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SU carb question

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tomgt6

Jedi Warrior
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I just picked up a new car with the 1147cc engine in it with SU carbs. The carbs were totally gone through by a professional SU guy.

Here is my issue the back SU keeps leaking. I can drive for about 5 minutes and then I start to smell gas. I open the hood and the back carb is wet with gas. The gas looks like it is going down the bowl. It looks like the float isn't shutting off the gas flow. I have set the float higher, (or lower depending on how you look at it) to shut the gas off sooner. It is almost like the bowl fills, gets shut off and then the pump builds up enough pressure to force gas into the bowl or maybe out one of the fittings. I can get back from a drive and find drips of gas coming out the side of the bowl at the seam. This is why I adjusted the float from the 1/8 inch gap to more like 1/4 gap. Also the needle is good. It moves freely and is brand new. The tip looks good as well. The float is good, it isn't filling with gas. I have also checked the hose and fittings filling the bowl.

My wife and I were going down the highway when we heard kind of a whoosh sound under the hood and a strong smell of gas. I started to pull over but then everything seemed to get better so I drove a mile to get off the highway and check things out. I found gas had sprayed around the engine, hood and fire wall but it wasn't dripping anymore out of the bowl like I have seen it in the past.

What do you guys think is going on with the carb.

The other thought I have is that it could be gas coming out the fuel line at the joint making it look like the fuel is coming out the bowl.
 
According to some of my acquaintances in the U.K. it's not unusual for the metal on metal float valves to have a burr on the outer profile of the needle. The "fix" is to use a knife to gently scrape the corners of the needle. Some go further after this and apply "toothpaste" to the tip of the needle and use that as lapping paste to develop a better seat for the needle to rest on.

When I was faced with this about two years ago I switched to the Viton tipped float valve needles. This has worked well for me.
 
As Doug is alluding to, the float valve may not be sealing tightly. This can be a defect in the valve, or crud in the fuel that gets caught and blocks the valve open. For me it's usually the latter, and means that it's time to replace all the soft fuel lines.

Also sometimes the float valve body doesn't seal tightly to the cover and so leaks into the fuel bowl. Easy check for this is to remove the lid and blow into the inlet while lifting the lever with your finger. It should take practically no pressure at all to totally stop the flow of air.

Another problem that seems to be common with the 'big' TRs is fuel pressure too high. Any more than about 4 psi will simply force the valve open against all the lift that the float can muster. Ethanol in the fuel, and high underhood temperatures seem to aggravate the problem. I don't know if Spits suffer the same disease, but it might be worth checking your fuel pressure to see.
 
"It's always th' dumb stuff" !!!

Try replacing the supply line first. Use good band clamps too. Check delivery pressure of the pump, then go for the Viton tipped needle/seat if those things are inside the parameters.
 
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