fuel leak/ onto intake manifold

[Robert C.]

I am seeing a fuel leak onto the center depressions of my alum. intake manifold after driving. I know this is common but I can't see where the fuel is comming from. The leaky stuff looks like oily gas, about a thimble full, both sides of the Holley. Car is 365hp 65. What's the common fix? Bob Cook


Texas Chapter NCRS

[Dale Pearman]

BOOM!

Hi Bob: Common fix is an AFB!

Varooom!

[G B.]

Might be perculation

Some Holleys absorb too much engine heat after you shut off. No fresh gas or air comes through for cooling the carburetor during this heat soak period. Eventually the gas already in the bowls will boil. It then bubbles over into the carb venturi tubes and drips out the throttle shafts. You can observe this happening if you remove the air cleaner after a hot drive and watch the carb.

I don't know of a sure cure, but installing a GM aluminum heat baffle plate wouldn't hurt. Also sealing off the exhaust heat riser passage in the intake would definitely help.

[Jerry Clark]

Re: fuel leak/ onto intake manifold

Hi Bob:

Given your description of the leaks location I would suspect the throttle shaft as the source. I am afraid I no longer remember how the fuel gets to this area of the carb but it can and under a combination of circumstances , does. The engines heat turns the gas into a tar like substance, fuel solids. the fix is to call Mr. Luck, I do remember, however it gets to the throttle bushings, remachining is involved in the resolution.

jer

[Patrick H.]

It's Possibly Oil

Bob,

One thing I learned from an engine rebuilder is that this puddle, which is somewhat common on small blocks, is often oil that follows the center intake bolts upward and then puddles onto the depression between cylinders 3 and 5 (or 4 and 6) on the intake. The pressurized oil and oil mist under your intake seeps up because the center intake bolts no longer have sealant on the threads (or at least not enough to do the job) and as these bolts go directly into the lifter valley area, they come in contact with this oil. Being next to the exhaust heat crossover makes it worse, as the heat cooks the thread sealant on these bolts faster than the outer intake bolts.

The cure I was told is to remove the center two intake bolts on each side, and seal the bolt threads with ARP bolt sealer or similar before reinstalling. This should cure the problem. If nothing else, it is a lot easier than playing with the carb for a week and trying to find the fix. One night of repair should tell you if this was the problem.

Good Luck,

Patrick Hulst

[Dale Pearman]

Re: It's Possibly Oil

Yessir, whenever I get a flat tire I change the spark plugs! This advice from an engine builder as well! Seriously Patrick, oil can be eaisly differentiated from gasoline. Your advice for oil puddles is very good. Few of us are aware that manifold cap screws are notorious "leakers" of oil. What sealant do you reccommend?

Varooom!

[Patrick H.]

Oil vs. Gas

Dale,

You differentiate oil from gasoline with a match, right?

Think about it: gas slowly dripping from a carb lands on the exhaust heat crossover, where the intake is nearly as hot as the exhaust and the paint ALWAYS burns off, and we expect gas to stay here for very long before evaporating? I don't think so, and I vote for the oil in this location. The few carb gas leaks I have had never leave around puddles much before they evaporate off. These puddles on the center of the intake will sit for days or weeks, which tends to go against them being gas.

Actually, I think that one of the best sealants for this is the stuff GM sells, or at least used to. When Joe comes back, you might ask him the number. Ever try and clean it off the bolt threads when you take apart a motor? Its even hard to take off then with a wire wheel.

I have used ARP thread sealer with good success (i.e. mine haven't leaked upwards yet).

Good luck,

Patrick Hulst

[Iron Duke NCRS #22045]

The engineer vs. the physician

It could be gasoline. (not "gas"- that's the stuff we breathe). I had a similar problem with the AFB on my '63 SHP. My conclusion was it was gasoline leaking past worn throttle shaft bores. It was easier to tell way back when because premeium leaded gasoline was dyed red and the dye stayed behind along with the heavy fraction of the fuel.

Gasoline is composed of a wide variety of hydrocarbon species with boiling points ranging from near room temperature to greater than the boiling point of water. If gasoline continuously seeps on a hot surface the heaviest fraction takes a long time to boil off and may appear similar to oil. Today's gasolines don't seem to have much if any die, so the telltale red is no longer there to help us, but they have more heavy fraction than in the carburetor era. I'm not sayiing it's gasoline for sure. Could be either, so both causes (carburetor and manifold bolts) have to be investigated, and I'd start by sealing the manifold bolts first because it's a quick and easy job. If a good sealilng job doesn't do the trick then it's probably seepage from the carburetor.

Duke

[Patrick H.]

The physician trained as a chemist

Duke,

If you look back, that's what I said originally. Seal the manifold bolts for a one night quick fix, and then go from there.

Actually, I was trained as a chemist, and worked my way through med school doing chemistry work. The thought of heavier gasoline hydrocarbons (other than oil) causing the puddle has crossed my mind, but I found that the leftover puddles from the small gas leak at the fuel line fitting of my Quadrajet didn't match the puddles I tended to find in the center of the intake on others' cars. I never took a sample of either and ran it through any testing (I still have contacts in the lab I used to work in) to see which it was. I just didn't want people to think that ONLY gasoline leaks caused puddles on the intakes, and that "backwards" oil leaks were also something to consider.

Good Luck,

Patrick Hulst

[Dale Pearman]

DUKE & PAT ARE BOTH RIGHT

I just learned a lot from you guys. Thanks. BUT gosh folks, the old timers can tell in a New York minute whatcha got. Gas or Earl! They don't SMELL the same nor do they TASTE the same!

The Rev.

P.S. My favorite pastime is putting bushings in carburetors and reaming them out for a tight throttle shaft fit!

[Patrick H.]

Re: DUKE & PAT ARE BOTH RIGHT

Dale,

I got gas. Too many beans at the NCRS picnic on Saturday

See you at Bloomington.

Patrick

P.S. Can't say I spend much time smelling or tasting it.....

[Gene M.]

Re: fuel leak/ onto intake manifold

Warped metering blocks are a common cause. Solution is to replace them or mill them flat. Best option is replacement. Be sure the face on the Airhorn is flat or the leak will persist. The air horn can also be milled flat by a good machine shop if found to be warped. The major cause is over tightening the screws.

[Dale Pearman]

Re: fuel leak/ onto intake manifold

Holley is a packaged LEAK at best! I can't get along with them and anything I ever wanted to do could be done with an AFB!

The Rev.

[motorman]

Re: fuel leak/ onto intake manifold

warped metering blocks are a problem but you can not mill them flat. you will remove the raised areas that cut into the gasket to help seal. there are fine ridges that surround the holes and passages on the metering blocks. i straighten them using a jig and this is the only way other than buying new ones, which i am sure the older ones are not available. you can mill or file the main bodies because they are just flat surfaces.