Bubbling Carb

High temperature maybe?

I used to get that all the time in my first speedster and also asked about it. It is heat and lots of it. I could hear it after i shut the motor down from a spirited drive.

Yep, fuel boiling in the carb from the heat. You could try a phenolic spacer to insulate the carb from the manifold. You might also check that your fuel line is away from major heat sources like the exhaust or get it insulated as well. But I'd also be double checking for excessive oil and cylinder head temperatures to make sure the engine's running ok. You can have this issue even on a perfectly running engine, hence the vw owner's manual instructions to hold the throttle to the floor to clear out this possible flooding that results on all hot starts, but best to verify that everything else is good.

Thanks for the help! 

Just curious, but do you have dual carbs or a single in the middle.

 

If they're duals, are both boiling or only on one side?

 

We might be able to isolate the cause and help you remedy it.

Hey Gordon! It's my dad's new VS...dual carb 1600. Both sides, but more on the left side. He says he can hear it "bubbling" when he comes home from driving. Does it for about 15 to 20 minutes after engine is shut off, and slows as the engine cools. He gets a black puff of smoke upon start up but engine runs fine after that.

Well, the smoke is because the bubbling carb bubbles fuel down into the carb/intake manifold and floods that side of the engine.  I've also heard of several (at least) VS cars on here with the exact, same problem.

 

OK< now we're getting somewhere.......

 

I assume that your fuel hose comes out of the transmission horn up towards the backside of the rear seat on the driver's side, then converts to a rubber hose, has a fuel filter in it somewhere between the steel fuel line going through the pan and the firewall.  

 

Then, after passing through the firewall, it can take one of two routes:

 

(a.) it runs along the driver's side of the engine compartment (often just lying on top of the driver's side engine tin) and up to the driver's side carb where it enters a "banjo" fitting with one hose in, and another hose out and across the back of the engine to a similar banjo fitting on the passenger side carb, OR

 

(b.) it goes through the firewall, drops down and lies close to the driver's side head, then around the engine to the fuel pump, then out of the top of the pump, Tees and has a hose running down onto the head tin and then up to the driver's side carb.

 

Somewhere back there the rubber fuel hose for the driver's side is located too close to the head on that side.  Find out where and re-route it away from the heat source and that should cure it.  

 

If not, report back and I'll let someone else try to figure it out, but the fact that the passenger side is OK while the driver's side bubbles tells me that THAT side is getting too much heated fuel entering the carb, rather than heat transfer through the manifold to the float bowl.

Thanks Gordon!

I relayed the possible scenarios to my dad and will have him take a look...will let you know what I find out.

STU

Gordon - as i said earlier in the thread i had the same issue in my first speedy.  Bubbling gas etc.  same size engine, my actual fuel filter sat in the engine bay which is nuts all in itself.  I did not have the foam H piece that encased the tins from the elements below it like the exhaust heat etc.  but my question is more for everyone who reAds this...is there a way to actually wrap those rubber lines in some sort of insulator or something? In my new car i am hoping this wont be the case.  Seems like a fairly standard thing that should be addressed?

Just being in the engine bay shouldn't cause a problem.  I had a hot restart problem when the fuel line with one of the filters was laying on the "shelf" very near the tin, and was being heated from underneath.  Rerouting the line and raising it a few inches off of the surface with a couple of tie-wraps solved my issue.

I say isolating the engine compartment from hot expelled air is mandatory and a first. If you keep your fuel lines, filter, etc away from heat sources that's all it takes. Should be pretty simple.

As Impala mentioned, isolating the engine compartment from exhaust and spent cooling air should be the first project. Then figure out why the fuel is overheating; Gordon's and Lane's suggestions should cover it. If the carbs are getting too hot because of the manifolds and heads, an insulator (I've used 1/8" bakelite between the heads and manifolds) will help.

 also check to be sure all fuel lines are tight, and the fuel level in the carbs is correct(float level), most of the time it is the float is set to high.you can take off the filter tops and look down into the carbs throats after shutting it down and look for a drip,trickel,or stream.also not havning good fuel filters can cause this.I run 3 filters on my stuff, 1 befor the fuel pump where the line exits the frame, and 1 before each carb to catch any debris that may come from the das fuelpumpkinhousen. all it take is a minute speck for the needel to not seat fully & close off the system.

Thanks for all of the possible causes to the carb issue. Dad moved a few lines away from some heat sources via zip ties and what do you know...no "bubblin" after driving for the past couple of days!!! You guys are awesome!! 

great, the heat was building pressure in the lines and it had to go somewhere. gladd you got it taken care of befor it killed the cylinders.