Carburetor Question?

[Ralph E.]

Is this normal? On my 1967 Corvette 327/300HP I heard some hissing, maybe even a bubbling sound, coming from the carburetor. This occurred about 15-20 minutes after I shut the car off. The car has a stock 3810 carb. The hissing started after taking a short ride. While working in my nice quiet garage next to the car I could hear noises coming from the carburetor, I see no gas leaks, but I definitely hear noises coming from the carb. Again, is this normal? Usually in my garage we have something going on, today all was quiet so I could hear it.

[Stuart F.]

Re: Carburetor Question?

Sounds like percolation to me. If you were to look into the carb you'd probably see some fuel dripping out of the Venturi clusters. When the drops hit the hot butterfly's they may sizzle. Check it out next time you duplicate the conditions. You may want to consider at least dropping your float levels a bit. Otherwise, there are many other things that can be done, but these would be non stock in appearance.

Stu Fox

[Michael G.]

Re: Carburetor Question?

Sounds like percolation to me. If you were to look into the carb you'd probably see some fuel dripping out of the Venturi clusters. When the drops hit the hot butterfly's they may sizzle. Check it out next time you duplicate the conditions. You may want to consider at least dropping your float levels a bit. Otherwise, there are many other things that can be done, but these would be non stock in appearance.

Stu Fox

Sounds about right.

[Ralph E.]

Re: Carburetor Question?

Sounds about right.

Thanks Guys. Will check the float level tomorrow morning and let you know.

[Michael G.]

Re: Carburetor Question?

Ralph,

Whether the float adjustment resolves the issue what your hearing like Stu indicated is percolation of the fuel at the carburetor. Hopefully a float adjustment will resolve the issue.

[Richard M.]

Re: Carburetor Question?

I hate to even bring it up......you know.....the "E" word, but in 28 years of ownership of my '59 dual-four WCFB 270hp..........

Ever sine that "E" stuff showed up in the fuel a few years a go, after a hot ride and shut down and a heat soak and I can hear and see percolation and bubbles in my glass bowl filter.

It seems like it's boiling. I think it IS boiling. It's a bit scary.

Rich
p.s. see, I was good. I did NOT use the word <> lonahtE .

[Edward J.]

Re: Carburetor Question?

Ralph, If your not seeing any fuel pooling on the intake, I am not sure I would start playing with float levels. Because its likely not going to correct the fuel boiling in the carb. It is only the engine going through a hot soak, You see after the engine is shut down engine temp. wll rise as there is no air flow or coolant circulation and the one thing ethanol does not like is higher temps. As you may have read there are many posts on the fuels of today.

[Ralph E.]

Re: Carburetor Question?

Ralph, If your not seeing any fuel pooling on the intake, I am not sure I would start playing with float levels. Because its likely not going to correct the fuel boiling in the carb. It is only the engine going through a hot soak, You see after the engine is shut down engine temp. with rise as there is no air flow or coolant circulation and the one thing ethanol does not like is higher temps. As you may have read there are many posts on the fuels of today.

Check for the carb hissing noise again this morning. Went for a short ride to get the car to normal running temp, about 180*.
Park car in my garage and waited and listened for the hissing. About 10 minutes after the engine reached about 192* (checked with IR) the hissing started.
I looked down into the carb and had a small about of fuel moisture on the on the butterfly's. Car runs and idle's fine.
On a 3810 Holley how do you adjust the floats? It looks like to remove the front bowel the acceleration pump also needs to be removed.

[Paul J.]

Re: Carburetor Question?

Ralph, If your not seeing any fuel pooling on the intake, I am not sure I would start playing with float levels. Because its likely not going to correct the fuel boiling in the carb. It is only the engine going through a hot soak, You see after the engine is shut down engine temp. with rise as there is no air flow or coolant circulation and the one thing ethanol does not like is higher temps. As you may have read there are many posts on the fuels of today.

Exactly. Ever seen an old radiator hose blow after the engine was shut off? You should hear my Harley ping and crack and snap when you shut it off.

[Duke W.]

Re: Carburetor Question?

The first thing to do is wire the heat riser valve open. The next step is to remove the carb and plug the heat slot under the front of the throttle body, if your '67 300 HP has one, which I think it does, and the final step if all else fails is to remove the manifold and block the driver's side heat riser package with a .010" stainless steel shim, maybe with a 1/2" hole drilled in it.

Insulating fuel lines beginning with the frame pipe where it passes near the exhaust manifold can also significantly mitigate the problem.

E10 causes percolation in non-pressurized fuel systems because the mid-range boiling points are up to 30 degrees lower than typical straight gasoline. It's a common problem in vintage cars in warm to hot weather.

This percolation commonly causes hard hot starting due to excessive richness... the engine is essentially "flooded" so you need to use the flooded engine start procedure in your owner's manual if this problem occurs.

Duke

[Edward J.]

Re: Carburetor Question?

Check for the carb hissing noise again this morning. Went for a short ride to get the car to normal running temp, about 180*.Park car in my garage and waited and listened for the hissing. About 10 minutes after the engine reached about 192* (checked with IR) the hissing started.I looked down into the carb and had a small about of fuel moisture on the on the butterfly's. Car runs and idle's fine.On a 3810 Holley how do you adjust the floats? It looks like to remove the front bowel the acceleration pump also needs to be removed.

Ralph. If its only just wet and not dripping fuel when you look inside the carb. that's ok. Just as a note to adjust float, the float bowl has to be removed. If your car is running fine don't tamper with anything, Hot weather and ethanol can sometimes bring out the worst on vintage cars. The hissing noise sound like a vacuum leak, If this hissing is happening while the engine is running. base gaskets or the plastic spacer plate between the Carburetor can cause this, or even just a idle mixture adjustment.

[Timothy B.]

Re: Carburetor Question?

The first thing to do is wire the heat riser valve open. The next step is to remove the carb and plug the heat slot under the front of the throttle body, if your '67 300 HP has one, which I think it does, and the final step if all else fails is to remove the manifold and block the driver's side heat riser package with a .010" stainless steel shim, maybe with a 1/2" hole drilled in it.

Insulating fuel lines beginning with the frame pipe where it passes near the exhaust manifold can also significantly mitigate the problem.

E10 causes percolation in non-pressurized fuel systems because the mid-range boiling points are up to 30 degrees lower than typical straight gasoline. It's a common problem in vintage cars in warm to hot weather.

This percolation commonly causes hard hot starting due to excessive richness... the engine is essentially "flooded" so you need to use the flooded engine start procedure in your owner's manual if this problem occurs.

Duke

I agree with Duke and Edward, wire the heat riser open, you will notice a big difference and make sure the hot slot under the carburetor is plugged.

If the engine idles and runs good don't lower the float level, this can cause some hesitation in the start up of the main fuel circuit while driving. It's OK to lower a bit but most times that does not cure the problem with the E gasoline.

[Ralph E.]

Re: Carburetor Question?

I agree with Duke and Edward, wire the heat riser open, you will notice a big difference and make sure the hot slot under the carburetor is plugged.

If the engine idles and runs good don't lower the float level, this can cause some hesitation in the start up of the main fuel circuit while driving. It's OK to lower a bit but most times that does not cure the problem with the E gasoline.

Heat riser is wired open, hot slot is plugged, been that way for years.
I don't think I am getting percolation or at least not like this video

http://www.youtube.com/watch?v=pKU91tRR7DY

[Edward J.]

Re: Carburetor Question?

Ralph, That video is a worst case. What I see is as Duke says longer cranking & harder starting after the car sits for a bit after a 20/30 minute drive. The hotter the outside temp the worst it gets. Another thing no has mentioned if your fuel is a ethanol area. some areas do not have ethanol blended fuels.

[Ralph E.]

Re: Carburetor Question?

Ralph, That video is a worst case. What I see is as Duke says longer cranking & harder starting after the car sits for a bit after a 20/30 minute drive. The hotter the outside temp the worst it gets. Another thing no has mentioned if your fuel is a ethanol area. some areas do not have ethanol blended fuels.

Here in CT we have ethanol blended fuels. So I guess this is normal for E fuels in the summer. Never notice this happening before. Like I said, the butterfly's are only slightly wet with fuel. Car always starts with minimal effort. Was going to lift the carb to verify the hot slot plugs are still in place, but will wait until tomorrow when everything has cooled off. The heat riser IS wired open.