Holley Carb Guru's - Boil over

[Keith B.]

My List # 3370 Holley carb for my 66 L-36 car continues to haunt me. I have the proper gaskets and baffle plate, etc. Driving during warm summer day in traffic and around town gas is leaking near the throttle idle adj screw. Actually when I turn the car off and remove air cleaner the gas is PERCOLATING over the venturi and hotter than hell.

Holley guy told me today to see if I have enough clearance for a 1/2" (bakelite material) spacer that goes onto my cast iron intake and dramtically reduces heat transfer and will stop the boil over of gas ??

I thought he was nuts............anyone heard of this. Not even sure If I have enough clearance with my 66 bubble hood..........hmmmm.

[Bill C.]

Re: Holley Carb Guru's - Boil over

that type of gasket will help.

But the real culprit is the ethanol laced fuel - it messes with the boiling point.

I have the same issue on my 72 when using ethanol fuels, if I get a tank of non ethanol, no problems.

I tried lowering the fuel bowl levels, but that led to a whole other set of problems.

You could always try the heat sheild they used on the CA emmisions LT1 cars - mounts below the carb base.

[Stuart F.]

Re: Holley Carb Guru's - Boil over

Responding just to whether a phenolic base plate/spacer (3/8") will help, I can say from testing I have done on my 63 L-76 with the Carter AFB is that I have seen a reduction in temperature at the carb (as measured with an IFR gun) of some 35 to 40 degrees under various operating conditions (hot idle after full warm operation, hot soak after shut down). I can't translate the other changes I made to address the 10% alcohol fuel problem with my application to that of your Holley, but the insulator accounts for the majority of the improvement I have seen.

Stu Fox

[Timothy B.]

Re: Holley Carb Guru's - Boil over

Keith,

Have you installed plugs in the exhaust crossover under the carburetor, this will reduce heat greatly. Next, set your idle timing at 12* BTDC and verify the vacuum advance is working properly with manifold vacuum. The vacuum advance control should be fully pulled to it's stop approx 2" vacuum below idle vacuum.

This should yield approx 27-28* idle timing and help with engine temperature. Double check that the heat riser is opening and float level is just below bottom of sight plug.

If this does not help I suspect there are some warped surfaces in the holley and when the aluminum gets hot there is internal leakage which is why gas is leaking out the throttle shaft.

[John H.]

Re: Holley Carb Guru's - Boil over

Keith -

These are the two holes you need to plug at the ends of the "hot-slot"; '66 and '67 L-36 both had them (as did '66-'67 small-blocks with iron intake manifolds).

[John D.]

Re: Holley Carb Guru's - Boil over

Along with what the others have said wiring the heat riser valve in the full open position help a lot with this issue on my 67 L79....

[Gerard F.]

Re: Holley Carb Guru's - Boil over

Keith,

I agree with others that blocking the heat is recommened to reduce heat to the carburetor and prevent the fuel from boiling in the bowls. I've done that on my 67 SB.

But the leak by the idle screw is probably a worn throttle bushing or the gasket between the throttle plate and main body. There is a little track in the top of the throttle plate on 4160's, which feeds the secondary idle from the primary metering block. It runs right along the edge of the throttle plate and if the gasket is deteriorated, you'll get a leak between the main body and throttle plate in the location you noticed.

Also you might check that the idle vents and high speed vents within the air horn are open and clear. In the photo below, this 3810 has one vent on each side blocked:



Even though the fuel bowls are vented directly or through the main body, if you take a look at the fuel circuit these vents service, if they are blocked, it could induce a siphoning of fuel to the air horn after shutdown. You should just check whether these vents are open.

I also agree with the ethanol comment. In my grappa still, the ethyl starts coming off at 180d and is just about all gone by 205d. This is just about your operating temperature on a hot day. When you shut down, all the heat rises to the carb at the top of the engine. It is the nature of the beast, and the best thing you can do is to try to vent the heat as quick as possible.

I notice on a hot day at a traffic light the idle goes way up then starts to sputter and cycle. You can correct this by giving it some revs and introducing some cooler fuel. But on shutdown on a hot day, you'll still percolate with the ethanol laced fuel.

[Stuart F.]

Re: Holley Carb Guru's - Boil over

Jerry;

You explained the situation very well, specially the part about the ethanol boil off. I'm sure a lot of the members with C-1's and 2's are living this through the summer months.

Stu Fox

[Ronald L.]

Re: Holley Carb Guru's - Boil over

John,
What plugs were used on the manifold?

That means removal and tapping the intake?

Sunoco Blue 110 will solve this problem.

[Clem Z.]

Re: Holley Carb Guru's - Boil over

John,
What plugs were used on the manifold?

That means removal and tapping the intake?

Sunoco Blue 110 will solve this problem.

use steel cup type expansion plugs and just tap them into the holes. check the hole size i think it is 7/16". the hole can be tapped for threaded plugs just use a shop vac while tapping and some grease on the tap

[Jeff S.]

Re: Holley Carb Guru's - Boil over

Keith

In addition to opening the heat riser & blocking off the hot-slot area -- which helps limit the convection heat source somewhat -- I would recommend dealing with the radiant heat generated by the intake runner(s) under the float bowls of the Holley, particularly when shut down & soaking is ongoing.

There's a Summit Racing aluminum heat-shield part that can be modified to present a dead air space between the intake runner & the float bowl. It needs few cuts, bends & a 'joggle', but works fine on my '66 base engine car. You can get an idea from photos, attached. Not sure what you'll run into obstruction wise on L36. I sandwich mine between two open Holley gaskets.

I doubt if those phenolic risers-things really do anything, since the conductive heat path is in the carb base, not the floats.

Jeff

[Joe C.]

Re: Holley Carb Guru's - Boil over

Keith,

Have you installed plugs in the exhaust crossover under the carburetor, this will reduce heat greatly. Next, set your idle timing at 12* BTDC and verify the vacuum advance is working properly with manifold vacuum. The vacuum advance control should be fully pulled to it's stop approx 2" vacuum below idle vacuum.

This should yield approx 27-28* idle timing and help with engine temperature. Double check that the heat riser is opening and float level is just below bottom of sight plug.

If this does not help I suspect there are some warped surfaces in the holley and when the aluminum gets hot there is internal leakage which is why gas is leaking out the throttle shaft.

Holley carb is not aluminum. It is pot metal, which retains heat more than pure aluminum.
Last year I replaced my 2818 with a Quick Fuel (Holley clone) race carburetor, which is aluminum, not pot metal. The difference in weight is SUBSTANTIAL..............IIRC, 7lbs versus 11 for the Holley. Among other problems solved, the percolation problem disappeared. Not to mention the light years of improvement in drivability!

The reason gas is leaking out the throttle shaft is because the throttle plates are (almost) closed, and the raw gas (which is not atomized) is flowing out the boosters, pooling on the throttle plates, and leaking out the shafts.

[Stuart F.]

Re: Holley Carb Guru's - Boil over

Jeff;

Therein, with what you say, may lie the big difference between the Holley and a Carter AFB. The float chambers do seem to be out and exposed to the heat from the runners much more with the Holley. However, it would make an interesting test to measure the temps at the carburetor with an IFR gun as I did with my AFB before and after your addition of the heat shield and after, and again using a phenolic spacer at the base. Don't ya think?

Stu Fox

[Joe C.]

Re: Holley Carb Guru's - Boil over

Keith,

I agree with others that blocking the heat is recommened to reduce heat to the carburetor and prevent the fuel from boiling in the bowls. I've done that on my 67 SB.

But the leak by the idle screw is probably a worn throttle bushing or the gasket between the throttle plate and main body. There is a little track in the top of the throttle plate on 4160's, which feeds the secondary idle from the primary metering block. It runs right along the edge of the throttle plate and if the gasket is deteriorated, you'll get a leak between the main body and throttle plate in the location you noticed.

Also you might check that the idle vents and high speed vents within the air horn are open and clear. In the photo below, this 3810 has one vent on each side blocked:



Even though the fuel bowls are vented directly or through the main body, if you take a look at the fuel circuit these vents service, if they are blocked, it could induce a siphoning of fuel to the air horn after shutdown. You should just check whether these vents are open.

I also agree with the ethanol comment. In my grappa still, the ethyl starts coming off at 180d and is just about all gone by 205d. This is just about your operating temperature on a hot day. When you shut down, all the heat rises to the carb at the top of the engine. It is the nature of the beast, and the best thing you can do is to try to vent the heat as quick as possible.

I notice on a hot day at a traffic light the idle goes way up then starts to sputter and cycle. You can correct this by giving it some revs and introducing some cooler fuel. But on shutdown on a hot day, you'll still percolate with the ethanol laced fuel.

As you know, Jerry, those are "bleeds", and they introduce air into the idle wells and the main wells in both metering blocks. The purpose of the bleeds is to introduce air bubbles into the fuel in the wells, which become entrained in the fuel being delivered either to the idle/off idle ports, or to the boosters when the main circuit is engaged. If one side bleed is closed, then the tendency will be to run very rich on the plugged side. AFAIK, both main bleeds should be of equal size, and matched to the main jet size. Both low speed bleeds should be matched as well, to the idle/off idle passage (which connects the idle well to the idle and off-idle ports) size. Sometimes, with a poorly designed dual plane intake manifold, the fuel distribution can become so unbalanced between 1,7,4,6 and 3,5,2,8, that different sized bleeds/jets are necessary on opposite sides of the carburetor (which feeds the upper and lower plenum halves, and their associated set of runners.

[Clem Z.]

Re: Holley Carb Guru's - Boil over

As you know, Jerry, those are "bleeds", and they introduce air into the idle well and the main well in both metering blocks. The purpose of the bleeds is to introduce air bubbles into the fuel in the wells, which become entrained in the fuel being delivered either to the idle/off idle ports, or to the boosters when the main circuit is engaged. If one side bleed is closed, then the tendency will be to run very rich on the plugged side. AFAIK, both main bleeds should be of equal size, and matched to the main jet size. Both low speed bleeds should be matched as well, to the idle/off idle passage (which connects the idle well to the idle and off-idle ports) size. Sometimes, with a poorly designed dual plane intake manifold, the fuel distribution can become so unbalanced between 1,7,4,6 and 3,5,2,8, that different sized bleeds/jets are necessary on opposite sides of the carburetor (which feeds the upper and lower plenum halves, and their associated set of runners.

those are also siphon breaks because if they are not there the fuel will keep flow after you shut off the engine. i had that happen on a carb used on a dirt track without well sealed air filter as you could hear the fuel running in the carb after shuting off the engine.