Not enough vacuum

[steve beddow]

My problem is not enough vacuum to open petcock (sp sorry). Usng a vacuum pump and placing a closed ended tube over where vacuum hose should go. Scooter runs super top end to low end. I have sprayed starter fluid spareingly around carb area, intake area and airbox area looking for vacuum leaks. There is no change in idle speed as if it was sucking in the fluid thus indicating a leak. The compression is not great but ok and the bike has plenty of pick up and starts easily cold or hot. This problem is on both my 50 cc sb,s. Fuel tanks full. I'm open to any suggestions

steve

[dun rite]

Hello. Is the filter inside the tank clean? or petcock diaphram could be dried out or gunked up. the vacuum pump probably pulls more vacuum than the engine. Have read of some here on the forum that have soaked the petcock in something( cant remember what exactly) and it works after. use the search function to look for how they did it

[mousewheels]

Yes, what 'dun rite' said.

Check out the 'wiki topic for petcocks at this link - petcock cleaning and anatomy

[steve beddow]

I made a new filter using 1 inch foam from a hobby shop. The air box is sealed with no leaks and the petcock is new--but I'm not sure if it's for a 49cc. I also have a 80 and 150.

Using my vacuum pump I only have to apply 0ne or two inchs of vacuum (bearly registers on the guage). Can the vacuum on the bike be measured? If so does anyone know what this measurement should be

[dun rite]

Hello I suppose you could install a T in the vacuum line, then hookup you vac. gauge to the tee and crank the engine.

The compression is not great but ok

What compression are you getting?

[Bear45-70]

The only issue here is the petcock and not the filter or fuel volume. How much vacuum (inches of mercury or just inches) are you pulling on the petcock to make it work? How much vacuum is there at the intake nipple with the engine running? If you are pulling like 10" of vacuum to get the petcock to work, your petcock is bad. If you engine has less than whatever the vacuum required to open the petcock you need to determine if the the engine is weak of the petcock is too stiff. I have never seen a vacuum spec or even a vacuum reading taken by anyone here on the forum for any model scooter. But I have check several petcocks and none needed more than 5" of vacuum (only one was that high) to operate and the average was about 3.5 ".

[mousewheels]

I have never seen a vacuum spec or even a vacuum reading taken by anyone here on the forum for any model scooter. But I have check several petcocks and none needed more than 5" of vacuum (only one was that high) to operate and the average was about 3.5 ".

Thanks for the numbers Bear.

The original poster gave his measurement at one to two inches:

Using my vacuum pump I only have to apply 0ne or two inchs of vacuum (bearly registers on the guage). Can the vacuum on the bike be measured? If so does anyone know what this measurement should be

[Bear45-70]

I have never seen a vacuum spec or even a vacuum reading taken by anyone here on the forum for any model scooter. But I have check several petcocks and none needed more than 5" of vacuum (only one was that high) to operate and the average was about 3.5 ".

Thanks for the numbers Bear.

The original poster gave his measurement at one to two inches:

Using my vacuum pump I only have to apply 0ne or two inchs of vacuum (bearly registers on the guage). Can the vacuum on the bike be measured? If so does anyone know what this measurement should be

Sure the vacuum on the scooter can be measured, just hook up a vacuum gauge to the petcock vacuum port on the manifold and crank the engine. It won't be a steady reading but whatever it peaks out at is what you have. Once the engine starts the vacuum will be higher and steadier.

[steve beddow]

I checked the vacuum directly at the nipple where the petcock vacuum tube is connected. Absolutely no guage movement. THis nipple is located at the throttle slide position. Any way I used a t fitting on another vacuun nipple and all works well.

Thanks everybody for your time


steve

[mousewheels]

I checked the vacuum directly at the nipple where the petcock vacuum tube is connected. Absolutely no guage movement. THis nipple is located at the throttle slide position.

If the tap looks similar to below, it's for oil injection. It's present on some Honda carbs, others inject oil into the intake - downstream of the carb.

[steve beddow]

The petcock vacuum nipple is located on the throttle slide tube. Infact, the slide has a slit in it so as it moves up and down it won't block what ever vacuun is available at this point. My vacuum guage placed on the nipple indicates 0 vacuum. I temprarily solved the problem by applying a tee at the oil injection nipple ( the petcock does not draw vacuum so oil is not sucked up the tubing to the petcock). My long term fix will be to install a 44 mm big bore kit I have on order.

[devenex]

I think you said this was a SB50 your working on, correct?

If so then there is only one nipple on the manifold and its for the vacuum line. The oil line connects to the carb on this model.

[Wheelman-111]

Greetings:

The throttle slide does indeed have a slit, but it's not for keeping the vacuum nipple clear. It is to mate with a nub in the slide bore to keep the slide from rotating.

You may be measuring "Vacuum" on a carb vent barb. The carb has a couple of vents so that fuel can flow.

What I'm not getting is how the bike can "run great" if the petcock isn't filling the carb bowl?

[mousewheels]

Steve,
On an Elite SB50 and Elite SA50's, the carb fitting is where the oil is injected.

On Spree's, there were two taps on the intake, the upstream tap was for vacuum, downstream for oil injection. On the SB50, Honda changed the intake to only the vacuum fitting. Then added the fitting for oil injection to the carb.

Your scooter has oil injection connected to the intake manifold, and the vacuum line to the carb. It is reversed from the factory connection.

-- Here is a photo from the Honda SB50 Service Manual.
Honda labeled the line from the intake to petcock incorrectly. It's a good enough photo to demonstrate this line does not route to the oil pump. The blue marked line I believe will route from the carb fitting to the oil pump

[Bear45-70]

I checked the vacuum directly at the nipple where the petcock vacuum tube is connected. Absolutely no guage movement. THis nipple is located at the throttle slide position. Any way I used a t fitting on another vacuun nipple and all works well.

Thanks everybody for your time


steve

The fitting for the petcock vacuum source is always on the intake manifold, NEVER on the carb. The nipple on the slide bore is for the oil injection pump output.