Carb Vent

[mmi16]

79 TR-7 - with Stromberg Carbs

Had the car in a shop for a severely leaking fuel pump. After replacing the fuel pump the shop reports the rear carb (nearest the firewall) has its vent pumping raw fuel to the charcoal canister.

Is this a stuck float or bad needle seat or something else?

[busheytrader]

In the UK, a standard 7 with SU carbs vents to air not a canister.

I had fuel spilling out a carb’s vent pipe back in the day. The needle valve had worn a circular groove around the top and it wouldn’t close off. A new set was easy to fit and cured it. Hopefully a Stromberg isn’t too different.

[dursleyman]

Sounds like the needle valve isn't shutting off when the float operates it, pretty easy job to replace them. It will likely make that carb run too rich as well as the fuel level will be too high.

[Beans]

What fuel pump was fitted? Could be it is producing too much pressure.

[Hasbeen]

As Beans says, it sounds like excessive fuel pump pressure, along with a less than perfect sealing of the carb float valve.

In Oz there were 2 different cam follower arm lengths on pumps sold for use on ours & other cars. The long arm gives far too high fuel pressure for either the zenith or SU carbs when fitted to our cylinder block.

The long arm pump should come with 2 phenolic spacers of different thickness to fit between the pump & the block to reduce the compression of the spring that actually pumps the fuel by the arm.

For either SU or Zenith carb you want 2 PSI maximum, or a little less. Too low will cause fuel starvation. Check your pressure, & check there is no fuel in the float, reducing it's buoyancy.

Hasbeen

[mmi16]

The car is US Spec with Stromberg CD-175 carbs that I have owned since new.

[saabfast]

If you have not changed the pump then, as above, it it normally the needle valve in the float chamber, either has a piece of crud in it or is worn and will not shut off. If you take it apart also check the float height is correct.

[mmi16]

What fuel pump was fitted? Could be it is producing too much pressure.

I don't know specifically what pump was applied by the garage that performed the service.

The service ticket does not show a part number, it only shows a price. It also shows a spacer was required for successful operation.

I have no idea of what the fuel output pressure is, I have no idea what the correct pressure range is.

[Hasbeen]

The correct fuel pressure for either Zenith or SU carbs is 2 PSI. Their is not much leeway above or below this pressure with either carb, too high will cause flooding, too low will give fuel starvation if driven hard. Incidentally the needle & seat valve is interchangeable between the Zenith & SU.

You must have excessive pressure if the rear carb is pouring out fuel, & wasn't before the change of pump. In earlier TRs the copper float could be adjusted to increase the sealing pressure, but that adjustability was lost with the change to plastic floats.

The bean counters also reduced the accuracy of the float level with the 7. With previous TRs the float bowl attachment to the carb body was adjusted from front to back carb, to allow for the downward tilt of the motor front to back. With the 7 they merely bolted the float bowl at right angles to the carb bodies, thus making the front float level somewhat higher than the rear. This means the front carb will flood before the rear, all conditions being equal.

This is fairly academic in the real world unless racing, but it means you definitely have a problem with your rear float bowl sealing, & probably with your new fuel pumps pressure.

Hasbeen

[mmi16]

The correct fuel pressure for either Zenith or SU carbs is 2 PSI. Their is not much leeway above or below this pressure with either carb, too high will cause flooding, too low will give fuel starvation if driven hard. Incidentally the needle & seat valve is interchangeable between the Zenith & SU.

You must have excessive pressure if the rear carb is pouring out fuel, & wasn't before the change of pump. In earlier TRs the copper float could be adjusted to increase the sealing pressure, but that adjustability was lost with the change to plastic floats.

The bean counters also reduced the accuracy of the float level with the 7. With previous TRs the float bowl attachment to the carb body was adjusted from front to back carb, to allow for the downward tilt of the motor front to back. With the 7 they merely bolted the float bowl at right angles to the carb bodies, thus making the front float level somewhat higher than the rear. This means the front carb will flood before the rear, all conditions being equal.

This is fairly academic in the real world unless racing, but it means you definitely have a problem with your rear float bowl sealing, & probably with your new fuel pumps pressure.

Hasbeen

With the front carb not spitting out fuel, I would expect that the fuel pressure from the new fuel pump is 'in the ballpark'. Guess I will have to dig into the needle valve and float settings on the rear carb.

[mmi16]

Took the car to a British Car specialty shop - they rebuilt the Strombergs.

Now that I have the car back - the cold idle is about 2500 RPM, the hot idle is between 1500 & 1900 RPM.

Both those figures seem high. Despite the high idle, everything seems to run fine.

[Hasbeen]

I get my Su carb 7 to idle nicely at 850/900 rpm. This is still higher than I would like, but it starts to hunt at lower idle.

I would not be happy with an "expert" who wanted me to accept anything over 1000 RPM.

Hasbeen

[Cobber]

Those idle speeds are excessively high….seems like a botched up job to me.
Are you sure the bloke you took it to was a mechanic and not a butcher……wasn’t wearing a striped apron and carrying a bloody great knife?

[FI Spyder]

I have a high idle on my FI that developed a while back. The electronics should fix that, made sure it wasn't something mechanical like stuck gas cable etc. It will idle at around 800 when first starting but after a run of any distance. idles at 1500, occasionally will fall back to 1,000. After consultation with some FI wedge owners, it is thought that the distributor advance weights aren't returning. I made sure they aren't worn, cleaned and lubbed them. I think (was suggested) the answer might be heavier weight return springs in the distributor as the springs can get weaker with time. I think that could happen to carb'd cars too which could send you down all sorts of garden paths.

[dursleyman]

Took the car to a British Car specialty shop - they rebuilt the Strombergs.

Now that I have the car back - the cold idle is about 2500 RPM, the hot idle is between 1500 & 1900 RPM.

Both those figures seem high. Despite the high idle, everything seems to run fine.

They may have rebuilt the carbs but they certainly didn't finish the job by setting them up properly!
You need to go back and ask them to finish it.