Vacuum leak?

cdnp6

cdnp6

Member
Noticed the smell of petrol in the engine bay today and found petrol on top of rear carbs float chamber.
Had a mechanic friend come for a butchers. Ran the engine and could not see any fuel leak, but engine running poorly even after warm up. The car was running so rough the ignition light was coming on and it was on the verge of stalling. Went for a ride and car runs well until you stop and come to idle. Brakes seem weak and seems to have a bit of travel before you get any action. At idle stepping on the brakes increases RPM`s. Any ideas guys.
 
Fuel on the float chamber generally means that the needle valve in the chamber is stuck or has a bit of dirt in it. This will also cause that carb to run rich as the fuel pump is pushing fuel straight out of the jet causing flooding . Either that or your float has sunk.
The brake issue could be a split booster diaphragm or air control diaphragm.
 
One of the shops here suggested trying to create a vaccum in the carb to pull out any debris before taking the carb apart.
He said to clamp off the fuel line on the affected carb until it quits to drain the flooded bowl, then
Give the car full throttle and pull the choke on.
I am not sure I want to do the latter with a fresh rebuilt engine.
What do you guys think?
 
That just sounds lazy! I presume you have HD8's with the separate float chamber alongside the carb? If so sorting out and cleaning the float chamber is very easy and you don't need to touch any of the rest of the carb. I suppose the advice you received could be appropriate for a multi jet carb like a Webber or Holley where there are a lot of places for crud to have migrated to, but an SU is so simple it is completely unnecessary. When you get into the float chamber there are only two things to look at. The needle valve that closes off the petrol supply and the float. It is possible tht the float is holed and therefore sinking instead of floating. But much the most likely is a sticky needle valve. This may be quite difficult to identify, so the best policy is simply to replace. Whilst you are in there you will be able to see whether there is any crud in the bottom of the chamber to clean out. Use this as a guide as to whether to do the other carb as well.

With too much petrol in one carb the settings will be all over the place! So ignore the engine speed up with the brakes until you have sorted it.

You're more general comment about the brakes might simply be the rears not self adjusting properly. Have you got excessive handbrake travel as well?

Lets re-assess the brakes once that carb is sorted!

Chris
 
cdnp6 said:
One of the shops here suggested trying to create a vaccum in the carb to pull out any debris before taking the carb apart.
He said to clamp off the fuel line on the affected carb until it quits to drain the flooded bowl, then
Give the car full throttle and pull the choke on.
I am not sure I want to do the latter with a fresh rebuilt engine.
What do you guys think?
Click to expand...
You can do the same without blocking off the fuel. Don't use the choke though, you just use your hand to cover the inlet on one carb after a rev and cover whilst the throttle is shut. You can be lucky but only on the odd occasion. If it's wear like Chris says it will stick again anyway :) Worth a try maybe :| and yes it's the lazy option :p
 
KiwiRover said:
Fuel on the float chamber generally means that the needle valve in the chamber is stuck or has a bit of dirt in it. This will also cause that carb to run rich as the fuel pump is pushing fuel straight out of the jet causing flooding . Either that or your float has sunk.
The brake issue could be a split booster diaphragm or air control diaphragm.
Click to expand...

In support of the split diaphragm suggestion, this may also explain the increasing rpm when applying the brakes, split diaphragm allowing air via the servo hose into the inlet manifold.
 
My carbs do have the float chamber beside the carb, but looking at what's written in the federal workshop manual supplement, it appears the carbs are an HS8 variant.
I hope to have the lid off tomorrow or Saturday to check the condition of the float and try to blow any debris out of the needle. I'm hoping this solves the problem. A replacement needle or float will take a week or two to get.
I also intend to clean the fuel pump gauze and install an inline filter between the fuel pump and carbs.
The brake servos are both brand new lockheed units so I hope the diaphragms are good. The vacuum line going from the servos over the top cover to the inlet manifold has some cracks so I will be replacing this hose.
Chris, my handbrake isn't optimally set. I haven't counted the number of clicks, but I'm sure it's more than the accepted amount. I do have to get under the car and set this properly.
 
So I opened up the chamber and found the float happily bouncing around in the chamber, blew out the needle seat, found and removed a strand of hair from the tip of the needle valve and reassembled. Running a little better now but still not great. I ordered new needles and seats. Will find out in a week or two if that will solve the problem.
I also took a look at the carb ID tags. Carbs are SU AUD 329's. From what I can find on the SU website these were used in US TC's in 1968 only. My car was manufactured December of 1969 so should have had AUD 411's. Maybe replaced at some time?
 
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