Mechanical Fuel Pump

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datsun69
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Mechanical Fuel Pump

Post by datsun69 »

Bought a new mechanical fuel pump but the fuel is still having a hard time flowing in the carbs? What could be the problem? What is the correct size of the hose? I was told that mechanical fuel pump works only when the engine is cranked. I experience hard starting when the engine in cold.

BTW i replaced the spark plugs with bosch platinum since the spark plugs were wet. What could be the cause of wet spark plugs? Just replaced the old ones 6 moths age. Is there too much fuel entering the engine?

Thanks

Arwin
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bakerjf
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Re: Mechanical Fuel Pump

Post by bakerjf »

As to the fuel flow - Did you check the two filter screens that are just upstream of the bowls? Also, make sure all the connections from the pump back to the tank are tight. But, as a initial matter...why do you think there is a fuel problem in the first place? Hard cold starts aren't all that uncommon, and there a bunch of more likely causes than lack of fuel flow to the carbs.
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datsun69
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Re: Mechanical Fuel Pump

Post by datsun69 »

I removed the hose from the fuel pump to the carb. There was no fuel flowing out of the hose. It only started when I poured gasoline directly to the carb valves. Once it started fuel starts to flow into the carb
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sunbeam590
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Re: Mechanical Fuel Pump

Post by sunbeam590 »

I find with my pump which is 8 yrs old that if I work on the solexs it wont pull the fuel up the first start, so I wrap a cloth around my air duster and pump a bit of air into my fuel tank, she fires up straight away after that and no more trouble till I work on the carbs etc again...
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Re: Mechanical Fuel Pump

Post by devo »

Where did you find this New pump, and do you have a pic of it. I bought one that was supposed to fit off of eBay, and found that it din't work at all.
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Re: Mechanical Fuel Pump

Post by msampsel »

Maybe this is a little off topic ..

One of the great reasons for the electric pump is easier starts when the car has been sitting.

Switch on the pump and watch the fuel fill into a see through filter. Pull the choke and crank over once or
twice and it starts.

With a new mechanical pump and an un-primed system or one runs out of gas, I recall needing to blow
on the fuel inlet to get the system to prime as it is cranked (two person job).

Never had to do this with an electric pump though.
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datsun69
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Re: Mechanical Fuel Pump

Post by datsun69 »

Bought the fuel pump from a Nissan dealer here in Manila. They still sell some parts for old datsun cars.
I think the pump works since once the engine cranks, fuel flows into the carbs. The problem is when the car is stalled for a couple of days. Is this common for owners of mechanical pumps? How can i solve this without converting to an electrical fuel pump?

Would this be related why my spark plugs are wet?
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Re: Mechanical Fuel Pump

Post by C.Costine »

datsun69 wrote:Bought the fuel pump from a Nissan dealer here in Manila. They still sell some parts for old datsun cars.
I think the pump works since once the engine cranks, fuel flows into the carbs. The problem is when the car is stalled for a couple of days. Is this common for owners of mechanical pumps? How can i solve this without converting to an electrical fuel pump?

Would this be related why my spark plugs are wet?
Wet plugs are a strong indication that your fuel system is working properly but your ignition is not. Replacing the plugs will not keep them from getting wet.
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Ed Mason
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Re: Mechanical Fuel Pump

Post by Ed Mason »

The mechanical fuel pump relies on an air tight seal from the fuel pump inlet back to the tank. If there is a leak anywhere from the tank to the pump then the pump won't prime and pump to the carbs. If you are running the original glass fuel filter check the bowl gasket and the bowl cast base. These cast bases can become warped from over tightening the glass bowl. The fastest way to check the fuel pump is to rig up a piece of fuel hose from a bottle of gasoline directly to the fuel pump inlet and another hose from the fuel pump outlet to another bottle to visually see fuel flow. This removes any variables. If the fuel pump works you have a vacuum leak. If it doesn't the fuel pump is the culprit.
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Roman
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Re: Mechanical Fuel Pump

Post by Roman »

^^^^
Yes, I have experienced this.
The tendency with the glass filter is for people to tighten it more instead of replacing the seal. Then it distorts more and seals less.

I've straightened them in a vise, put in a new seal, and then it works.
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Re: Mechanical Fuel Pump

Post by Linda »

Thread revival:
My rebuilt pump is not pulling any gas, none flowing into the filter. I replaced all hoses. Then shot a bit of air back to the tank. I read above that blowing air from tank to filter to pump can improve priming.
Before I would just prime the fuel bowls and it would run and prime the pump.
I did prime the carbs and it runs but still no fuel.
Just a couple of shots of air in a rag wrapped around the fuel filler neck should work?
Also I assume no leaks at the tank inspection / fuel guage cover.
I do have a vented cap.
Thx
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Re: Mechanical Fuel Pump

Post by nismou20 »

Linda, Is there gas between tank and fuel pump inlet when pulling hose off? Is the pump arm engaging the cam eccentric? Was car running with another pump before this? Can you pull vacuum from outlet side on pump?
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Linda
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Re: Mechanical Fuel Pump

Post by Linda »

Tom,
Car was running, pump was starting to go out so I removed it and rebuilt with valves and diaphram. Pump arm moves. I was careful to get the arm on the cam. Spacer block intact w gasket.
Car will run with priming the carbs, but dies when fuel runs out.
I used some air to clear input hose from filter back to tank. I think I need to use some air from tank to filter to help it prime. But usually carb priming gets everything going but not this time...
Perhaps there is a vacuum leak somewhere along the input fuel line, or pump screws. Will try to check.
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Re: Mechanical Fuel Pump

Post by cktrap »

Check valves reversed?
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Linda
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Re: Mechanical Fuel Pump

Post by Linda »

Good question. Input valve is flat, output is raised, viewed from the pump fuel bowls. I think that is right.
I can switch the hose easily.
Valves are correct.
Last edited by Linda on Sun Feb 04, 2018 5:07 pm, edited 1 time in total.