Gas Blowing Out Carburetor When Running

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Mercury01
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Gas Blowing Out Carburetor When Running

Post by Mercury01 » Tue Feb 04, 2020 7:52 pm

I have a 1925 Fordor. Once the engine is running it blows a mist of gas out the inlet to the carb.

I have done the following:

1) New timer (anderson flapper style)
2) Set timing....with the timing fully retarded, spark occurs at each cylinder at TDC. Good strong spark at each cylinder.
3) Fresh Valve Job, Cut seats & new valves
4) New Head Gasket
5) Compression test is ~45 psi, & each cylinder is within 3-5 psi of each other
6) Rebuilt carburetor: cleaning and seals

Once the engine starts, it also sounds like it it is not firing on all cylinders.

When I crank the engine over with the starter, the engine pulls a good vacuum at the intake manifold.

What should I try next?

Thanks!


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Re: Gas Blowing Out Carburetor When Running

Post by got10carz » Tue Feb 04, 2020 8:18 pm

Do you have the correct firing order? Or perhaps carbon track in coil box.

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Mark Gregush
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Re: Gas Blowing Out Carburetor When Running

Post by Mark Gregush » Tue Feb 04, 2020 8:21 pm

Does gas run out of the carb when it is not running?
RE: " spark occurs at each cylinder at TDC" needs to be AFTER not at TDC on full retard at startup.
What do you have your valve stem gaps set at?
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Mercury01
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Re: Gas Blowing Out Carburetor When Running

Post by Mercury01 » Tue Feb 04, 2020 8:37 pm

The valves are set to ~.012" Inch

The carb doesn't leak any fuel when not running.

I haven't pulled the coil box yet. Is it common for carbon tracking to occur? Is there anything else to check with the coil box?

Thanks!


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Mercury01
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Re: Gas Blowing Out Carburetor When Running

Post by Mercury01 » Tue Feb 04, 2020 8:38 pm

Forgot to mention, The firing order has been checked and is 1-2-4-3.


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Re: Gas Blowing Out Carburetor When Running

Post by Scott_Conger » Tue Feb 04, 2020 8:50 pm

Pay attention to what Mark said about timing, for your starter's sake and your forearm's sake.
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Re: Gas Blowing Out Carburetor When Running

Post by Allan » Tue Feb 04, 2020 8:56 pm

Mark is correct re the timing. With full retard at the lever, and the timing set at TDC, you stand the chance of the car firing up as normal, or Backfiring. I believe the figure is 15 degees past TDC. That way the car will always fire on the downstroke, eliminating backfiring, starter damage and possible broken cranking arms.

Allan from down under.


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Mercury01
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Re: Gas Blowing Out Carburetor When Running

Post by Mercury01 » Tue Feb 04, 2020 9:21 pm

I can definitely retard a bit more. Once the engine is running, I do have to advance the timing. It doesn't try to "kick back" against the starter.

Thanks


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Re: Gas Blowing Out Carburetor When Running

Post by Scott_Conger » Tue Feb 04, 2020 9:25 pm

Now that the spark timing issue is out of the way, you have defined exactly when spark occurs (correct or not). What you have not determined is if the cam itself is a tooth off. You can set the timer to be "perfect" to the cam, but if your valve events are off, as in the intake valve not closed at the point of beginning compression (cam retarded by a tooth), you will get blowing at the carb and a terrible running engine as the intake flow is going in and out as opposed to a relatively smooth flow toward the cylinders. All valve events will of course, be late, which will include a barking exhaust note and a very hot exhaust system.

Have you had the cam out, since the car last ran correctly?

Special ground cams have considerable overlap which mimic this situation to a degree, but carburetion, swept area of the piston, compression, and a need for a higher idle all go into the equation to minimize the downside of this condition.

I do not get that you are dealing with a high performance engine, but simply a mal-adjusted stock one.
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Tyranny under the guise of law is still Tyranny

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Re: Gas Blowing Out Carburetor When Running

Post by Mercury01 » Tue Feb 04, 2020 9:37 pm

Scott,

Thanks for the reply. The engine was running fine ~1 year ago. It began to start "hard" and then I started to dig into it and made the changes I mentioned. I first thought it was actually the cam gear, but after removing the timing cover, the fiber gear looked fine, and I left it as is. The timer that was installed was an anderson, and had quite a bit of wear on the flaps. I chose to replace with an anderson timer again.

Thanks


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Re: Gas Blowing Out Carburetor When Running

Post by Scott_Conger » Tue Feb 04, 2020 11:23 pm

If in fact the valve events ARE occuring at the correct time, the timing gear is not damaged or misaligned, then you may really not have one or more valves sealing as well as you think. While even, your compression could be better, though not bad for a worn engine.

The only other likely way fuel is misting out the intake is there is a disruption of air flow or a seriously maladjusted carb which is doing things we cannot predict with certainty.

Guessing this is an NH carb, remember that this is a very primitive puddle-type carb and if the float level is seriously mal-adjusted, float valve is not closing off properly or more likely (based on symptoms, not because it is common), the main jet is missing a gasket and allowing volumes of fuel to puddle while running rather than being metered past the needle.

Easiest thing to do right now is to at least borrow a known good carb and see if you can pinpoint or eliminate that as the source.
Scott Conger

Tyranny under the guise of law is still Tyranny

NH Full Flow Float Valves
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Re: Gas Blowing Out Carburetor When Running

Post by Steve Jelf » Wed Feb 05, 2020 1:05 am

In regard to setting ignition timing, I find it easiest to refer to the crank pin. Putting a thumb over the spark plug hole and pulling up the crank until you feel compression gets you close to TDC. Then pull the crank slowly and watch the pin. When the pin is perfectly horizontal you are at TDC. If you think of the pin as clock hands, they're pointing at 9 and 3. You want to turn a little past that until the hands are between 9 and 10, and between 3 and 4. At that point you are at 15º past TDC. That's where the fully retarded timer should be set to fire on BAT.
The inevitable often happens.
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Re: Gas Blowing Out Carburetor When Running

Post by ABoer » Wed Feb 05, 2020 5:05 am

Keith , Did you try an other model Timer ? Like an old style roller , or a New Day ??
Toon

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Re: Gas Blowing Out Carburetor When Running

Post by Ed Baudoux » Wed Feb 05, 2020 7:34 am

Check for a restriction in the exhaust.
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Re: Gas Blowing Out Carburetor When Running

Post by Humblej » Wed Feb 05, 2020 8:10 am

Keith, for gas vapor to go the wrong way out the back of the carb you have a valve or valve timing problem, for it to be an ignition timing problem it would be so far off the engine wouldnt run. Could be heavy carbon build up that is glowing hot enough to ignite the fuel vapor during the intake stroke, or some freak ignition problem that is causing a spark plug to fire more than once a cycle...a short, or carbon tracks in coil box could cause that.
Last edited by Humblej on Wed Feb 05, 2020 10:03 am, edited 1 time in total.

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Re: Gas Blowing Out Carburetor When Running

Post by Charlie B in N.J. » Wed Feb 05, 2020 8:39 am

"Is a puzzlement" as Yul Brenner famously said. I'm off the ignition business and on to leaking valve/blocked exhaust (Good guess Ed).
Forget everything you thought you knew.


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Re: Gas Blowing Out Carburetor When Running

Post by Scott_Conger » Wed Feb 05, 2020 11:10 am

And finally, I would start the car, idle as best as it can for a few seconds and then shut off the fuel supply line (you do have a shut-off, right?). If after a very short while the car smoothes out and stops spitting gas (and then dies for lack of fuel), you have a bad carb. Anything else is valves/timing or as Ed suggested, and I concur, a restricted exhaust.
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Re: Gas Blowing Out Carburetor When Running

Post by Norman Kling » Wed Feb 05, 2020 11:28 am

I think all the bases have been covered by the other posters, however one more thing which could be causing the problem. If the wires between the coil box and the timer are grounded anywhere on route, the coil will spark. If on battery the spark could be continuous. If on magneto it could still spark every time the AC wave peaks. Sometimes a wire of post can contact a bolt at the front of the crankcase. The bolt near the timer should be turned with the nut downward. Other places where it could ground would be against the spring holding the timer to the plate. So check carefully all those wires for one or more which might be grounded.
Norm


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Re: Gas Blowing Out Carburetor When Running

Post by StanHowe » Wed Feb 05, 2020 11:48 am

Sounds to me like you have a sticky intake valve. Take the plugs out, hand crank it over until each intake opens and spray some good carb cleaner down the inlet area under the valve. Let it set for a few minutes, do it again, put the plugs back in and go for a drive. Should take care of it.
Today's gas is very sticky


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Re: Gas Blowing Out Carburetor When Running

Post by Roz » Wed Feb 05, 2020 5:44 pm

If it is ignition related, it would be a backfire, or for some reason igniting when the intake valve is open. If it’s not firing back out of the carb and just chuffing raw gas spray out, it has to be valve related. The only way gas can flow backward through the carb is through an open intake valve. Either an intake valve is not closing, or the valve timing has changed. An intake valve would have to be open on the piston upstroke after the intake stroke.

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Re: Gas Blowing Out Carburetor When Running

Post by Ruxstel24 » Wed Feb 05, 2020 6:09 pm

Another possibility is a bad cam lobe on an exhaust valve. If the exhaust valve doesn’t open enough, the pressure will release through the intake valve when it opens.
Usually runs fair till you get a load or open the throttle much.
And will show decent on a compression test.

Good luck ;)


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Re: Gas Blowing Out Carburetor When Running

Post by Scott_Conger » Wed Feb 05, 2020 7:02 pm

Dave

I once had to deal with a '16 with a broken-off exhaust lifter on #2 from a friend who bought a "great running car" off of eBay. The lifter piece was missing, and while too large to get out of the valve chamber...it wasn't there, so the valve cover had been off and the eBay lister/owner knew, and there was clearly no way this was a "great running car" when put on the transport. The explosive release through the intake valve/carb was horrendous and it had been run so long that way, the center main was beaten out of it. Pulling the pan, the center main cap was brand new and beaten to pieces. So it had been run long enough to wipe out 2 different center main caps.

Great running car, huh?

a new lifter and a carefully scraped new center main cap solved the problem until it was resold upon my friend's death. Fortunately for all, the block side of the center main survived unscathed, which I guess isn't all that big of a surprise when you do a failure analysis of the root cause of the situation.

That car nearly hopped off the ground while running in that condition. It had a starter, and when he first started it for me to listen, it sure made odd sounds while cranking and once fired up, I couldn't get to the key quickly enough to shut it down.
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Tyranny under the guise of law is still Tyranny

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