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Old 05-20-2020, 12:18 PM   #1
gtoger
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Default Weber carb conversion power loss & backfire

Looking for some advice from someone experienced with the Weber conversion.



I've recently acquired a 1930 Town Sedan. The previous owner was unfortunately sick for a while and the car sat for a couple years before coming home with me. He made some upgrades:
- 12V conversion

- Weber 2 barrel
- Performance intake
- Exhaust header
- Fuel pump to feed Weber
- Pertronix distributor conversion (w/ appropriate coil & wires)
- Mitchel overdrive
- shock suspension
- air conditioning (great for Texas!)



When I first got it, the performance was less than my 31 with standard induction. I could smell the bad gas. I've since run a couple tanks of fresh gas and Berrymans and replaced the fuel filter. I've disassembled the top half of the carb and cleaned out the orifices. Now the car makes much better power at low end and during acceleration.



One problem persists, however, which is if I'm cruising in 3rd at 45mph or better, the car will cruise along for a while then lose power and cough & backfire. If I let off the gas and coast for a couple seconds it will come back and be OK again. Until it isn't. Rinse, repeat.



Doesn't seem to be a rhyme or reason to the amount of time it takes to cough and sputter. I think it feels like fuel starvation.



I've adjusted the mixture and it runs great until it doesn't. I'll replace the fuel pump after lunch and try again.



Maybe someone else has experience with a similar setup. Any thoughts / words of wisdom?
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Old 05-20-2020, 12:58 PM   #2
30 Closed Cab PU
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Default Re: Weber carb conversion power loss & backfire

1st try running with the gas cap loose. If issue disappears you have a clogged vent hole in the gas cap.


Have you removed the fuel shut off and flushed the tank? Could be a partially clogged aftermarket pencil filter in the inlet of the Fuel shutoff, or trash (rust/Grit) in the gas tank.


Check that the gas line into your carb is not screwed into the carb too far. The end of the gas line can push into too far blocking gas flow. Check the carb gas filter. Check that the Sediment bowl screen is not full of varnish by back lighting it with a flashlight - a little carb clean or just replace the screen.


Try using a small gas tank supported from the Radiator support rod into the carb, see if issue is still there, if it is try a different carb or clean yours, make sure the float level is set properly.


If after all this and still have the issue possible the issue is electrical.
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Old 05-20-2020, 01:01 PM   #3
BRENT in 10-uh-C
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Default Re: Weber carb conversion power loss & backfire

My first instinct is to suggest you have a fuel pump that either is failing, -or about to fail. Begin by checking to ensure you have a good flow of fuel to the pump. If good, you might want to check fuel pressure and fuel volume from the pump. Let us know what you find...
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Old 05-20-2020, 01:08 PM   #4
tinkirk
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Default Re: Weber carb conversion power loss & backfire

Night want to check float level
And make sure fuel pump is working
I found out that float level makes a big difference


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Old 05-20-2020, 01:19 PM   #5
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Default Re: Weber carb conversion power loss & backfire

Quote:
Originally Posted by BRENT in 10-uh-C View Post
My first instinct is to suggest you have a fuel pump that either is failing, -or about to fail. Begin by checking to ensure you have a good flow of fuel to the pump. If good, you might want to check fuel pressure and fuel volume from the pump. Let us know what you find...
What Brent said!
This is exactly what happened to me. Fuel pump was cutting in and out, then finally quit working.
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Old 05-20-2020, 01:34 PM   #6
gtoger
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Default Re: Weber carb conversion power loss & backfire

Thanks for the words of advice. I'll start checking things off the list and see what we can do!
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Old 05-20-2020, 01:45 PM   #7
Kurt in NJ
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Default Re: Weber carb conversion power loss & backfire

Filter??? , debris in fuel tank???, tank venting??? Along with other posts--- similar carburetor used in 70s cars also can have screens above float valve or between fuel inlet and float valve
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Old 05-20-2020, 02:14 PM   #8
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Default Re: Weber carb conversion power loss & backfire

The previous owner may or may not have installed a "pencil " or "Micro-Filter" at the bottom of the gas tank which would be located where the fuel shutoff screws into the gas tank. It is a press fit and filters all the contaminants found inside the 90 year old tank. My car did exactly what yours is doing at highway speeds. Drain the tank and remove the gas iine and pencil filter and fill and flush the tank with a gallon of fresh gas and install a new pencil filter. I was amazed how much sediment came out of my tank and it was clogging the filter causing the lean condition.At least this is a starting point to insure that the tank/filter are clean.
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Old 05-20-2020, 04:30 PM   #9
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Default Re: Weber carb conversion power loss & backfire

I would agree with earlier post suggesting the fuel pump or gas filter could be your problem. But seeing how your issue is with a Weber carburetor at higher speeds I might suggest checking the accelerator pump on the Weber. It is a diaphragm that can dry out and get hard and crack. It is found on the near the fuel connection to the carburetor and is connected with four screws. The carburetor normally used on Model A's is the Weber 32/36. I would suggest you purchase a new one before disassembly. The are not very expensive and can be found at most auto parts stores.
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Old 05-20-2020, 04:43 PM   #10
gtoger
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Default SOLVED: Weber carb conversion power loss & backfire

SOLVED!



Thank you, everyone for the helpful suggestions.



The issue turned out to be a weak fuel pump. It made plenty of noise and if I released the line from the carb it would pump fuel into a pony tank. I have no way to gauge the PSI or flow rate but the old pump (when connected to the carb) would trickle gas through the filter, similar to what I would expect on a gravity-feed setup. With the fuel line disconnected from the carb the gas would "pulse" through the filter. The new pump will SWIRL and WHOOSH gas through the filter. You can tell when the floats in the carb bowl open the valve.



I just took it on a quick test ride and got her up past 60 MPH for some extended runs (my shop is right by the highway). No shudder, coughing or loss of power. I've got a ~25 mile trip to take this afternoon but I'm pretty confident it's going to go smoothly.



This particular installation does not have the "pencil filter", but does have an inline filter between the glass sediment bowl and the fuel pump. I changed the filter on the voyage home (50+ miles from the previous owner's barn to my house) and have put about 200 miles on the car in total so far and monitor no new sediment or dirt in the filter or glass bowl.


I used a small piece of wire cut from a wire brush to clean out the orifices on the carb then blew it out with cleaner. Since doing that and then re-tuning the mixture the car has plenty of low-end grunt and seems to be receptive to high-speed cruise.


Thanks again, everyone!
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Old 05-20-2020, 05:01 PM   #11
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Default Re: Weber carb conversion power loss & backfire

If the inline filter is for a mower or similar type filter, at times may have issues, better to go with an automotive higher flow filter.
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Old 05-20-2020, 05:05 PM   #12
30 Closed Cab PU
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Default Re: Weber carb conversion power loss & backfire

A main reason for the pencil filter is to keep rust/grit out of the shutoff valve. Rust/grit is a main cause of bad fuel shutoffs since the internals of the valve can get scored.
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Old 05-20-2020, 08:33 PM   #13
gtoger
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Default Re: Weber carb conversion power loss & backfire

Just as a follow up, I drove home and it was like a different car. I was keeping up with freeway traffic and the car didn't sputter or skip a beat.


'll have another look for some sort of filter in front of the shutoff. The shutoff valve on this car is a different style petcock than what I'm accustomed to seeing. It's been reworked and relocated to accommodate the air conditioner. I didn't see any filter but then again I haven't really crawled around underneath the dash. I'm sort of knocking around getting the big items crossed off the list.


The starter button has also been relocated to the dash, so there's a lot to get used to.
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