question about bench syncing.

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juntjoo
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question about bench syncing.

Postby juntjoo » Sat Jan 13, 2018 3:50 pm

if you can get all the plates to close on a drill bit or whatever the guide uses accurately enough by hand what would be the need of a carb tuner? Do other factors come into play that affect the vacuum that require a mechanical DEtuning to get the vacuum consistent? Is that what it is? If not, and I can get my plates to turn together with a bench sync, I'd like to stop spending my money this month lol. What do you think? Bench sync then see what happens? What else should I know?
82" cb650 Nighthawk

juntjoo
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Re: question about bench syncing.

Postby juntjoo » Sat Jan 13, 2018 9:10 pm

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GeorgeSweety
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Re: question about bench syncing.

Postby GeorgeSweety » Sun Jan 14, 2018 5:18 am

Hi juntjoo, I can't comment on VB carbs yet as mine are on my "to do" list, but on my PD slide carbs I did a bench sync by raising the idle screw and inserting a small drill bit under the number 2 slide (base slide with no adjuster), then lowering the idle screw until the drill bit was a nice sliding fit, then the other three slides were adjusted either up or down to achieve the same sliding fit.
It was good enough to start the bike but it was a bit lacking in throttle response. Bench syncing is just a ballpark setting but it doesn't take into account wear and tear of individual components in each carb, if only it was as simple as that :(
After using the vacuum gauges the throttle response was a lot crisper even though there was only one carb that was noticeably out of sync, the other two required only very minor adjustment. When I last removed the carb rack I checked on the differences between the slide heights and the carb slide that needed the most adjustment was approx 1-1.5mm higher than the other three. No amount of bench syncing would allow you to compensate for what the vacuum gauge will tell you.
By all means use the bench sync to get the bike running and see if you are happy with the result, only you can make the decision whether you want to, or even need to buy a vacuum gauge tool.

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Folsoml
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Re: question about bench syncing.

Postby Folsoml » Sun Jan 14, 2018 8:05 am

I bench synced the first few bikes I rebuilt and they seemed to run fine. With your VB carbs, the sync only effects the idle. Once you get on the throttle, the sync no longer matters. I was, however, pleasantly surprised by how much more smoothly the engine idled once I got a vacuum tuner.

I've never heard of this "drill bit" method of bench syncing before. With VB carbs (I have very little experience with PB carbs), if you look in the throat of the carb, you will see one tiny hole, and then a little further in, a set of three more tiny holes arranged in a triangle. Adjust the #2 carb until the butterfly is covering 1/2 of the first hole in the 3 hole set. Use the black throttle adjustment knob to do this. Then use the sync screws to do the same with #1, #3, and #4 (in that order). Your carbs are now bench synced.
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juntjoo
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Re: question about bench syncing.

Postby juntjoo » Sun Jan 14, 2018 10:51 am

Folsoml wrote:I bench synced the first few bikes I rebuilt and they seemed to run fine. With your VB carbs, the sync only effects the idle. Once you get on the throttle, the sync no longer matters. I was, however, pleasantly surprised by how much more smoothly the engine idled once I got a vacuum tuner.

I've never heard of this "drill bit" method of bench syncing before. With VB carbs (I have very little experience with PB carbs), if you look in the throat of the carb, you will see one tiny hole, and then a little further in, a set of three more tiny holes arranged in a triangle. Adjust the #2 carb until the butterfly is covering 1/2 of the first hole in the 3 hole set. Use the black throttle adjustment knob to do this. Then use the sync screws to do the same with #1, #3, and #4 (in that order). Your carbs are now bench synced.


ya know, the drill method might be something for the "pb" ones. And your method is the one I'm seeing in the macgregor pdf which doesn't use a drill. thanks for going over the. The pdf around here gets complicated. I still don't quite understand what we're doing and why just lining them up in the closed position isn't good enough but I'll figure it out eventually
82" cb650 Nighthawk

juntjoo
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Re: question about bench syncing.

Postby juntjoo » Sun Jan 14, 2018 10:55 am

GeorgeSweety wrote:Hi juntjoo, I can't comment on VB carbs yet as mine are on my "to do" list, but on my PD slide carbs I did a bench sync by raising the idle screw and inserting a small drill bit under the number 2 slide (base slide with no adjuster), then lowering the idle screw until the drill bit was a nice sliding fit, then the other three slides were adjusted either up or down to achieve the same sliding fit.
It was good enough to start the bike but it was a bit lacking in throttle response. Bench syncing is just a ballpark setting but it doesn't take into account wear and tear of individual components in each carb, if only it was as simple as that :(
After using the vacuum gauges the throttle response was a lot crisper even though there was only one carb that was noticeably out of sync, the other two required only very minor adjustment. When I last removed the carb rack I checked on the differences between the slide heights and the carb slide that needed the most adjustment was approx 1-1.5mm higher than the other three. No amount of bench syncing would allow you to compensate for what the vacuum gauge will tell you.
By all means use the bench sync to get the bike running and see if you are happy with the result, only you can make the decision whether you want to, or even need to buy a vacuum gauge tool.


interesting. thanks. so indeed it has nothing to do with actual phyical parts alignment, but as a matter of fact it's sounds like the special tools for this are for when wear /tear takes parts OUT of physical alignment and you can't go by it any more. So I'm guessing then we don't see people using sync gauges on new bikes
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GeorgeSweety
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Re: question about bench syncing.

Postby GeorgeSweety » Sun Jan 14, 2018 11:58 am

juntjoo wrote:interesting. thanks. so indeed it has nothing to do with actual phyical parts alignment, but as a matter of fact it's sounds like the special tools for this are for when wear /tear takes parts OUT of physical alignment and you can't go by it any more. So I'm guessing then we don't see people using sync gauges on new bikes


"it has nothing to do with actual phyical parts alignment". Well bench syncing is aligning the physical parts, it is a start point not the end point :D you could chuck your carbs together with slides all over the place but you would (probably) end up in a right old mess :roll: you'd have cylinders getting lots of fuel and other cylinders getting none.
Vacuum gauges are designed to read the amount of air pressure inside the engine; whenever the engine is running, there is a vacuum created, the vacuum created controls the amount of suction through each carb. Carb bench syncing cannot measure air pressure in your engine but it is a good place to start, only vacuum gauges can measure air pressure inside an engine.
Vacuum gauges wouldn't only be of use in cases of wear and tear, small things being slightly out of adjustment can add up to making carbs being out of sync with each other, so I would still use them on a new bike if I were tuning it.

juntjoo
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Re: question about bench syncing.

Postby juntjoo » Sun Jan 14, 2018 12:15 pm

alright fine Ill get one. and my carbs better be off. and a couple times a year at that. don't like tools wasting space lol.
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GeorgeSweety
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Re: question about bench syncing.

Postby GeorgeSweety » Sun Jan 14, 2018 1:14 pm

juntjoo wrote:alright fine Ill get one. and my carbs better be off. and a couple times a year at that. don't like tools wasting space lol.


Get the bike running first, then decide if you need to buy one. Maybe bench syncing will be good enough with VB carbs, I don't know until I mess about with my own. I have used vacuum gauges on numerous other carbs from other manufacturers and I can tell you that the gauges are a necessity to smooth running.

juntjoo
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Re: question about bench syncing.

Postby juntjoo » Sun Jan 14, 2018 1:18 pm

Folsoml wrote:I bench synced the first few bikes I rebuilt and they seemed to run fine. With your VB carbs, the sync only effects the idle. Once you get on the throttle, the sync no longer matters. I was, however, pleasantly surprised by how much more smoothly the engine idled once I got a vacuum tuner.

I've never heard of this "drill bit" method of bench syncing before. With VB carbs (I have very little experience with PB carbs), if you look in the throat of the carb, you will see one tiny hole, and then a little further in, a set of three more tiny holes arranged in a triangle. Adjust the #2 carb until the butterfly is covering 1/2 of the first hole in the 3 hole set. Use the black throttle adjustment knob to do this. Then use the sync screws to do the same with #1, #3, and #4 (in that order). Your carbs are now bench synced.


thanks. good info. wondered what that black screw was for. and now I understand a bit better how idle works. doesn't the throttle need to be open a bit to idle tho? that pilot hole must be supplying air then correct? I wasnt sure if it was air or fuel going through that hole. must be air... right?
82" cb650 Nighthawk

juntjoo
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Re: question about bench syncing.

Postby juntjoo » Sun Jan 14, 2018 1:19 pm

GeorgeSweety wrote:
juntjoo wrote:alright fine Ill get one. and my carbs better be off. and a couple times a year at that. don't like tools wasting space lol.


Get the bike running first, then decide if you need to buy one. Maybe bench syncing will be good enough with VB carbs, I don't know until I mess about with my own. I have used vacuum gauges on numerous other carbs from other manufacturers and I can tell you that the gauges are a necessity to smooth running.


thanks. yeah, ill probably end up getting one. if not now then later.
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Folsoml
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Re: question about bench syncing.

Postby Folsoml » Sun Jan 14, 2018 6:15 pm

juntjoo wrote:doesn't the throttle need to be open a bit to idle tho??


It will be "open," but just a tiny bit. The black knob can be screwed out so far that the bike would not idle. If that is the case, screw it in until the idle is where it needs to be (screwing it in opens the butterflies).
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juntjoo
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Re: question about bench syncing.

Postby juntjoo » Sun Jan 14, 2018 6:28 pm

Folsoml wrote:
juntjoo wrote:doesn't the throttle need to be open a bit to idle tho??


It will be "open," but just a tiny bit. The black knob can be screwed out so far that the bike would not idle. If that is the case, screw it in until the idle is where it needs to be (screwing it in opens the butterflies).


wow, okay, we're dealing with tiny spaces here. and that single tiny hole opposite the pilot screw is letting in fuel? or air? thanks
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Folsoml
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Re: question about bench syncing.

Postby Folsoml » Sun Jan 14, 2018 9:23 pm

Air.
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