MGB: Checking for Vacuum Leaks

I've heard that I should be spraying water near the carburettor unit to find out if there are vacuum leaks. What should I be seeing though? Im guessing the area where there is a leak there will be bubbling of some sort? Also on the HIF's where should I be spraying and checking for vacuum leaks. The reason I think I may have a vacuum leak is because when I tuned the carbs I had to set the carburettor closest to the fuel filter at full richness and it still stumbles when I lift the piston.

I have tuned the carbs and it runs much better than before. The car was only able to run when the choke was out and now I can push it in after a couple of minutes. I know the car is running too rich because I can hear 'popping' coming from the exhaust off throttle when I am in gear (sounds like unburnt fuel being burnt off). And friends driving behind have said they can smell the gas coming from the exhaust.

The car runs much better now than it did before, I would just like to see if I can make it run even better (lean it out a bit) and not have any stumbling (like in the John Twist videos).

So in summation:

Where should I be checking for leaks?
What should I be seeing that tells me there are leaks?
How can I fix potential problems that some of you have seen before?


Cheers and thanks in advance!

-Andrew

I think the water will pull in and cause a drop in idle. If you are looking for outward pressure add a little dishwashing soap to encourage bubbles. Good luck!

I don't know if this is the accepted procedure, but I would check for leaks with carburetor cleaner spray. You wouldn't spray loads on there (like you would spray water on a tire to check for bubbling at a puncture). You'd spray enough to get the air hovering in that area full of carb cleaner mist. If there's a vacuum leak the spray will be sucked in and, being a volatile substance, will be like adding fuel to your existing fuel-air mixture - your engine will speed up a bit.

So, in answer:
1. Where? All around the carbs and their connections to vacuum hoses. All around vacuum hoses
2. You won't _see_ anything but rather hear the engine pick up a little bit (this is all done with the engine running).
3. How can you fix stuff? Replace old rubber hoses!

Good luck with it!

Would brake/parts cleaner work alright as well?
The only reason I ask how can I fix whatever I find to be the problem is because I remember seeing someone mentioning that the throttle and choke spindles could be a problem area and need to be reseated? That's where the confusion is...I know to replace problem hoses (if that is the case)...its just that spindle potential leak that I am a little worried (if there is a leak there) about.

-Andrew

hmmm....

Well I used the piston lifting pin on the side of the HIF's....
And I held the pin and assumed the rpms were supposed to rise then fall a bit?

So am I just supposed to be lifting the pin for a slight amount of time???

I sure hope this is the problem!

Thanks for the help so far guys!

-Andrew

arahaman Wrote:

arahaman Wrote:

So my second question is then....if I hold it for more than a couple of seconds...should it stumble?

Uh, I dunno. I always have to look at the book and double-check! There are two or three different conditions that will happen when you hold the piston up. You adjust (or don't adjust) depending on which condition you get. Sorry I can't be more helpful at the moment; consult a Haynes or Bentley manual. Cheers!

a vacuum leak would not usually bubble outward, but suck inward.

propane (from an unliighted torch, set low--be quick with the valve, I suppose)) works better than carb cleaner, I'm told.

if you get substantial rpm rise at the throttle shafts it is an indication the carbs need to be reconditioned.

In the quest for leaks most any spray will do that when sprayed in the intake makes the rpm go up. I use carb spray. Propane works. etc. I would leave the air cleaners on for the start. Those gaskets too.

The most important things to spray are the gaskets on the air cleaner, header, carbs, heat shield intake etc. Often these need to be tightened up. Also check hoses however on older cars there are few.

After you locate these leaks and fix them spray the throttle shafts. If there are substantial leaks it takes a little machining work to correct the issue. Usually a carb rebuild of the set. Small leaks can be ignored.

Larry

And if you use the lifting pin, I believe the manual states to lift it only a little bit (like 1/16"). Don't lift it the whole distance it can travel.

If you want to know what works for checking vacuum leaks, test it. If you'r eusing brake cleaner, spray a bit into the carbs. If it makes a change, you can expect half that reaction when you find a vacuum leak (more or less.) See what works for you - I use an unlit propane torch. Some folks use starting fluid. Others use brake or carb cleaner. It depends on whether you're running rich or lean which will work best.

ingoldsb Wrote:

A mechanic told me WD 40 is the way to go for a spray to use for checking vacuum leaks. There will be a noticible rise in RPM as the fluid get pulled in and burned. My 2 bits...