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SU Carb Float Valves

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Jim_Gruber

Jim_Gruber

Yoda
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Every time I start Bugsy, ease off choke after less than 1/2 mile in the summer, I catch a whiff of raw gas. Cannot find a leak anywhere and suspect might be gas escaping from the overflow on the carbs. Too small an amount to see if I stop the car and pop the bonnet but the smell of raw gas is there for a moment and goes away. It will come back occasionally after a hard pull but much lighter smell not nearly as intense.

Situation 2, stopped at Subway to grab something to eat and came out 5 minutes later. Car was slightly warmed up but only 2-3 miles from home, 80 degree temps. Turned the key and a tremendous kaboom from the muffler. Last night turned key cold after sitting 3 days, another small kaboom not normal starting sounds.

Can I raise the question that I might have a leaky float valve. Now adjustable floats were shimmed properly by my LBC Mechanic last summer so I don't think floats are set too high but perhaps the valves themselves are leaking and not shutting off tightly hence small of gas but I cannot see evidence of gas under the bonnet. Also could account for backfire/kaboom if gas was leaking into the cylinders from a leaky flat valve could it not.

I have a spare set of SU's, may simply change out the top of the carbs and change floats to one with an adjustable type to see if gas small goes away. Thoughts anyone?

Leaking float valve, time to change the oil as well if that is the case.
 
what do your plugs look like? you might be running a bit rich.
 
If we are talking HS2 carbs, and it sounds like we are, the all plastic flaots can be set as for float height, by using samll shims under the needle and seat valve where it threads into the float lid, Joe Curto has them. You can remove your flaot lid and pull the flaot hinge pin out and check the flaot easy enough, I sumerge them in a bucket of water and cheack for bubbles, hardly ever do I find a bad flaot, but it does happen. My guess if you got fuel coming out of the vent tubes is you have a sticking needle and seat valve. I would guess as far as the popping/backfiring to check the ignition over, I would start by checking the timing.
 
Raw fuel in the exhaust, leaky float needle or choke linkage not set quite right?

Plug inspection, and IMHO the idea that ethanol is involved: "Schmaze" from the interaction of atmosphere and ethanol at the needle and seat of the float bowls. A thorough cleaning of the needle, the bore it moves in and the mixture needles would be my first move.

"Stabil" additive would be the second thing.
 
I once blew the muffler completely off my '80 Monza by turning the key off and on and pumping the gas. It was GREAT in traffic. If you wanted someone to get out of your way; flip the switch a few times and they would move over. I got really good at it. I could pull of 8 bangs in as many seconds. Dad always got a kick out of it. Oh to be young and stupid again. Being old and stupid isn't nearly as much fun amd much more tiresome
 
Thanks guys I think you've confirmed my suspicions. I checked plugs and they are indeed black. I decreased 1 Flat and next plan will be to change floats out off of my spare set of carbs and see if I can get rid of the gas smell exiting through the overflow. That alone makes me nervous. Floats were shimmed to spec already but I do believe I have an issue with the needle valves. Hopefully changing the carb tops out will fix the issue. More to follow after I do some more cogitating on the subject. Thanks again.
 
When you release the choke -fastidle make sure both jets are all the way seated up. Give them a little push upwards into the carb body My rear carb will some times allow the jet to HANG.
 
Bringing this one back up again. I had a chance to pull apart the float valves and clean them with some decent 3M carb/fuel injector cleaner stuff. Put things back together and leaned another 1 flat. Took Bugsy out and did a hard pull and let off the gas as usual and no gas smell anymore. I'm going to theorize that as someone theorize and I had suspicions that I had crud on the seat of the ball valve. Blew it out thoroughly and it fixed the issue.

Plugs are no longer dark black but tending towards dark grey so I did again lean it out one flat. Bugsy running great once again.
 
Jim_Gruber said:
I had suspicions that I had crud on the seat of the ball valve.
Click to expand...

Sounds like you have a Grose jet rather than traditional needle seat valve, many folks have had trouble with Grose jets.
 
Hap there is a Ball Valve type fitting in there. The once on my other set of SU's are different. I understand the Gross Jets are easily swapped out to a better type just tap out the float pin, and with a 5/16" wrench can be swapped out. At this point some 3M Carb / Fuel injector cleaner took care of things. Going to see what happens after some more driving and testing.

Everything appears to be bright and shiny in there on the ball valve. Tolerances are extremely tight and I can see how even a slight buildup of gum and varnish could cause float bowls to overfill and excess to run out of the overflow. A little bit of gas can be smelled a long way away and at speed.

Right now thankful all is fixed for now.
 
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