Bench sync'ing carbs

[Old guy]

Well I was thinking and....... sometimes that's a good thing, sometimes, not so much.

I never thought setting the sync with an Allen bit or small drill bit could be that good as far as getting the carbs close.

I have a shop vac, some old carb boots, and a vac gauge.......... You see where this is going now

With a large enough sweep on the gauge it seems like the pilot screws might be able to be checked for uniformity too.

IDK, it's late, random late night thoughts.

[posplayr]

The reason bench sink is good is because it is good enough. There are cyclinder to cylindr variations that you will be ignoring if you basically create a flow bench for your carbs. Get you head ported and balanced and the car. Balance will work better but not as well as the carb balance of a running engine.

[Steve]

Old guy, you are REALLY over-thinking it. Skip all the hardware, most of it is rather innacurate anyway.

Just hold the carbs up to a light, look at the sliver of light at the bottom of the butterfly on #3. Adjust #2 to match, then #1, finally, #4.

Here is what you are looking for:


If your Allen key, drill bit, paper clip or whatever you prefer is ever-so-slightly off-center, or even just in a different position in the throat, it will give you a different setting, because the distance from the edge of the butterfly to teh carb throat varies as you move from the bottom to the side. Skipping the inserted hardware and simply looking at a sliver of light works GREAT.

In the picture, the butterflies are open quite a bit more than necessary. Just use the idle speed control to open or close the butterflies, you will easily see which one opens first, then adjust the rest to match.

.

[Dogma]

Steve, you like a bigger sliver of light than I do. I adjust the idle until I can just barely find some light sneaking through #2. Then #3, then the rest, just looking for the tiniest sliver of light. I figure that's easier to match than trying match the amount of light coming through larger gaps. Of course, then I still depend on you for the vacuum sync. I'll grow up someday.

[Old guy]

I tried this out, and it worked great. You know it's good when it involves duct tape and zip ties. Sometimes I just have to try things.

I drilled and then fitted a barb to the shop vac hose end.

Took up a little gap around the end of the hose when fitted into the throat of the carb with duct tape wrapped around the hose end.

Ran a clear hose up to the ceiling, one end into a cup of water, the other to the barb.

Put a zip tie around the hose loose enough to slide up and down to mark cyl. 2 vacuum, matched the rest, opened and closed the throttle a few times, rechecked. Done.

[Old guy]

I give it 2 thumbs up. About $4. 20 minutes of fiddling time, spot on results.

The slivers of light, not the same on all carbs.

[tkent02]

To bad you have to change everything to synch them on your engine.

[posplayr]

To bad you have to change everything to synch them on your engine.

Yep .......


But just watch it will just happen to be close and we will have a new fundamental law of the gs universe to contend with based on a sample of ONE!

[Steve]

Steve, you like a bigger sliver of light than I do.

Nope, I like tiny little slivers, they are just a little hard to photograph.

The slivers of light, not the same on all carbs.

The purpost of the bench sync is to MAKE them all the same.

.

[tkent02]

The purpost of the bench sync is to MAKE them all the same.

.

The real purpose it to get it to run well enough to do a vacuum synch.

It still ain't rocket science.

[Steve]

True, but getting them at least CLOSE to the same is what does it.

.

[JTGS850GL]

Sometimes you guys WAY over think things.

[Steve]

Sometimes you guys WAY over think things.

I would rather over think them than over WORK them.

.

[Old guy]

I'd say it's far from overthinking.

And..... There was execution. A far cry from spouting and proposing and never doing like the internet is so famous for.

Outside the box, proof of concept....... Sometimes it's just about being able to tinker, which is, for me, at times the whole point.

[JTGS850GL]

Come on guys... Can't you see that it was meant as light humor?

My point was simply that a bench sync is just a short term thing to get the bike started. Weather you use a pin, a flow bench or your eyes is irrelevant. All you're really trying to achieve is a "close enough" sync so you can do the proper sync for your engine's condition using a proper tool. It's not rocket science at that level. I use a thin feeler gauge and that seems to work for me.

Now if you want to go into great detail, then discussing methods to get your final sync would seem more productive. That's where things get more critical. Lots of potential science there to entertain the mind as well.