The $1.55 Carb Syncronizer
by Marty Ignazito
The objective of the vacuum measuring carb synchronizer is to see that the vacuum signals from both carbs are the same. It is the difference between the signals and not the actual signals we are interested in however. The Rotax two stroke engine shows a signal of about 6" of mercury at idle measured against the atmosphere at its test signal port and a difference of about 1/16th to 1/20th of an inch of mercury can be read easily by eye. If lighter liquids are used like oil this signal would read as about 97 inches of oil requiring a very tall manometer gauge. With a senstivity of some 16 times greater than a mercury manometer, an oil manometer only needs to look at the difference. By hooking each side of the manometer to each of the carbs we have a very sensitive tool for synchronization. A 1/20th of an inch of mercury difference would show up as about 13/16ths of an inch of oil on the oil manometer allowing for even finer adjustment than possible with mercury.
Below is a photo of an oil manometer I made up with some cheap materials from the local Ace hardware store. I filled mine with air compressor oil since I did not have any two stroke oil around, but two stroke oil would be a better safety measure in the event of oil getting into a carb.
Here is the bill of materials:
12 ft of 1/8" ID clear PVC tubing $0.84
1 wooden yardstick $0.59
2 wire ties $0.12
For a total cost of $1.55 plus tax.
The tubing was taped to the yardstick with transparent packing tape. The wire ties were put thru the hole in the yardstick to secure the tubing on each side at the top. If desired a machinists or carpenters square and a pencil can be used to extend the 1/8" markings to both sides of the stick or the level of the oil on one side can simply be noted before testing begins since it must return to this point to be in balance. Fill with oil to about the 26 or 27 inch mark (36 at the bottom). You may have to leave the unit sit for a day or so to get all the bubbles out. If you are careful not to introduce air when filling this is less of a problem.
When in use with the engine running both lines must be connected, one to each carb. The oil will be sucked up and out of the manometer if only a single carb is connected and the other end is left open to the atmosphere.
Marty
I just got a length of clear vinyl tubing about 10 feet long, taped a "U" to a yardstick with clear packaging tape, filled it about 1/2 full (on the yardstick) with ATF, hung the stick/tubes on the handlebar, pulled the rubber covers on the carb runners and stuck the ends of the vinyl tubing on, started the engine and leveled the atf in the tubes. The adjustment screw is really sensitive. Just a wiggle will change the levels. Use the fan, open the garage door and don't burn yourself. The pipes and engine get HOT.
If you suck in some ATF, the garage gets smoky so leave the door OPEN.
Total cost was about $2.00. I stole my wife’s yardstick (she don't have a clue where it went). I "borrowed" her clear packaging tape and the tubing came from the pet department at WallyWorld.
A little hint, I seemed to have a vacuum leak at the rubber covers. They fit loose so I put some RTV in each one and then I put them back on.
Just make sure both carbs give identical readings on identical instrumentation, then swap feeds to be 100% sure.
Always do a good tune-up first (valve clearances within tolerance, air filter clean, fresh plugs, recent oil change). Some people may poo-poo the oil change, but every detail figures in.
Warm up your bike, set your idle at 1,000 RPM, then start your sync run.
It helps to use the biggest fan you have pointing at the face of the engine to keep things relatively cool while you sync.
by Marty Ignazito
The objective of the vacuum measuring carb synchronizer is to see that the vacuum signals from both carbs are the same. It is the difference between the signals and not the actual signals we are interested in however. The Rotax two stroke engine shows a signal of about 6" of mercury at idle measured against the atmosphere at its test signal port and a difference of about 1/16th to 1/20th of an inch of mercury can be read easily by eye. If lighter liquids are used like oil this signal would read as about 97 inches of oil requiring a very tall manometer gauge. With a senstivity of some 16 times greater than a mercury manometer, an oil manometer only needs to look at the difference. By hooking each side of the manometer to each of the carbs we have a very sensitive tool for synchronization. A 1/20th of an inch of mercury difference would show up as about 13/16ths of an inch of oil on the oil manometer allowing for even finer adjustment than possible with mercury.
Below is a photo of an oil manometer I made up with some cheap materials from the local Ace hardware store. I filled mine with air compressor oil since I did not have any two stroke oil around, but two stroke oil would be a better safety measure in the event of oil getting into a carb.
Here is the bill of materials:
12 ft of 1/8" ID clear PVC tubing $0.84
1 wooden yardstick $0.59
2 wire ties $0.12
For a total cost of $1.55 plus tax.
The tubing was taped to the yardstick with transparent packing tape. The wire ties were put thru the hole in the yardstick to secure the tubing on each side at the top. If desired a machinists or carpenters square and a pencil can be used to extend the 1/8" markings to both sides of the stick or the level of the oil on one side can simply be noted before testing begins since it must return to this point to be in balance. Fill with oil to about the 26 or 27 inch mark (36 at the bottom). You may have to leave the unit sit for a day or so to get all the bubbles out. If you are careful not to introduce air when filling this is less of a problem.
When in use with the engine running both lines must be connected, one to each carb. The oil will be sucked up and out of the manometer if only a single carb is connected and the other end is left open to the atmosphere.
Marty
I just got a length of clear vinyl tubing about 10 feet long, taped a "U" to a yardstick with clear packaging tape, filled it about 1/2 full (on the yardstick) with ATF, hung the stick/tubes on the handlebar, pulled the rubber covers on the carb runners and stuck the ends of the vinyl tubing on, started the engine and leveled the atf in the tubes. The adjustment screw is really sensitive. Just a wiggle will change the levels. Use the fan, open the garage door and don't burn yourself. The pipes and engine get HOT.
If you suck in some ATF, the garage gets smoky so leave the door OPEN.
Total cost was about $2.00. I stole my wife’s yardstick (she don't have a clue where it went). I "borrowed" her clear packaging tape and the tubing came from the pet department at WallyWorld.
A little hint, I seemed to have a vacuum leak at the rubber covers. They fit loose so I put some RTV in each one and then I put them back on.
Just make sure both carbs give identical readings on identical instrumentation, then swap feeds to be 100% sure.
Always do a good tune-up first (valve clearances within tolerance, air filter clean, fresh plugs, recent oil change). Some people may poo-poo the oil change, but every detail figures in.
Warm up your bike, set your idle at 1,000 RPM, then start your sync run.
It helps to use the biggest fan you have pointing at the face of the engine to keep things relatively cool while you sync.
.........No, make that Great!
- is it critical to stay on the air screws only to effect the vac ?, and do the stop screws have to be exactly the same through the whole process - in other words can one be a bit different from the other to achieve the same vac and rpm as the other cylinder ?.
...........just joking.
