How do you know when your carbs need to be synched? - Motorcycle Forum
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post #1 of 8 (permalink) Old 04-28-2011, 01:13 AM Thread Starter
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How do you know when your carbs need to be synched?

So I just recently swapped motors out on my cb 750 and I took the carbs off of my old motor, its been a long project but it's done and yesterday I got to go for the first ride in the last few years. the bike backfires a little bit when I rev it above 5000 rpms (The gas is old.. I know I need to burn it up).
It idles fine and runs like the clock that it is otherwise.

When putting the carbs back on I gave them a quick once over and they looked clean enough IMHO,

Bottom line is I got into the argument with my brother tonight that I don't need to synch my carbs. He claims they should be synched and I may feel otherwise because I want to ride this thing and it appears ready.

Opinions?
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post #2 of 8 (permalink) Old 04-28-2011, 02:34 PM
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Whenever you disturb the throttle cables, if you have one to each carb, a sync is required. If the throttles are ganged together on a common control shaft, if you think you may have disturbed that connection in any way, the sync should be checked. If you're just cruising on a level stretch, and you feel the bike isn't as smooth as it should be, like vibration or slight surge, you need to check the sync. With most bikes, it's fairly simple to check, if you have the gauges. Any time I disturb the carbs, I like to do a quick sync test, just because.

Rick
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'68-7? CB450 and '98 Valkyrie Tourer, too
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post #3 of 8 (permalink) Old 04-29-2011, 09:39 AM
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I wonder if you have a very slight air leak?
Over time -especially with over-tightening- the carb and head flangesd sort of mold to each other and they warp to mate. (very slight) Putting the carbs on different heads, they won't be mating as well- despite a new gasket. (You DID use a new gasket?)
You can check the surfaces using a snall piece of plate glass. If there is any wobbling- the surface is not flat. You can halp of even completely alleviate that by using wet-dry sandpaper over the glass and rubbing it on the surface to bring down any high spots.

FWIW, it wouldn't hurt to turn the whole tank up-side down and get rid of what's in the bottom. You can run that old gas through a car with no ill effects.
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post #4 of 8 (permalink) Old 04-29-2011, 02:46 PM Thread Starter
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MMmm yea. I put on 200 miles today. and it still sounds sick. I don't think its the gas anymore. I'm going to have to borrow the gages from a friend and make an attempt at synchronizing the carbs myself.

I also noticed oil on the forks... blown seal... DOH!
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post #5 of 8 (permalink) Old 04-30-2011, 07:46 AM Thread Starter
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PAIR check valve was uncapped... put a cap on it and it sounds and runs the way that its supposed to. I'm not touching the carbs.
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post #6 of 8 (permalink) Old 05-08-2011, 01:33 AM
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I synch my carbs at least once a season whether they need it or not.
Only takes a minute to check; if they're good, get on & ride...
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post #7 of 8 (permalink) Old 08-08-2011, 05:16 PM
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has anyone had any expeirience with the twinmax synchronizer? It looks like it will either work really well or be a gigantic PITA especially with a 4 cyl
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post #8 of 8 (permalink) Old 08-08-2011, 05:17 PM
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Quote:
Originally Posted by ChriswithaHawk View Post
MMmm yea. I put on 200 miles today. and it still sounds sick. I don't think its the gas anymore. I'm going to have to borrow the gages from a friend and make an attempt at synchronizing the carbs myself.

I also noticed oil on the forks... blown seal... DOH!
Quote:
Originally Posted by ChriswithaHawk View Post
PAIR check valve was uncapped... put a cap on it and it sounds and runs the way that its supposed to. I'm not touching the carbs.
Good glad you found the problem!


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