Topic: Checking for carb leaks with a Mityvac

[JDrider]

1975 cb550k1
I rebuilt my carbs and saw YouTube videos where they use the Mityvac, set to pressure, to check for leaks.
This seems like a nice concept.
I was concerned with making a mess with gas lines
Run to it to test for gas leak.

After trying it, it wouldn’t hold air at all in either gas line.
Is there something I’m doing wrong or would this idea not work with my carbs?
I made sure the floats were tilted back. I even pushed down them lightly to see.

[newday777]

I've not seen any videos or heard of testing with the mighty vac that way. It doesn't make sense to try to suck backwards on inlet lines when the float valves operates on the push(gravity) direction.

[JDrider]

I’m not using vacuum, this gun does positive pressure also.

[ekpent]

Do you own a slave tank for gas ? They are easy to make. I use an old riding mower gas tank with a cheap small engine petcock. Can use it outdoors to bench test the carbs with real fuel. As hard as 550 carbs are to remove and install it would be nice to know they really work first.

[JDrider]

I dont, I was just going to use my tank off the bike. Thought that would be messy so I was trying this method first.

I just dont know why it wasn't holding any pressure.

The carbs have dried out over the last week and a half.
I sprayed WD40 on the seat and needles.

[JDrider]

https://youtu.be/nAe_5YwWoV0

https://youtu.be/1e3z9mo_kvA

these videos are where I got the idea

[BenelliSEI]

Good idea, never tried it. I wonder if you are just creating too much pressure? The float valves just hold back a gravity feed from a few inches above. Don’t know the actual pressure, but it must be a very small number…….

[Deltarider]

Sorry, but those video's just show how to make things complicated. Holy cow! It's sooo simple: if there's a leak, the overflow pipe of that carb will tell you. It's as simple as that. What's so difficult to have the rack on the bench in its normal position and pour gas in the fuellines? Have you seen the vids I have posted?
Talking about fuellines. In the picture I have seen you have the rack that has two fuellines.
The correct dimensions for these lines are: for the CB500 and earlier CB550s: 18 and 30 cm length for the models with the bowltype petcock and 17 and 28 cm for models that have the newer style petcock. Do not be tempted to have them even one cm longer. For the tube that supplies carbs 3+4, use the soft metal clamp in the middle front side as shown in the pic (btw, horizontal is good enough) for proper routing. Ideal inside ⌀ of the fuel lines is 5,5 mm. Outside ⌀ will be around 10mm. Have this and the lines will not kink. Abstain from extra inline filters. Some have been lucky with them, but my experience is they can't be trusted. Often it isn't that filter itself, but the rerouting of the fuel line that - sometimes intermittent - impedes a proper delivery of the gas.

[Deltarider]

BTW, this method to check for a correct fuel height will also work for your model CB550 and can be done with the rack on the bench in its normal position. http://forums.sohc4.net/index.php/topic,185754.msg2151647.html#msg2151647

[Flyin900]

I use that system every time I rebuild a set of carbs for myself or a customer. You must have the carbs with the fuel needles sitting flat into the brass seat 100%. Which requires them being upside down not sideways in your picture.

With the float bowls off if it leaks vacuum you can then spray each fuel valve with a soap/water solution to find the leaker. I still do a leak test afterwards with isopropyl alcohol rather than gasoline, as it is nicer to deal with than gas.
Hook up my IV gas bottle and fill the carbs and bowls and let sit for an hour to see if anything leaks. Some will also do a float bowl fuel level check to ensure the fuel valves are shutting off and leaving that 1-2 mm level below the base of the carb body.

[JDrider]

Yes, I had the floats upside down when doing the pressure test, so the needle was seated.

Well I figured out where the air leak was.
It’s where the fuel line T’s in both sides.

So now I have to separate them and replace the o rings

[Flyin900]

Age is the culprit there as the double O rings on the fuel connector pipes rarely leak with that double ring set up. Keihin went to a single O ring on each side of the pipes later in the late 70’s and onward, so more leakers in that style.

[Deltarider]

Age is the culprit there as the double O rings on the fuel connector pipes rarely leak with that double ring set up. Keihin went to a single O ring on each side of the pipes later in the late 70’s and onward, so more leakers in that style.

This ^. When carbs are always in a natural state which is wet, these O-rings last forever. Mine are 47 years old.

[JDrider]

Just an update.
Once the o rings were replaced on the fuel line T
I was able to use the Mityvac to see that it is holding pressure.

So for all the people who don’t have a dedicated workbench and
Extra fuel tanks to test your carbs. This Mityvac really came in handy. (The one that does vacuum and pressure)

I was working on my kitchen counter and my basement freezer as work benches.

If I can do this stuff, anyone can.
Thanks for all the help on this forum.

[Flyin900]

Looks good and congrats on the success