Weber Carbs gurgling after shutdown

[Mike Rohach]

Hello All, I am running 48 IDAs on a 408 Windsor in my CAV. I have a good handle on the tuning. No problems with popping or leaking. I'm using an A/F meter so my mixture is right around the 12 range throughout. I have 38 deg. of total timing at 17deg. of initial advance. No excessive heat in the intake.
The issue is, after shut down the fuel in the carbs sounds like it is boiling. But, the temp of the carbs and intake has never registered more than 120 deg. F with an inferred thermometer. Would changing my pump bypass from a 0 to a 30 relieve some pressure in the pumps if this is in fact the issue.
Any feed back would be helpful.:laugh:

 

[Tim Kay]

When the fuel boils do you get excess fuel dripping down on to the butterfly from the accelerator jet?
Do you have a turkey pan or some other barrier from heat off the headers?
Are all four carbs boiling?

I would not suggest changing the pump relief valve (jet at the bottom of the bowl). That may have an adverse effect on your already tuned setup. And even if it doesn’t effect the tune I doubt it would help a boiling issue.

On my motor, usually just the two carbs at the rear of the motor boil due to excessive heat near the bundle of snakes (I do not have a turkey pan but I do have a heat shield at the rear of the intake). I simply turn off the fuel pump and run the bowls down before turning the motor off. In my experience, the downside to boiling is the dripping from the accel jets making it hard to start due to pooling of fuel in the barrel. Other than that it’s livable.

 

[Mike Rohach]

Hello Tim,
I do have good heat separation, an insulated pan and wrapped headers. I am almost certain the fuel is not boiling, only expanding. The temp of the intake at the base of the carbs never goes above 130deg F, the temp of the carb bodies never registers more than 120deg F.
I spoke with Jim Inglese yesterday, he recommended lowering my fuel pressure to 2.5 lbs from 3.5 lbs, and double checking all my float levels,
I also try to run the carbs dry of fuel when I shut it down. The sound and drip still seem to occur. The situation is livable, but as you said, It makes restart a bit more difficult.

 

[Neal]

I had the same issue. I fabricated a "turkey pan" much like the originals. Less thermal transfer. Problem solved.

 

[Tim Kay]

Mike,

Then I'm no further than you to realizing a resolve.......:lurker:

BTW, I followed Jim Inglese's advice and run less than 3lb fuel pressure also.

 

[Ron Scarboro]

My Cobra does the same thing if the fuel pressure is above 3lb. I keep it just over 2lb.

 

[Rick]

Yes, change the pump bypass. The fuel within the acclerator pump chamber has nowhere to go after shutdown and while absorbing heat it expands, boils and then exits the Pump Squirters and drips onto the throttle plates and into the combustion chambers or onto the backs of the intake valves if they are closed. I doubt you will see any affect to your tune up. Tim's comment above regarding burning fuel after shutting the pump off has some merrit too, but thats a BAND AID.

 

[Mike Rohach]

Thanks Rick, I am going to try a 30 or 35 pump exhaust. While I have the carbs off the motor for an intake change, I'll give them a thorough cleaning. Then, I'll tune the whole set-up again.

 

[Rick]

???????

 

[Randy V]

Re: Shutting off the pump being a band-aide

Doubtful.. It's actually a good policy to shut the fuel pump down and let the engine idle until it dies. Why? With the methanol content in today's fuels, the hygroscopic properties of the fuel can easily pull moisture out of the air and cause corrosion in the carburetors. Running till out of fuel removes this possibility, plus the latent heat in the engine will cause any fuel residue to evaporate. The only thing this is hard on at all would be possibly the accelerator pump and that would depend on the material the pump is made of.

Now I am curious how there could have still been this gurgling after the pump had been shut down and the engine run out of fuel in the carb? There is simply nothing left to percolate!

 

[Cliff Beer]

Mike,

I'd recommend trying to get some heat away from the residual fuel in the carb...there's going to be some residual even with shutting down the pumps and running the engine until it dies.

The turkey pan Neil suggests will help a lot. You might also try some insulators if you don't have them already. If you already have insulators, I might try doubling the insulators with a heavy duty paper gasket in between. I know it's tight under the rear clamshell of a CAV, particularly with IDAs though.

 

[Jac Mac]

Now I am curious how there could have still been this gurgling after the pump had been shut down and the engine run out of fuel in the carb? There is simply nothing left to percolate!

Maybe its nothing to do with fuel/carbs, could be the coolant being moved out or back from the header/overflow tanks after shutdown, especially if motor is fitted in nose-down attitude or car parked in that fashion.

 

[Mark Clapp]

I think Jac Mac is onto something (as usual). I don't have Webers but I hear gurgling coolant sometimes after normal shut down even though the temp stays at 180F.

 

[Tim Kay]

........ The sound and drip still seem to occur. The situation is livable, but as you said, It makes restart a bit more difficult.

According to Mike part of the issue is when it "gurgles" it drips, indicative of the fuel boiling in the carb.

 

[Mike Rohach]

Hello All,
This winter has really been hampering my time in the drivers seat. But, after spending many hours grinding and fitting the new Pro-comp intake. Going through each carb with a thorough cleaning, installing a #30 pump exhaust, and double checking all my float settings. I have been able to start tuning again. The new pump exhaust jets seem to have done the trick. No more dripping or gurgling. I do have to tune out a slight stumble(leaning out) at about 3800 rpm, but I will address that when the weather warms up a bit. I do have an extra set(4) of #30 pump exhaust jets if any is interested. Thanks for the help.

 

[Tim Kay]

Mike, is the pump exhaust the only change that was mad? You 'checked' the floats but did they take any adjustment at all?

Tim

 

[Mike Rohach]

Hello Tim,
The exhaust jets were the only mechanical change. I had initially set the floats on the car. I am guessing that because of the pitch of the motor in the chassis I had some slight variations in all the settings. Setting them dry on the bench, I was able to match each one exactly. I will have to richen either the accelerator pump nozzle or my main jets to overcome the slight stumble at the start of the main circuit.

 

[Jac Mac]

Mike, I see your using a Pro Comp intake manifold, is that the one with balance runners connecting all the ports? There has not been a lot of info with these in regards to actual usage with carbs & I wonder whether the design might act as a 'heat sink' after shutdown with the plenum area being more exposed to heat from the valley etc...just another thing to consider. Also be aware that with these manifolds you are asking the webers to operate in what is essentially a common plenum enviroment rather than I/R and jetting requirements particularly idle & progression will need to be altered with that in mind.

 

[Mike Rohach]

Jac Mac,
Yes, this intake does have a 3/8 hole in each runner connected to a common vacuum plenum. The Redline intake that I had installed also had the common plenum set-up machine into the casting. The basic difference between the two is the size and position of the runners. The Pro-Comp intake has a great deal more material in the casting, which is where all the grinding and fitting was necessary. As far as heat sinking is concerned, if you have your timing set properly you should never have such issues.

 

[Jac Mac]

Timing has nothing to do with what I was suggesting. The point I am trying to make is the large amount of casting area exposed to the heat from the valley/lifter area which will be ~200°c from the coolant and 250°c from the oil shortly after shutdown, due to the plenum being an integral part of the manifold with no air gap feature like most performance 4bbl intakes now use. Something that might help is the factory one piece pressed steel intake plate gasket fitted to late model 351w FI that would act as a heat sheild and might allow you to drill a couple of holes in the intake to allow some air circulation between it and the manifold.