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Old 05-12-2014, 05:11 PM   #1
howie123
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Default Need help with persistent problem.

When driving my Model A speedster it starts easily and runs well. At about 30 mph I start to get a spitting or light popping noise from the exhaust. It blows black soot out the tailpipe and the spark plugs have a light coating of soft black soot. Fuel mileage is also quite poor. I am running a fuel pump and regulator. I checked the pressure at 1 psi. I replaced the Zenith carburetor with a Marvel. It still runs fine but the same symptoms persist. I am using W18 spark plugs. Tried some W14 that I had just for fun with no change. I also tried a different condensor with no change. I have double and triple checked the timing. The last time I checked the spark looked strong. I am running out of ideas. Any help would be appreciated. Thanks. Howie.
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Old 05-12-2014, 05:16 PM   #2
Gunmetal blue2
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Default Re: Need help with persistent problem.

are you running a bigger main jet or gav set to far open.
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Old 05-12-2014, 06:23 PM   #3
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Default Re: Need help with persistent problem.

The main jet is what was in the carburetor. I start the engine with GAV one turn open and dial it down. At 1/4 turn open it loses power so I have been leaving it about 1/2 turn open. Howie
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Old 05-12-2014, 06:29 PM   #4
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Default Re: Need help with persistent problem.

Are you running the stock Model A distributor and timing lever?
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Old 05-12-2014, 06:52 PM   #5
ford3
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Default Re: Need help with persistent problem.

black smoke and soot means your running rich, hook carb up with out the electric fuel pump, I would normally say float to high but you said the problem happened with different carbs, so go around the fuel pump and see what happens
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Old 05-12-2014, 07:07 PM   #6
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Yes, distributor is all stock. Howie.
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Old 05-12-2014, 09:31 PM   #7
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Default Re: Need help with persistent problem.

Just to help rule out weak ignition, I would check the air gaps between the rotor tip and the contacts in the distributor cap. If the gaps are too close the spark will be weaker. I try to equalize the gaps at .035 for hotter spark. If the manifold seems to be getting hotter, check points gap, it may be closing up .
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Old 05-13-2014, 08:35 PM   #8
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Default Re: Need help with persistent problem.

Thanks for the suggestions. How much fuel pressure is too much? I installed a pressure gauge temporarily and it showed 1psi. With the engine off and the fuel pump on there is no leakage from the carburetor. I assume this means the float valve is closing and not being forced open by the pump pressure. Is that a safe assumption? I checked the air gap between the rotor and the contacts in the distributor cap body. They range from.028-.040+. The copper tip on the rotor is bent down and passes very low on the contacts. Is that normal? Also, the flat spring on the top of the rotor has gotten very weak and the carbon button in the cap is flattened. Should I look for better quality parts? Which supplier should I go with? I checked the point gap and it is right at .020. Thanks again. Howie
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Old 05-14-2014, 12:18 PM   #9
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Default Re: Need help with persistent problem.

It does sound like you are running rich and starting to flood out. If you are running a regular updraft carburetor, just remember that is was originally gravity fed so putting a fuel pump in the mix could add more fuel. I would try, as mentioned, either trying to eliminate the fuel pump from the circuit and see if the problem goes away, or continue trying to dial down the regulator until it either starves out or gets better.
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Old 05-14-2014, 04:21 PM   #10
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Default Re: Need help with persistent problem.

I just tried dialing down the regulator. It didn't seem to make any difference in how the car runs, still spitting when I get up to about 30mph. I still don't understand how the pump can cause flooding if the float valve is working. And if it is not working, shouldn't there be leakage from the carburetor? It does appear that even in the short distance I have driven that the Marvel is getting better mileage than the Zenith. It may be just too early to tell. I am committed to keeping the fuel pump since I need it on hills. I just really want to figure this out. Thanks again. Howie
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Old 05-14-2014, 04:44 PM   #11
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Default Re: Need help with persistent problem.

Try setting up a gravity feed to your carb. Yunno. Sort of like the "temporary" tanks that people make out of lawnmower gas tanks and attached to a temporary firewall - so they can go out and test drive the chassis?

Watch your windage though. Whatever you set up has to be wind resistant so you don't create an accident.

If the problem goes away perhaps turning down isn't addressing the issue?

And if the problem remains - then you know where NOT to look.

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Old 05-14-2014, 05:04 PM   #12
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Default Re: Need help with persistent problem.

Do a compression check, if you have a wayward cylinder, check your valve lash adjustments.
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Old 05-14-2014, 05:24 PM   #13
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Default Re: Need help with persistent problem.

Try a new Battery
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Old 05-14-2014, 08:22 PM   #14
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Default Re: Need help with persistent problem.

Are you using an air cleaner? If so remove it for a while to test if it is causing an air restriction.
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Old 05-14-2014, 08:32 PM   #15
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Default Re: Need help with persistent problem.

Check the jet that can be seen in the side near the bottom of the bowl in the carb. Sometimes people install the brass GAV seat as this jet, the engine will run very rich if that is done. (Not all carbs use a brass GAV seat, so it is "spare parts" on some carbs which can cause confusion for some.)
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Old 05-14-2014, 08:45 PM   #16
howie123
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Default Re: Need help with persistent problem.

No air cleaner so that's out. I'm using the Marvel carburetor so it only has the one removable jet. I've thought about the valves but just turning the engine by hand feels like good compression on all four cylinders and all four spark plugs have the same soft black sooty coating. I will say that the engine seems to run smoother with the Marvel carburetor in spite of this persistent problem.
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Old 05-14-2014, 09:21 PM   #17
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Default Re: Need help with persistent problem.

hello
i don't know what the pressure should be for the carb but maybe it is starving for fuel when incressing speed try going 1.5 psi or 1.25 and see how it drives just a thought may not help
kevin
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Old 05-14-2014, 09:47 PM   #18
Purdy Swoft
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Default Re: Need help with persistent problem.

I once had a problem with a distributor that caused a similiar problem. The problem was that the plug wire-strip was touching the distributor cap body on number three cylinder. Where the wire-strip layed over the distributor cap had melted a small spot and was causing number three to cross fire with number four. The car cranked and ran good untill speed increased,. When speed increased the spark heat increased and the number three would begin to cross fire. I finally discovered the problem at night with the hood up and reving the engine, I saw sparks under the number three plug wire strip. I changed caps and used longer plug wire-strips and the problem solved. In fine tuning I would want the air gaps to be equal. I prefer a gap of .035. It sounds that a new rotor and upper cap couldn't hurt. When the carbon contact in the cap wears flat, it doesn't hold good contact with the rotor.

The carburetor may have a partial blockage that would require running richer on the gav adjustment. I would rev it up pretty good and almost choke it down with my hand. I would do this a few times and see if it helped free up the carb. I once had a rig with the gas tank mounted a bit low for some steep hills. I installed an electric fuel pump and hooked it up to a toggle switch. I could reach over and flip the switch when I needed it for a steep hill. The gas would flow through the pump even when the switch was turned off. I didn't use a pressure regulator and run this setup with a Tillotson model X carb

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Old 05-15-2014, 04:11 PM   #19
howie123
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Default Re: Need help with persistent problem.

Thanks for all the suggestions. I will just make a systematic list and start checking things out. I will report back if I figure it out. It may not be real soon as car show season is just starting. Have to get it cleaned up for the first one on Sunday. Fortunately this problem doesn't keep me from driving it. Thanks again. Howie
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