just sync ta idle.

Good luck Tom
 

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Carb sync becomes less important as the throttle plates open. Doing a carb sync is mainly to make sure the plates begin to open all at the same time as so that all four cylinders are pulling about the same at low rpms. At higher rpms the throttle plates are so far open that a few degrees difference in their position doesn't have much effect on the air flow.

The easiest way to think of it is to consider the engine with full throttle applied. With the throttle plates full open they'll be parallel to the airflow in the carb. A degree or two off of parallel obviously isn't going to change the open area of the carb throat. At idle when the plates are almost closed, a slight difference of the plate position will make a much larger change in the flow through a carb. Basically a throttle plate isn't a linear device. So you sync at idle where the setting has the most effect on the engine. If you have major differences in vacuum at higher rpms it might be worth while checking the cam timing.
 
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