81 GS1100G Carb Syncronization

[Supercub]

I brought four round dial vac gauges and vac tubing, I want to sync my carbs after us cleaning. The service manual describes the use of a bar gauge setup with increments but not "Hg values, and that the inner cylinders are to be set differently than the outers. Does anyone know what this difference would translate to in "Hg?

[Nessism]

The vacuum level doesn't matter, just make them all the same.

Don't forget to block off the vacuum port for the petcock, and when running lines to the gauges, install restrictors in the lines. A small plug, with a tiny hole drilled through, will work.

You would do well to test each gauge on the same cylinder, to verify they all read the same.

[Supercub]

I have screw-in manifold barbs with .087 dia holes, is that small enough? I checked each gauge with a calibrated vacuum source and all check good. What reading should I expect to see at 1650rpm?

[Brendan W]

I have screw-in manifold barbs with .087 dia holes, is that small enough? I checked each gauge with a calibrated vacuum source and all check good. What reading should I expect to see at 1650rpm?

The restrictor Nessism is talking about is to damp out the pulses making it easier to read the gauge.
Mercury columns have little or no damping and need restrictor plugs.
Your gauges may be smart enough to damp or not.
Only one way to find out. Is .087 small enough, maybe.
Watch out for the thread size on the intake. The barb you have could be slightly different. 4 and 5mm are typical. Be real sure you have it right or threads will get damaged.
A previous owner may have already done this, as in my case with the 850, so assume nothing.
The Suzuki manual uses floating ball gauges and sets them to a zero.
I have a strong memory of my setting the middle two cylinders about 1cm Hg below the outers for some reason?
1 bar is 75cm Hg

[unfocused]

Let me add - when I sync my tool, I use a "T" for all four gauges so I can sync them together before plugging them into the carbs. Actually it's 5 connections.
I prefer syncing my gauges this way. I do use a liquid gauge but I also have and sync my vacuum gauges this way.
I use restrictors either way and the restrictors need to be on the carb end, not the tool end. You'll see some 4 gauge set ups with restrictors.
RPM's I sync at
1500
2500
4000

My carbs are synced all the same.
If you see a vacuum gauge boucing from end to end, assume valve issues. Get out a single vacuum gauge and test if you see this.
Just ran into this similar situation on a friends bike he thought was ready for carb sync...

[Supercub]

The bike has 4,000 miles on it and has had stumbling issues since I brought it. Valves could be pitted from sitting. I imagine I will see an erratic gauge in that case. I checked valve clearances and reshimmed two tight exhaust valves that were at the low limit. Replaced plug caps due to one open one. Ultrasonic cleaned the carbs and chased passages with wire and carb spray. Cyl 3 has a faint blue header pipe, just noticed yesterday. The bike idles fine and takes off smoothly under normal takeoffs but flattens out and then builds up power under stonger acceleration.

[Brendan W]

Can't take anything for granted on an old machine, irrespective of mileage.
At that age you could well have corrosion on the ign and kill switch contacts and coil supply plugs.
All bets are off if the electrical system is not in good shape.
A lot of carburettor issues are, in fact, electrical.

[Supercub]

I'll check the connections. So far I havent found any corrosion on any contacts but I shoukd pull the ignition connections apart.

[unfocused]

I'd put a vacuum gauge on and check. As mentioned my friends bike would run good but always had a dead spot or he would see an issue of loss of power at speed.
As for the ignition caps, I tend to toss those as they're only needed for radio interference... however I've switched to a Dyna and cheap 3 ohm online coils so I build my own wires.
My GS had a cylinder head issue, bought a replacement and I couldn't believe how bad the valves were on both heads. Just purchased a GS550 head and am expecting to see a mess here as well. The original had a drink of water and took out the valve seat, valve, and piston rings were rusted to the cylinder...

However, corrosion has been mentioned. Check the points plate, timing plate and condensor/points mounting.

[Supercub]

Its got plenty of power past the flat spot and I can bury the 85'r in seconds.
Its electronic ignition. I read that the resistors are for protection of the ignition module and smooth out the spark. The caps were $94 total and I could have put brass slugs in place of the resistors and used R plugs but I want to keep it stock so I dont have to second guess anything.
I should have replaced the manifold orings but the manifolds look like they are faced right up to the head so I didnt question them, the rubber is good and pliable and the carbs fit tight inside of them, but anything is possible.

[Brendan W]

........ but anything is possible.

The ingenuity of previous owners or their mechanics knows no bounds.
I don't believe it possible to overstate just how many ways thing can be done wrong, left disconnected, parts omitted etc etc.
Many times it the reason a bike changes hands.

[Supercub]

So far no signs of bad wrenching, no rounded screw heads, tool marks, pod filters, etc. same owner since 2008.

[Supercub]

I need smaller restrictors in the lines as the needles jump like crazy. If I partially pinch the lines I get steady readings but only 4 to 8" Hg.

[Brendan W]

I need smaller restrictors in the lines as the needles jump like crazy. If I partially pinch the lines I get steady readings but only 4 to 8" Hg.

Would clothes pins do it?
The actual readings are not important. Balance is.
The motor is an air pump. Against a closed throttle plate it's pulling a vacuum and the manifold pressure should drop way below atmospheric.
Talking in inches of mercury, one atmosphere is just under 30" Hg.
When you see 4" Hg, the pressure in the inlets has fallen to about 13% of atmospheric as you would expect a pump to do pulling against a closed throttle plate.
Balancing at higher revs, the throttle plates are partially open and you typically the pressure rise.

[Grimly]

I need smaller restrictors in the lines as the needles jump like crazy. If I partially pinch the lines I get steady readings but only 4 to 8" Hg.

You need aquarium air restrictors in line.

https://www.amazon.com/ALEGI-Aquarium-Control-Plastic-Regulator/dp/B09W8WJRL1/ref=sr_1_4?