• Hey Guest!
    British Car Forum has been supporting enthusiasts for over 25 years by providing a great place to share our love for British cars. You can support our efforts by upgrading your membership for less than the dues of most car clubs. There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Upgraded members don't see this banner, nor will you see the Google ads that appear on the site.)
Tips
Tips

SU Carb Question

Share this page

Number_6

Number_6

Jedi Warrior
Country flag
Offline
My TR3A is the first LBC that I've owned with SU's. All my past LBCs had ZS CDSE carbs.

My question, when I leave my car sit for a week or so without starting it up I find that the float chambers are empty. I don't smell any gas or see any dampness under them but obviously the gas in the chambers is leaking out. Is this normal for a side float chamber SU Carb? Or are the seals gone on the carbs and they need a rebuild?
 
Pretty normal I think. The original fuel pump had a lever on it to prime the carbs. The little cork (Gland seals) on the jet dry out, and I would bet that it leaks a little bit there. You might make sure that the bolt that holds the float chamber to the carb body is snug. If you see signs of leakage when it is running, you should plan on an overhaul. Gasoline and hot exhaust manifold can be a bad combo.
 
Somewhere between those two extremes, IMO. You probably have small leaks at both the jet glands, and the float bowl washers. But I wouldn't call that grounds for a full rebuild.

If it bugs you, replace the seals. My experience was that the original type seals would only survive a year or two of daily driving before they started to leak enough to notice (liquid fuel on the outside of the jet/bolt). Moss (and others) offer an O-ring jet seal that lasts longer than the cork (but still not forever). And someone (Joe Curto? Mike Macy? I forget offhand, should be in the archives) offers a polyurethane replacement for the bowl seal that should last much longer as well.

Both seals can be replaced without even removing the carbs from the car.
 
I've had my car for 15 years and the float bowls are always low after a few weeks sitting in the garage. I don't think I would go for a rebuild either. I have rebuilt my carbs in the past, but the bowl level problem was unaffected by the rebuild. I think it is normal, if undesireable, and probably why there is a priming lever on the fuel pump.
 
Fuel is going to evaporate over sometime.

Don
 
donbmw said:
Fuel is going to evaporate over sometime.

Don
Click to expand...
:iagree:

The float chambers are essentially open to the atmosphere. My carbs only have a few hours running on them and show no signs of leakage, but if I let the car sit for a week or two, the level in the bowls drop considerably. As a matter of course, when the car sits for awhile I always pump them up with the primer on the fuel pump. You can feel the resistance change when the bowls get full. The longer the car sits, the more I have to pump the priming lever.
 
I have sometimes thought this was a brilliant design feature... when the car has sat for a long spell the engine (& fuel pump) have to crank a bit to fill the bowls and start -- meanwhile oil pressure builds up in advance of the engine running.
 
I have HIF6's on my rover v8 engine, I got into a habit of turning on the ignition for about 5 seconds to let the electric fuel pump pump up the fuel carbs before I try to start the first time of the day.
 
Just for peace of mind, make sure your oil level is the same everytime. Sometimes the fuel will leak down into the crank case.
 
Since my TR3 has been back on the road Oct 98. This has been like that. Set for week or more it has to turn over a few times before it will start. I installed a electric fuel pump inline with the normal pump because of a fuel vapor lock problem. Now I will turn it on before starting to fill the bowls.

Don
 
Thanks guys for the help, As I say, I have no real experience with SU's just Zenith Strombergs. I don't recall having the same issue with the ZS's but it's also been about 20 years since I last drove a car with ZS's so my memory is probably fuzzy, the same may have happened with the ZS's also. I don't know yet what's normal and to be expected with SU carbs.

Sounds like there's nothing unusual going on with these carbs so I wont sweat it. A DPO at some point repalced the Fuel pump with a pump that doesn't have the priming handle on it on so I can't prime the carbs by hand. I guess a squirt of starting fluid when I fire it up. I still have a long way to go before I bring the car back in to Have the rebuilt engine given it's first 500 mile checkup and have the head retorqued.
 
The ZS carbs were a later design that eliminated the common leakage points of the early SUs. SU also eventually solved the problem with the HS series (and others) ... likely due to competition from ZS!

Look carefully at that fuel pump. Some of the repros without priming levers have the pivot pin only staked in place into the soft alloy body. The pins sometimes work loose, with potentially catastrophic results.
 
martx-5 said:
The float chambers are essentially open to the atmosphere.
Click to expand...
Hmm, that's an interesting thought. I long ago changed my bowl vents to long pieces of fuel line that hang down to the frame rather than pointing into the air filters. (I also use TR4A air filters, so there's no room for the original vents anyway.) Possibly that's part of why I was able to leave the car sit for several weeks and have it still start easily. I hardly ever used the priming lever, except when testing something.
 
TR3driver said:
I long ago changed my bowl vents to long pieces of fuel line that hang down to the frame rather than pointing into the air filters. Possibly that's part of why I was able to leave the car sit for several weeks and have it still start easily. I hardly ever used the priming lever, except when testing something.
Click to expand...
That's interesting, That's the same arrangement that my carbs have also. Two long copper breather pipes that hang way down to below the frame.
 
Aloha,

The bowl vents on my MG are that way as an original design. The potential problem with this design is that road grime may some times collect and close off the vent. This can lead to a vapor lock condition when starting a warm car. It rarely happens but can be a vexing problem to discover based on personal experience.
 
Back
Top