Carb Syncing - DIY Manometer vs "sound and feel"

79josh81

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So today I went outside to (in this order) set timing on my points ignition, then sync the carbs, and then set my mixture screws. I decided to do all this because I went up a size on my pilots and 2 sizes on my main jets for the Uni filters and open headers that I'm running. I had already did my valve lash, cam chain tension, and breaker point gaps to spec and also re-set my mixture screws to the factory spec to give me a good starting point. My timing went well with the light and everything was right on with both cylinders and my advance.

I then moved on and hooked up my DIY manometer to sync the carbs. I have aftermarket carb holders without the vacuum barb but I used the 2 vacuum ports on the sides of the carbs by removing the screw and threading in barbs that I actually pulled off of the old vacuum petcocks that I'm not using anymore (they fit perfect in the carbs in case anyone is curious). I notice right away that neither cylinder was pulling more vacuum than the other enough to suck in fluid but the vacuum readings being shown on my manometer were erratic, where each side seemed to pull harder at different times.

After messing with the sync screw for a little while, I noticed that the closer I got to getting the fluid levels to be even with each other in the tubes, the shittier the bike ran and the fluid levels would still jump around where one side would pull and then the other side would pull. So at that point I decided to just adjust the sync screw by sound/feel and I watched where the fluid in the manometer ended up when it sounded good.

When I got the engine running good (meaning the sound/feel from each exhaust was identical), the fluid levels were no where even to each other. I wanna say that the manometer fluid level for the right cylinder was probably 12 inches higher than the fluid level for the left cylinder. BUT...the fluid stopped being erratic at that level and it did not move at all, even when throttling up to the higher RPM's. That tells me, regardless of where the fluid levels were in the tubes, that the vacuum being pulled on both sides was equal because neither could budge the other. So that's where I set it.

My question is, what is a better method to go by (and this is going to cause a shit storm), the DIY manometer or by sound and feel? Now after doing this with my DIY manometer, I'm smart enough to understand that the reason why my fluid wasn't even on both sides when I got it running good was probably because by the time I found the sweet spot in the sync, the fluid was already out of whack in the hoses and when I finally hit equal vacuum it just locked the fluid levels in their current positions (because of the equal vacuum on both sides). But most of the videos I've seen using the DIY manometer method, seem to show guys who have their fluid levels completely even to each other at perfect sync. I just wonder how many people chase getting the levels even like that and, in-turn, run their bikes slightly out of sync. Thoughts???
 
I can tell you from experience with my homemade glass bottle manometer that the engine is definitely the best when the levels were equal...I have used my yard stick manometer as well and found the best results with equel levels as well...I do like the bottle manometer better as it way smoother to get levels perfect as opposed to the erratic fluid movement of the yard stick.... although inserting old carb jets up into the vacuum tube does slow the erratic fluid motion down alot
 
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I don't think so. Equal vacuum pressure on each side should push/pull the fluid so it's level side to side. Are you sure the vac nipples you installed are well sealed into the carbs? Maybe one side is leaking? Usually you put a soft sealing washer or o-ring on the nipple to seal it once it's screwed in.
 
I’m gonna go try it again. I’ll add an o ring but I’m thinking as long as the levels don’t move on either side the vacuum should be equal. The only reason the liquid is level before starting the engine is because of gravity. Once you introduce vacuum, gravity shouldn’t have any influence on the fluid.

Example: Let’s say my carbs are perfectly in sync. And instead of hooking up both carbs to my diy manometer I just decide to connect only one side at first. I start the engine and the side with the connected vacuum hose starts pulling the fluid (like we all know would happen) If I then quickly plugged in the other side, and both sides have equal vacuum now, the fluid should just stop wherever it is in the line. Because if “x”psi is pulling one way and the same “x” psi is also pulling the other way, they should equalize and hold the fluid in place.
 
Once you introduce vacuum, gravity shouldn’t have any influence on the fluid.
Not how it works. Vacuum pulls the fluid up, gravity pulls it back down. If fluid is higher on one side, then vacuum is pulling harder against gravity on that side. Fluids need to be equal.
 
.....but I used the 2 vacuum ports on the sides of the carbs by removing the screw and threading in barbs......
Any chance you can put up some pics of where you tapped into the carbs? Might help solve this dilemma.
 
Totally false.

Not how it works. Vacuum pulls the fluid up, gravity pulls it back down. If fluid is higher on one side, then vacuum is pulling harder against gravity on that side. Fluids need to be equal.
Totally false is right. I’ve been outside proving myself wrong. I’m no physicist, but it appears that when vacuum is equal on both sides, then gravity DOES come back into play at that point because everything is equalized.

I’m going with all you guys now that there’s an air leak where I’m connecting to the barbs. I found that at idle I could get it pretty close, but the more rpm’s (more vacuum) the fluids would separate more. It made even more sense that there was a leak when I switched the hoses to the opposite sides and the symptoms of a leak followed.

Unfortunately I was only able to get it close because I couldn’t find any o-rings laying around that were small enough for the barbs.

That’s why I love this forum though. Open discussions to help make sense of things are better than any service manual.
 
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I’m using the usual spot...
 
I've also read somewhere, I think on here, that because of how thin the fluid is we use compared to mercury, that our manometers are loads more sensitive. So I'm thinking if my bike sounds good and the fluid levels stay within an inch of each other and there's no gross leaks, I should be alright for now.
 
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On the bright side of all this (or the not so bright side), I realized my push rod seal is leaking pretty good after running the bike at idle in the same spot for so long. The small puddle told me so. :doh:
 
- - - My question is, what is a better method to go by (and this is going to cause a shit storm), the DIY manometer or by sound and feel? Now after doing this with my DIY manometer, I'm smart enough to understand that the reason why my fluid wasn't even on both sides when I got it running good was probably because by the time I found the sweet spot in the sync, the fluid was already out of whack in the hoses and when I finally hit equal vacuum it just locked the fluid levels in their current positions (because of the equal vacuum on both sides). But most of the videos I've seen using the DIY manometer method, seem to show guys who have their fluid levels completely even to each other at perfect sync. I just wonder how many people chase getting the levels even like that and, in-turn, run their bikes slightly out of sync. Thoughts???

Hi josh,
it depends on the DIY manometer, don't it?
And there's only needs to be one tube in a twin's DIY manometer.
U-shaped thing at least 6 feet tall with the upper ends of the tube going to the vacuum connectors.
That's if the manometer is using a liquid with an SG of about 0.8,compared to water's 1.0
( antifreeze for it's green colour or better yet ATF for it's red colour and flammability.)
Note that as an ATF manometer is 13 times as accurate as a mercury manometer a reading that's within a 1/2" or so is good enough.
I'd say your manometer was reading wrong due to an air leak some place and your carbs were actually balanced OK.
 
The manometer and "sound and feel" are both crazy. The sane choice is Harbor Freight vacuum meter.
 
That's if the manometer is using a liquid with an SG of about 0.8,compared to water's 1.0
( antifreeze for it's green colour or better yet ATF for it's red colour and flammability.)
Note that as an ATF manometer is 13 times as accurate as a mercury manometer a reading that's within a 1/2" or so is good enough.
I'd say your manometer was reading wrong due to an air leak some place and your carbs were actually balanced OK.

Yeah that’s what I was thinking. I used Sta-bil in my manometer when I made it so it’s pretty thin stuff. If I had to do it again though I’d probably choose to use ATF or 2 stroke oil.
 
I have an original mercury filled "Carb Stix" unit and I think it works very well. The 4 tubes draw from a central, shared reservoir at the bottom of the "sticks". When you rev the bike, the mercury levels in the tubes do change, and I think that's why. More mercury can be drawn up or the excess can drain back down into the reservoir. I've thought of building a "DIY" one but if I did, it would be the reservoir type as well, not just the single long tube type. I would use a plastic jar about half full of ATF (or similar liquid) with 2 brass tubes stuck down through the lid into the liquid. A yard stick would be attached to the lid and plastic hoses plugged on the brass tubes.
 
I know this is off subject but I mentioned earlier about my push rod seal leaking. I have a couple quick questions that I can’t find answers to and I prefer not to start a whole other thread:

-Does the oil level in the crank case sit below the clutch push rod when the engine is cold that I can change that seal without draining the oil?

-I was thinking about going to the one long push rod while I’m going in there but really don’t want to pop the right cover off. Does the compressed air in the crankcase actually work to push the inner push rod out? I know I’ve been able to pull the ball bearing out with a magnet in the past but I never tried any techniques to pull the inner rod out through the left side.
 
Yes, the oil level is below the seal so no need to drain it when changing the seal. You can withdraw the inner rod and ball with a magnet from the left side while the seal is out. I never could pull the ball out through the seal. Some say they have but I never could. That's why I think it's best to do it while the seal is out.
 
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