Drippy Webers after about 1/2 hour of sitting when warm

[tomeric914]

I've got Weber 40 IDTP 3C's on my 914-6 sitting on PMO manifolds and insulators designed for use on CIS heads. I rebuilt and rejetted the carbs earlier this year and they run and idle great.

Problem is, roughly 1/2 hour after I shut the engine off, there is a fairly strong fuel smell sometimes accompanied by furl dripping from the center throttle shafts.

I used the PMO float tool to properly set the floats.

It seems like even with the insulators, I'm boiling fuel in the carbs. That's the only thing I can think of. Any thoughts?


thanks

[john walker's workshop]

did you vent the carb tops yet? with the PMO jig?

[1QuickS]

The "PMO" drilled top covers is actually a Weber fix that Richard adopted for the benefit of Weber users. The "fix" is twofold: 1) install insulators at the interface between the intake manifolds and the heads and 2) drill little holes into the inner baffles of the top cover of the throttle body. These little holes act as a "relief" to the fuel which is boiling "percolating" within the fuel wells. The fuel is directed to flow into the cylinders of the engine as opposed to flowing up the vent pipes and possibly down onto a hot exhaust manifold; better to let raw fuel drain into your engine than to catch your engine bay on fire. Whatever fuel is spilled down the throttle bores will land on the closed butterflies and hence drain along the throttle shaft which is where you are seeing the fuel come from.

An item of interest to make this system most effective is to be sure your vent pipes are tight and not loose with their fit into the top covers. If they are loose then fuel can still escape the throttle body at the loose fit.

I have found that dirty fuel bowls or fuel bowls that have been corroded create nodes for the fuel to vaporize on and accelerate the boiling process. I tested a throttle body that had one side free of corrosion/debris and the other side was very pitted from corrosion. I put fuel into both bowls and mounted the carb onto a steel plate which I heated from below. The bowl with corrosion damage boiled much earlier than the clean bowl.

The pictures show the two bowls (clean and clean but damaged by corrosion), the throttle body with clear top cover sitting on top of the steel plate and the view through the top cover showing the right bowl boiling and the left bowl sitting still.

[Joe Bob]

Do you vent to the return line?

[KTL]

How do the carb bodies feel after the engine is warm? My Weber 46s feel cool to the touch even after running hard on the track. If yours are cool, fuel boiling shouldn't be the problem.

How are your accelerator pump housings? Mine were leaky, despite having new gaskets in them. I ended up having to true-up both the pump cover and intermediate piece with some sandpaper on a sheet of glass. Worked perfect. No more leaking from the bottom of the carb

[1QuickS]

The Ideal Gas Law states that for a given volume; T2 = (P2 x T1)/P1 where T is temperature and P is pressure. Downstream from the waist of the main venturi is a low pressure area compared to atmospheric pressure so if P2 is less than P1 then T2 is less than T1. This is the cooling effect of carburetors during operation and is why carbureted aircraft engines have a heat source, "Carb De-ice" to prevent icing during flight.

Webers will be cooler than ambient during operation but will heat soak once parked as the cylinder heads conduct their heat to the manifolds and to ambient air.

[tomeric914]

Originally Posted by john walker's workshop

did you vent the carb tops yet? with the PMO jig?

I have not vented the carb tops, and I don't believe the PO did either yet they seem to drain internally with some drippage along the throttle shaft as Paul mentions

Originally Posted by 1QuickS

An item of interest to make this system most effective is to be sure your vent pipes are tight and not loose with their fit into the top covers. If they are loose then fuel can still escape the throttle body at the loose fit.

I don't think that is where they are leaking from though I know that a couple are slightly loose.

Originally Posted by 1QuickS

I have found that dirty fuel bowls or fuel bowls that have been corroded create nodes for the fuel to vaporize on and accelerate the boiling process. I tested a throttle body that had one side free of corrosion/debris and the other side was very pitted from corrosion. I put fuel into both bowls and mounted the carb onto a steel plate which I heated from below. The bowl with corrosion damage boiled much earlier than the clean bowl.

I do have some corrosion in a couple of the bowls though not as bad as in your example. That is fantastic research Paul!

Originally Posted by Joe Bob

Do you vent to the return line?

I don't have a return line at all. Should I with carbs? Where would I vent from?

Originally Posted by KTL

How are your accelerator pump housings? Mine were leaky, despite having new gaskets in them. I ended up having to true-up both the pump cover and intermediate piece with some sandpaper on a sheet of glass. Worked perfect. No more leaking from the bottom of the carb

Mine were leaky at first because I didn't want to overtighten them. Once tightened up, they were fine.

This is all great information!

[1QuickS]

I forgot another source of fuel dripping from the throttle shaft: the idle air jets that are pressed into the top of the main throttle body and look like little brass pills with a small hole drilled into them may become blocked which will cause a siphon action to drain the main fuel bowl into that cylinder. These air correction jets are located inboard from each idle jet and provide emulsifying air to mix with the fuel delivered through the idle jet. If this air jet is blocked by either a piece of gasket (hole not cut-out) or by a piece of crud then fuel will siphon from the main fuel well past the idle mixture screw and into that cylinder, not stopping until the fuel bowl is drained. Normally the idle air jet will break this siphon when the engine is stopped. You will still drain the fuel gallery from the idle jet down to the mixture screw but this is a small amount of fuel.

[Steve@Rennsport]

Originally Posted by tomeric914

I used the PMO float tool to properly set the floats.

thanks

Did you check the float levels with the engine running?

Is the meniscus sitting precisely between the two lines on the float guage?

Do you have 3.5 psi of fuel pressure?

[tomeric914]

Originally Posted by 1QuickS

I forgot another source of fuel dripping from the throttle shaft: the idle air jets that are pressed into the top of the main throttle body and look like little brass pills with a small hole drilled into them may become blocked which will cause a siphon action to drain the main fuel bowl into that cylinder...

They were clear of crud and gasket when I rebuilt them, but I will check.

Originally Posted by Steve@Rennsport

Did you check the float levels with the engine running?

Is the meniscus sitting precisely between the two lines on the float guage?

Do you have 3.5 psi of fuel pressure?

I checked the float levels with the engine warmed up and running. The settings were precisely between the two lines on the float gauge.

I have a Carter fuel pump which is supposed to be preset to 3.5 psi that has never been verified. However, if there was more than 3.5 psi of pressure, I would think that fuel leakage would have been more prevalent while the engine was running or it would begin to drip immediately after being shut off. What's weird is that it takes about 1/2 hour to start dripping and stinking.

[Grady Clay]

tommeric914,

Am I correct, you have a 3.0?
What external oil cooling?
What fan size and crank-to-fan ratio?

When I had carbureted engines in my 914-6 (MFI now), I used three insulating spacers (from ‘71T) under my manifolds (ported to match manifold to head).

Make sure you don’t have the ’78-’79 911SC 226 mm fan (or a 5-blade).
The combination of the 134 mm crank pulley (’78-’79 SC & more) with the 80-83 mm fan pulley (’80 SC-> & more) gives 1.82:1 ratio on a 245 mm fan.
This tends to have the heads cooler when the engine is to be shut off.

Another ‘trick’ is to plumb the fresh air blower to blow backwards through the heaters.
When you park, turn on the fan (with a timer) and open the heater controls.
This will tend to cool the engine faster, exhausting heat backwards through the engine fan.

A modified engine lid with a full-length grill will also help.

Best,
Grady

[tomeric914]

The engine is a 3.0 with a 12" x 11" Setrab coil under the rear deck. The cooler is triggered to come on at 180 deg which does ok on short trips. Typically, the highest I see is 220. I put it out back as a trial to see if it could keep temps down without introducing more air under the car, but it's not good enough. That cooler will be moved up front this winter and ducted through the front air dam.

The engine fan is a 245mm 11 blade with the earlier upper pulley so I'm not getting the maximum airflow out of it. I do have the smaller pulley, but it appears to be damaged so I haven't used it yet. That's something I need to source and try out.

There is no heater in the car so I don't have fresh air blower to run backwards. That is a great idea though!

The GT style grill is on my list for this winter.

It sounds like the concern is higher than normal head temps. Get the temps down and end the percolation.

[Grady Clay]

tomeric914,

As a ‘quick trial’, temporarily install a “Rubbermaid Solution”, less than $100.
This will really cool the heads and lead you toward identifying the problem.


How do you intend to plumb the oil cooler system when you move the cooler to the front?
What thermostat/safety pressure relief valve will you use?
I’ll encourage you to use an electric fan also.


If you simply change to the larger crank pulley, that will make a noticeable increase in engine cooling air.
What engine mount are you using – single-point 914-6 or modified 911?
Make sure none of the cooling air is being wasted and there is no recirculation of hot air back to the fan.


Yes, engine (head) temperatures are critical. However, don’t forget to deal with the carburetor float chamber venting noted above.


Dealing with carburetor ‘heat soak’ is so critical, my racer (SCCA GT2 with Weber 46IDA and two phenolic insulators) has two ‘scoops’ alongside the carbs.
Most look at them and assume they are for air flow.
Not so – I pour a 10# bag of ice cubes against the carbs and manifolds between track sessions.
No fuel percolation!
The engine (91x66, 11.5:1, single plug, Sprint cams) starts and runs perfectly from the first ¼-turn.

Best,
Grady

[lindy 911]

I had a plugged tank vent that would let the fuel tank build pressure after driving. This pressure would push past the pump (Holley Red), the fuel pressure regulator and the needle and seat of my 46 mm Webers. It pushed so much fuel into the #4 cylinder that it hydro locked. An easy test is to run the car, let it sit until you smell fuel, then crack open the fuel filler cap. If you hear a pressure relief sound, you found your problem.

[tomeric914]

Grady,

That is fantastic! An evaporative cooling system! I could have used it for the last DE I went to. I won't say what my oil temps were for fear of being banned from this forum.

I've thought about keeping the shrouded fan, but I'm not sure I'll need it. I can't remember the last time I had to sit in traffic. My plan is to run two hard lines under the center tunnel to the front of the car holding them in place every 8-12" or so. There will be a thermostat back at the engine.

For engine mount, I am using the Rich Johnson mount with 914-6 engine tin and new seals. There should be very little if any recirculation of engine air.

I'll do the bowl venting this winter if it hasn't already been done.

I'd love to see a picture of those carburetor "scoops"!


Lindy,

I think my vent is ok, but it is worth checking. I've run across that problem before.


Thanks to everyone who has contributed here! What a great wealth of knowledge!