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Author Topic: how to sync carbs with vacume gauges  (Read 3680 times)

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Offline manic2stroke

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how to sync carbs with vacume gauges
« on: March 10, 2009, 12:38:27 PM »
i was going to bring my bike to a really well known shop for 2 strokes to have the carbs synced because i have a flat spot and i beleave that it might be the problem.  i see these gauge kits and they are not that expensive.  is there a book on how to do it.  like carb syncing for dumbies?  the guys the shop said that there is a sync adjustment screw.  is this just the idle mix screw. thanks 

Offline Matty

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Re: how to sync carbs with vacume gauges
« Reply #1 on: March 10, 2009, 12:47:44 PM »
Is it an RD350?

On an RD350, you just sync the carbs with the idle screw.  You can usually get it damn close by just eyeballing it with no filters on.  Also, there is no place to hook up a vacuum gauge on those carbs.

I'd be seriously shocked if your carbs are far enough out of sync to create a flat spot, its usually not a huge issue on parallel twins like it is on I4's and the like.
« Last Edit: March 10, 2009, 12:49:08 PM by Mattydiah »
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Offline olz431

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Re: how to sync carbs with vacume gauges
« Reply #2 on: March 10, 2009, 12:52:22 PM »
with the vacuum gauges, you just match the vacuum between the carbs

But i do it by comparing slide height by eye with the idle screw, and also make sure the cables pull the slides at the same time...

Offline manic2stroke

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Re: how to sync carbs with vacume gauges
« Reply #3 on: March 10, 2009, 01:01:06 PM »
i was actually thinking.  wouldnt it be hard to use a vacume gauge on these carbs sense they have a cross tube connecting both of them.  what would you do. plug the hole then compare readings till they are same?

Offline m in sc

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Re: how to sync carbs with vacume gauges
« Reply #4 on: March 10, 2009, 01:04:23 PM »
i have an old school bellmouth carb sync tool, it works okay, i use it on the triples... but it works no better at part throttle than looking in to make sure the slides are raising evenly. tip: use mirror, you can see both slides at same time.

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Offline olz431

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Re: how to sync carbs with vacume gauges
« Reply #5 on: March 10, 2009, 01:05:48 PM »
Along those lines, why would the carbs need to be sync'd with a crossover tube? LOL Yeah Id plug them if you are going the vacuum gauge route

Offline manic2stroke

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Re: how to sync carbs with vacume gauges
« Reply #6 on: March 10, 2009, 01:19:42 PM »
Along those lines, why would the carbs need to be sync'd with a crossover tube? LOL Yeah Id plug them if you are going the vacuum gauge route

ya  the guy on the phone got me thinking that i was doing something wrong.  guess i wont be bringing him the bike.  gona go get it dyno'd and see where the flat spot is occuring. and go from there.

Offline m in sc

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Re: how to sync carbs with vacume gauges
« Reply #7 on: March 10, 2009, 01:28:03 PM »
i always tune the bike with the crossover plugged. the bad thing about the crossover is it can mask a problem one carb might be having. 
when tuned, re-insert crossover, drop idle stop screws down equal amount on each carb till correct idle is achieved, done.
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Offline mkane63t

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Re: how to sync carbs with vacume gauges
« Reply #8 on: March 10, 2009, 08:33:59 PM »
Take 2 pieces of tubing, brass, alum. or 2 drill bits and place them under the slides while the throttle is closed. They will stick out of the bellmouths enough that U can see them. Adjust cables so that both pieces move at the same time while working throttle. I adjust my idle in a strange way. I mess with one side until i see a change on the tach, then do the other side the same way. Can get the idle to 1200 with great response and she idles down rapidly, or sometimes just have no idle at all. This doesn't make sense on a 2smoke because it has zero compression braking.
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Offline jrodenhiser

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Re: how to sync carbs with vacume gauges
« Reply #9 on: March 11, 2009, 10:55:46 AM »
Only the Daytonas have vacuum ports for syncing. Older RD's are all mechanical syncing.
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Offline dirtybird

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Re: how to sync carbs with vacume gauges
« Reply #10 on: March 11, 2009, 12:57:21 PM »
Only the Daytonas have vacuum ports for syncing. Older RD's are all mechanical syncing.

They do??? Where???

Offline dirtybird

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Re: how to sync carbs with vacume gauges
« Reply #11 on: March 11, 2009, 01:08:13 PM »
I start with the idle, first I poen and close the throttle while turning just one idle screw until I can feel the exact moment the slide hits the idle screw tip. I then scribe a mark on the idle screw half way vertically from the top to the middke of the screw head. This mark is like a clock hand and is just an indicator for balancing. I then repete the proscess for the other carb, again making the scribe. Then I raise eack side one turn making sure the marks line up back at their regular spots. I repete this until I am close and then use less than a turn until the idle is correct. The trick here is to make sure you move them excattally the same and that is what the scribes let you do.... Oh and if you do not want to scribe them then use a magic marker.

As for balacing them, the balance can be done 3 ways I know of (non Daytona) and that is by feel (putting your fingers in each bell and feeling when they rise. By sight such as the drill bit in the bell mentioned bu Mkane63t or the Yamaha way of removeing the view screw and aligning the slide markers with the bottom of the windows. I use the Yamaha meathod because I am more concerned with the balance when the throttle is open and complete tension is on the cables and slides. If you decide to use the Yamaha method then I would suggest you use a little engine paint and a tooth pick and paint the inside of the dot on the slide so you can see it at our old ages.

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Offline dirtybird

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Re: how to sync carbs with vacume gauges
« Reply #12 on: March 11, 2009, 01:10:57 PM »
Oh and in case you are working on an RD400, you sink them at the cable end on the top of the carb. Care should be taken when tightening down the set nut. Use 2 wrenches one to hold the setting and one to tighten the backing nut, thus maintianing the setting. Check the alignment even after tightening the backing nut for changes.

Offline twinlinemoto

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Re: how to sync carbs with vacume gauges
« Reply #13 on: March 11, 2009, 03:50:39 PM »
I sync twins all the time. Unless they have a port specifically blocked off for vacuum guages, I do the safety wire method:
Take two pieces of safety wire (or welding wire) about 8"long and bend them both in half 90 degrees. open the throttle and insert the piece of wire down the barrel of the carb and close the throttle. This should make the slide rest on the piece of wire. Then, slowly turn the throttle watching both protruding ends of wire. When one falls before the other, raise the opposite slide up with the cable adjuster at the top of the carburetor. Repeat until both pieces of wire fall at the same time.
It sounds a little complicated, but if you do it once, you will understand it better. good luck!
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Re: how to sync carbs with vacume gauges
« Reply #14 on: March 11, 2009, 04:29:10 PM »
I sync twins all the time. Unless they have a port specifically blocked off for vacuum guages, I do the safety wire method:
Take two pieces of safety wire (or welding wire) about 8"long and bend them both in half 90 degrees. open the throttle and insert the piece of wire down the barrel of the carb and close the throttle. This should make the slide rest on the piece of wire. Then, slowly turn the throttle watching both protruding ends of wire. When one falls before the other, raise the opposite slide up with the cable adjuster at the top of the carburetor. Repeat until both pieces of wire fall at the same time.
It sounds a little complicated, but if you do it once, you will understand it better. good luck!

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